FEMA 273 NEHRP Guidelines for the Seismic Rehabilitation of ...

BSSCSeismicRehabilitationProjectNEHRP GUIDELINES FOR THESEISMIC REHABILITATION OF BUILDINGS(FEMA Publication 273)Prepared for theBUILDING SEISMIC SAFETY COUNCILWashington, D.C.By theAPPLIED TECHNOLOGY COUNCIL (ATC-33 Project)Redwood City, CaliforniaWith funding byFEDERAL EMERGENCY MANAGEMENT AGENCYWashington, D.C.October 1997Washington, D.C.

PROJECT OVERSIGHTCOMMITTEEEugene Zeller, ChairmanThomas G. Atkinson, ATCGerald Jones, BSSCChristopher Rojahn, ATCPaul Seaburg, ASCEAshvin Shah, ASCEJames R. Smith, BSSCBUILDING SEISMICSAFETY COUNCILPROJECT MANAGERJames R. SmithDEPUTY PROJECT MANAGERThomas HollenbachTECHNICAL WRITER-EDITORClaret HeiderSEISMIC REHABILITATIONADVISORY PANELGerald Jones, ChairmanDavid AllenJohn BattlesDavid BreiholzMichael CaldwellGregory L. F. ChiuTerry DooleySusan DowtySteven J. EderS. K. GhoshBarry J. GoodnoCharles G. GutberletWarner HoweHoward KunreutherHarry W. MartinRobert McCluerMargaret Pepin-DonatWilliam PetakHoward SimpsonWilliam StewartJames ThomasL. Thomas TobinPROJECT COMMITTEEWarner Howe, ChairmanGerald H. JonesAllan R. PorushF. Robert PreeceWilliam W. StewartSOCIETAL ISSUESRobert A. OlsonFEDERAL EMERGENCYMANAGEMENTAGENCYPROJECT OFFICERUgo MorelliTECHNICAL ADVISORDiana ToddPARTICIPANTSAPPLIED TECHNOLOGYCOUNCILPRINCIPAL INVESTIGATORChristopher RojahnPROJECT DIRECTORDaniel ShapiroCO-PROJECT DIRECTORLawrence D. ReaveleySENIOR TECHNICAL ADVISORWilliam T. HolmesTECHNICAL ADVISORJack P. MoehleATC BOARDREPRESENTATIVEThomas G. AtkinsonGENERAL REQUIREMENTSRonald O. Hamburger, Team LeaderSigmund A. FreemanPeter Gergely (deceased)Richard A. ParmeleeAllan R. PorushMODELING AND ANALYSISMike Mehrain, Team LeaderRonald P. GallagherHelmut KrawinklerGuy J. P. NordensonMaurice S. PowerAndrew S. WhittakerGEOTECHNICAL &FOUNDATIONSJeffrey R. Keaton, Team LeaderCraig D. ComartinPaul W. GrantGeoffrey R. MartinMaurice S. PowerCONCRETEJack P. Moehle, Co-Team LeaderLawrence D. Reaveley, Co-TeamLeaderJames E. CarpenterJacob GrossmanPaul A. MurrayJoseph P. NicolettiKent B. SoelbergJames K. WightMASONRYDaniel P. Abrams, Team LeaderSamy A. AdhamGregory R. KingsleyOnder KustuJohn C. TheissSTEELDouglas A. Foutch, Team LeaderNavin R. AminJames O. MalleyCharles W. RoederThomas Z. ScarangelloWOODJohn M. Coil, Team LeaderJeffery T. MillerRobin ShepherdWilliam B. VaughnNEW TECHNOLOGIESCharles A. Kircher, Team LeaderMichael C. ConstantinouAndrew S. WhittakerNONSTRUCTURALChristopher Arnold, Team LeaderRichard L. HessFrank E. McClureTodd W. PerbixSIMPLIFIED REHABILITATIONChris D. Poland, Team LeaderLeo E. ArgirisThomas F. HeauslerEvan ReisTony TschanzQUALIFICATION OFIN-PLACE MATERIALSCharles J. Hookham, Lead ConsultantRichard Atkinson (deceased)Ross EsfandiariLANGUAGE & FORMATJames R. HarrisREPORT PREPARATIONRoger E. Scholl (deceased),Lead ConsultantRobert K. ReithermanA. Gerald Brady, Copy EditorPatty Christofferson, CoordinatorPeter N. Mork, IllustrationsAMERICAN SOCIETY OFCIVIL ENGINEERSREHABILITATION STEERINGCOMMITTEEVitelmo V. BerteroPaul SeaburgRoland L. SharpeJon S. TrawClarkson W. PinkhamWilliam J. HallUSERS WORKSHOPSTom McLane, ManagerDebbie Smith, CoordinatorRESEARCH SYNTHESISJames O. JirsaSPECIAL ISSUESMelvyn Green

In MemoriamThe Building Seismic Safety Council, theApplied Technology Council, the AmericanSociety of Civil Engineers, and the FederalEmergency Management Agency wish toacknowledge the significant contribution to theGuidelines and to the overall field ofearthquake engineering of the participants inthe project who did not live to see this effortcompleted:Richard AtkinsonPeter GergelyRoger SchollThe built environment has benefited greatlyfrom their work.

ForewordThe volume you are now holding in your hands, theNEHRP Guidelines for the Seismic Rehabilitation ofBuildings, and its companion Commentary volume, arethe culminating manifestation of over 13 years of effort.They contain systematic guidance enabling designprofessionals to formulate effective and reliablerehabilitation approaches that will limit the expectedearthquake damage to a specified range for a specifiedlevel of ground shaking. This kind of guidanceapplicable to all types of existing buildings and in allparts of the country has never existed before.Since 1984, when the Federal Emergency ManagementAgency (FEMA) first began a program to address therisk posed by seismically unsafe existing buildings, thecreation of these Guidelines has been the principaltarget of FEMA’s efforts. Prior preparatory steps,however, were much needed, as was noted in the 1985Action Plan developed at FEMA’s request by the ABEJoint Venture. These included the development of astandard methodology for identifying at-risk buildingsquickly or in depth, a compendium of effectiverehabilitation techniques, and an identification ofsocietal implications of rehabilitation.By 1990, this technical platform had been essentiallycompleted, and work could begin on these Guidelines.The $8 million, seven-year project required the variedtalents of over 100 engineers, researchers and writers,smoothly orchestrated by the Building Seismic SafetyCouncil (BSSC), overall manager of the project; theApplied Technology Council (ATC); and the AmericanSociety of Civil Engineers (ASCE). Hundreds moredonated their knowledge and time to the project byreviewing draft documents at various stages ofdevelopment and providing comments, criticisms, andsuggestions for improvements. Additional refinementsand improvements resulted from the consensus reviewof the Guidelines document and its companionCommentary through the balloting process of the BSSCduring the last year of the effort.No one who worked on this project in any capacity,whether volunteer, paid consultant or staff, receivedmonetary compensation commensurate with his or herefforts. The dedication of all was truly outstanding. Itseemed that everyone involved recognized themagnitude of the step forward that was being taken inthe progress toward greater seismic safety of ourcommunities, and gave his or her utmost. FEMA andthe FEMA Project Officer personally warmly andsincerely thank everyone who participated in thisendeavor. Simple thanks from FEMA in a Foreword,however, can never reward these individualsadequately. The fervent hope is that, perhaps, havingthe Guidelines used extensively now and improved byfuture generations will be the reward that they so justlyand richly deserve.The Federal Emergency Management AgencyFEMA 273 Seismic Rehabilitation Guidelines vii

viii Seismic Rehabilitation Guidelines FEMA 273

PrefaceIn August 1991, the National Institute of BuildingSciences (NIBS) entered into a cooperative agreementwith the Federal Emergency Management Agency(FEMA) for a comprehensive seven-year programleading to the development of a set of nationallyapplicable guidelines for the seismic rehabilitation ofexisting buildings. Under this agreement, the BuildingSeismic Safety Council (BSSC) served as programmanager with the American Society of Civil Engineers(ASCE) and the Applied Technology Council (ATC)working as subcontractors. Initially, FEMA providedfunding for a program definition activity designed togenerate the detailed work plan for the overall program.The work plan was completed in April 1992 and inSeptember FEMA contracted with NIBS for theremainder of the effort.The major objectives of the project were to develop aset of technically sound, nationally applicableguidelines (with commentary) for the seismicrehabilitation of buildings; develop building communityconsensus regarding the guidelines; and develop thebasis of a plan for stimulating widespread acceptanceand application of the guidelines. The guidelinesdocuments produced as a result of this project areexpected to serve as a primary resource on the seismicrehabilitation of buildings for the use of designprofessionals, educators, model code and standardsorganizations, and state and local building regulatorypersonnel.As noted above, the project work involved the ASCEand ATC as subcontractors as well as groups ofvolunteer experts and paid consultants. It was structuredto ensure that the technical guidelines writing effortbenefited from a broad section of considerations: theresults of completed and ongoing technical efforts andresearch activities; societal issues; public policyconcerns; the recommendations presented in an earlierFEMA-funded report on issues identification andresolution; cost data on application of rehabilitationprocedures; reactions of potential users; and consensusreview by a broad spectrum of building communityinterests. A special effort also was made to use theresults of the latest relevant research.While overall management has been the responsibilityof the BSSC, responsibility for conduct of the specificproject tasks is shared by the BSSC with ASCE andATC. Specific BSSC tasks were completed under theguidance of a BSSC Project Committee. To ensureproject continuity and direction, a Project OversightCommittee (POC) was responsible to the BSSC Boardof Direction for accomplishment of the projectobjectives and the conduct of project tasks. Further, aSeismic Rehabilitation Advisory Panel reviewed projectproducts as they developed and advised the POC on theapproach being taken, problems arising or anticipated,and progress made.Three user workshops were held during the course ofthe project to expose the project and various drafts ofthe Guidelines documents to review by potential usersof the ultimate product. The two earlier workshopsprovided for review of the overall project structure andfor detailed review of the 50-percent-complete draft.The last workshop was held in December 1995 whenthe Guidelines documents were 75 percent complete.Participants in this workshop also had the opportunityto attend a tutorial on application of the guidelines andto comment on all project work done to date.Following the third user workshop, written and oralcomments on the 75-percent-complete draft of thedocuments received from the workshop participants andother reviewers were addressed by the authors andincorporated into a pre-ballot draft of the Guidelinesand Commentary. POC members were sent a reviewcopy of the 100-percent-complete draft in August 1996and met to formulate a recommendation to the BSSCBoard of Direction concerning balloting of thedocuments. Essentially, the POC recommended that theBoard accept the documents for consensus balloting bythe BSSC member organization. The Board, havingreceived this recommendation in late August, votedunanimously to proceed with the balloting.The balloting of the Guidelines and Commentaryoccurred between October 15 and December 20, 1996,and a ballot symposium for the voting representatives ofBSSC member organizations was held in Novemberduring the ballot period. Member organization votingrepresentatives were asked to vote on each majorsubsection of the Guidelines document and on eachchapter of the Commentary. As required by BSSCprocedures, the ballot provided for four responses:FEMA 273 Seismic Rehabilitation Guidelines ix

“yes,” “yes with reservations,” “no,” and “abstain.” All“yes with reservations” and “no” votes were to beaccompanied by an explanation of the reasons for thevote and the “no” votes were to be accompanied byspecific suggestions for change if those changes wouldchange the negative vote to an affirmative.Although all sections of the Guidelines andCommentary documents were approved in the balloting,the comments and explanations received with “yes withreservations” and “no” votes were compiled by theBSSC for delivery to ATC for review and resolution.The ATC Senior Technical Committee reviewed thesecomments in detail and commissioned members of thetechnical teams to develop detailed responses and toformulate any needed proposals for change reflectingthe comments. This effort resulted in 48 proposals forchange to be submitted to the BSSC memberorganizations for a second ballot. In April 1997, theATC presented its recommendations to the ProjectOversight Committee, which approved them forforwarding to the BSSC Board. The BSSC Boardsubsequently gave tentative approval to the reballotingpending a mail vote on the entire second ballot package.This was done and the reballoting was officiallyapproved by the Board. The second ballot package wasmailed to BSSC member organizations on June 10 withcompleted ballots due by July 28.All the second ballot proposals passed the ballot;however, as with the first ballot results, commentssubmitted with ballots were compiled by the BSSC forreview by the ATC Senior Technical Committee. Thiseffort resulted in a number of editorial changes and sixadditional technical changes being proposed by theATC. On September 3, the ATC presented itsrecommendations for change to the Project OversightCommittee that, after considerable discussion, deemedthe proposed changes to be either editorial or ofinsufficient substance to warrant another ballot.Meeting on September 4, the BSSC Board received therecommendations of the POC, accepted them, andapproved preparation of the final documents fortransmittal to the Federal Emergency ManagementAgency. This was done on September 30, 1997.It should be noted by those using this document thatrecommendations resulting from the concept work ofthe BSSC Project Committee have resulted in initiationof a case studies project that will involve thedevelopment of seismic rehabilitation designs for atleast 40 federal buildings selected from an inventory ofbuildings determined to be seismically deficient underthe implementation program of Executive Order 12941and determined to be considered “typical of existingstructures located throughout the nation.” The casestudies project is structured to:• Test the usability of the NEHRP Guidelines for theSeismic Rehabilitation of Buildings in authenticapplications in order to determine the extent towhich practicing design engineers and architectsfind the Guidelines documents themselves and thestructural analysis procedures and acceptancecriteria included to be presented in understandablelanguage and in a clear, logical fashion that permitsvalid engineering determinations to be made, and toevaluate the ease of transition from currentengineering practices to the new concepts presentedin the Guidelines.• Assess the technical adequacy of the Guidelinesdesign and analysis procedures. Determine ifapplication of the procedures results (in thejudgment of the designer) in rational designs ofbuilding components for corrective rehabilitationmeasures. Assess whether these designs adequatelymeet the selected performance levels whencompared to existing procedures and in light of theknowledge and experience of the designer. Evaluatewhether the Guidelines methods provide a betterfundamental understanding of expected seismicperformance than do existing procedures.• Assess whether the Guidelines acceptance criteriaare properly calibrated to result in componentdesigns that provide permissible values of such keyfactors as drift, component strength demand, andinelastic deformation at selected performance levels.• Develop empirical data on the costs of rehabilitationdesign and construction to meet the Guidelines“basic safety objective” as well as the higherperformance levels included. Assess whether theanticipated higher costs of advanced engineeringanalysis result in worthwhile savings compared tothe cost of constructing more conservative designsolutions necessary with a less systematicengineering effort.x Seismic Rehabilitation Guidelines FEMA 273

• Compare the acceptance criteria of the Guidelineswith the prevailing seismic design requirements fornew buildings in the building location to determinewhether requirements for achieving the Guidelines“basic safety objective” are equivalent to or more orless stringent than those expected of new buildings.Feedback from those using the Guidelines outside thiscase studies project is strongly encouraged. Further,the curriculum for a series of education/trainingseminars on the Guidelines is being developed and anumber of seminars are scheduled for conduct in early1998. Those who wish to provide feedback or with adesire for information concerning the seminars shoulddirect their correspondence to: BSSC, 1090 VermontAvenue, N.W., Suite 700, Washington, D.C. 20005;phone 202-289-7800; fax 202-289-1092; e-mailbssc@nibs.org. Copies of the Guidelines andCommentary can be obtained by phone from the FEMADistribution Facility at 1-800-480-2520.The BSSC Board of Direction gratefully acknowledgesthe contribution of all the ATC and ASCE participantsin the Guidelines development project as well as thoseof the BSSC Seismic Rehabilitation Advisory Panel, theBSSC Project Committee, and the User Workshopparticipants. The Board also wishes to thank UgoMorelli, FEMA Project Officer, and Diana Todd,FEMA Technical Advisor, for their valuable input andsupport.Eugene ZellerChairman, BSSC Board of DirectionFEMA 273 Seismic Rehabilitation Guidelines xi

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Table of ContentsForeword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11.2 Significant New Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11.2.1 Seismic Performance Levels and Rehabilitation Objectives . . . . . . . . . . . . . . . .1-11.2.2 Simplified and Systematic Rehabilitation Methods . . . . . . . . . . . . . . . . . . . . . . .1-31.2.3 Varying Methods of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-31.2.4 Quantitative Specifications of Component Behavior . . . . . . . . . . . . . . . . . . . . . .1-31.2.5 Procedures for Incorporating New Information and Technologies into Rehabilitation1-41.3 Scope, Contents, and Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-41.3.1 Buildings and Loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-41.3.2 Activities and Policies Associated with Seismic Rehabilitation . . . . . . . . . . . . . .1-41.3.3 Seismic Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-51.3.4 Technical Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-61.4 Relationship to Other Documents and Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-61.5 Use of the Guidelines in the Seismic Rehabilitation Process . . . . . . . . . . . . . . . . . . . . . . . .1-71.5.1 Initial Considerations for Individual Buildings . . . . . . . . . . . . . . . . . . . . . . . . . .1-81.5.2 Initial Risk Mitigation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-101.5.3 Simplified Rehabilitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-101.5.4 Systematic Rehabilitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-111.5.5 Verification and Economic Acceptance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-111.5.6 Implementation of the Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-111.6 Use of the Guidelines for Local or Directed Risk Mitigation Programs . . . . . . . . . . . . . . .1-121.6.1 Initial Considerations for Mitigation Programs . . . . . . . . . . . . . . . . . . . . . . . . .1-121.6.2 Use in Passive Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-151.6.3 Use in Active or Mandated Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-151.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-162. General Requirements (Simplified and SystematicRehabilitation) . . . . . . . . . . . . . . . . . . . . . .2-12.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-12.2 Basic Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-12.3 Design Basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-22.4 Rehabilitation Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-42.4.1 Basic Safety Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-52.4.2 Enhanced Rehabilitation Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-62.4.3 Limited Rehabilitation Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-62.5 Performance Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-72.5.1 Structural Performance Levels and Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-72.5.2 Nonstructural Performance Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-82.5.3 Building Performance Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-10FEMA 273 Seismic Rehabilitation Guidelines xiii

2.6 Seismic Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-182.6.1 General Ground Shaking Hazard Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-192.6.2 Site-Specific Ground Shaking Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-232.6.3 Seismicity Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-242.6.4 Other Seismic Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-242.7 As-Built Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-252.7.1 Building Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-252.7.2 Component Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-252.7.3 Site Characterization and Geotechnical Information . . . . . . . . . . . . . . . . . . . . . .2-272.7.4 Adjacent Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-272.8 Rehabilitation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-282.8.1 Simplified Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-282.8.2 Systematic Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-282.9 Analysis Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-282.9.1 Linear Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-292.9.2 Nonlinear Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-312.9.3 Alternative Rational Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-312.9.4 Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-322.10 Rehabilitation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-352.10.1 Local Modification of Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-352.10.2 Removal or Lessening of Existing Irregularities and Discontinuities . . . . . . . . .2-352.10.3 Global Structural Stiffening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-352.10.4 Global Structural Strengthening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-362.10.5 Mass Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-362.10.6 Seismic Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-362.10.7 Supplemental Energy Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-362.11 General Analysis and Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-362.11.1 Directional Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-372.11.2 P-∆ Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-372.11.3 Torsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-372.11.4 Overturning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-372.11.5 Continuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-392.11.6 Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-392.11.7 Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-402.11.8 Nonstructural Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-402.11.9 Structures Sharing Common Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-402.11.10 Building Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-402.11.11 Vertical Earthquake Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-412.12 Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-412.12.1 Construction Quality Assurance Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-412.12.2 Construction Quality Assurance Requirements . . . . . . . . . . . . . . . . . . . . . . . . . .2-422.12.3 Regulatory Agency Responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-422.13 Alternative Materials and Methods of Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-432.13.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-432.13.2 Data Reduction and Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-442.13.3 Design Parameters and Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-442.14 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-46xiv Seismic Rehabilitation Guidelines FEMA 273

4.6.3 Piers and Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-184.7 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-194.8 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-194.9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-215. Steel and Cast Iron (Systematic Rehabilitation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-15.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-15.2 Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-15.3 Material Properties and Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-15.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-15.3.2 Properties of In-Place Materials and Components . . . . . . . . . . . . . . . . . . . . . . . .5-25.3.3 Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-45.3.4 Knowledge (κ) Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-85.4 Steel Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-95.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-95.4.2 Fully Restrained Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-95.4.3 Partially Restrained Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-185.5 Steel Braced Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-255.5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-255.5.2 Concentric Braced Frames (CBF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-255.5.3 Eccentric Braced Frames (EBF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-295.6 Steel Plate Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-315.6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-315.6.2 Stiffness for Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-315.6.3 Strength and Deformation Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . .5-325.6.4 Rehabilitation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-325.7 Steel Frames with Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-325.8 Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-325.8.1 Bare Metal Deck Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-325.8.2 Metal Deck Diaphragms with Structural Concrete Topping . . . . . . . . . . . . . . . .5-345.8.3 Metal Deck Diaphragms with Nonstructural Concrete Topping . . . . . . . . . . . . .5-355.8.4 Horizontal Steel Bracing (Steel Truss Diaphragms) . . . . . . . . . . . . . . . . . . . . . .5-365.8.5 Archaic Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-375.8.6 Chord and Collector Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-385.9 Steel Pile Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-395.9.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-395.9.2 Stiffness for Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-395.9.3 Strength and Deformation Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . .5-395.9.4 Rehabilitation Measures for Steel Pile Foundations . . . . . . . . . . . . . . . . . . . . . .5-395.10 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-395.11 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-415.12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-436. Concrete (Systematic Rehabilitation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-16.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-16.2 Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-16.3 Material Properties and Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-2xvi Seismic Rehabilitation Guidelines FEMA 273

6.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26.3.2 Properties of In-Place Materials and Components . . . . . . . . . . . . . . . . . . . . . . . . 6-26.3.3 Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-86.3.4 Knowledge (κ ) Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-106.3.5 Rehabilitation Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-106.3.6 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-116.4 General Assumptions and Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-116.4.1 Modeling and Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-116.4.2 Design Strengths and Deformabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-136.4.3 Flexure and Axial Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-146.4.4 Shear and Torsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-146.4.5 Development and Splices of Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-156.4.6 Connections to Existing Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-166.5 Concrete Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-166.5.1 Types of Concrete Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-166.5.2 Reinforced Concrete Beam-Column Moment Frames . . . . . . . . . . . . . . . . . . . . 6-176.5.3 Post-Tensioned Concrete Beam-Column Moment Frames . . . . . . . . . . . . . . . . 6-266.5.4 Slab-Column Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-276.6 Precast Concrete Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-316.6.1 Types of Precast Concrete Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-316.6.2 Precast Concrete Frames that Emulate Cast-in-Place Moment Frames . . . . . . . 6-326.6.3 Precast Concrete Beam-Column Moment Frames other thanEmulated Cast-in-Place Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-326.6.4 Precast Concrete Frames Not Expected to Resist Lateral Loads Directly . . . . . 6-336.7 Concrete Frames with Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-336.7.1 Types of Concrete Frames with Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-336.7.2 Concrete Frames with Masonry Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-346.7.3 Concrete Frames with Concrete Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-376.8 Concrete Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-396.8.1 Types of Concrete Shear Walls and Associated Components . . . . . . . . . . . . . . 6-396.8.2 Reinforced Concrete Shear Walls, Wall Segments, Coupling Beams, andRC Columns Supporting Discontinuous Shear Walls . . . . . . . . . . . . . . . . . . . . 6-406.9 Precast Concrete Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-486.9.1 Types of Precast Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-486.9.2 Precast Concrete Shear Walls and Wall Segments . . . . . . . . . . . . . . . . . . . . . . . 6-496.10 Concrete Braced Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-516.10.1 Types of Concrete Braced Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-516.10.2 General Considerations in Analysis and Modeling . . . . . . . . . . . . . . . . . . . . . . . 6-516.10.3 Stiffness for Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-526.10.4 Design Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-526.10.5 Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-526.10.6 Rehabilitation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-536.11 Concrete Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-536.11.1 Components of Concrete Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-536.11.2 Analysis, Modeling, and Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . 6-536.11.3 Rehabilitation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-546.12 Precast Concrete Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-54FEMA 273 Seismic Rehabilitation Guidelines xvii

6.12.1 Components of Precast Concrete Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . .6-546.12.2 Analysis, Modeling, and Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . .6-546.12.3 Rehabilitation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-556.13 Concrete Foundation Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-556.13.1 Types of Concrete Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-556.13.2 Analysis of Existing Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-556.13.3 Evaluation of Existing Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-566.13.4 Rehabilitation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-566.14 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-576.15 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-576.16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-597. Masonry (SystematicRehabilitation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-17.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-17.2 Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-17.3 Material Properties and Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-27.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-27.3.2 Properties of In-Place Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-27.3.3 Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-47.3.4 Knowledge (κ) Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-57.4 Engineering Properties of Masonry Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-57.4.1 Types of Masonry Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-67.4.2 URM In-Plane Walls and Piers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-87.4.3 URM Out-of-Plane Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-107.4.4 Reinforced Masonry In-Plane Walls and Piers . . . . . . . . . . . . . . . . . . . . . . . . . .7-117.4.5 RM Out-of-Plane Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-147.5 Engineering Properties of Masonry Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-147.5.1 Types of Masonry Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-177.5.2 In-Plane Masonry Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-187.5.3 Out-of-Plane Masonry Infills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-217.6 Anchorage to Masonry Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-227.6.1 Types of Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-227.6.2 Analysis of Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-227.7 Masonry Foundation Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-227.7.1 Types of Masonry Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-227.7.2 Analysis of Existing Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-227.7.3 Rehabilitation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-237.8 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-237.9 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-247.10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-268. Wood and Light Metal Framing (Systematic Rehabilitation) . . . . . . . . . . . . . . . . . . . . . . . . . . .8-18.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-18.2 Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-18.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-18.2.2 Building Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-18.2.3 Evolution of Framing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-1xviii Seismic Rehabilitation Guidelines FEMA 273

8.3 Material Properties and Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28.3.2 Properties of In-Place Materials and Components . . . . . . . . . . . . . . . . . . . . . . . . 8-38.3.3 Condition Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-68.3.4 Knowledge (κ) Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-88.3.5 Rehabilitation Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-88.4 Wood and Light Frame Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-88.4.1 Types of Light Frame Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-98.4.2 Light Gage Metal Frame Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-128.4.3 Knee-Braced and Miscellaneous Timber Frames . . . . . . . . . . . . . . . . . . . . . . . . 8-128.4.4 Single Layer Horizontal Lumber Sheathing or Siding Shear Walls . . . . . . . . . . 8-128.4.5 Diagonal Lumber Sheathing Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-158.4.6 Vertical Wood Siding Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-188.4.7 Wood Siding over Horizontal Sheathing Shear Walls . . . . . . . . . . . . . . . . . . . . 8-188.4.8 Wood Siding over Diagonal Sheathing Shear Walls . . . . . . . . . . . . . . . . . . . . . 8-188.4.9 Structural Panel or Plywood Panel Sheathing Shear Walls . . . . . . . . . . . . . . . . 8-198.4.10 Stucco on Studs, Sheathing, or Fiberboard Shear Walls . . . . . . . . . . . . . . . . . . . 8-198.4.11 Gypsum Plaster on Wood Lath Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-208.4.12 Gypsum Plaster on Gypsum Lath Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . 8-208.4.13 Gypsum Wallboard Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-218.4.14 Gypsum Sheathing Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-218.4.15 Plaster on Metal Lath Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-218.4.16 Horizontal Lumber Sheathing with Cut-In Braces orDiagonal Blocking Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-218.4.17 Fiberboard or Particleboard Sheathing Shear Walls . . . . . . . . . . . . . . . . . . . . . . 8-228.4.18 Light Gage Metal Frame Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-228.5 Wood Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-228.5.1 Types of Wood Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-228.5.2 Single Straight Sheathed Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-248.5.3 Double Straight Sheathed Wood Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . 8-258.5.4 Single Diagonally Sheathed Wood Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . 8-268.5.5 Diagonal Sheathing with Straight Sheathing or Flooring AboveWood Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-268.5.6 Double Diagonally Sheathed Wood Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . 8-268.5.7 Wood Structural Panel Sheathed Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . 8-278.5.8 Wood Structural Panel Overlays on Straight orDiagonally Sheathed Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-288.5.9 Wood Structural Panel Overlays on Existing WoodStructural Panel Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-288.5.10 Braced Horizontal Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-298.5.11 Effects of Chords and Openings in Wood Diaphragms . . . . . . . . . . . . . . . . . . . 8-298.6 Wood Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-298.6.1 Wood Piling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-298.6.2 Wood Footings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-308.6.3 Pole Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-308.7 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-308.8 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-33FEMA 273 Seismic Rehabilitation Guidelines xix

10.7 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1610.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10-1611. Architectural, Mechanical, and Electrical Components(Simplifiedand Systematic Rehabilitation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-111.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-111.2 Procedural Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-111.3 Historical and Component Evaluation Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-511.3.1 Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-511.3.2 Component Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-511.4 Rehabilitation Objectives, Performance Levels, and Performance Ranges . . . . . . . . . . . .11-511.4.1 Performance Levels for Nonstructural Components . . . . . . . . . . . . . . . . . . . . . .11-511.4.2 Performance Ranges for Nonstructural Components . . . . . . . . . . . . . . . . . . . . .11-611.4.3 Regional Seismicity and Nonstructural Components . . . . . . . . . . . . . . . . . . . . .11-611.4.4 Means of Egress: Escape and Rescue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-611.5 Structural-Nonstructural Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-711.5.1 Response Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-711.5.2 Base Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-711.6 Acceptance Criteria for Acceleration-Sensitive and Deformation-Sensitive Components .11-711.6.1 Acceleration-Sensitive Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-711.6.2 Deformation-Sensitive Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-811.6.3 Acceleration- and Deformation-Sensitive Components . . . . . . . . . . . . . . . . . . .11-811.7 Analytical and Prescriptive Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-911.7.1 Application of Analytical and Prescriptive Procedures . . . . . . . . . . . . . . . . . . .11-911.7.2 Prescriptive Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-911.7.3 Analytical Procedure: Default Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-911.7.4 Analytical Procedure: General Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-911.7.5 Drift Ratios and Relative Displacements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1011.7.6 Other Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1011.8 Rehabilitation Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1311.9 Architectural Components: Definition, Behavior, and Acceptance Criteria . . . . . . . . . . .11-1311.9.1 Exterior Wall Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1311.9.2 Partitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1611.9.3 Interior Veneers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1711.9.4 Ceilings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1711.9.5 Parapets and Appendages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1811.9.6 Canopies and Marquees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1911.9.7 Chimneys and Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1911.9.8 Stairs and Stair Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-1911.10 Mechanical, Electrical, and Plumbing Components: Definition, Behavior, andAcceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2011.10.1 Mechanical Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2011.10.2 Storage Vessels and Water Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2111.10.3 Pressure Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2211.10.4 Fire Suppression Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2211.10.5 Fluid Piping other than Fire Suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2211.10.6 Ductwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-23FEMA 273 Seismic Rehabilitation Guidelines xxi

11.10.7 Electrical and Communications Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2411.10.8 Electrical and Communications Distribution Components . . . . . . . . . . . . . . . .11-2411.10.9 Light Fixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2511.11 Furnishings and Interior Equipment: Definition, Behavior, and Acceptance Criteria . . . .11-2511.11.1 Storage Racks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2511.11.2 Bookcases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2611.11.3 Computer Access Floors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2611.11.4 Hazardous Materials Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2711.11.5 Computer and Communication Racks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2711.11.6 Elevators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2711.11.7 Conveyors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2811.12 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2811.13 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-2911.14 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11-29A. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1B. Seismic Rehabilitation Guidelines Project Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .I-1xxii Seismic Rehabilitation Guidelines FEMA 273

List of FiguresFigure 1-1 Rehabilitation Process Flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9Figure 2-1 General Response Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-23Figure 2-2 In-Plane Discontinuity in Lateral System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-30Figure 2-3 Typical Building with Out-of-Plane Offset Irregularity . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-30Figure 2-4 General Component Behavior Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-32Figure 2-5 Idealized Component Load versus Deformation Curves for Depicting ComponentModeling and Acceptability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-33Figure 2-6 Backbone Curve for Experimental Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-45Figure 2-7 Alternative Force Deformation Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-46Figure 3-1 Calculation of Effective Stiffness, K e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12Figure 4-1 (a) Idealized Elasto-Plastic Load-Deformation Behavior for Soils (b) UncoupledSpring Model for Rigid Footings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8Figure 4-2 Elastic Solutions for Rigid Footing Spring Constants (based on Gazetas, 1991 andLam et al., 1991) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-11Figure 4-3 (a) Foundation Shape Correction Factors (b) Embedment Correction Factors . . . . . . . . . . 4-12Figure 4-4 Lateral Foundation-to-Soil Stiffness for Passive Pressure (after Wilson, 1988) . . . . . . . . . 4-13Figure 4-5 Vertical Stiffness Modeling for Shallow Bearing Footings . . . . . . . . . . . . . . . . . . . . . . . . 4-14Figure 4-6 Idealized Concentration of Stress at Edge of Rigid Footings Subjected toOverturning Moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15Figure 5-1 Definition of the a, b, c, d, and e Parameters in Tables 5-4, 5-6, and 5-8, andthe Generalized Load-Deformation Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10Figure 5-2 Definition of Chord Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11Figure 5-3 Full-Pen Connection in FR Connection with Variable Behavior . . . . . . . . . . . . . . . . . . . . 5-15Figure 5-4 Clip Angle Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-22Figure 5-5 T-Stub Connection may be FR or PR Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23Figure 5-6 Flange Plate Connection may be FR or PR Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24Figure 5-7 End Plate Connection may be FR or PR Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24Figure 5-8 Two Configurations of PR Composite Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24Figure 6-1 Generalized Load-Deformation Relation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12Figure 6-2 Plastic Hinge Rotation in Shear Wall where Flexure Dominates Inelastic Response . . . . . 6-42Figure 6-3 Story Drift in Shear Wall where Shear Dominates Inelastic Response . . . . . . . . . . . . . . . . 6-42Figure 6-4 Chord Rotation for Shear Wall Coupling Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-42Figure 7-1 Idealized Force-Deflection Relation for Walls, Pier Components, and Infill Panels . . . . . 7-10Figure 8-1 Normalized Force versus Deformation Ratio for Wood Elements . . . . . . . . . . . . . . . . . . . 8-15Figure 9-1 Calculation of Secant Stiffness, K s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-20Figure 10-1 Comparison of FEMA 178 (BSSC, 1992a) and Guidelines Acceptance Criteria . . . . . . . 10-17FEMA 273 Seismic Rehabilitation Guidelines xxiii

xxiv Seismic Rehabilitation Guidelines FEMA 273

List of TablesTable 2-1 Guidelines and Criteria in Chapter 2 and Relation to Guidelines and Criteria inOther Chapters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3Table 2-2 Rehabilitation Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5Table 2-3 Damage Control and Building Performance Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11Table 2-4 Structural Performance Levels and Damage—Vertical Elements . . . . . . . . . . . . . . . . . . . 2-11Table 2-5 Structural Performance Levels and Damage—Horizontal Elements . . . . . . . . . . . . . . . . . 2-14Table 2-6 Nonstructural Performance Levels and Damage—Architectural Components . . . . . . . . . . 2-15Table 2-7 Nonstructural Performance Levels and Damage—Mechanical, Electrical, andPlumbing Systems/Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-16Table 2-8 Nonstructural Performance Levels and Damage—Contents . . . . . . . . . . . . . . . . . . . . . . . . 2-17Table 2-9 Building Performance Levels/Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-17Table 2-10 Values of Exponent n for Determination of Response Acceleration Parameters atHazard Levels between 10%/50 years and 2%/50 years; Sites whereMapped BSE-2 Values of S S ≥ 1.5g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20Table 2-11 Values of Exponent n for Determination of Response Acceleration Parameters atProbabilities of Exceedance Greater than 10%/50 years; Sites whereMapped BSE-2 Values of S S < 1.5g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21Table 2-12 Values of Exponent n for Determination of Response Acceleration Parameters atProbabilities of Exceedance Greater than 10%/50 years; Sites whereMapped BSE-2 Values of S S ≥ 1.5g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21Table 2-13 Values of F a as a Function of Site Class and Mapped Short-Period Spectral ResponseAcceleration S S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21Table 2-14 Values of F v as a Function of Site Class and Mapped Spectral Response Acceleration atOne-Second Period S 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21Table 2-15 Damping Coefficients B S and B 1 as a Function of Effective Damping β . . . . . . . . . . . . . . 2-23Table 2-16 Calculation of Component Action Capacity—Linear Procedures . . . . . . . . . . . . . . . . . . . 2-34Table 2-17 Calculation of Component Action Capacity—Nonlinear Procedures . . . . . . . . . . . . . . . . . 2-34Table 2-18 Coefficient χ for Calculation of Out-of-Plane Wall Forces . . . . . . . . . . . . . . . . . . . . . . . . 2-40Table 3-1 Values for Modification Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9Table 3-2 Values for Modification Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13Table 4-1 Estimated Susceptibility to Liquefaction of Surficial Deposits During Strong GroundShaking (after Youd and Perkins, 1978) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3Table 4-2 Presumptive Ultimate Foundation Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7Table 4-3 Effective Shear Modulus and Shear Wave Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9Table 4-4 Soil Foundation Acceptability Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17Table 5-1 Default Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5Table 5-2 Default Expected Material Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6Table 5-3 Acceptance Criteria for Linear Procedures—Fully Restrained (FR) Moment Frames . . . . 5-14Table 5-4 Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—FullyRestrained (FR) Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16Table 5-5 Acceptance Criteria for Linear Procedures—Partially Restrained (PR) Moment Frames . 5-20Table 5-6 Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—PartiallyRestrained (PR) Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21FEMA 273 Seismic Rehabilitation Guidelines xxv

Table 5-7 Acceptance Criteria for Linear Procedures—Braced Frames andSteel Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-26Table 5-8 Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—BracedFrames and Steel Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-28Table 6-1 Tensile and Yield Properties of Concrete Reinforcing Bars for Various Periods . . . . . . . . .6-2Table 6-2 Tensile and Yield Properties of Concrete Reinforcing Bars for Various ASTMSpecifications and Periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-3Table 6-3 Compressive Strength of Structural Concrete (psi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-4Table 6-4 Effective Stiffness Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-12Table 6-5 Component Ductility Demand Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-14Table 6-6 Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-19Table 6-7 Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-20Table 6-8 Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete Beam-Column Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-21Table 6-9 Values of γ for Joint Strength Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-22Table 6-10 Numerical Acceptance Criteria for Linear Procedures—Reinforced Concrete Beams . . . .6-23Table 6-11 Numerical Acceptance Criteria for Linear Procedures—Reinforced Concrete Columns . .6-24Table 6-12 Numerical Acceptance Criteria for Linear Procedures—Reinforced ConcreteBeam-Column Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-25Table 6-13 Modeling Parameters and Numerical Acceptance Criteria for NonlinearProcedures—Two-Way Slabs and Slab-Column Connections . . . . . . . . . . . . . . . . . . . . . .6-29Table 6-14 Numerical Acceptance Criteria for Linear Procedures—Two-Way Slabs andSlab-Column Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-30Table 6-15 Modeling Parameters and Numerical Acceptance Criteria for NonlinearProcedures—Reinforced Concrete Infilled Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-35Table 6-16 Numerical Acceptance Criteria for Linear Procedures—Reinforced ConcreteInfilled Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-36Table 6-17 Modeling Parameters and Numerical Acceptance Criteria for NonlinearProcedures—Members Controlled by Flexure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-43Table 6-18 Modeling Parameters and Numerical Acceptance Criteria for NonlinearProcedures—Members Controlled by Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-44Table 6-19 Numerical Acceptance Criteria for Linear Procedures—Members Controlledby Flexure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-46Table 6-20 Numerical Acceptance Criteria for Linear Procedures—Members Controlledby Shear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-47Table 7-1 Linear Static Procedure—m Factors for URM In-Plane Walls and Piers . . . . . . . . . . . . . .7-10Table 7-2 Nonlinear Static Procedure—Simplified Force-Deflection Relations for URMIn-Plane Walls and Piers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-11Table 7-3 Permissible h/t Ratios for URM Out-of-Plane Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-11Table 7-4 Linear Static Procedure—m Factors for Reinforced Masonry In-Plane Walls . . . . . . . . . .7-15Table 7-5 Nonlinear Static Procedure—Simplified Force-Deflection Relations forReinforced Masonry Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-16Table 7-6 Linear Static Procedure—m Factors for Masonry Infill Panels . . . . . . . . . . . . . . . . . . . . . .7-20Table 7-7 Nonlinear Static Procedure—Simplified Force-Deflection Relations forMasonry Infill Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-20xxvi Seismic Rehabilitation Guidelines FEMA 273

Table 7-8 Maximum h inf /t inf Ratios for which No Out-of-Plane Analysis is Necessary . . . . . . . . . . 7-21Table 7-9 Values of λ 2 for Use in Equation 7-21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-22Table 8-1 Numerical Acceptance Factors for Linear Procedures—Wood Components . . . . . . . . . . . 8-13Table 8-2 Normalized Force-Deflection Curve Coordinates for Nonlinear Procedures—WoodComponents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16Table 8-3 Ultimate Capacities of Structural Panel Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-20Table 10-1 Limitations on Use of Simplified Rehabilitation Method . . . . . . . . . . . . . . . . . . . . . . . . . 10-18Table 10-2 Description of Model Building Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-20Table 10-3 W1: Wood Light Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-23Table 10-4 W1A: Multistory, Multi-Unit, Wood Frame Construction . . . . . . . . . . . . . . . . . . . . . . . . 10-23Table 10-5 W2: Wood, Commercial, and Industrial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-23Table 10-6 S1 and S1A: Steel Moment Frames with Stiff or Flexible Diaphragms . . . . . . . . . . . . . . 10-23Table 10-7 S2 and S2A: Steel Braced Frames with Stiff or Flexible Diaphragms . . . . . . . . . . . . . . . 10-24Table 10-8 S3: Steel Light Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-24Table 10-9 S4: Steel Frames with Concrete Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-24Table 10-10 S5, S5A: Steel Frames with Infill Masonry Shear Walls and Stiff orFlexible Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-25Table 10-11 C1: Concrete Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-25Table 10-12 C2, C2A: Concrete Shear Walls with Stiff or Flexible Diaphragms . . . . . . . . . . . . . . . . . 10-26Table 10-13 C3, C3A: Concrete Frames with Infill Masonry Shear Walls and Stiff orFlexible Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-26Table 10-14 PC1: Precast/Tilt-up Concrete Shear Walls with Flexible Diaphragms . . . . . . . . . . . . . . 10-27Table 10-15 PC1A: Precast/Tilt-up Concrete Shear Walls with Stiff Diaphragms . . . . . . . . . . . . . . . . 10-27Table 10-16 PC2: Precast Concrete Frames with Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-28Table 10-17 PC2A: Precast Concrete Frames Without Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-28Table 10-18 RM1: Reinforced Masonry Bearing Wall Buildings with Flexible Diaphragms . . . . . . . 10-28Table 10-19 RM2: Reinforced Masonry Bearing Wall Buildings with Stiff Diaphragms . . . . . . . . . . 10-29Table 10-20 URM: Unreinforced Masonry Bearing Wall Buildings with Flexible Diaphragms . . . . . 10-29Table 10-21 URMA: Unreinforced Masonry Bearing Walls Buildings with Stiff Diaphragms . . . . . . 10-29Table 10-22 Cross Reference Between the Guidelines and FEMA 178 (BSSC, 1992a)Deficiency Reference Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-30Table 11-1 Nonstructural Components: Applicability of Life Safety and Immediate OccupancyRequirements and Methods of Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2Table 11-2 Nonstructural Component Amplification and Response Modification Factors . . . . . . . . 11-11FEMA 273 Seismic Rehabilitation Guidelines xxvii

1.Introduction1.1 PurposeThe primary purpose of this document is to providetechnically sound and nationally acceptable guidelinesfor the seismic rehabilitation of buildings. TheGuidelines for the Seismic Rehabilitation of Buildingsare intended to serve as a ready tool for designprofessionals, a reference document for buildingregulatory officials, and a foundation for the futuredevelopment and implementation of building codeprovisions and standards.This document consists of two volumes. The Guidelinesvolume details requirements and procedures, which theCommentary volume explains. A companion volumetitled Example Applications contains information ontypical deficiencies, rehabilitation costs, and otheruseful explanatory information.This document is intended for a primary user group ofarchitects, engineers, and building officials, specificallythose in the technical community responsible fordeveloping and using building codes and standards, andfor carrying out the design and analysis of buildings.Parts of the document will also be useful andinformative to such secondary audiences beyond thetechnical community as building owners, governmentagencies, and policy makers.The engineering expertise of a design professional is aprerequisite to the appropriate use of the Guidelines,and most of the provisions of the following chapterspresume the expertise of a professional engineerexperienced in building design, as indicated in specificreferences to “the engineer” found extensivelythroughout this document.An engineer can use this document to help a buildingowner select seismic protection criteria when theowner’s risk reduction efforts are purely voluntary. Theengineer can also use the document for the design andanalysis of seismic rehabilitation projects. However,this document should not be considered to be a designmanual, textbook, or handbook. Notwithstanding theinstructional examples and explanations found in theCommentary and Example Applications volume, othersupplementary information and instructional resourcesmay well be required to use this documentappropriately.This document is neither a code nor a standard. It isintended to be suitable both for voluntary use by ownersand design professionals as well as for adaptation andadoption into model codes and standards. Conversion ofmaterial from the Guidelines into a code or standardwill require, as a minimum, a) careful study as to theapplicability of acceptance criteria to the specificsituation and building type, b) reformatting into codelanguage, c) the addition of rules of applicability or“triggering” policies, and d) modification or addition ofrequirements relating to specific building departmentoperations within a given jurisdiction.See Section 1.3 for important descriptions of the scopeand limitations of this document.1.2 Significant New FeaturesThis document contains several new features that departsignificantly from previous seismic design proceduresused to design new buildings.1.2.1 Seismic Performance Levels andRehabilitation ObjectivesMethods and design criteria to achieve several differentlevels and ranges of seismic performance are defined.The four Building Performance Levels are CollapsePrevention, Life Safety, Immediate Occupancy, andOperational. (The Operational Level is defined, butspecification of complete design criteria is not includedin the Guidelines. See Chapter 2.) These levels arediscrete points on a continuous scale describing thebuilding’s expected performance, or alternatively, howmuch damage, economic loss, and disruption mayoccur.Each Building Performance Level is made up of aStructural Performance Level that describes the limitingdamage state of the structural systems and aNonstructural Performance Level that describes thelimiting damage state of the nonstructural systems.Three Structural Performance Levels and fourNonstructural Performance Levels are used to form thefour basic Building Performance Levels listed above.In addition, two ranges of structural performance aredefined to provide a designation for uniquerehabilitations that may be intended for specialpurposes and therefore will fall between the ratherFEMA 273 Seismic Rehabilitation Guidelines 1-1

Chapter 1: IntroductionBuilding Performance Levels and RangesPerformance Level: the intended post-earthquakecondition of a building; a well-defined point on a scalemeasuring how much loss is caused by earthquakedamage. In addition to casualties, loss may be in termsof property and operational capability.Performance Range: a range or band of performance,rather than a discrete level.Designations of Performance Levels and Ranges:Performance is separated into descriptions of damageof structural and nonstructural systems; structuraldesignations are S-1 through S-5 and nonstructuraldesignations are N-A through N-D.Building Performance Level: The combination of aStructural Performance Level and a NonstructuralPerformance Level to form a complete description ofan overall damage level.Rehabilitation Objective: The combination of aPerformance Level or Range with Seismic DemandCriteria.higher performanceless lossOperational LevelBackup utility servicesmaintain functions; very littledamage. (S1+NA)well-defined structural levels. Other structural andnonstructural categories are included to describe a widerange of seismic rehabilitation intentions. In fact, oneof the goals of the performance level system employedin this document is to enable description of allperformance objectives previously designated in codesand standards and most objectives used in voluntaryrehabilitation efforts.The three Structural Performance Levels and twoStructural Performance Ranges consist of:• S-1: Immediate Occupancy Performance Level• S-2: Damage Control Performance Range(extends between Life Safety and ImmediateOccupancy Performance Levels)• S-3: Life Safety Performance Level• S-4: Limited Safety Performance Range(extends between Life Safety and CollapsePrevention Performance Levels)• S-5: Collapse Prevention Performance LevelIn addition, there is the designation of S-6, StructuralPerformance Not Considered, to cover the situationwhere only nonstructural improvements are made.The four Nonstructural Performance Levels are:Immediate Occupancy LevelThe building receives a “greentag” (safe to occupy) inspectionrating; any repairs are minor.(S1+NB)Life Safety LevelStructure remains stable andhas significant reservecapacity; hazardousnonstructural damage iscontrolled. (S3+NC)Collapse Prevention LevelThe building remains standing,but only barely; any otherdamage or loss is acceptable.(S5+NE)lower performancemore loss• N-A: Operational Performance Level• N-B: Immediate Occupancy Performance Level• N-C: Life Safety Performance Level• N-D: Hazards Reduced Performance LevelIn addition, there is the designation of N-E,Nonstructural Performance Not Considered, to coverthe situation where only structural improvements aremade.A description of “what the building will look like afterthe earthquake” raises the questions: Whichearthquake? A small one or a large one? A minor-tomoderatedegree of ground shaking severity at the sitewhere the building is located, or severe ground motion?Ground shaking criteria must be selected, along with adesired Performance Level or Range, for the Guidelines1-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: Introductionto be applied; this can be done either by reference tostandardized regional or national ground shaking hazardmaps, or by site-specific studies.Once a desired Building Performance Level for aparticular ground shaking severity (seismic demand) isselected, the result is a Rehabilitation Objective (seeSection 1.5.1.3 for a detailed discussion). With theexception of the Basic Safety Objective (BSO), thereare no preset combinations of performance and groundshaking hazard. The Basic Safety Objective is metwhen a building can satisfy two criteria: (1) the LifeSafety Building Performance Level, which is thecombination of the Structural and Nonstructural LifeSafety Performance Levels, for the Basic SafetyEarthquake 1 (BSE-1), and (2) the Collapse PreventionPerformance Level, which only pertains to structuralperformance, for the stronger shaking that occurs lessfrequently as defined in the Basic Safety Earthquake 2(BSE-2). One or more of these two levels of earthquakemotion may be used in the design process to meet otherRehabilitation Objectives as well, but they have beenselected as the required ground shaking criteria for theBSO. While the margin against failure may be smallerand the reliability less, the primary goal of the BSO is toprovide a level of safety for rehabilitated buildingssimilar to that of buildings recently designed to USseismic code requirements. In fact, the strongestargument for using similar ground motions to thoseused for new buildings is to enable a direct comparisonof expected performance. It should be remembered,however, that economic losses from damage are notexplicitly considered in the BSO, and these losses inrehabilitated existing buildings should be expected to belarger than in the case of a newly constructed building.Using various combinations of Performance Levels andground shaking criteria, many other RehabilitationObjectives can be defined. Those objectives that exceedthe requirements for the BSO, either in terms ofPerformance Level, ground shaking criteria, or both, aretermed Enhanced Objectives, and similarly, those thatfail to meet some aspect of the BSO are termed LimitedObjectives.1.2.2 Simplified and SystematicRehabilitation MethodsSimplified Rehabilitation may be applied to certainsmall buildings specified in the Guidelines. Theprimary intent of Simplified Rehabilitation is to reduceseismic risk efficiently where possible and appropriateby seeking Limited Objectives. Partial rehabilitationmeasures, which target high-risk building deficienciessuch as parapets and other exterior falling hazards, areincluded as Simplified Rehabilitation techniques.Although limited in scope, Simplified Rehabilitationwill be applicable to a large number of buildingsthroughout the US. The Simplified RehabilitationMethod employs equivalent static force analysisprocedures, which are found in most seismic codes fornew buildings.Systematic Rehabilitation may be applied to anybuilding and involves thorough checking of eachexisting structural element or component (an elementsuch as a moment-resisting frame is composed of beamand column components), the design of new ones, andverification of acceptable overall interaction forexpected displacements and internal forces. TheSystematic Rehabilitation Method focuses on thenonlinear behavior of structural response, and employsprocedures not previously emphasized in seismic codes.1.2.3 Varying Methods of AnalysisFour distinct analytical procedures can be used inSystematic Rehabilitation: Linear Static, LinearDynamic, Nonlinear Static, and Nonlinear DynamicProcedures. The choice of analytical method is subjectto limitations based on building characteristics. Thelinear procedures maintain the traditional use of a linearstress-strain relationship, but incorporate adjustments tooverall building deformations and material acceptancecriteria to permit better consideration of the probablenonlinear characteristics of seismic response. TheNonlinear Static Procedure, often called “pushoveranalysis,” uses simplified nonlinear techniques toestimate seismic structural deformations. The NonlinearDynamic Procedure, commonly known as nonlineartime history analysis, requires considerable judgmentand experience to perform, and may only be used withinthe limitations described in Section 2.9.2.2 of theGuidelines.1.2.4 Quantitative Specifications ofComponent BehaviorInherent in the concept of Performance Levels andRanges is the assumption that performance can bemeasured using analytical results such as story driftratios or strength and ductility demands on individualcomponents or elements. To enable structuralverification at the selected Performance Level, stiffness,strength, and ductility characteristics of many commonFEMA 273 Seismic Rehabilitation Guidelines 1-3

Chapter 1: Introductionelements and components have been derived fromlaboratory tests and analytical studies and put in astandard format in the Guidelines.1.2.5 Procedures for Incorporating NewInformation and Technologies intoRehabilitationIt is expected that testing of existing materials andelements will continue and that additional correctivemeasures and products will be developed. It is alsoexpected that systems and products intended to modifystructural response beneficially will be advanced. Theformat of the analysis techniques and acceptabilitycriteria of the Guidelines allows rapid incorporation ofsuch technology. Section 2.13 gives specific guidancein this regard. It is expected that the Guidelines willhave a significant impact on testing and documentationof existing materials and systems as well as newproducts. In addition, an entire chapter (Chapter 9) hasbeen devoted to two such new technologies, seismicisolation and energy dissipation.1.3 Scope, Contents, and LimitationsThis section describes the scope and limitations of thecontents of this document pertaining to the following:• buildings and loadings• activities and policies associated with seismicrehabilitation• seismic mapping• technical content1.3.1 Buildings and LoadingsThis document is intended to be applied to allbuildings—regardless of importance, occupancy,historic features, size, or other characteristics—that bysome criteria are deficient in their ability to resist theeffects of earthquakes. In addition to the direct effectsof ground shaking, this document also considers theeffects on buildings of local ground failure such asliquefaction. With careful extrapolation, the proceduresherein can also be applied to many nonbuildingstructures such as pipe racks, steel storage racks,structural towers for tanks and vessels, piers, wharves,and electrical power generating facilities. Theapplicability of the procedures has not been examinedfor each and every structural type, particularly thosethat have generally been covered by their own codes orstandards, such as bridges and nuclear power plants. Itis important to note that, as written, the provisions arenot intended to be mandatory. Careful consideration ofthe applicability to any given group of buildings orstructures should be made prior to adoption of anyportion of these procedures for mandatory use.This document applies to the seismic resistance of boththe overall structural system of a building and itselements—such as shear walls or frames—and theconstituent components of elements, such as a columnin a frame or a boundary member in a wall. It alsoapplies to nonstructural components of existingbuildings—ceilings, partitions, and mechanical/electrical systems. In addition to techniques forincreasing strength and ductility of systems, thisdocument provides rehabilitation techniques forreducing seismic demand, such as the introduction ofisolation or damping devices. And, although thisdocument is not intended to address the design of newbuildings, it does cover new components or elements tobe added to existing buildings. Evaluation ofcomponents for gravity and wind forces in the absenceof earthquake demands is beyond the scope of thedocument.1.3.2 Activities and Policies Associated withSeismic RehabilitationThere are several significant steps in the process ofreducing seismic risk in buildings that this documentdoes not encompass. The first step, deciding whether ornot to undertake a rehabilitation project for a particularbuilding, is beyond the scope of the Guidelines. Oncethe decision to rehabilitate a building has been made,the Guidelines’ detailed engineering guidance on howto conduct seismic rehabilitation analysis can beapplied.Another step, determining when the Guidelines shouldbe applicable in a mandatory way to a remodeling orstructural alteration project (the decision as to when theprovisions are “triggered”), is also beyond the scope ofthis document. Finally, methods of reducing seismicrisk that do not physically change the building—such asreducing the number of occupants—are not coveredhere.Recommendations regarding the selection of aRehabilitation Objective for any building are also1-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: Introductionbeyond the scope of this document. As noted above, alife safety risk often considered acceptable, is definedby a specific objective, termed the Basic SafetyObjective (BSO). Higher and lower objectives can alsobe defined by the user. The Commentary discussesissues to consider when combining various performanceand seismic hazard levels; it should be noted that not allcombinations constitute reasonable or cost-effectiveRehabilitation Objectives. The Guidelines were writtenunder the premise that greater flexibility is required inseismic rehabilitation than in the design of newbuildings. However, even with the flexibility providedby various Rehabilitation Objectives, once aRehabilitation Objective is decided upon, theGuidelines provide internally consistent procedures thatinclude the necessary analysis and constructionspecifications.Featured in the Guidelines are descriptions of damagestates with relation to specific Performance Levels.These descriptions are intended to aid designprofessionals and owners when selecting appropriatePerformance Levels for rehabilitation design. They arenot intended to be used directly for conditionassessment of earthquake-damaged buildings. Althoughthere are similarities in damage descriptions that areused for selection of rehabilitation design criteria anddescriptions used for post-earthquake damageassessment, many factors enter into the design andassessment processes. No single parameter should becited as defining either a Performance Level or thesafety or usefulness of an earthquake-damagedbuilding.Techniques of repair for earthquake-damaged buildingsare not included in the Guidelines. However, if themechanical properties of repaired components areknown, acceptability criteria for use in this documentcan be either deduced by comparison with other similarcomponents, or derived. Any combination of repairedelements, undamaged existing elements, and newelements can be modeled using this document, and eachchecked against Performance Level acceptance criteria.Although the Guidelines were not written for thepurpose of evaluating the expected performance of anunrehabilitated existing building, they may be used as areference for evaluation purposes in deciding whether abuilding requires rehabilitation, similarly to the waycode provisions for new buildings are sometimes usedas an evaluation tool.1.3.3 Seismic MappingSpecial or new mapping of expected seismic groundshaking for the country has not been developed for theGuidelines. However, new national earthquake hazardmaps were developed in 1996 by the United StatesGeological Survey (USGS) as part of a joint project(known as Project ’97) with the Building SeismicSafety Council to update the 1997 NEHRPRecommended Provisions for new buildings. Nationalprobabilistic maps were developed for ground motionswith a 10% chance of exceedance in 50 years, a 10%chance of exceedance in 100 years (which can also beexpressed as a 5% chance of exceedance in 50 years)and a 10% chance of exceedance in 250 years (whichalso can be expressed as a 2% chance of exceedance in50 years). These probabilities correspond to motionsthat are expected to occur, on average, about once every500, 1000, and 2500 years. In addition, in certainlocations with well-defined earthquake sources, localground motions for specific earthquakes weredeveloped, known as deterministic motions. Keyordinates of a ground motion response spectrum forthese various cases allow the user to develop a completespectrum at any site. The Guidelines are written to usesuch a response spectrum as the seismic demand inputfor the various analysis techniques.The responsibility of the Building Seismic SafetyCouncil in Project ’97 was to develop a national mapand/or analytical procedure to best utilize the newseismic hazard information for the design of newbuildings. As part of that process, rules were developedto combine portions of both the USGS probabilistic anddeterministic maps to create a map of ground motionsrepresenting the effects of large, rare events in all partsof the country. This event is called the MaximumConsidered Earthquake (MCE). New buildings are to bedesigned, with traditional design rules, for two-thirds ofthese ground motion values with the purpose ofproviding an equal margin against collapse for thevaried seismicity across the country.For consistency in this document, ground motionprobabilities will be expressed with relationship to50-year exposure times, and in a shorthand format; i.e.,10%/50 years is a 10% chance of exceedance in 50years, 5%/50 years is a 5% chance of exceedance in 50years, and 2%/50 years is a 2% chance of exceedance in50 years.The variable Rehabilitation Objectives featured in theGuidelines allows consideration of any ground motionFEMA 273 Seismic Rehabilitation Guidelines 1-5

Chapter 1: Introductionthat may be of interest, the characteristics of which canbe determined specifically for the site, or taken from anational or local map. However, specifically for usewith the BSO, and generally for convenience indefining the ground motion for other RehabilitationObjectives, the 10%/50 year probabilistic maps and theMCE maps developed in Project 97 are in the mappackage distributed with the Guidelines. For additionalmap packages, call FEMA at 1-800-480-2520.New ground motion maps specifically related to theseismic design procedures of the 1997 NEHRPRecommended Provisions are expected to be available.These maps plot key ordinates of a ground motionresponse spectrum, allowing development by the userof a complete spectrum at any site. The Guidelines arewritten to use such a response spectrum as the seismicdemand input for the various analysis techniques. Whilethe NEHRP maps provide a ready source for this type ofinformation, the Guidelines may be used with seismichazard data from any source as long as it is expressed asa response spectrum.1.3.4 Technical ContentThe Guidelines have been developed by a large team ofspecialists in earthquake engineering and seismicrehabilitation. The most advanced analytical techniquesthat were considered practical for production use havebeen incorporated, and seismic Performance Levelcriteria have been specified using actual laboratory testresults, where available, supplemented by theengineering judgment of the various developmentteams. Certain buildings damaged in the 1994Northridge earthquake and a limited number of designsusing codes for new buildings have been checked withthe procedures of this document. There has not yet beenthe opportunity, however, for comprehensivecomparisons with other codes and standards, nor forevaluation of the accuracy in predicting the damagelevel under actual earthquake ground motions. As ofthis writing (1997), significant case studies are alreadyunderway to test more thoroughly the various analysistechniques and acceptability criteria. Thereundoubtedly will also be lessons learned from futuredamaging earthquakes by studying performance of bothunrehabilitated buildings and buildings rehabilitated tothese or other standards. A structured program will alsobe instituted to gather and assess the new knowledgerelevant to the data, procedures, and criteria containedin the Guidelines, and make recommendations forfuture refinements. Engineering judgment should beexercised in determining the applicability of variousanalysis techniques and material acceptability criteria ineach situation. It is suggested that results obtained forany individual building be validated by additionalchecks using alternative methodologies and carefulanalysis of any differences. Information contained inthe Commentary will be valuable for such individualvalidation studies.The concepts and terminology of performance-baseddesign are new and should be carefully studied anddiscussed with building owners before use. Theterminology used for Performance Levels is intended torepresent goals of design. The actual ground motionwill seldom be comparable to that specified in theRehabilitation Objective, so in most events, designstargeted at various damage states may only determinerelative performance. Even given a ground motionsimilar to that specified in the Rehabilitation Objectiveand used in design, variations from stated performancesshould be expected. These could be associated withunknown geometry and member sizes in existingbuildings, deterioration of materials, incomplete sitedata, variation of ground motion that can occur within asmall area, and incomplete knowledge andsimplifications related to modeling and analysis.Compliance with the Guidelines should therefore not beconsidered a guarantee of the specified performance.Determination of statistical reliability of therecommendations in the Guidelines was not a part of thedevelopment project. Such a study would requiredevelopment of and consensus acceptance of a newmethodology to determine reliability. However, theexpected reliability of achieving various PerformanceLevels when the requirements of a given Level arefollowed is discussed in the Commentary for Chapter 2.1.4 Relationship to Other Documentsand ProceduresThe Guidelines contain specific references to manyother documents; however, the Guidelines are alsorelated generically to the following publications.• FEMA 222A and 223A, NEHRP RecommendedProvisions for Seismic Regulations for NewBuildings (BSSC, 1995): For the purposes of thedesign of new components, the Guidelines havebeen designed to be as compatible as possible withthe companion Provisions for new buildings and itsreference design documents. Detailed references tothe use of specific sections of the Provisions1-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: Introductiondocument will be found in subsequent sections ofthe Guidelines.• FEMA 302 and 303, 1997 NEHRP RecommendedProvisions for Seismic Regulations for NewBuildings and Other Structures (BSSC, 1997),referred to herein as the 1997 NEHRPRecommended Provisions, have been in preparationfor the same time as the later versions of theGuidelines. Most references are to the 1994 NEHRPRecommended Provisions.• FEMA 237, Development of Guidelines for SeismicRehabilitation of Buildings, Phase I: IssuesIdentification and Resolution (ATC, 1992), whichunderwent an American Society of Civil Engineers(ASCE) consensus approval process, providedpolicy direction for this document.• Proceedings of the Workshop To Resolve SeismicRehabilitation Sub-issues (ATC, 1993) providedrecommendations to the writers of the Guidelines onmore detailed sub-issues.• FEMA 172, NEHRP Handbook of Techniques forthe Seismic Rehabilitation of Existing Buildings(BSSC, 1992a), originally produced by URS/Blumeand reviewed by the BSSC, contains constructiontechniques for implementing engineering solutionsto the seismic deficiencies of existing buildings.• FEMA 178, NEHRP Handbook for the SeismicEvaluation of Existing Buildings (BSSC, 1992b),which was originally developed by ATC andunderwent the consensus approval process of theBSSC, covers the subject of evaluating existingbuildings to decide if they are seismically deficientin terms of life safety. The model building types andother information from that publication are used orreferred to extensively in the Guidelines inChapter 10 and in the Example Applicationsdocument (ATC, 1997). FEMA 178, 1992 edition, isbeing updated to include additional performanceobjectives as well as to be more compatible with theGuidelines.• FEMA 156 and 157, Second Edition, Typical Costsfor Seismic Rehabilitation of Existing Buildings(Hart, 1994 and 1995), reports statistical analysis ofthe costs of rehabilitation of over 2000 buildings,based on construction costs or detailed studies.Several different seismic zones and performancelevels are included in the data. Since the data weredeveloped in 1994, none of the data is based onbuildings rehabilitated specifically in accordancewith the current Guidelines document. PerformanceLevels defined in the Guidelines are not intended tobe significantly different from parallel levels usedpreviously, and costs should still be reasonablyrepresentative.• FEMA 275, Planning for Seismic Rehabilitation:Societal Issues (VSP, 1996), discusses societal andimplementation issues associated with rehabilitation,and describes several case histories.• FEMA 276, Guidelines for the SeismicRehabilitation of Buildings: Example Applications(ATC, 1997), intended as a companion document tothe Guidelines and Commentary, describes examplesof buildings that have been seismically rehabilitatedin various seismic regions and for differentRehabilitation Objectives. Costs of the work aregiven and references made to FEMA 156 and 157.Since the document is based on previous casehistories, none of the examples were rehabilitatedspecifically in accordance with the currentGuidelines document. However, PerformanceLevels defined in the Guidelines are not intended tobe significantly different than parallel levels usedpreviously, and the case studies are thereforeconsidered representative.• ATC 40, Seismic Evaluation and Retrofit ofConcrete Buildings, (ATC, 1996), incorporatesperformance levels almost identical to those shownin Table 2-9 and employs “pushover” nonlinearanalysis techniques. The capacity spectrum methodfor determining the displacement demand is treatedin detail. This document covers only concretebuildings.1.5 Use of the Guidelines in theSeismic Rehabilitation ProcessFigure 1-1 is an overview of the flow of procedurescontained in this document as well as an indication ofthe broader scope of the overall seismic rehabilitationprocess for individual buildings. In addition to showinga simplified flow diagram of the overall process,Figure 1-1 indicates points at which input from thisdocument is likely, as well as potential steps outside thescope of the Guidelines. Specific chapter references areFEMA 273 Seismic Rehabilitation Guidelines 1-7

Chapter 1: Introductionnoted at points in the flow diagram where input fromthe Guidelines is to be obtained. This is a very generaldepiction of this process, which can take many formsand may include steps more numerous and in differentorder than shown.As indicated in Section 1.3, the Guidelines are writtenwith the assumption that the user has already concludedthat a building needs to be seismically improved;evaluation techniques for reaching this decision are notspecifically prescribed. However, the use of the detailedanalysis and verification techniques associated withSystematic Rehabilitation (Section 1.5.4) may indicatethat some buildings determined to be deficient by otherevaluation or classification systems are actuallyacceptable without modification. This might occur, forexample, if a Guidelines analysis method reveals that anexisting building has greater capacity than wasdetermined by use of a less exact evaluation method.1.5.1 Initial Considerations for IndividualBuildingsThe use of the Guidelines will be simplified and mademore efficient if certain base information is obtainedand considered prior to beginning the process.The building owner should be aware of the range ofcosts and impacts of rehabilitation, including both thevariation associated with different RehabilitationObjectives and the potential add-on costs oftenassociated with seismic rehabilitation, such as other lifesafety upgrades, hazardous material removal, workassociated with the Americans with Disabilities Act,and nonseismic building remodeling. Also to beconsidered are potential federal tax incentives for therehabilitation for historic buildings and for some otherolder nonresidential buildings.The use of the building must be considered in weighingthe significance of potential temporary or permanentdisruptions associated with various risk mitigationschemes. Other limitations on modifications to thebuilding due to historic or aesthetic features must alsobe understood. The historic status of every building atleast 50 years old should be determined (see the sidebar,Considerations for Historic Buildings, later in thischapter). This determination should be made early,because it could influence the choices of rehabilitationapproaches and techniques.This document is focused primarily on the technicalaspects of rehabilitation. Basic information specificallyincluded in the Guidelines is discussed below.1.5.1.1 Site Hazards Other than SeismicGround ShakingThe analysis and design procedures of the Guidelinesare primarily aimed at improving the performance ofbuildings under the loads and deformations imposed byseismic shaking. However, other seismic hazards couldexist at the building site that could damage the buildingregardless of its ability to resist ground shaking. Thesehazards include fault rupture, liquefaction or othershaking-induced soil failures, landslides, andinundation from offsite effects such as dam failure ortsunami.The risk and possible extent of damage from such sitehazards should be considered before undertakingrehabilitation aimed solely at reducing shaking damage.In some situations, it may be feasible to mitigate the sitehazard. In many cases, the likelihood of the site hazardoccurring will be sufficiently small that rehabilitatingthe building for shaking alone is appropriate. Where asite hazard exists, it may be feasible to mitigate it, eitherby itself or in connection with the buildingrehabilitation project. It is also possible that the riskfrom a site hazard is so extreme and difficult to controlthat rehabilitation will not be cost-effective.Chapter 2 describes the applicability of seismic groundfailure hazards to this document’s seismic rehabilitationrequirements, and Chapter 4 describes correspondinganalysis procedures and mitigation measures.1.5.1.2 Characteristics of the ExistingBuildingChapter 2 discusses investigation of as-built conditions.Efficient use of the Guidelines requires basicknowledge of the configuration, structuralcharacteristics, and seismic deficiencies of the building.Much of this information will normally be availablefrom a seismic evaluation of the building. For situationswhere seismic rehabilitation has been mandated bylocal government according to building constructionclassification, familiarity with the building type and itstypical seismic deficiencies is recommended. Suchinformation is available from several sources, includingFEMA 178 (BSSC, 1992b) and the companion ExampleApplications document.1-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: IntroductionInterest in reducing seismic risk1 Review initial considerations• Structural characteristics (Chapter 2)• Site seismic hazards (Chapters 2 and 4)• Occupancy (not considered in Guidelines; see Section 1.3)• Historic status (see Section 1.6.1.3)• Economic considerations: See Example Applications volume (FEMA 276)for cost information• Societal Issues: See Planning for Seismic Rehabilitation: Societal Issues(FEMA 275)2 Select Rehabilitation Objective (Chapter 2)• Earthquake ground motion• Performance level3 Select initial approach to risk mitigation (Chapter 2)3A Simplified rehabilitation(Chapters 2, 10 and 11)• Identify building model type• Consider deficiencies• Select full or partialrehabilitation(Note: Simplified Rehabilitation can beused for Limited Objectives only.)3BSystematic rehabilitation(Chapters 2–9 and 11)• Consider deficiencies• Select rehabilitation strategy(Chapter 2)• Select analysis procedure(Chapters 2 and 3)• Consider general requirements(Chapter 2)3COther choices(not in Guidelines)• Reduce occupancy•Demolish4ADesign rehabilitationmeasures• Determine and designcorrective measures tomeet applicableFEMA 178 requirements4BPerform rehabilitation design• Develop mathematical model (Chapters 3 through 9 for stiffness andstrength)• Perform force and deformation response evaluation(Chapters 2 through 9 and 11)• Size elements, components, and connections(Chapters 2, 5 through 9, and 11)5AVerify rehabilitation design measures• Reevaluate building to assurethat rehabilitation measuresremove all deficiencies withoutcreating new ones• Review for economic acceptability5BVerify rehabilitation measures• Apply component acceptance criteria (Chapters 2 through 9and 11)• Review for conformance with requirements of Chapter 2• Review for economic acceptability6A1 If not acceptable• Return to 3A and reviserehabilitation goal or to 4Aand revise correctivemeasures6A2 If acceptable• Develop constructiondocuments• Begin rehabilitation• Exercise quality control(Chapter 2)6B1 If not acceptable• Return to 3B to refineanalysis and design or to2 to reconsiderRehabilitation Objective6B2 If acceptable• Develop constructiondocuments• Begin rehabilitation• Exercise quality control(Chapter 2)Figure 1-1Rehabilitation Process FlowchartFEMA 273 Seismic Rehabilitation Guidelines 1-9

Chapter 1: IntroductionBasic information about the building is needed todetermine eligibility for Simplified Rehabilitation(Step 3 in Figure 1-1), if its use is desired, or to developa preliminary design (Step 4 in Figure 1-1). It is prudentto perform preliminary calculations to select keylocations or parameters prior to establishing a detailedtesting program, in order to obtain knowledge costeffectivelyand with as little disruption as possible ofconstruction features and materials properties atconcealed locations.If the building is historic, additional as-built conditionsshould be more thoroughly investigated and analyzed.Publications dealing with the specialized subject of thecharacter-defining spaces, features, and details ofhistoric buildings should be consulted, and the servicesof a historic preservation expert may be required.1.5.1.3 Rehabilitation ObjectiveA Rehabilitation Objective must be selected, at least ona preliminary basis, before beginning to use theprocedures of the Guidelines. A RehabilitationObjective is a statement of the desired limits of damageor loss (Performance Level) for a given seismicdemand. The selection of a Rehabilitation Objectivewill be made by the owner and engineer in voluntaryrehabilitation cases, or by relevant public agencies inmandatory programs. If the building is historic, thereshould be an additional goal to preserve its historicfabric and character in conformance with the Secretaryof the Interior’s Standards for Rehabilitation.Whenever possible, the Rehabilitation Objective shouldmeet the requirements of the BSO, which consists oftwo parts: 1, the Life Safety Building PerformanceLevel for BSE-1 (the earthquake ground motion with a10% chance of exceedance in 50 years (10%/50 year),but in no case exceeding two-thirds of the groundresponse expressed for the Maximum ConsideredEarthquake) and 2, the Collapse Prevention BuildingPerformance Level for the earthquake ground motionrepresenting the large, rare event, called the MaximumConsidered Earthquake (described in the Guidelines asBSE-2). Throughout this document, the BSO provides anational benchmark with which lower or higherRehabilitation Objectives can be compared.Due to the variation in performance associated withunknown conditions in existing buildings, deteriorationof materials, incomplete site data, and large variationexpected in ground shaking, compliance with theGuidelines should not be considered a guarantee of thespecified performance. The expected reliability ofachieving various Performance Levels when therequirements of a given Level are followed is discussedin the Commentary to Chapter 2.1.5.2 Initial Risk Mitigation StrategiesThere are many ways to reduce seismic risk, whetherthe risk is to property, life safety, or post-earthquake useof the building. The occupancy of vulnerable buildingscan be reduced, redundant facilities can be provided,and nonhistoric buildings can be demolished andreplaced. The risks posed by nonstructural componentsand contents can be reduced. Seismic site hazards otherthan shaking can be mitigated.Most often, however, when all alternatives areconsidered, the options of modifying the building toreduce the risk of damage must be studied. Suchcorrective measures include stiffening or strengtheningthe structure, adding local elements to eliminateirregularities or tie the structure together, reducing thedemand on the structure through the use of seismicisolation or energy dissipation devices, and reducing theheight or mass of the structure. These modificationstrategies are discussed in Chapter 2.Modifications appropriate to the building can bedetermined using either the Simplified RehabilitationMethod or Systematic Rehabilitation Method.1.5.3 Simplified RehabilitationSimplified Rehabilitation will apply to many smallbuildings of regular configuration, particularly inmoderate or low seismic zones. SimplifiedRehabilitation requires less complicated analysis and insome cases less design than the complete analyticalrehabilitation design procedures found underSystematic Rehabilitation. In many cases, SimplifiedRehabilitation represents a cost-effective improvementin seismic performance, but often does not requiresufficiently detailed or complete analysis andevaluation to qualify for a specific Performance Level.Simplified Rehabilitation techniques are described forcomponents (e.g., parapets, wall ties), as well as entiresystems. Simplified Rehabilitation of structural systemsis covered in Chapter 10, and the combinations ofseismicity, Model Building, and other considerationsfor which it is allowed are provided in Section 2.8 andin Table 10-1. Simplified rehabilitation of nonstructuralcomponents is covered in Chapter 11.1-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: Introduction1.5.4 Systematic RehabilitationThe Systematic Rehabilitation Method is intended to becomplete and contains all requirements to reach anyspecified Performance Level. Systematic Rehabilitationis an iterative process, similar to the design of newbuildings, in which modifications of the existingstructure are assumed for the purposes of a preliminarydesign and analysis, and the results of the analysis areverified as acceptable on an element and componentbasis. If either new or existing components or elementsstill prove to be inadequate, the modifications areadjusted and, if necessary, a new analysis andverification cycle is performed. SystematicRehabilitation is covered in Chapters 2 through 9,and 11.1.5.4.1 Preliminary DesignA preliminary design is needed to define the extent andconfiguration of corrective measures in sufficient detailto estimate the interaction of the stiffness, strength, andpost-yield behavior of all new, modified, or existingelements to be used for lateral force resistance. Thedesigner is encouraged to include all elements withsignificant lateral stiffness in a mathematical model toassure deformation capability under realistic seismicdrifts. However, just as in the design of new buildings,it may be determined that certain components orelements will not be considered part of thelateral-force-resisting system, as long as deformationcompatibility checks are made on these components orelements to assure their adequacy. In Figure 1-1, thepreliminary design is in Steps 3 and 4.1.5.4.2 AnalysisA mathematical model, developed for the preliminarydesign, must be constructed in connection with one ofthe analysis procedures defined in Chapter 3. These arethe linear procedures (Linear Static and LinearDynamic) and the nonlinear procedures (NonlinearStatic and Nonlinear Dynamic). With the exception ofthe Nonlinear Dynamic Procedure, the Guidelinesdefine the analysis and rehabilitation design proceduressufficiently that compliance can be checked by abuilding department in a manner similar to designreviews for new buildings. Modeling assumptions to beused in various situations are given in Chapters 4through 9, and Chapter 11 for nonstructuralcomponents, and guidance on required seismic demandis given in Chapter 2. Guidance is given for the use ofthe Nonlinear Dynamic Procedure; however,considerable judgment is required in its application.Criteria for applying ground motion for various analysisprocedures is given, but definitive rules for developingground motion input are not included in the Guidelines.1.5.5 Verification and Economic AcceptanceFor systematic rehabilitation, the effects of forces anddisplacements imposed on various elements by theseismic demand must be checked for acceptability forthe selected Performance Level. These acceptabilitycriteria, generally categorized by material, are given inChapters 4 through 9. In addition, certain overalldetailing, configuration, and connectivity requirements,covered in Chapter 2 and in Chapter 10 for simplifiedrehabilitation, must be satisfied prior to completeacceptance of the rehabilitation design. Nonstructuralcomponents are covered in Chapter 11. At this stage acost estimate can be made to review the design’seconomic acceptability.If the design proves uneconomical or otherwiseunfeasible, different Rehabilitation Objectives or riskmitigation strategies may have to be considered, and theprocess would begin anew at Step 2 or 3 in Figure 1-1.The process would return to Step 3 or 4 if onlyrefinements were needed in the design, or if a differentscheme were to be tested.1.5.6 Implementation of the DesignWhen a satisfactory design is completed, the importantimplementation phase may begin. Chapter 2 containsprovisions for a quality assurance program duringconstruction. While detailed analysis of constructioncosts and scheduling is not covered by the procedures inthe Guidelines, these important issues are discussed inthe Example Applications volume (ATC, 1997). Othersignificant aspects of the implementation process—including details of the preparation of constructiondocuments by the architectural and engineering designprofessionals, obtaining a building permit, selection of acontractor, details of historic preservation techniquesfor particular kinds of materials, and financing—are notpart of the Guidelines.Social, Economic, and Political ConsiderationsFEMA 273 Seismic Rehabilitation Guidelines 1-11

Chapter 1: IntroductionThe scope of the Guidelines is limited to the engineeringbasis for seismically rehabilitating a building, but theuser should also be aware of significant nonengineeringissues and social and economic impacts. These problemsand opportunities, which vary with each situation, arediscussed in a separate publication, Planning for SeismicRehabilitation: Societal Issues (FEMA 275).Construction CostIf seismic rehabilitation were always inexpensive, the socialand political costs and controversies would largely disappear.Unfortunately, seismic rehabilitation often requires removal ofarchitectural materials to access the vulnerable portions of thestructure, and nonseismic upgrading (e.g., electrical,handicapped access, historic restoration) is frequently“triggered” by a building code’s remodeling permitrequirements or is desirable to undertake at the same time.HousingWhile seismic rehabilitation ultimately improves the housingstock, units can be temporarily lost during the constructionphase, which may last more than a year. This can requirerelocation of tenants.Impacts on Lower-Income GroupsLower-income residents and commercial tenants can bedisplaced by seismic rehabilitation. Often caused byupgrading unrelated to earthquake concerns, seismic upgradingalso tends to raise rents and real estate prices, because of theneed to recover the costs of the investment.RegulationsAs with efforts to impose safety regulations in other fields,mandating seismic rehabilitation is often controversial. TheGuidelines are not written as mandatory code provisions, butone possible application is to adapt them for that use. In suchcases political controversy should be expected, andnonengineering issues of all kinds should be carefullyconsidered.ArchitectureEven if a building is not historic, there are often significantarchitectural impacts. The exterior and interior appearance maychange, and the division of spaces and arrangement ofcirculation routes may be altered.Community RevitalizationSeismic rehabilitation not only poses issues and implies costs,it also confers benefits. In addition to enhanced public safetyand economic protection from earthquake loss, seismicrehabilitation can play a leading role in the revitalization ofolder commercial and industrial areas as well as residentialneighborhoods.1.6 Use of the Guidelines for Local orDirected Risk MitigationProgramsThe Guidelines have been written to accommodate usein a wide variety of situations, including both local riskmitigation programs and directed programs created bybroadly based organizations or governmental agenciesthat have jurisdiction over many buildings. Theseprograms may target certain building types forrehabilitation or require complete rehabilitation coupledwith other remodeling work. The incorporation ofvariable Rehabilitation Objectives and use of ModelBuilding Types in the Guidelines allows creation ofsubsets of rehabilitation requirements to suit localconditions of seismicity, building inventory, social andeconomic considerations, and other factors. Provisionsappropriate for local situations can be extracted, putinto regulatory language, and adopted into appropriatecodes, standards, or local ordinances.1.6.1 Initial Considerations for MitigationProgramsLocal or directed programs can either target high-riskbuilding types or set overall priorities. These decisionsshould be made with full consideration of physical,social, historic, and economic characteristics of thebuilding inventory. Although financial incentives caninduce voluntary risk mitigation, carefully plannedmandatory or directed programs, developed incooperation with those whose interests are affected, aregenerally more effective. Potential benefits of suchprograms include reduction of direct earthquakelosses—such as casualties, costs to repair damage, andloss of use of buildings—as well as more rapid overallrecovery. Rehabilitated buildings may also increase invalue and be assigned lower insurance rates. Additionalissues that should be considered for positive or negativeeffects include the interaction of rehabilitation withoverall planning goals, historic preservation, and thelocal economy. These issues are discussed in Planningfor Seismic Rehabilitation: Societal Issues (VSP, 1996).1-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: Introduction1.6.1.1 Potential Costs of Local or DirectedProgramsThe primary costs of seismic rehabilitation—theconstruction work itself, including design, inspection,and administration—are normally paid by the owner.Additional costs that should be weighed when creatingseismic risk reduction programs are those associatedwith developing and administering the program, such asthe costs of identification of high-risk buildings,environmental or socioeconomic impact reports,training programs, plan checking and constructioninspection.The construction costs include not only the cost of thepure structural rehabilitation but also the costsassociated with new or replaced finishes that may berequired. In some cases, seismic rehabilitation workwill trigger other local jurisdictional requirements, suchas hazardous material removal or partial or fullcompliance with the Americans with Disabilities Act.The costs of seismic or functional improvements tononstructural systems should also be considered. Theremay also be costs to the owner associated withtemporary disruption or loss of use of the buildingduring construction. To offset these costs, there may below-interest earthquake rehabilitation loans availablefrom state or local government, or historic building taxcredits.If seismic rehabilitation is the primary purpose ofconstruction, the costs of the various nonseismic workthat may be required should be included as directconsequences. On the other hand, if the seismic work isan added feature of a major remodel, the nonseismicimprovements probably would have been requiredanyway, and therefore should not be attributed toseismic rehabilitation.A discussion of these issues, as well as guidance on therange of costs of seismic rehabilitation, is included inFEMA 156 and 157, Second Edition, Typical Costs forSeismic Rehabilitation of Buildings (Hart, 1994 and1995) and in FEMA 276, Guidelines for the SeismicRehabilitation of Buildings: Example Applications(ATC, 1997). Since the data for these documents weredeveloped prior to the Guidelines, the information is notbased on buildings rehabilitated specifically inaccordance with the current document. However,Performance Levels defined in the Guidelines are notintended to be significantly different than parallel levelsused previously, and costs should still be reasonablyrepresentative.1.6.1.2 Timetables and EffectivenessPresuming that new buildings are being constructedwith adequate seismic protection and that olderbuildings are occasionally demolished or replaced, theinventory of seismically hazardous buildings in anycommunity will be gradually reduced. This attrition rateis normally small, since the structures of manybuildings have useful lives of 100 years or more andvery few buildings are actually demolished. If buildingsor districts become historically significant, they maynot be subject to attrition at all. In many cases, then,doing nothing (or waiting for an outside influence toforce action) may present a large cumulative risk to theinventory.It has often been pointed out that exposure time is asignificant element of risk. The time aspect of riskreduction is so compelling that it often appears as partof book and workshop titles; for example, Between TwoEarthquakes: Cultural Property in Seismic Zones(Feilden, 1987); Competing Against Time (CaliforniaGovernor’s Board of Inquiry, 1990); and “In Wait forthe Next One” (EERI, 1995). Therefore, an importantconsideration in the development of programs is thetime allotted to reach a certain risk reduction goal. It isgenerally assumed that longer programs create lesshardship than short ones by allowing more flexibility inplanning for the cost and possible disruption ofrehabilitation, as well as by allowing natural oraccelerated attrition to reduce undesirable impacts. Onthe other hand, the net reduction of risk is smaller due tothe increased exposure time of the seismically deficientbuilding stock.Given a high perceived danger and certainadvantageous characteristics of ownership, size, andoccupancy of the target buildings, mandatory programshave been completed in as little as five to ten years.More extensive programs—involving complexbuildings such as hospitals, or with significant fundinglimitations—may have completion goals of 30 to 50years. Deadlines for individual buildings are also oftendetermined by the risk presented by building type,occupancy, location, soil type, funding availability, orother factors.1.6.1.3 Historic PreservationSeismic rehabilitation of buildings can affect historicpreservation in two ways. First, the introduction of newelements that will be associated with the rehabilitationmay in some way impact the historic fabric of theFEMA 273 Seismic Rehabilitation Guidelines 1-13

Chapter 1: IntroductionIt must be determined early in the process whether abuilding is “historic.” A building is historic if it is atleast 50 years old and is listed in or potentiallyeligible for the National Register of Historic Placesand/or a state or local register as an individualstructure or as a contributing structure in a district.Structures less than 50 years old may also be historicif they possess exceptional significance. For historicbuildings, users should develop and evaluatealternative solutions with regard to their effect on theloss of historic character and fabric, using theSecretary of the Interior’s Standards forRehabilitation (Secretary of the Interior, 1990).In addition to rehabilitation, the Secretary of theInterior also has standards for preservation,restoration, and reconstruction. These are published inthe Standards for the Treatment of Historic Properties(Secretary of the Interior, 1992). A seismicrehabilitation project may include work that fallsunder the Rehabilitation Standards, the TreatmentStandards, or both.For historic buildings as well as for other structures ofarchitectural interest, it is important to note that theSecretary of the Interior’s Standards definerehabilitation as “the process of returning a propertyto a state of utility, through repair or alteration, whichmakes possible an efficient contemporary use whilepreserving those portions and features of the propertywhich are significant to its historic, architectural andcultural values.” The Secretary has also publishedstandards for “preservation,” “restoration,” and“reconstruction.” Further guidance on the treatment ofhistoric properties is contained in the publications inthe Catalog of Historic Preservation Publications(NPS, 1995).Rehabilitation ObjectivesIf seismic rehabilitation is required by the governingbuilding jurisdiction, the minimum seismicrequirements should be matched with a RehabilitationObjective defined in the Guidelines. It should beConsiderations for Historic Buildingsnoted that many codes covering historic buildingsallow some amount of flexibility in requiredperformance, depending on the effect of rehabilitationon important historic features.If a building contains items of unusual architecturalinterest, consideration should be given to the value ofthese items. It may be desirable to rehabilitate thebuilding to the Damage Control Performance Rangeto ensure that the architectural fabric survives certainearthquakes.Rehabilitation StrategiesIn development of initial risk mitigation strategies,consideration must be given to the architectural andhistoric value of the building and its fabric.Development of a Historic Structure Reportidentifying the primary historic fabric may beessential in the preliminary planning stages for certainbuildings. Some structurally adequate solutions maynevertheless be unacceptable because they involvedestruction of historic fabric or character. Alternaterehabilitation methods that lessen the impact on thehistoric fabric should be developed for consideration.Partial demolition may be inappropriate for historicstructures. Elements that create irregularities may beessential to the historic character of the structure. Theadvice of historic preservation experts may benecessary.Structural rehabilitation of historic buildings may beaccomplished by hiding the new structural membersor by exposing them as admittedly new elements inthe building’s history. Often, the exposure of newstructural members is preferred, because alterations ofthis kind are “reversible”; that is, they couldconceivably be undone at a future time with no loss ofhistoric fabric to the building. The decision to hide orexpose structural members is a complex one, bestmade by a preservation professional.building. Second, the seismic rehabilitation work canserve to better protect the building from possiblyunrepairable future earthquake damage. The effects ofany seismic risk reduction program on historicbuildings or preservation districts should be carefullyconsidered during program development, andsubsequent work should be carefully monitored toassure compliance with previously mentioned nationalpreservation guidelines. (See the sidebar,“Considerations for Historic Buildings.”)1-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: Introduction1.6.2 Use in Passive ProgramsPrograms that only require seismic rehabilitation inassociation with other activity on the building are oftenclassified as “passive.” “Active” programs, on the otherhand, are those that mandate seismic rehabilitation fortargeted buildings in a certain time frame, regardless ofother activity associated with the building (seeSection 1.6.3). Activities in a building that maypassively generate a requirement to seismicallyrehabilitate—such as an increase in occupancy,structural modification, or a major remodeling thatwould significantly extend the life of the building—arecalled “triggers.” The concept of certain activitiestriggering compliance with current standards is wellestablished in building codes. However, the details ofthe requirements have varied widely. These issues havebeen documented with respect to seismic rehabilitationin California (Hoover, 1992). Passive programs reducerisk more slowly than active programs.1.6.2.1 Selection of Seismic RehabilitationTriggersThe Guidelines do not cover triggers for seismicrehabilitation. The extent and detail of seismic triggerswill greatly affect the speed, effectiveness, and impactsof seismic risk reduction, and the selection of triggers isa policy decision expected to be done locally, by theperson or agency responsible for the inventory. Triggersthat have been used or considered in the past includerevision of specified proportions of the structure,remodeling of specified percentages of the buildingarea, work on the building that costs over a specifiedpercentage of the building value, change in use thatincreases the occupancy or importance of the building,and changes of ownership.1.6.2.2 Selection of Passive SeismicRehabilitation StandardsThe Guidelines purposely afford a wide variety ofoptions that can be adopted into standards for seismicrehabilitation to facilitate risk reduction. Standards canbe selected with varying degrees of risk reduction andvarying costs by designating different RehabilitationObjectives. As described previously, a RehabilitationObjective is created by specifying a desired BuildingPerformance Level for specified earthquake groundmotion criteria. A jurisdiction can thus specifyappropriate standards by extracting applicablerequirements and incorporating them into its own codeor standard, or by reference.A single Rehabilitation Objective could be selectedunder all triggering situations (the BSO, for example),or more stringent objectives can be used for importantchanges to the building, less stringent objectives forminor changes. For example, it is sometimes necessaryfor design professionals, owners, and building officialsto negotiate the extent of seismic improvements done inassociation with building alterations. Completerehabilitation is often required by local regulation forcomplete remodels or major structural alterations. It isthe intent of the Guidelines to provide a commonframework for all of these various uses.1.6.3 Use in Active or Mandated ProgramsActive programs are most often targeted at high-riskbuilding types or occupancies. Active seismic riskreduction programs are those that require owners torehabilitate their buildings in a certain time frame or, inthe case of government agencies or other owners oflarge inventories, to set self-imposed deadlines forcompletion.1.6.3.1 Selection of Buildings to be IncludedPrograms would logically target only the highest-riskbuildings or at least create priorities based on risk. Riskcan be based on the likelihood of building failure, theoccupancy or importance of buildings, soil types, orother factors. The Guidelines are primarily written to beused in the process of rehabilitation and do not directlyaddress the comparative risk level of various buildingtypes or other risk factors. Certain building types, suchas unreinforced masonry bearing wall buildings andolder improperly detailed reinforced concrete framebuildings, have historically presented a high risk,depending on local seismicity and building practice.Therefore, these building types have sometimes beentargeted in active programs.A more pragmatic consideration is the ease of locatingtargeted buildings. If certain building types cannot beeasily identified, either by the local jurisdiction or bythe owners and their engineers, enforcement couldbecome difficult and costly. In the extreme, everybuilding designed prior to a given acceptable code cyclewould require a seismic evaluation to determinewhether targeted characteristics or other risk factors arepresent, the cost of which may be significant. Analternate procedure might be to select easily identifiablebuilding characteristics to set timelines, even if moreaccurate building-by-building priorities are somewhatcompromised.FEMA 273 Seismic Rehabilitation Guidelines 1-15

Chapter 1: Introduction1.6.3.2 Selection of Active SeismicRehabilitation StandardsAs discussed for passive programs (Section 1.6.2.2), theGuidelines are written to facilitate a wide variation inrisk reduction. Factors used to determine an appropriateRehabilitation Objective include local seismicity, thecosts of rehabilitation, and local socioeconomicconditions.It may be desirable to use Simplified RehabilitationMethods for active or mandated programs. OnlyLimited Performance Objectives are included in theGuidelines for this method. However, if a program hasidentified a local building type with few variations inmaterial and configuration, a study of a sample oftypical buildings using Systematic Methods mayestablish that compliance with the requirements ofSimplified Rehabilitation meets the BSO, or better, forthis building type in this location. Such risk andperformance decisions can only be made at the locallevel.1.7 ReferencesATC, 1992, Development of Guidelines for SeismicRehabilitation of Buildings, Phase I: IssuesIdentification and Resolution, developed by the AppliedTechnology Council (Report No. ATC-28) for theFederal Emergency Management Agency (Report No.FEMA 237), Washington, D.C.ATC, 1993, Proceedings of the Workshop to ResolveSeismic Rehabilitation Subissues—July 29 and 30,1993; Development of Guidelines for SeismicRehabilitation of Buildings, Phase I: IssuesIdentification and Resolution, Report No. ATC-28-2,Applied Technology Council, Redwood City,California.ATC, 1996, Seismic Evaluation and Retrofit of ConcreteBuildings, prepared by the Applied TechnologyCouncil, (Report No. ATC-40), Redwood City,California, for the California Seismic SafetyCommission (Report No. SSC 96-01).ATC, 1997, Guidelines for the Seismic Rehabilitation ofBuildings: Example Applications, prepared by theApplied Technology Council, for the Building SeismicSafety Council and the Federal EmergencyManagement Agency (Report No. FEMA 276),Washington, D.C.BSSC, 1992a, NEHRP Handbook of Techniques for theSeismic Rehabilitation of Existing Buildings, developedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 172), Washington, D.C.BSSC, 1992b, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 178), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos.FEMA 222A and 223A), Washington, D.C.BSSC, 1997, NEHRP Recommended Provisions forSeismic Regulations for New Buildings and OtherStructures, 1997 Edition, Part 1: Provisions and Part 2:Commentary, prepared by the Building Seismic SafetyCouncil for the Federal Emergency ManagementAgency (Report Nos. FEMA 302 and 303),Washington, D.C.California Governor’s Board of Inquiry on the 1989Loma Prieta Earthquake, 1990, Competing AgainstTime, report to Governor George Deukmejian, State ofCalifornia, Office of Planning and Research,Sacramento, California.EERI, 1995, “In Wait for the Next One,” Proceedings ofthe Fourth Japan/U.S. Workshop on Urban EarthquakeHazard Reduction, Earthquake Engineering ResearchInstitute and Japan Institute of Social Safety Science,sponsors, Osaka, Japan.Feilden, Bernard M., 1987, Between Two Earthquakes:Cultural Property in Seismic Zones, Getty ConservationInstitute, Marina del Rey, California.1-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 1: IntroductionHart, 1994, Typical Costs for Seismic Rehabilitation ofBuildings, Second Edition, Volume 1: Summary,prepared by the Hart Consultant Group for the FederalEmergency Management Agency (Report No. FEMA156), Washington, D.C.Hart, 1995, Typical Costs for Seismic Rehabilitation ofBuildings, Second Edition, Volume II: SupportingDocumentation, prepared by the Hart Consultant Groupfor the Federal Emergency Management Agency(Report No. FEMA 157), Washington, D.C.Hoover, C. A., 1992, Seismic Retrofit Policies: AnEvaluation of Local Practices in Zone 4 and TheirApplication to Zone 3, Earthquake EngineeringResearch Institute, Oakland, California.Secretary of the Interior, 1990, Standards forRehabilitation and Guidelines for RehabilitatingHistoric Buildings, National Park Service,Washington, D.C.Secretary of the Interior, 1992, Standards for theTreatment of Historic Properties, National ParkService, Washington, D.C.VSP, 1996, Planning for Seismic Rehabilitation:Societal Issues, prepared by VSP Associates for theBuilding Seismic Safety Council and FederalEmergency Management Agency (Report No.FEMA 275), Washington, D.C.NPS, 1995, Catalog of Historic PreservationPublications, National Park Service, Washington, D.C.FEMA 273 Seismic Rehabilitation Guidelines 1-17

2.General Requirements(Simplified and Systematic Rehabilitation)2.1 ScopeThis chapter presents the Guidelines’ generalrequirements for rehabilitating existing buildings. Theframework in which these requirements are specified ispurposefully broad in order to accommodate buildingsof many different types, satisfy a broad range ofperformance levels, and include consideration of thevariety of seismic hazards throughout the United Statesand Territories.Criteria for the following general issues regarding theseismic rehabilitation of buildings are included in thischapter:• Rehabilitation Objectives: Selection of desiredperformance levels for given earthquake severitylevels– Performance Levels: Definition of theexpected behavior of the building in the designearthquake(s) in terms of limiting levels ofdamage to the structural and nonstructuralcomponents– Seismic Hazard: Determination of the designground shaking and other site hazards, such aslandsliding, liquefaction, or settlement• As-Built Characteristics: Determination of thebasic construction characteristics and earthquakeresistive capacity of the existing building• Rehabilitation Methods: Selection of theSimplified or Systematic Method• Rehabilitation Strategies: Selection of a basicstrategy for rehabilitation, e.g., providing additionallateral-load-carrying elements, seismic isolation, orreducing the mass of the building• Analysis and Design Procedures: For SystematicRehabilitation approaches, selection among LinearStatic, Linear Dynamic, Nonlinear Static, orNonlinear Dynamic Procedures• General Analysis and Design: Specification ofthe force and deformation actions for which givencomponents of a building must be evaluated, andminimum design criteria for interconnection ofstructural components• Building Interaction: Guidelines for buildingsthat share elements with neighboring structures, andbuildings with performance affected by the presenceof adjacent structures• Quality Assurance: Guidelines for ensuring thatthe design intent is appropriately implemented in theconstruction process• Alternative Materials and Methods: Guidelinesfor evaluating and designing structural componentsnot specifically covered by other sections of theGuidelines2.2 Basic ApproachThe basic approach for seismic rehabilitation design includesthe steps indicated below. Note that these stepsare presented here in the order in which they would typicallybe followed in the rehabilitation process. However,the guidelines for actually performing these steps arepresented in a somewhat different order, to facilitate presentationof the concepts.• Obtain as-built information on the building anddetermine its characteristics, including whether thebuilding has historic status (Section 2.7).• Select a Rehabilitation Objective for the building(Section 2.4).• Select an appropriate Rehabilitation Method(Section 2.8).• If a Simplified Method is applicable, follow theprocedures of Chapter 10; or,• If a Systematic Method is to be followed:– Select a Rehabilitation Strategy (Section 2.10)and perform a preliminary design of correctivemeasures.– Select an appropriate Analysis Procedure(Section 2.9).FEMA 273 Seismic Rehabilitation Guidelines 2-1

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)– Perform an analysis of the building, including thecorrective measures, to verify its adequacy tomeet the selected Rehabilitation Objective(Chapter 3).– If the design is inadequate, revise the correctivemeasures or try an alternative strategy and repeatthe analysis until an acceptable design solution isobtained.Prior to embarking on a rehabilitation program, anevaluation should be performed to determine whetherthe building, in its existing condition, has the desiredlevel of seismic resistance. FEMA 178 (BSSC, 1992) isan example of an evaluation methodology that may beused for this purpose. However, FEMA 178 currentlydoes not address objectives other than the Life SafetyPerformance Level for earthquakes with a 10%probability of exceedance in 50 years (10%/50 year),whereas these Guidelines may be used for otherperformance levels and ground shaking criteria. FEMA178 is being revised to include the Damage ControlPerformance Range.Table 2-1 gives an overview of guidelines and criteriaincluded in this chapter and their relation to guidelinesand criteria in other chapters of the Guidelines.2.3 Design BasisThe Guidelines are intended to provide a nationallyapplicable approach for the seismic rehabilitation ofbuildings. It is expected that most buildingsrehabilitated in accordance with the Guidelines wouldperform within the desired levels when subjected to thedesign earthquakes. However, compliance with theGuidelines does not guarantee such performance. Thepractice of earthquake engineering is rapidly evolving,and both our understanding of the behavior of buildingssubjected to strong earthquakes and our ability topredict this behavior are advancing. In the future, newknowledge and technology will provide more reliablemethods of accomplishing these goals.The procedures contained in the Guidelines arespecifically applicable to the rehabilitation of existingbuildings and are, in general, more appropriate for thatpurpose than are building codes for the seismic designof new buildings. Building codes are primarily intendedto regulate the design and construction of newbuildings; as such, they include many provisions thatencourage the development of designs with featuresimportant to good seismic performance, includingregular configuration, structural continuity, ductiledetailing, and materials of appropriate quality. Manyexisting buildings were designed and constructedwithout these features, and contain characteristics—such as unfavorable configuration and poor detailing—that preclude application of building code provisions fortheir seismic rehabilitation.A Rehabilitation Objective must be selected as the basisfor a rehabilitation design in order to use the provisionsof these Guidelines. Each Rehabilitation Objectiveconsists of one or more specifications of a seismicdemand (hazard level) and corresponding damage state(building performance level). The Guidelines present aBasic Safety Objective (BSO), which has performanceand hazard levels consistent with seismic risktraditionally considered acceptable in the United States.Alternative objectives that provide lower levels(Limited Objectives) and higher levels (EnhancedObjectives) of performance are also described in theGuidelines.Each structural component and element of the building,including its foundations, shall be classified as eitherprimary or secondary. In a typical building, nearly allelements, including many nonstructural components,will contribute to the building’s overall stiffness, mass,and damping, and consequently its response toearthquake ground motion. However, not all of theseelements are critical to the ability of the structure toresist collapse when subjected to strong groundshaking. For example, exterior cladding and interiorpartitions can add substantial initial stiffness to astructure, yet this stiffness is not typically considered inthe design of new buildings for lateral force resistancebecause the lateral strength of these elements is oftensmall. Similarly, the interaction of floor framingsystems and columns in shear wall buildings can addsome stiffness, although designers typically neglectsuch stiffness when proportioning the building’s shearwalls. In the procedures contained in these Guidelines,the behavior of all elements and components thatparticipate in the building’s lateral response isconsidered, even if they are not normally considered aspart of the lateral-force-resisting system. This is toallow evaluation of the extent of damage likely to beexperienced by each of these elements. The concept ofprimary and secondary elements permits the engineer todifferentiate between the performance required ofelements that are critical to the building’s ability toresist collapse and of those that are not.2-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-1Guidelines and Criteria in Chapter 2 and Relation to Guidelines and Criteria in Other ChaptersChapter 2 Criteria SectionDetailed Implementation Criteria in Other ChaptersInformationActionSection Presented Chapter(s) Information PresentedSelect Rehabilitation Section 2.4 Detailed GuidelinesObjectiveSelect Performance Section 2.5 Detailed GuidelinesLevelSelect Shaking Hazard Section 2.6 Detailed CriteriaEvaluate Other SeismicHazardsObtain As-BuiltInformation, IncludingHistoric StatusSection 2.6 General Discussion Chapter 4 Evaluation and MitigationMethodsSection 2.7 Detailed Criteria Chapters 4–8 and 11 Material PropertyGuidelinesTesting GuidelinesSelect Rehabilitation Section 2.8 Rehabilitation MethodsMethodSimplified Chapters 10 and 11 Detailed GuidelinesSystematic Section 2.11 General Analysis andDesign CriteriaSelect AnalysisSection 2.9 Detailed CriteriaProcedureSelect Rehabilitation Section 2.10 Detailed GuidelinesStrategyCreate MathematicalModelPerform Force andDeformation EvaluationApply ComponentAcceptance CriteriaApply QualityAssuranceUse AlternativeMaterials and Methodsof ConstructionSection 2.11Section 2.11General Analysis andDesign CriteriaGeneral Analysis andDesign CriteriaChapters 3–9 and 11Chapter 3Chapters 4–9 and 11Chapter 3Section 2.9 General Criteria Chapter 3Chapters 4–9 and 11Section 2.12Section 2.13Detailed CriteriaDetailed CriteriaImplementation ofSystematic MethodDetailed RequirementsStiffness and Strength ofComponentsDetailed CriteriaDetailed CriteriaComponent Strength andDeformation Criteria• The primary elements and components are those thatprovide the structure’s overall ability to resistcollapse under earthquake-induced ground motion.Although damage to these elements, and somedegradation of their strength and stiffness, may bepermitted to occur, the overall function of theseelements in resisting structural collapse should notbe compromised.• Other elements and components of the existingbuilding are designated as secondary. For somestructural performance levels, substantialdegradation of the lateral-force-resisting stiffnessand strength of secondary elements and componentsis permissible, as this will not inhibit the entirebuilding’s capacity to withstand the design groundmotions. However, the ability of these secondaryelements and components to support gravity loads,under the maximum deformations that the designearthquake(s) would induce in the building, must bepreserved.FEMA 273 Seismic Rehabilitation Guidelines 2-3

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)For a given performance level, acceptance criteria forprimary elements and components will typically bemore restrictive (i.e., less damage is permissible) thanthose for secondary elements and components.In order to comply with the BSO or any EnhancedRehabilitation Objective, the rehabilitated buildingshall be provided with a continuous load path, or paths,with adequate strength and stiffness to transferseismically induced forces caused by ground motion inany direction, from the point of application to the finalpoint of resistance. It shall be demonstrated that allprimary and secondary elements of the structure arecapable of resisting the forces and deformationscorresponding to the earthquake hazards within theacceptance criteria contained in the Guidelines for theapplicable performance levels. Nonstructuralcomponents and building contents shall also beadequately anchored or braced to the structure tocontrol damage as required by the acceptance criteriafor the applicable performance level.2.4 Rehabilitation ObjectivesAs stated earlier, a Rehabilitation Objective shall beselected as the basis for design. RehabilitationObjectives are statements of the desired buildingperformance (see Section 2.5) when the building issubjected to earthquake demands of specified severity(see Section 2.6).Building performance can be described qualitatively interms of the safety afforded building occupants, duringand after the event; the cost and feasibility of restoringthe building to pre-earthquake condition; the length oftime the building is removed from service to effectrepairs; and economic, architectural, or historic impactson the larger community. These performancecharacteristics are directly related to the extent ofdamage sustained by the building.In these Guidelines, the extent of damage to a buildingis categorized as a Building Performance Level. Abroad range of Building Performance Levels may beselected when determining Rehabilitation Objectives.Each Building Performance Level consists of aStructural Performance Level, which defines thepermissible damage to structural systems, and aNonstructural Performance Level, which defines thepermissible damage to nonstructural buildingcomponents and contents.Section 2.5.1 defines a series of three discrete StructuralPerformance Levels that may be used in constructingproject Rehabilitation Objectives. These are ImmediateOccupancy (S-1), Life Safety (S-3), and CollapsePrevention (S-5). Two Structural Performance Rangesare defined to allow design for structural damage statesintermediate to those represented by the discreteperformance levels. These are Damage Control (S-2)and Limited Safety (S-4). In addition, there is thedesignation of S-6, Structural Performance NotConsidered, to cover the situation where onlynonstructural improvements are made.Section 2.5.2 defines a series of three discreteNonstructural Performance Levels. These are:Operational Performance Level (N-A), ImmediateOccupancy Performance Level (N-B), and Life SafetyPerformance Level (N-C). There is also a HazardsReduced Performance Range (N-D) and a fifth level orcategory (N-E) in which nonstructural damage is notlimited.Section 2.5.3 indicates how Structural andNonstructural Performance Levels may be combined toform designations for Building Performance Levels.Numerals indicate the Structural Performance Leveland letters the Nonstructural Performance Level. FourPerformance Levels commonly used in the formation ofBuilding Rehabilitation Objectives are described; theseare the Operational Performance Level (1-A),Immediate Performance Occupancy Level (1-B), LifeSafety Performance Level (3-C), and CollapsePrevention Performance Level (5-E).Section 2.6, Seismic Hazard, presents methods fordetermining earthquake shaking demands andconsidering other seismic hazards, such as liquefactionand landsliding. Earthquake shaking demands areexpressed in terms of ground motion response spectra,discrete parameters that define these spectra, or suitesof ground motion time histories, depending on theanalysis procedure selected. For sites with significantpotential for ground failure, demands should also beexpressed in terms of the anticipated permanentdifferential ground deformations.Earthquake demands are a function of the location ofthe building with respect to causative faults, theregional and site-specific geologic characteristics, andthe ground motion hazard level(s) selected in theRehabilitation Objective. In the Guidelines, hazardlevels may be defined on either a probabilistic or2-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)deterministic basis. Probabilistic hazards are defined interms of the probability that more severe demands willbe experienced (probability of exceedance) in a 50-yearperiod. Deterministic demands are defined within alevel of confidence in terms of a specific magnitudeevent on a particular fault, which is most appropriate forbuildings located within a few miles of a major activefault. Probabilistic hazard levels frequently used inthese Guidelines and their corresponding mean returnperiods (the average number of years between events ofsimilar severity) are as follows:Earthquake HavingProbability of ExceedanceMean Return Period(years)50%/50 year 7220%/50 year 22510%/50 year 4742%/50 year 2,475These mean return periods are typically rounded to 75,225, 500, and 2,500 years, respectively. The Guidelinesmake frequent reference to two levels of earthquakehazard that are particularly useful for the formation ofRehabilitation Objectives. These are defined in terms ofboth probabilistic and deterministic approaches. Theyare termed a Basic Safety Earthquake 1 (BSE-1) andBasic Safety Earthquake 2 (BSE-2). The BSE-1 andBSE-2 earthquakes are typically taken as 10%/50 and2%/50 year events, respectively, except in regions nearmajor active faults. In these regions the BSE-1 andBSE-2 may be defined based on deterministic estimatesof earthquakes on these faults. More detailed discussionof ground motion hazards is presented in Section 2.6.The Rehabilitation Objective selected as a basis fordesign will determine, to a great extent, the cost andfeasibility of any rehabilitation project, as well as thebenefit to be obtained in terms of improved safety,reduction in property damage, and interruption of use inthe event of future earthquakes. Table 2-2 presents amatrix indicating the broad range of RehabilitationObjectives that may be used in these Guidelines. (SeeSection 2.5.3 for definitions of Building PerformanceLevels.) Each cell in this matrix represents a singleRehabilitation Objective. The goal of a rehabilitationproject may be to satisfy a single RehabilitationObjective—for example, Life Safety for the BSE-1earthquake—or multiple Rehabilitation Objectives—forexample, Life Safety for the BSE-1 earthquake,Collapse Prevention for the BSE-2 earthquake, andImmediate Occupancy for an earthquake with a 50%probability of exceedance in 50 years. A specificanalytical evaluation should be performed to confirmthat a rehabilitation design is capable of meeting eachdesired Rehabilitation Objective selected as a goal forthe project.Table 2-2Earthquake HazardLevelRehabilitation Objectives2.4.1 Basic Safety ObjectiveBuilding Performance LevelsOperational PerformanceLevel (1-A)Immediate Occupancy PerformanceLevel (1-B)Life Safety PerformanceLevel (3-C)Collapse Prevention PerformanceLevel (5-E)50%/50 year a b c d20%/50 year e f g hBSE-1(~10%/50 year)BSE-2(~2%/50 year)i j k lm n o pk + p = BSOk + p + any of a, e, i, m; or b, f, j, or n = Enhanced Objectiveso = Enhanced Objectivek alone or p alone = Limited Objectivesc, g, d, h = Limited ObjectivesA desirable goal for rehabilitation is to achieve theBasic Safety Objective (BSO). In order to achieve thisobjective, building rehabilitation must be designed toachieve both the Life Safety Performance Level (3-C)for BSE-1 earthquake demands and the CollapsePrevention Level (5-E) for BSE-2 earthquake demands.Buildings that have been properly designed andconstructed in conformance with the latest edition of theNational Building Code (BOCA, 1993), StandardBuilding Code (SBCC, 1994), or Uniform BuildingFEMA 273 Seismic Rehabilitation Guidelines 2-5

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Code (ICBO, 1994), including all applicable seismicprovisions of those codes, may be deemed by codeenforcement agencies to meet the BSO.Building rehabilitation programs designed to the BSOare intended to provide a low risk of danger for anyearthquake likely to affect the site. This approximatelyrepresents the earthquake risk to life safety traditionallyconsidered acceptable in the United States. Buildingsmeeting the BSO are expected to experience littledamage from the relatively frequent, moderateearthquakes that may occur, but significantly moredamage from the most severe and infrequentearthquakes that could affect them.The level of damage to buildings rehabilitated to theBSO may be greater than that expected in properlydesigned and constructed new buildings.When it is desired that a building be able to resistearthquakes with less damage than implied by the BSO,rehabilitation may be designed to one or more of theEnhanced Rehabilitation Objectives of Section 2.4.2.2.4.2 Enhanced Rehabilitation ObjectivesAny Rehabilitation Objective intended to provideperformance superior to that of the BSO is termed anEnhanced Objective. An Enhanced Objective mustprovide better than BSO-designated performance ateither the BSE-1 or BSE-2, or both. Enhancedperformance can be obtained in two ways:• Directly, by design for the BSE-1 or BSE-2earthquakes. Examples include designing for ahigher Performance Level than Life Safety for theBSE-1 or a higher Performance Level than CollapsePrevention for the BSE-2.• Indirectly, by having the design controlled by someother selected Performance Level and hazard thatwill provide better than BSO performance at theBSE-1 or BSE-2. For example, if providingImmediate Occupancy for a 50%/50 year eventcontrolled the rehabilitation acceptability criteria insuch a way that deformation demand were less thanthat allowed by the BSO, the design would beconsidered to have an Enhanced Objective.The Guidelines do not incorporate EnhancedRehabilitation Objectives in any formal procedure, butthe definition is included to facilitate discussion of theconcept of variable Performance Levels both in theGuidelines and the Commentary.2.4.3 Limited Rehabilitation ObjectivesAny Rehabilitation Objective intended to provideperformance inferior to that of the BSO is termed aLimited Objective. A Limited Objective may consist ofeither Partial Rehabilitation (Section 2.4.3.1) orReduced Rehabilitation (Section 2.4.3.2). LimitedRehabilitation Objectives should be permissible if thefollowing conditions are met:• The rehabilitation measures do not create astructural irregularity or make an existing structuralirregularity more severe;• The rehabilitation measures do not result in areduction in the capability of the structure to resistlateral forces or deformations;• The rehabilitation measures do not result in anincrease in the seismic forces to any component thatdoes not have adequate capacity to resist theseforces, unless this component’s behavior is stillacceptable considering overall structuralperformance;• All new or rehabilitated structural elements aredetailed and connected to the existing structure, asrequired by the Guidelines;• An unsafe condition is not created or made moresevere by the rehabilitation measures; and• Locally adopted and enforced building regulationsdo not preclude such rehabilitation.2.4.3.1 Partial RehabilitationAny rehabilitation program that does not fully addressthe lateral-force-resisting capacity of the completestructure is termed Partial Rehabilitation. The portion ofthe structure that is addressed in Partial Rehabilitationshould be designed for a target Rehabilitation Objectiveand planned so that additional rehabilitation could beperformed later to meet fully that objective.2.4.3.2 Reduced RehabilitationReduced Rehabilitation programs address the entirebuilding’s lateral-force-resisting capacity, but not at thelevels required for the BSO. Reduced Rehabilitation2-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)may be designed for one or more of the followingobjectives:• Life Safety Performance Level (3-C) for earthquakedemands that are less severe (more probable) thanthe BSE-1• Collapse Prevention Performance Level (5-E) forearthquake demands that are less severe (moreprobable) than the BSE-2• Performance Levels 4-C, 4-D, 4-E, 5-C, 5-D, 5-E,6-D, or 6-E for BSE-1 or less severe (more probable)earthquake demands2.5 Performance LevelsBuilding performance is a combination of theperformance of both structural and nonstructuralcomponents. Table 2-3 describes the overall levels ofstructural and nonstructural damage that may beexpected of buildings rehabilitated to the levels definedin the Guidelines. For comparative purposes, theestimated performance of a new building subjected tothe BSE-1 level of shaking is indicated. Theseperformance descriptions are estimates rather thanprecise predictions, and variation among buildings ofthe same Performance Level must be expected.Independent performance definitions are provided forstructural and nonstructural components. Structuralperformance levels are identified in these Guidelines byboth a name and numerical designator (following S-) inSection 2.5.1. Nonstructural performance levels areidentified by a name and alphabetical designator(following N-) in Section 2.5.2.2.5.1 Structural Performance Levels andRangesThree discrete Structural Performance Levels and twointermediate Structural Performance Ranges aredefined. Acceptance criteria, which relate to thepermissible earthquake-induced forces anddeformations for the various elements of the building,both existing and new, are tied directly to theseStructural Performance Ranges and Levels.A wide range of structural performance requirementscould be desired by individual building owners. Thethree Structural Performance Levels defined in theseGuidelines have been selected to correlate with the mostcommonly specified structural performancerequirements. The two Structural Performance Rangespermit users with other requirements to customize theirbuilding Rehabilitation Objectives.The Structural Performance Levels are the ImmediateOccupancy Level (S-1), the Life Safety Level (S-3), andthe Collapse Prevention Level (S-5). Table 2-4 relatesthese Structural Performance Levels to the limitingdamage states for common vertical elements of lateralforce-resistingsystems. Table 2-5 relates theseStructural Performance Levels to the limiting damagestates for common horizontal elements of buildinglateral-force-resisting systems. Later sections of theseGuidelines specify design parameters (such as mfactors, component capacities, and inelasticdeformation demands) recommended as limiting valuesfor calculated structural deformations and stresses fordifferent construction components, in order to attainthese Structural Performance Levels for a knownearthquake demand.The drift values given in Table 2-4 are typical valuesprovided to illustrate the overall structural responseassociated with various performance levels. They arenot provided in these tables as drift limit requirementsof the Guidelines, and they do not supersede thespecific drift limits or related component or elementdeformation limits that are specified in Chapters 5through 9, and 11. The expected post-earthquake stateof the buildings described in these tables is for designpurposes and should not be used in the post-earthquakesafety evaluation process.The Structural Performance Ranges are the DamageControl Range (S-2) and the Limited Safety Range (S-4). Specific acceptance criteria are not provided fordesign to these intermediate performance ranges. Theengineer wishing to design for such performance needsto determine appropriate acceptance criteria.Acceptance criteria for performance within the DamageControl Range may be obtained by interpolating theacceptance criteria provided for the ImmediateOccupancy and Life Safety Performance Levels.Acceptance criteria for performance within the LimitedSafety Range may be obtained by interpolating theacceptance criteria for performance within the LifeSafety and Collapse Prevention Performance Levels.FEMA 273 Seismic Rehabilitation Guidelines 2-7

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.5.1.1 Immediate Occupancy PerformanceLevel (S-1)Structural Performance Level S-1, ImmediateOccupancy, means the post-earthquake damage state inwhich only very limited structural damage hasoccurred. The basic vertical-, and lateral-force-resistingsystems of the building retain nearly all of their preearthquakestrength and stiffness. The risk of lifethreateninginjury as a result of structural damage isvery low, and although some minor structural repairsmay be appropriate, these would generally not berequired prior to reoccupancy.2.5.1.2 Life Safety Performance Level (S-3)Structural Performance Level S-3, Life Safety, meansthe post-earthquake damage state in which significantdamage to the structure has occurred, but some marginagainst either partial or total structural collapse remains.Some structural elements and components are severelydamaged, but this has not resulted in large falling debrishazards, either within or outside the building. Injuriesmay occur during the earthquake; however, it isexpected that the overall risk of life-threatening injuryas a result of structural damage is low. It should bepossible to repair the structure; however, for economicreasons this may not be practical. While the damagedstructure is not an imminent collapse risk, it would beprudent to implement structural repairs or installtemporary bracing prior to reoccupancy.2.5.1.3 Collapse Prevention PerformanceLevel (S-5)Structural Performance Level S-5, Collapse Prevention,means the building is on the verge of experiencingpartial or total collapse. Substantial damage to thestructure has occurred, potentially including significantdegradation in the stiffness and strength of the lateralforce-resistingsystem, large permanent lateraldeformation of the structure, and—to a more limitedextent—degradation in vertical-load-carrying capacity.However, all significant components of the gravityload-resistingsystem must continue to carry theirgravity load demands. Significant risk of injury due tofalling hazards from structural debris may exist. Thestructure may not be technically practical to repair andis not safe for reoccupancy, as aftershock activity couldinduce collapse.2.5.1.4 Damage Control Performance Range(S-2)Structural Performance Range S-2, Damage Control,means the continuous range of damage states that entailless damage than that defined for the Life Safety level,but more than that defined for the ImmediateOccupancy level. Design for Damage Controlperformance may be desirable to minimize repair timeand operation interruption; as a partial means ofprotecting valuable equipment and contents; or topreserve important historic features when the cost ofdesign for Immediate Occupancy is excessive.Acceptance criteria for this range may be obtained byinterpolating between the values provided for theImmediate Occupancy (S-1) and Life Safety (S-3)levels.2.5.1.5 Limited Safety Performance Range(S-4)Structural Performance Range S-4, Limited Safety,means the continuous range of damage states betweenthe Life Safety and Collapse Prevention levels. Designparameters for this range may be obtained byinterpolating between the values provided for the LifeSafety (S-3) and Collapse Prevention (S-5) levels.2.5.1.6 Structural Performance NotConsidered (S-6)Some owners may desire to address certainnonstructural vulnerabilities in a rehabilitationprogram—for example, bracing parapets, or anchoringhazardous materials storage containers—withoutaddressing the performance of the structure itself. Suchrehabilitation programs are sometimes attractivebecause they can permit a significant reduction inseismic risk at relatively low cost. The actualperformance of the structure with regard to Guidelinesrequirements is not known and could range from apotential collapse hazard to a structure capable ofmeeting the Immediate Occupancy Performance Level.2.5.2 Nonstructural Performance LevelsFour Nonstructural Performance Levels are defined inthese Guidelines and are summarized in Tables 2-6through 2-8. Nonstructural components addressed inthese performance levels include architecturalcomponents, such as partitions, exterior cladding, andceilings; and mechanical and electrical components,including HVAC systems, plumbing, fire suppressionsystems, and lighting. Occupant contents and2-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)furnishings (such as inventory and computers) areincluded in these tables for some levels but aregenerally not covered with specific Guidelinesrequirements. Design procedures and acceptancecriteria for rehabilitation of nonstructural componentsto the Life Safety Performance Level are contained inChapter 11. General guidance only is provided for otherperformance levels.2.5.2.1 Operational Performance Level (N-A)Nonstructural Performance Level A, Operational,means the post-earthquake damage state of the buildingin which the nonstructural components are able tosupport the building’s intended function. At this level,most nonstructural systems required for normal use ofthe building—including lighting, plumbing, HVAC, andcomputer systems—are functional, although minorcleanup and repair of some items may be required. Thisperformance level requires considerations beyond thosethat are normally within the sole province of thestructural engineer. In addition to assuring thatnonstructural components are properly mounted andbraced within the structure, in order to achieve thisperformance it is often necessary to provide emergencystandby utilities. In addition, it may be necessary toperform rigorous qualification testing of the ability ofkey electrical and mechanical equipment items tofunction during or after strong shaking.Specific design procedures and acceptance criteria forthis performance level are not included in theGuidelines. Users wishing to design for thisperformance level will need to refer to appropriatecriteria from other sources, such as equipmentmanufacturers’ data, to ensure the performance ofmechanical and electrical systems.2.5.2.2 Immediate Occupancy Level (N-B)Nonstructural Performance Level B, ImmediateOccupancy, means the post-earthquake damage state inwhich only limited nonstructural damage has occurred.Basic access and life safety systems, including doors,stairways, elevators, emergency lighting, fire alarms,and suppression systems, remain operable, providedthat power is available. There could be minor windowbreakage and slight damage to some components.Presuming that the building is structurally safe, it isexpected that occupants could safely remain in thebuilding, although normal use may be impaired andsome cleanup and inspection may be required. Ingeneral, components of mechanical and electricalsystems in the building are structurally secured andshould be able to function if necessary utility service isavailable. However, some components may experiencemisalignments or internal damage and be nonoperable.Power, water, natural gas, communications lines, andother utilities required for normal building use may notbe available. The risk of life-threatening injury due tononstructural damage is very low.2.5.2.3 Life Safety Level (N-C)Nonstructural Performance Level C, Life Safety, is thepost-earthquake damage state in which potentiallysignificant and costly damage has occurred tononstructural components but they have not becomedislodged and fallen, threatening life safety eitherwithin or outside the building. Egress routes within thebuilding are not extensively blocked, but may beimpaired by lightweight debris. HVAC, plumbing, andfire suppression systems may have been damaged,resulting in local flooding as well as loss of function.While injuries may occur during the earthquake fromthe failure of nonstructural components, it is expectedthat, overall, the risk of life-threatening injury is verylow. Restoration of the nonstructural components maytake extensive effort.2.5.2.4 Hazards Reduced Level (N-D)Nonstructural Performance Level D, Hazards Reduced,represents a post-earthquake damage state level inwhich extensive damage has occurred to nonstructuralcomponents, but large or heavy items that pose a fallinghazard to a number of people—such as parapets,cladding panels, heavy plaster ceilings, or storageracks— are prevented from falling. While isolatedserious injury could occur from falling debris, failuresthat could injure large numbers of persons—eitherinside or outside the structure—should be avoided.Exits, fire suppression systems, and similar life-safetyissues are not addressed in this performance level.2.5.2.5 Nonstructural Performance NotConsidered (N-E)In some cases, the decision may be made to rehabilitatethe structure without addressing the vulnerabilities ofnonstructural components. It may be desirable to do thiswhen rehabilitation must be performed withoutinterruption of building operation. In some cases, it ispossible to perform all or most of the structuralrehabilitation from outside occupied building areas,while extensive disruption of normal operation may berequired to perform nonstructural rehabilitation. Also,FEMA 273 Seismic Rehabilitation Guidelines 2-9

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)since many of the most severe hazards to life safetyoccur as a result of structural vulnerabilities, somemunicipalities may wish to adopt rehabilitationordinances that require structural rehabilitation only.2.5.3 Building Performance LevelsBuilding Performance Levels are obtained bycombining Structural and Nonstructural PerformanceLevels. A large number of combinations is possible.Each Building Performance Level is designated alphanumericallywith a numeral representing the StructuralPerformance Level and a letter representing theNonstructural Performance Level (e.g. 1-B, 3-C).Table 2-9 indicates the possible combinations andprovides names for those that are most likely to beselected as a basis for design. Several of the morecommon Building Performance Levels are describedbelow.2.5.3.1 Operational Level (1-A)This Building Performance Level is a combination ofthe Structural Immediate Occupancy Level and theNonstructural Operational Level. Buildings meetingthis performance level are expected to sustain minimalor no damage to their structural and nonstructuralcomponents. The building is suitable for its normaloccupancy and use, although possibly in a slightlyimpaired mode, with power, water, and other requiredutilities provided from emergency sources, and possiblywith some nonessential systems not functioning.Buildings meeting this performance level pose anextremely low risk to life safety.Under very low levels of earthquake ground motion,most buildings should be able to meet or exceed thisperformance level. Typically, however, it will not beeconomically practical to design for this performanceunder severe levels of ground shaking, except forbuildings that house essential services.2.5.3.2 Immediate Occupancy Level (1-B)This Building Performance Level is a combination ofthe Structural and Nonstructural Immediate Occupancylevels. Buildings meeting this performance level areexpected to sustain minimal or no damage to theirstructural elements and only minor damage to theirnonstructural components. While it would be safe toreoccupy a building meeting this performance levelimmediately following a major earthquake,nonstructural systems may not function due to either alack of electrical power or internal damage toequipment. Therefore, although immediate reoccupancyof the building is possible, it may be necessary toperform some cleanup and repair, and await therestoration of utility service, before the building couldfunction in a normal mode. The risk to life safety at thisperformance level is very low.Many building owners may wish to achieve this level ofperformance when the building is subjected to moderatelevels of earthquake ground motion. In addition, someowners may desire such performance for very importantbuildings, under severe levels of earthquake groundshaking. This level provides most of the protectionobtained under the Operational Level, without the costof providing standby utilities and performing rigorousseismic qualification of equipment performance.2.5.3.3 Life Safety Level (3-C)This Building Performance Level is a combination ofthe Structural and Nonstructural Life Safety levels.Buildings meeting this level may experience extensivedamage to structural and nonstructural components.Repairs may be required before reoccupancy of thebuilding occurs, and repair may be deemedeconomically impractical. The risk to life in buildingsmeeting this performance level is low.This performance level entails somewhat more damagethan anticipated for new buildings that have beenproperly designed and constructed for seismicresistance when subjected to their design earthquakes.Many building owners will desire to meet thisperformance level for a severe level of ground shaking.2.5.3.4 Collapse Prevention Level (5-E)This Building Performance Level consists of theStructural Collapse Prevention Level with noconsideration of nonstructural vulnerabilities, exceptthat parapets and heavy appendages are rehabilitated.Buildings meeting this performance level may pose asignificant hazard to life safety resulting from failure ofnonstructural components. However, because thebuilding itself does not collapse, gross loss of lifeshould be avoided. Many buildings meeting this levelwill be complete economic losses.This level has sometimes been selected as the basis formandatory seismic rehabilitation ordinances enacted bymunicipalities, as it results in mitigation of the mostsevere life-safety hazards at relatively low cost.2-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-3Damage Control and Building Performance LevelsCollapse PreventionLevelLife SafetyLevelBuilding Performance LevelsImmediateOccupancyLevelOperationalLevelOverall Damage Severe Moderate Light Very LightGeneralLittle residual stiffnessand strength, but loadbearingcolumns andwalls function. Largepermanent drifts.Some exits blocked.Infills and unbracedparapets failed or atincipient failure.Building is nearcollapse.Some residualstrength and stiffnessleft in all stories.Gravity-load-bearingelements function. Noout-of-plane failure ofwalls or tipping ofparapets. Somepermanent drift.Damage to partitions.Building may bebeyond economicalrepair.No permanent drift.Structure substantiallyretains originalstrength and stiffness.Minor cracking offacades, partitions,and ceilings as well asstructural elements.Elevators can berestarted. Fireprotection operable.No permanent drift;structure substantiallyretains originalstrength and stiffness.Minor cracking offacades, partitions,and ceilings as well asstructural elements. Allsystems important tonormal operation arefunctional.NonstructuralcomponentsComparison withperformance intendedfor buildings designed,under the NEHRPProvisions, for theDesign EarthquakeExtensive damage.Significantly moredamage and greaterrisk.Falling hazardsmitigated but manyarchitectural,mechanical, andelectrical systems aredamaged.Somewhat moredamage and slightlyhigher risk.Equipment andcontents are generallysecure, but may notoperate due tomechanical failure orlack of utilities.Much less damageand lower risk.Negligible damageoccurs. Power andother utilities areavailable, possiblyfrom standby sources.Much less damageand lower risk.Table 2-4Structural Performance Levels and Damage 1 —Vertical ElementsStructural Performance LevelsCollapse PreventionElementsType S-5Concrete Frames Primary Extensive cracking andhinge formation in ductileelements. Limitedcracking and/or splicefailure in some nonductilecolumns. Severe damagein short columns.SecondaryDrift 2Extensive spalling incolumns (limitedshortening) and beams.Severe joint damage.Some reinforcing buckled.4% transientor permanentLife SafetyS-3Extensive damage tobeams. Spalling of coverand shear cracking (< 1/8"width) for ductile columns.Minor spalling innonductile columns. Jointcracks < 1/8" wide.Extensive cracking andhinge formation in ductileelements. Limitedcracking and/or splicefailure in some nonductilecolumns. Severe damagein short columns.2% transient;1% permanentImmediate OccupancyS-1Minor hairline cracking.Limited yielding possibleat a few locations. Nocrushing (strains below0.003).Minor spalling in a fewplaces in ductile columnsand beams. Flexuralcracking in beams andcolumns. Shear crackingin joints < 1/16" width.1% transient;negligible permanentFEMA 273 Seismic Rehabilitation Guidelines 2-11

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-4Structural Performance Levels and Damage 1 —Vertical Elements (continued)Collapse PreventionElementsType S-5Steel Moment Frames Primary Extensive distortion ofbeams and columnpanels. Many fractures atmoment connections, butshear connections remainintact.Hinges form. Localbuckling of some beamelements. Severe jointdistortion; isolatedmoment connectionfractures, but shearconnections remain intact.A few elements mayexperience partialfracture.Secondary Same as primary. Extensive distortion ofbeams and columnpanels. Many fractures atmoment connections, butshear connections remainintact.Drift 2 5% transientor permanentBraced Steel Frames Primary Extensive yielding andbuckling of braces. Manybraces and theirconnections may fail.2.5% transient;1% permanentMany braces yield orbuckle but do not totallyfail. Many connectionsmay fail.Minor local yielding at afew places. No fractures.Minor buckling orobservable permanentdistortion of members.Same as primary.0.7% transient;negligible permanentMinor yielding or bucklingof braces.Secondary Same as primary. Same as primary. Same as primary.Drift 2 2% transient1.5% transient;0.5% transient;or permanent0.5% permanentnegligible permanentConcrete Walls Primary Major flexural and shearcracks and voids. Slidingat joints. Extensivecrushing and buckling ofreinforcement. Failurearound openings. Severeboundary elementdamage. Coupling beamsshattered and virtuallydisintegrated.SecondaryDrift 2Panels shattered andvirtually disintegrated.2% transientor permanentStructural Performance LevelsLife SafetyS-3Some boundary elementdistress, including limitedbuckling of reinforcement.Some sliding at joints.Damage aroundopenings. Some crushingand flexural cracking.Coupling beams:extensive shear andflexural cracks; somecrushing, but concretegenerally remains inplace.Major flexural and shearcracks. Sliding at joints.Extensive crushing.Failure around openings.Severe boundary elementdamage. Coupling beamsshattered and virtuallydisintegrated.1% transient;0.5% permanentImmediate OccupancyS-1Minor hairline cracking ofwalls, < 1/16" wide.Coupling beamsexperience cracking< 1/8" width.Minor hairline cracking ofwalls. Some evidence ofsliding at constructionjoints. Coupling beamsexperience cracks < 1/8"width. Minor spalling.0.5% transient;negligible permanent2-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-4Structural Performance Levels and Damage 1 —Vertical Elements (continued)Structural Performance LevelsCollapse PreventionElementsType S-5Unreinforced Masonry Primary Extensive cracking andInfill Walls 3 crushing; portions of facecourse shed.UnreinforcedMasonry (Noninfill)WallsReinforced MasonryWallsSecondaryDrift 2PrimarySecondaryDrift 2PrimarySecondaryExtensive crushing andshattering; some wallsdislodge.0.6% transientor permanentExtensive cracking; facecourse and veneer maypeel off. Noticeable inplaneand out-of-planeoffsets.Nonbearing panelsdislodge.1% transientor permanentCrushing; extensivecracking. Damage aroundopenings and at corners.Some fallen units.Panels shattered andvirtually disintegrated.Drift 2 1.5% transientor permanentWood Stud Walls Primary Connections loose. Nailspartially withdrawn. Somesplitting of members andpanels. Veneersdislodged.SecondaryDrift 2Sheathing sheared off.Let-in braces fracturedand buckled. Framing splitand fractured.3% transientor permanentLife SafetyS-3Extensive cracking andsome crushing but wallremains in place. Nofalling units. Extensivecrushing and spalling ofveneers at corners ofopenings.Same as primary.0.5% transient;0.3% permanentExtensive cracking.Noticeable in-planeoffsets of masonry andminor out-of-plane offsets.Same as primary.0.6% transient;0.6% permanentExtensive cracking(< 1/4") distributedthroughout wall. Someisolated crushing.Crushing; extensivecracking; damage aroundopenings and at corners;some fallen units.0.6% transient;0.6% permanentModerate loosening ofconnections and minorsplitting of members.Connections loose. Nailspartially withdrawn. Somesplitting of members andpanels.2% transient;1% permanentImmediate OccupancyS-1Minor (

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-4Structural Performance Levels and Damage 1 —Vertical Elements (continued)Structural Performance LevelsElementsPrecast ConcreteConnectionsTypePrimaryCollapse PreventionS-5Some connection failuresbut no elementsdislodged.Local crushing andspalling at connections,but no gross failure ofconnections.Secondary Same as primary. Some connection failuresbut no elementsdislodged.Foundations General Major settlement andtilting.Life SafetyS-3Total settlements < 6" anddifferential settlements

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-6Nonstructural Performance Levels and Damage—Architectural ComponentsComponentCladdingGlazingPartitionsCeilingsParapets andOrnamentationHazards ReducedLevel N-DSevere damage toconnections andcladding. Many panelsloosened.General shatteredglass and distortedframes. Widespreadfalling hazards.Severe racking anddamage in manycases.Most ceilingsdamaged. Lightsuspended ceilingsdropped. Severecracking in hardceilings.Extensive damage;some fall innonoccupied areas.Life SafetyN-CNonstructural Performance LevelsSevere distortion inconnections.Distributed cracking,bending, crushing, andspalling of claddingelements. Somefracturing of cladding,but panels do not fall.Extensive crackedglass; little brokenglass.Distributed damage;some severe cracking,crushing, and rackingin some areas.Extensive damage.Dropped suspendedceiling tiles. Moderatecracking in hardceilings.Extensive damage;some falling innonoccupied areas.ImmediateOccupancy N-BConnections yield;minor cracks (< 1/16"width) or bending incladding.Some cracked panes;none broken.Cracking to about1/16" width atopenings. Minorcrushing and crackingat corners.Minor damage. Somesuspended ceiling tilesdisrupted. A fewpanels dropped. Minorcracking in hardceilings.Minor damage.OperationalN-AConnections yield;minor cracks (< 1/16"width) or bending incladding.Some cracked panes;none brokenCracking to about1/16" width atopenings. Minorcrushing and crackingat corners.Generally negligibledamage. Isolatedsuspended paneldislocations, or cracksin hard ceilings.Minor damage.Canopies & Marquees Extensive distortion. Moderate distortion. Minor damage. Minor damage.Chimneys & Stacks Extensive damage. No Extensive damage. No Minor cracking. Negligible damage.collapse.collapse.Stairs & Fire EscapesLight FixturesDoorsExtensive racking.Loss of use.Extensive damage.Falling hazards occur.Distributed damage.Many racked andjammed doors.Some racking andcracking of slabs,usable.Many broken lightfixtures. Fallinghazards generallyavoided in heavierfixtures (> 20 pounds).Distributed damage.Some racked andjammed doors.Minor damage.Minor damage. Somependant lights broken.Minor damage. Doorsoperable.Negligible damage.Negligible damage.Minor damage. Doorsoperable.FEMA 273 Seismic Rehabilitation Guidelines 2-15

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-7Nonstructural Performance Levels and Damage—Mechanical, Electrical, and PlumbingSystems/ComponentsSystem/ComponentElevatorsHVAC EquipmentDuctsPipingFire Sprinkler SystemsFire Alarm SystemsEmergency LightingElectrical DistributionEquipmentPlumbingHazards ReducedN-DElevators out ofservice;counterweights offrails.Most units do notoperate; many slide oroverturn; somesuspended units fall.Ducts break loose ofequipment andlouvers; somesupports fail; someducts fall.Some lines rupture.Some supports fail.Some piping falls.Many sprinkler headsdamaged bycollapsing ceilings.Leaks develop atcouplings. Somebranch lines fail.Ceiling mountedsensors damaged.System nonfunctional.Some lights fall.Power may not beavailable.Units slide and/oroverturn, rupturingattached conduit.Uninterruptable PowerSource systems fail.Diesel generators donot start.Some fixtures broken;lines broken; mainsdisrupted at source.Life SafetyN-CNonstructural Performance LevelsElevators out ofservice;counterweights do notdislodge.Units shift onsupports, rupturingattached ducting,piping, and conduit,but do not fall.Minor damage at jointsof sections andattachment toequipment; somesupports damaged,but ducts do not fall.Minor damage atjoints, with someleakage. Somesupports damaged,but systems remainsuspended.Some sprinkler headsdamaged by swayingceilings. Leaksdevelop at somecouplings.ImmediateOccupancy N-BElevators operable;can be started whenpower available.Units are secure andmost operate if powerand other requiredutilities are available.Minor damage atjoints, but ductsremain serviceable.Minor leaks develop ata few joints.Minor leakage at a fewheads or pipe joints.System remainsoperable.OperationalN-AElevators operate.Units are secure andoperate; emergencypower and otherutilities provided, ifrequired.Negligible damage.Negligible damage.Negligible damage.May not function. System is functional. System is functional.System is functional. System is functional. System is functional.Units shift on supportsand may not operate.Generators providedfor emergency powerstart; utility servicelost.Some fixtures broken,lines broken; mainsdisrupted at source.Units are secure andgenerally operable.Emergencygenerators start, butmay not be adequateto service all powerrequirements.Fixtures and linesserviceable; however,utility service may notbe available.Units are functional.Emergency power isprovided, as needed.System is functional.On-site water supplyprovided, if required.2-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-8Nonstructural Performance Levels and Damage—ContentsNonstructural Performance LevelsContents TypeComputer SystemsManufacturingEquipmentHazards ReducedN-DUnits roll and overturn,disconnect cables.Raised access floorscollapse.Units slide andoverturn; utilitiesdisconnected. Heavyunits requirereconnection andrealignment. Sensitiveequipment may not befunctional.Life Safety N-CUnits shift and maydisconnect cables, butdo not overturn. Powernot available.Units slide, but do notoverturn; utilities notavailable; somerealignment requiredto operate.Desktop Equipment Units slide off desks. Some equipmentslides off desks.File CabinetsBook ShelvesHazardous MaterialsArt ObjectsCabinets overturn andspill contents.Shelves overturn andspill contents.Severe damage; nolarge quantity ofmaterial released.Objects damaged byfalling, water, dust.Drawers slide open;cabinets tip.Books slide offshelves.Minor damage;occasional materialsspilled; gaseousmaterials contained.Objects damaged byfalling, water, dust.ImmediateOccupancy N-BUnits secure andremain connected.Power may not beavailable to operate,and minor internaldamage may occur.Units secure, andmost operable if powerand utilities available.Some equipmentslides off desks.Drawers slide open,but cabinets do not tip.Books slide onshelves.Negligible damage;materials contained.Some objects may bedamaged by falling.Operational N-AUnits undamaged andoperable; poweravailable.Units secure andoperable; power andutilities available.Equipment secured todesks and operable.Drawers slide open,but cabinets do not tip.Books remain onshelves.Negligible damage;materials contained.Objects undamaged.Table 2-9Building Performance Levels/RangesNonstructuralPerformanceLevelsN-AOperationalN-B ImmediateOccupancyS-1 ImmediateOccupancyOperational1-AImmediateOccupancy 1-BS-2 DamageControl RangeStructural Performance Levels/Ranges2-A NotrecommendedS-3 Life Safety S-4 LimitedSafety RangeNotrecommended2-B 3-B NotrecommendedS-5 CollapsePreventionNotrecommendedNotrecommendedN-C Life Safety 1-C 2-C Life Safety 3-C 4-C 5-C 6-CN-D HazardsReducedN-E NotConsideredNotrecommendedNotrecommended2-D 3-D 4-D 5-D 6-DNotrecommendedNotrecommended4-E 5-E CollapsePreventionS-6 NotConsideredNotrecommendedNotrecommendedNorehabilitationFEMA 273 Seismic Rehabilitation Guidelines 2-17

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.6 Seismic HazardThe most common and significant cause of earthquakedamage to buildings is ground shaking; thus, the effectsof ground shaking form the basis for most building coderequirements for seismic design. As stated inSection 2.4, two levels of earthquake shaking hazardare used to satisfy the BSO for these Guidelines. Theseare termed Basic Safety Earthquake 1 (BSE-1) andBasic Safety Earthquake 2 (BSE-2). BSE-2 earthquakeground shaking, also termed Maximum ConsideredEarthquake (MCE) ground shaking, is similar to thatdefined for the MCE in the 1997 NEHRPRecommended Provisions (BSSC, 1997). In most areasof the United States, BSE-2 earthquake ground motionhas a 2% probability of exceedance in 50 years (2%/50 year). In regions close to known faults withsignificant slip rates and characteristic earthquakes withmagnitudes in excess of about 6.0, the BSE-2 groundshaking is limited by a conservative estimate (150% ofthe median attenuation) of the shaking likely to beexperienced as a result of such a characteristic event.Ground shaking levels determined in this manner willtypically correspond to a probability of exceedance thatis greater than 2% in 50 years. The BSE-1 earthquake issimilar, but not identical to the concept of a designearthquake contained in the NEHRP Provisions. It isdefined as that ground shaking having a 10%probability of exceedance in 50 years (10%/50 year).The motions need not exceed those used for newbuildings, defined as 2/3 of the BSE-2 motion.In addition to the BSE-1 and BSE-2 levels of groundmotion, Rehabilitation Objectives may be formedconsidering earthquake ground shaking hazards withany defined probability of exceedance, or based on anydeterministic event on a specific fault.Response spectra are used to characterize earthquakeshaking demand on buildings in the Guidelines. Groundshaking response spectra for use in seismicrehabilitation design may be determined in accordancewith either the General Procedure of Section 2.6.1 orthe Site-Specific Procedure of Section 2.6.2. Seismiczones are defined in Section 2.6.3. Other seismichazards (e.g., liquefaction) are discussed inSection 2.6.4.In the General Procedure, ground shaking hazard isdetermined from available response spectrumacceleration contour maps. Maps showing 5%-dampedresponse spectrum ordinates for short-period (0.2second) and long-period (1 second) response distributedwith the Guidelines can be used directly with theGeneral Procedure of Section 2.6.1 for developingdesign response spectra for either or both the BSE-1 andBSE-2, or for earthquakes of any desired probability ofexceedance. Alternatively, other maps and otherprocedures can be used, provided that 5%-dampedresponse spectra are developed that represent theground shaking for the desired earthquake returnperiod, and the site soil classification is considered. Inthe Site-Specific Procedure, ground shaking hazard isdetermined using a specific study of the faults andseismic source zones that may affect the site, as well asevaluation of the regional and geologic conditions thataffect the character of the site ground motion caused byevents occurring on these faults and sources.The General Procedure may be used for any building.The Site-Specific Procedure may also be used for anybuilding and should be considered where any of thefollowing apply:• Rehabilitation is planned to an EnhancedRehabilitation Objective, as defined in Section 2.4.2.• The building site is located within 10 kilometers ofan active fault.• The building is located on Type E soils (as definedin Section 2.6.1.4) and the mapped BSE-2 spectralresponse acceleration at short periods (S S ) exceeds2.0g.• The building is located on Type F soils as defined inSection 2.6.1.4.Exception: Where S S , determined in accordancewith Section 2.6.1.1, < 0.20g. In these cases, a TypeE soil profile may be assumed.• A time history response analysis of the building willbe performed as part of the design.Other site-specific seismic hazards that may causedamage to buildings include:• surface fault rupture• differential compaction of the foundation material• landsliding2-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)• liquefaction• lateral spreading• floodingIf the potential for any of these, or other, seismichazards exists at a given site, then they also should beconsidered in the rehabilitation design, in accordancewith Section 2.6.4 and Chapter 4.2.6.1 General Ground Shaking HazardProcedureThe general procedures of this section may be used todetermine acceleration response spectra for any of thefollowing hazard levels:• Basic Safety Earthquake 1 (BSE-1)• Basic Safety Earthquake 2 (BSE-2)• Earthquake with any defined probability ofexceedance in 50 yearsDeterministic estimates of earthquake hazard, in whichan acceleration response spectrum is obtained for aspecific magnitude earthquake occurring on a definedfault, shall be made using the Site-Specific Proceduresof Section 2.6.2.The basic steps for determining a response spectrumunder this general procedure are:s1. Determine whether the desired hazard levelcorresponds to one of the levels contained in theground shaking hazard maps distributed with theGuidelines. The package includes maps for BSE-2(MCE) ground shaking hazards as well as forhazards with 10%/50 year exceedance probabilities.2. If the desired hazard level corresponds with one ofthe mapped hazard levels, obtain spectral responseacceleration parameters directly from the maps, inaccordance with Section 2.6.1.1.3. If the desired hazard level is the BSE-1, then obtainthe spectral response acceleration parameters fromthe maps, in accordance with Section 2.6.1.2.4. If the desired hazard level does not correspond withthe mapped levels of hazard, then obtain the spectralresponse acceleration parameters from the availablemaps, and modify them to the desired hazard level,either by logarithmic interpolation or extrapolation,in accordance with Section 2.6.1.3.5. Obtain design spectral response accelerationparameters by adjusting the mapped, or modifiedmapped spectral response acceleration parametersfor site class effects, in accordance withSection 2.6.1.4.6. Using the design spectral response accelerationparameters that have been adjusted for site classeffects, construct the response spectrum inaccordance with Section 2.6.1.5.2.6.1.1 BSE-2 and 10%/50 ResponseAcceleration ParametersThe mapped short-period response accelerationparameter, S S , and mapped response accelerationparameter at a one-second period, S 1 , for BSE-2 groundmotion hazards may be obtained directly from the mapsdistributed with the Guidelines. The mapped shortperiodresponse acceleration parameter, S S , and mappedresponse acceleration parameter at a one-second period,S 1 , for 10%/50 year ground motion hazards may also beobtained directly from the maps distributed with theGuidelines.Parameters S S and S 1 shall be obtained by interpolatingbetween the values shown on the response accelerationcontour lines on either side of the site, on theappropriate map, or by using the value shown on themap for the higher contour adjacent to the site.2.6.1.2 BSE-1 Response AccelerationParametersThe mapped short-period response accelerationparameter, S S , and mapped response accelerationparameter at a one-second period, S 1 , for BSE-1 groundshaking hazards shall be taken as the smaller of thefollowing:• The values of the parameters S S and S 1 , respectively,determined for 10%/50 year ground motion hazards,in accordance with Section 2.6.1.1.• Two thirds of the values of the parameters S S and S 1 ,respectively, determined for BSE-2 ground motionhazards, in accordance with Section 2.6.1.1.FEMA 273 Seismic Rehabilitation Guidelines 2-19

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.6.1.3 Adjustment of Mapped ResponseAcceleration Parameters for OtherProbabilities of ExceedanceWhen the mapped BSE-2 short period responseacceleration parameter, S S , is less than 1.5g, themodified mapped short period response accelerationparameter, S S , and modified mapped responseacceleration parameter at a one-second period, S 1 , forprobabilities of exceedance between 2%/50 years and10%/50 years may be determined from the equation:where:1n(S i )1n(S i ) = 1n(S i10/50 ) + [1n(S iBSE-2 ) – 1n(S i10/50 )][0.606 1n(P R ) – 3.73] (2-1)1n(S i10/50 )1n(S iBSE-2 )1n(P R )= Natural logarithm of the spectralacceleration parameter (“i” = “s” forshort period or “i” = 1 for 1 secondperiod) at the desired probability ofexceedance= Natural logarithm of the spectralacceleration parameter (“i” = “s” forshort period or “i” = 1 for 1 secondperiod) at a 10%/50 year exceedancerate= Natural logarithm of the spectralacceleration parameter (“i” = “s” forshort period or “i” = 1 for 1 secondperiod) for the BSE-2 hazard level= Natural logarithm of the mean returnperiod corresponding to theexceedance probability of the desiredhazard leveland the mean return period P R at the desired exceedanceprobability may be calculated from the equation:1P R = -----------------------------------------------1 e 0.02 1n ( 1 – P E50)–(2-2)where P E50 is the probability of exceedance in 50 yearsof the desired hazard level.When the mapped BSE-2 short period responseacceleration parameter, S S , is greater than or equal to1.5g, the modified mapped short period responseacceleration parameter, S S , and modified mappedresponse acceleration parameter at a one-second period,S 1 , for probabilities of exceedance between 2%/50years and 10%/50 years may be determined from theequation:(2-3)where S i , S i10/50 , and P R are as defined above and n maybe obtained from Table 2-10.Table 2-10 and the two following specify five regions,three of which are not yet specifically defined, namelyIntermountain, Central US, and Eastern US. For statesor areas that might lie near the regional borders, carewill be necessary.Table 2-10PS i S ⎛ Ri10 ⁄ 50-------- ⎞ n=⎝475⎠Values of Exponent n forDetermination of ResponseAcceleration Parameters at HazardLevels between 10%/50 years and2%/50 years; Sites where MappedBSE-2 Values of S S ≥ 1.5gValues of Exponent n forRegion S S S 1California 0.29 0.29Pacific Northwest 0.56 0.67Intermountain 0.50 0.60Central US 0.98 1.09Eastern US 0.93 1.05When the mapped BSE-2 short period responseacceleration parameter, S S , is less than 1.5g, themodified mapped short period response accelerationparameter, S S , and modified mapped responseacceleration parameter at a one-second period, S 1 , forprobabilities of exceedance greater than 10%/50 yearsmay be determined from Equation 2-3, where theexponent n is obtained from Table 2-11.When the mapped BSE-2 short period responseacceleration parameter, S S , is greater than or equal to1.5g, the modified mapped short period responseacceleration parameter, S S , and modified mappedresponse acceleration parameter at a one-second period,S 1 , for probabilities of exceedance greater than 10%/502-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Table 2-11Values of Exponent n forDetermination of ResponseAcceleration Parameters atProbabilities of Exceedance Greaterthan 10%/50 years; Sites whereMapped BSE-2 Values of S S < 1.5gValues of Exponent n forRegion S S S 1California 0.44 0.44Pacific Northwest and 0.54 0.59IntermountainCentral and Eastern US 0.77 0.80Table 2-13SiteClassValues of F a as a Function of SiteClass and Mapped Short-PeriodSpectral Response Acceleration S SMapped Spectral Acceleration at Short Periods S SS S ≤ 0.25 S S = 0.50 S S = 0.75 S S = 1.00 S S ≥ 1.25A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.2 1.2 1.1 1.0 1.0D 1.6 1.4 1.2 1.1 1.0E 2.5 1.7 1.2 0.9 ∗F ∗ ∗ ∗ ∗ ∗Table 2-12Values of Exponent n forDetermination of ResponseAcceleration Parameters atProbabilities of Exceedance Greaterthan 10%/50 years; Sites whereMapped BSE-2 Values of S S ≥ 1.5gValues of Exponent n forNOTE: Use straight-line interpolation for intermediate values of S S .* Site-specific geotechnical investigation and dynamic site responseanalyses should be performed.Table 2-14Values of F v as a Function of SiteClass and Mapped SpectralResponse Acceleration at One-Second Period S 1Region S S S 1California 0.44 0.44Pacific Northwest 0.89 0.96Intermountain 0.54 0.59Central US 0.89 0.89Eastern US 1.25 1.25years may be determined from Equation 2-3, where theexponent n is obtained from Table 2-12.2.6.1.4 Adjustment for Site ClassThe design short-period spectral response accelerationparameter, S XS , and the design spectral responseacceleration parameter at one second, S X1 , shall beobtained respectively from Equations 2-4 and 2-5 asfollows:S XS=F a S SS X1 = F v S 1where F a and F v are site coefficients determinedrespectively from Tables 2-13 and 2-14, based on the(2-4)(2-5)Mapped Spectral Acceleration at One-Second Period S 1SiteClass S 1 ≤ 0.1 S 1 = 0.2 S 1 = 0.3 S 1 = 0.4 S 1 ≥ 0.50A 0.8 0.8 0.8 0.8 0.8B 1.0 1.0 1.0 1.0 1.0C 1.7 1.6 1.5 1.4 1.3D 2.4 2.0 1.8 1.6 1.5E 3.5 3.2 2.8 2.4 ∗F ∗ ∗ ∗ ∗ ∗NOTE: Use straight-line interpolation for intermediate values of S 1 .* Site-specific geotechnical investigation and dynamic site responseanalyses should be performed.site class and the values of the response accelerationparameters S S and S 1 .Site classes shall be defined as follows:• Class A: Hard rock with measured shear wavevelocity, v s > 5,000 ft/sec• Class B: Rock with 2,500 ft/sec < v s < 5,000 ft/secFEMA 273 Seismic Rehabilitation Guidelines 2-21

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)• Class C: Very dense soil and soft rock with1,200 ft/sec < v s≤ 2,500 ft/sec or with eitherstandard blow count N > 50 or undrained shearstrength s u > 2,000 psf• Class D: Stiff soil with 600 ft/sec < ≤ 1,200 ft/sec or with 15 < N ≤ 50 or 1,000 psf ≤ s u < 2,000psf• Class E: Any profile with more than 10 feet ofsoft clay defined as soil with plasticity index PI >20, or water content w > 40 percent, and s u < 500psf or a soil profile with v s < 600 ft/sec. Ifinsufficient data are available to classify a soilprofile as type A through D, a type E profile shouldbe assumed.• Class F: Soils requiring site-specific evaluations:– Soils vulnerable to potential failure or collapseunder seismic loading, such as liquefiable soils,quick and highly-sensitive clays, collapsibleweakly-cemented soils– Peats and/or highly organic clays (H > 10 feet ofpeat and/or highly organic clay, where H =thickness of soil)– Very high plasticity clays (H > 25 feet with PI >75 percent)– Very thick soft/medium stiff clays (H > 120 feet)The parameters v s , N , and s u are, respectively, theaverage values of the shear wave velocity, StandardPenetration Test (SPT) blow count, and undrained shearstrength of the upper 100 feet of soils at the site. Thesevalues may be calculated from Equation 2-6, below:v s , N,s un∑ d i= -----------------------------------i = 1nd----- d i i d---- i∑ , , -----v si N i s uii = 1v s(2-6)where:N ind is uiv siand= SPT blow count in soil layer “i”= Number of layers of similar soil materials forwhich data is available= Depth of layer “i”= Undrained shear strength in layer “i”= Shear wave velocity of the soil in layer “i”n∑i = 1d i100 ft(2-7)Where reliable v s data are available for the site, suchdata should be used to classify the site. If such data arenot available, N data should preferably be used forcohesionless soil sites (sands, gravels), and s u data forcohesive soil sites (clays). For rock in profile classes Band C, classification may be based either on measuredor estimated values of v s . Classification of a site asClass A rock should be based on measurements of v seither for material at the site itself, or for similar rockmaterials in the vicinity; otherwise, Class B rock shouldbe assumed. Class A or B profiles should not beassumed to be present if there is more than 10 feet ofsoil between the rock surface and the base of thebuilding.2.6.1.5 General Response Spectrum=A general, horizontal response spectrum may beconstructed by plotting the following two functions inthe spectral acceleration vs. structural period domain, asshown in Figure 2-1. Where a vertical responsespectrum is required, it may be constructed by takingtwo-thirds of the spectral ordinates, at each period,obtained for the horizontal response spectrum.S a = ( S XS ⁄ B S )( 0.4 + 3T ⁄ T o )for 0 < T ≤ 0.2T oS a = ( S X1 ⁄ ( B 1 T)) for T > T o(2-8)(2-9)2-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Spectral response acceleration, S aFigure 2-1S a = (S XS /B S )(0.4 + 3T /T o )S a = S XS /B SS a = S X1 /B 1 TS X1 /B 10.4S XS /B S1.0General Response Spectrumwhere T o is given by the equationwhere B S and B 1 are taken from Table 2-15.Table 2-150.2T oT oPeriod, TT o = ( S X1 B S ) ⁄ ( S XS B 1 )(2-10)Damping Coefficients B S and B 1 as aFunction of Effective Damping βEffective Damping β(percentage of critical) 1 B S B 1< 2 0.8 0.85 1.0 1.010 1.3 1.220 1.8 1.530 2.3 1.740 2.7 1.9> 50 3.0 2.01. The damping coefficient should be based on linear interpolation foreffective damping values other than those given.In general, it is recommended that a 5% dampedresponse spectrum be used for the rehabilitation designof most buildings and structural systems. Exceptionsare as follows:• For structures without exterior cladding an effectiveviscous damping ration, β, of 2% should beassumed.• For structures with wood diaphragms and a largenumber of interior partitions and cross walls thatinterconnect the diaphragm levels, an effectiveviscous damping ratio, β, of 10% may be assumed.• For structures rehabilitated using seismic isolationtechnology or enhanced energy dissipationtechnology, an equivalent effective viscous dampingratio, β, should be calculated using the procedurescontained in Chapter 9.In Chapter 9 of the Guidelines, the analyticalprocedures for structures rehabilitated using seismicisolation and/or energy dissipation technology makespecific reference to the evaluation of earthquakedemands for the BSE-2 and user-specified designearthquake hazard levels. In that chapter, theparameters: S aM , S MS , S M1 , refer respectively to thevalue of the spectral response acceleration parametersS a , S XS , and S X1 , evaluated for the BSE-2 hazard level,and the parameters S aD , S DS , S D1 in Chapter 9, referrespectively to the value of the spectral responseacceleration parameters S a , S XS , and S X1 , evaluated forthe user-specified design earthquake hazard level.2.6.2 Site-Specific Ground Shaking HazardWhere site-specific ground shaking characterization isused as the basis of rehabilitation design, thecharacterization shall be developed in accordance withthis section.2.6.2.1 Site-Specific Response SpectrumDevelopment of site-specific response spectra shall bebased on the geologic, seismologic, and soilcharacteristics associated with the specific site.Response spectra should be developed for an equivalentviscous damping ratio of 5%. Additional spectra shouldbe developed for other damping ratios appropriate to theindicated structural behavior, as discussed inSection 2.6.1.5. When the 5% damped site-specificspectrum has spectral amplitudes in the period range ofgreatest significance to the structural response that areless than 70 percent of the spectral amplitudes of theGeneral Response Spectrum, an independent third-partyreview of the spectrum should be made by an individualwith expertise in the evaluation of ground motion.FEMA 273 Seismic Rehabilitation Guidelines 2-23

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)When a site-specific response spectrum has beendeveloped and other sections of these Guidelinesrequire values for the spectral response parameters, S XS ,S X1 , or T 0 , they may be obtained in accordance with thissection. The value of the design spectral responseacceleration at short periods, S XS , shall be taken as theresponse acceleration obtained from the site-specificspectrum at a period of 0.2 seconds, except that itshould be taken as not less than 90% of the peakresponse acceleration at any period. In order to obtain avalue for the design spectral response accelerationparameter S X1 , a curve of the form S a = S X1 /T should begraphically overlaid on the site-specific spectrum suchthat at any period, the value of S a obtained from thecurve is not less than 90% of that which would beobtained directly from the spectrum. The value of T 0shall be determined in accordance with Equation 2-11.Alternatively, the values obtained in accordance withSection 2.6.1 may be used for all of these parameters.T 0 = S X1 ⁄ S XS(2-11)2.6.2.2 Acceleration Time HistoriesTime-History Analysis shall be performed with nofewer than three data sets (two horizontal componentsor, if vertical motion is to be considered, two horizontalcomponents and one vertical component) of appropriateground motion time histories that shall be selected andscaled from no fewer than three recorded events.Appropriate time histories shall have magnitude, faultdistances, and source mechanisms that are consistentwith those that control the design earthquake groundmotion. Where three appropriate recorded groundmotiontime history data sets are not available,appropriate simulated time history data sets may beused to make up the total number required. For eachdata set, the square root of the sum of the squares(SRSS) of the 5%-damped site-specific spectrum of thescaled horizontal components shall be constructed. Thedata sets shall be scaled such that the average value ofthe SRSS spectra does not fall below 1.4 times the5%-damped spectrum for the design earthquake forperiods between 0.2T seconds and 1.5T seconds (whereT is the fundamental period of the building).Where three time history data sets are used in theanalysis of a structure, the maximum value of eachresponse parameter (e.g., force in a member,displacement at a specific level) shall be used todetermine design acceptability. Where seven or moretime history data sets are employed, the average valueof each response parameter may be used to determinedesign acceptability.2.6.3 Seismicity ZonesIn these Guidelines, seismicity zones are defined asfollows.2.6.3.1 Zones of High SeismicityBuildings located on sites for which the 10%/50 year,design short-period response acceleration, S XS , is equalto or greater than 0.5g, or for which the 10%/50 yeardesign one-second period response acceleration, S X1 , isequal to or greater than 0.2g shall be considered to belocated within zones of high seismicity.2.6.3.2 Zones of Moderate SeismicityBuildings located on sites for which the 10%/50 year,design short-period response acceleration, S XS , is equalto or greater than 0.167g but is less than 0.5g, or forwhich the 10%/50 year, design one-second periodresponse acceleration, S X1 , is equal to or greater than0.067g but less than 0.2g shall be considered to belocated within zones of moderate seismicity.2.6.3.3 Zones of Low SeismicityBuildings located on sites that are not located withinzones of high or moderate seismicity, as defined inSections 2.6.3.1 and 2.6.3.2, shall be considered to belocated within zones of low seismicity.2.6.4 Other Seismic HazardsIn addition to ground shaking, seismic hazards caninclude ground failure caused by surface fault rupture,liquefaction, lateral spreading, differential settlement,and landsliding. Earthquake-induced flooding, due totsunami, seiche, or failure of a water-retaining structure,can also pose a hazard to a building site. The process ofrehabilitating a building shall be based on theunderstanding that either the site is not exposed to asignificant earthquake-induced flooding hazard orground failure, or the site may be stabilized or protectedfrom such hazards at a cost that is included along withthe other rehabilitation costs. Chapter 4 describes, andprovides guidance for evaluating and mitigating, theseand other on-site and off-site seismic hazards.2-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.7 As-Built InformationExisting building characteristics pertinent to its seismicperformance—including its configuration, and the type,detailing, material strengths, and condition of thevarious structural and nonstructural elements, includingfoundations and their interconnections—shall bedetermined in accordance with this section. The projectcalculations should include documentation of thesecharacteristics in drawings or photographs,supplemented by appropriate descriptive text. Existingcharacteristics of the building and site should beobtained from the following sources, as appropriate:• Field observation of exposed conditions andconfiguration• Available construction documents, engineeringanalyses, reports, soil borings and test logs,maintenance histories, and manufacturers’ literatureand test data• Reference standards and codes from the period ofconstruction as cited in Chapters 5 through 8• Destructive and nondestructive examination andtesting of selected building components• Interviews with building owners, tenants, managers,the original architect and engineer, contractor(s), andthe local building officialAs a minimum, at least one site visit should beperformed to obtain detailed information regardingbuilding configuration and condition, site andgeotechnical conditions, and any issues related toadjacent structures, and to confirm that the availableconstruction documents are generally representative ofexisting conditions. If the building is a historicstructure, it is also important to identify the locations ofhistorically significant features and fabric. Care shouldbe taken in the design and investigation process tominimize the impact of work on these features. Refer tothe Secretary of the Interior’s Standards for theTreatment of Historic Properties as discussed inChapter 1.2.7.1 Building ConfigurationThe as-built building configuration consists of the typeand arrangement of existing structural elements andcomponents composing the gravity- and lateral-loadresistingsystems, and the nonstructural components.The structural elements and components shall beidentified and categorized as either primary orsecondary, using the criteria described in Section 2.3,with any structural deficiencies potentially affectingseismic performance also identified.It is important, in identifying the building configuration,to account for both the intended load-resisting elementsand components and the effective elements andcomponents. The effective load-resisting systems mayinclude building-code-conforming structural elements,nonconforming structural elements, and thosenonstructural elements that actually participate inresisting gravity, lateral, or combined gravity and lateralloads, whether or not they were intended to do so by theoriginal designers. Existing load paths should beidentified, considering the effects of any modifications(e.g., additions, alterations, rehabilitation, degradation)since original construction. Potential discontinuities andweak links should also be identified, as well asirregularities that may have a detrimental effect on thebuilding’s response to lateral demands. FEMA 178(BSSC, 1992) offers guidance for these aspects ofbuilding evaluation.2.7.2 Component PropertiesMeaningful structural analysis of a building’s probableseismic behavior and reliable design of rehabilitationmeasures requires good understanding of the existingcomponents (e.g., beams, columns, diaphragms), theirinterconnection, and their material properties (strength,deformability, and toughness). The strength anddeformation capacity of existing components should becomputed, as indicated in Chapters 4 through 9 and 11,based on derived material properties and detailedcomponent knowledge. Existing component actionstrengths must be determined for two basic purposes: toallow calculation of their ability to deliver load to otherelements and components, and to allow determinationof their capacity to resist forces and deformations.Component deformation capacity must be calculated toallow validation of overall element and buildingdeformations and their acceptability for the selectedRehabilitation Objectives. In general, componentcapacities are calculated as “expected values” thataccount for the mean material strengths as well as theprobable effects of strain hardening and/or degradation.The exception to this is the calculation of strengths usedto evaluate the adequacy of component force actionswith little inherent ductility (force-controlledbehaviors). For these evaluations, lower-bound strengthFEMA 273 Seismic Rehabilitation Guidelines 2-25

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)estimates—taking into account the possible variation inmaterial strengths—are used for determination ofcapacity. Guidance on how to obtain these expected andlower-bound values is provided in Chapters 5 through 8for the commonly used structural materials andsystems.Knowledge of existing component configuration,quality of construction, physical condition, andinterconnection to other structural components isnecessary to compute strength and deformationcapacities. This knowledge should be obtained byvisual surveys of condition, destructive andnondestructive testing, and field measurement ofdimensions, as appropriate. Even with an exhaustiveeffort to maximize knowledge, uncertainty will remainregarding the validity of computed component strengthand deformation capacities. To account for thisuncertainty, a knowledge factor, κ, is utilized in thecapacity evaluations. Two possible values exist for κ,based on the reliability of available knowledge—classified as either minimum or comprehensive.When only a minimum level of knowledge is available,a κ value of 0.75 shall be included in componentcapacity and deformation analyses. The followingcharacteristics represent the minimum appropriate levelof effort in gaining knowledge of structuralconfiguration:• Records of the original construction and anymodifications, including structural and architecturaldrawings, are generally available. In the absence ofstructural drawings, a set of record drawings and/orsketches is prepared, documenting both gravity andlateral systems.• A visual condition survey is performed on theaccessible primary elements and components, withverification that the size, location, and connection ofthese elements is as indicated on the availabledocumentation.• A limited program of in-place testing is performed,as indicated in Chapters 5 through 8, to quantify thematerial properties, component condition, anddimensions of representative primary elements withquantification of the effects of any observabledeterioration. Alternatively, default values providedin Chapters 5 through 8 are utilized for materialstrengths, taking into account the observed conditionof these materials; if significant variation is found inthe condition or as-tested properties of materials,consideration should be given to grouping thosecomponents with similar condition or properties sothat the coefficient of variation within a group doesnot exceed 30%.• Knowledge of any site-related concerns—such aspounding from neighboring structures, party walleffects, and soil or geological problems includingrisks of liquefaction—has been gained through fieldsurveys and research.• Specific foundation- and material-related concernscited in Chapters 4 through 8, as applicable, havebeen examined, and knowledge of their influence onbuilding performance has been gained.A κ value of 1.0 may be used where comprehensiveknowledge and understanding of componentconfiguration has been obtained. Comprehensiveknowledge may be assumed when all of the followingfactors exist:• Original construction records, including drawingsand specifications, as well as any post-constructionmodification data, are available and explicitly depictas-built conditions. Where such documents are notavailable, drawings and sketches are developedbased on detailed surveys of the primary structuralelements. Such surveys include destructive and/ornondestructive investigation as required todetermine the size, number, placement, and type ofobscured items such as bolts and reinforcing bars. Inaddition, documentation is developed forrepresentative secondary elements.• Extensive in-place testing is performed as indicatedin Chapters 4 through 8 to quantify materialproperties, and component conditions anddimensions or records of the results of qualityassurance tests constructed during testing areavailable. Coefficients of variability for materialstrength test results are less than 20%, orcomponents are grouped and additional testing isperformed such that the material strength test resultsfor each group have coefficients of variation withinthis limit.• Knowledge of any site-related concerns—such aspounding from neighboring structures, party walleffects, and soil or geological problems including2-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)risks of liquefaction—has been gained fromthorough visual survey and research efforts.• Specific foundation- and material-related concernscited in Chapters 4 through 8, as applicable, havebeen examined and knowledge of their influence onbuilding performance has been gained.Whenever practical, investigation should be sufficientlythorough to allow the use of a single value of κ for allbuilding components and elements. If extenuatingcircumstances prevent use of a common κ value forcertain components, multiple κ values should be used inthe analysis, as appropriate to the available knowledgeof the individual components. When a nonlinearanalysis procedure is employed, the level ofinvestigation should be sufficient to allowcomprehensive knowledge of the structure (κ = 1.0).2.7.3 Site Characterization andGeotechnical InformationData on surface and subsurface conditions at the site,including the configuration of foundations, shall beobtained for use in building analyses. Data shall beobtained from existing documents, visual sitereconnaissance, or a program of subsurfaceinvestigation. If adequate geotechnical data are notavailable from previous investigations, a program ofsite-specific subsurface investigation should beconsidered for sites in areas subject to liquefaction,lateral spreading, or landsliding, and for all buildingswith an Enhanced Rehabilitation Objective. Additionalguidelines for site characterization and subsurfaceinvestigation are contained in Chapter 4.A site reconnaissance should always be performed. Inthe course of this reconnaissance, variances from thebuilding drawings should be noted. Such variancescould include foundation modifications that are notshown on the existing documentation. Off-sitedevelopment that should be noted could includebuildings or grading activities that may impose a load orreduce the level of lateral support to the structure.Indicators of poor foundation performance—such assettlements of floor slabs, foundations or sidewalks,suggesting distress that could affect buildingperformance during a future earthquake—should benoted.2.7.4 Adjacent BuildingsData should be collected on the configuration ofadjacent structures when such structures have thepotential to influence the seismic performance of therehabilitated building. Data collected should besufficient to permit analysis of the potential interactionissues identified below, as applicable. In some cases, itmay not be possible to obtain adequate information onadjacent structures to permit a meaningful evaluation.In such cases, the owner should be notified of thepotential consequences of these interactions.2.7.4.1 Building PoundingData on adjacent structures should be collected topermit investigation of the potential effects of buildingpounding whenever the side of the adjacent structure islocated closer to the building than 4% of the buildingheight above grade at the location of potential impacts.Building pounding can alter the basic response of thebuilding to ground motion, and impart additionalinertial loads and energy to the building from theadjacent structure. Of particular concern is the potentialfor extreme local damage to structural elements at thezones of impact. (See Section 2.11.10.)2.7.4.2 Shared Element ConditionData should be collected on all adjacent structures thatshare elements in common with the building. Buildingssharing common elements, such as party walls, haveseveral potential problems. If the buildings attempt tomove independently, one building may pull the sharedelement away from the other, resulting in a partialcollapse. If the buildings behave as an integral unit, theadditional mass and inertial loads of one structure mayresult in extreme demands on the lateral-force-resistingsystem of the other. (See Section 2.11.9.)2.7.4.3 Hazards from Adjacent StructureData should be collected on all structures that have thepotential to damage the building with falling debris, orother earthquake-induced physical hazards such asaggressive chemical leakage, fire, or explosion.Consideration should be given to hardening thoseportions of the building that may be impacted by debrisor other hazards from adjacent structures. WhereImmediate Occupancy of the building is desired, andingress to the building may be impaired by suchhazards, consideration should be given to providing forFEMA 273 Seismic Rehabilitation Guidelines 2-27

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)suitably resistant access to the building. Sufficientinformation must be collected on adjacent structures toallow preliminary evaluation of the likelihood andnature of hazards such as potential falling debris, fire,and blast pressures. Evaluations similar to those inFEMA 154, Rapid Visual Screening of Buildings forSeismic Hazards: A Handbook (ATC, 1988), should beadequate for this purpose.2.8 Rehabilitation MethodsThe scope of building structural alterations andmodifications required to meet the selectedRehabilitation Objective shall be determined inaccordance with one of the methods described in thissection. In addition, rehabilitation of historic buildingsshould be carefully considered in accordance with thediscussion in Chapter 1.2.8.1 Simplified MethodThe Simplified Method allows for design of buildingrehabilitation measures without requiring analyses ofthe entire building’s response to earthquake hazards.This method is not applicable to all buildings and can beused only to achieve Limited Rehabilitation Objectives(Section 2.4.3).The Simplified Method may be used to achieve aRehabilitation Objective consisting of the Life SafetyPerformance Level (3-C) for a BSE-1 earthquake forbuildings meeting all of the following conditions:• The building conforms to one of the Model BuildingTypes indicated in Table 10-1, as well as alllimitations indicated in that table with regard tonumber of stories, regularity, and seismic zone; and• A complete evaluation of the building is performedin accordance with FEMA 178 (BSSC, 1992), andall deficiencies identified in that evaluation areaddressed by the selected Simplified RehabilitationMethods.Any building may be partially rehabilitated to achieve aLimited Rehabilitation Objective using the SimplifiedMethod, subject to the limitations of Section 2.4.3.The Simplified Method may not be used for buildingsintended to meet the BSO or any EnhancedRehabilitation Objectives. For those buildings and otherbuildings not meeting the limitations for the SimplifiedMethod, the Systematic Method shall be used.2.8.2 Systematic MethodRehabilitation programs for buildings and objectivesthat do not qualify for Simplified Rehabilitation underSection 2.8.1 shall be designed in accordance with thissection. The basic approach shall include the following:• The structure shall be analyzed to determine if it isadequate to meet the selected RehabilitationObjective(s) and, if it is not adequate, to identifyspecific deficiencies. If initial analyses indicate thatkey elements or components of the structure do notmeet the acceptance criteria, it may be possible todemonstrate acceptability by using more detailedand accurate analytical procedures. Section 2.9provides information on alternative analyticalprocedures that may be used.• One or more rehabilitation strategies shall bedeveloped to address the deficiencies identified inthe preliminary evaluation. Alternative rehabilitationstrategies are presented in Section 2.10.• A preliminary rehabilitation design shall bedeveloped that is consistent with the rehabilitationstrategy.• The structure and the preliminary rehabilitationmeasures shall be analyzed to determine whether therehabilitated structure will be adequate to meet theselected Rehabilitation Objective(s).• The process shall be repeated as required until adesign solution is obtained that meets the selectedRehabilitation Objective(s), as determined by theanalysis.2.9 Analysis ProceduresAn analysis of the structure shall be conducted todetermine the distribution of forces and deformationsinduced in the structure by the design ground shakingand other seismic hazards corresponding with theselected Rehabilitation Objective(s). The analysis shalladdress the seismic demands and the capacity to resistthese demands for all elements in the structure thateither:2-28 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)• Are essential to the lateral stability of the structure(primary elements); or• Are essential to the vertical load-carrying integrityof the building; or• Are otherwise critical to meeting the RehabilitationObjective and could be subject to damage as a resultof the building’s response to the earthquake hazards.The analysis procedure shall consist of one of thefollowing:• Linear analysis, in accordance with Section 3.3,including Linear Static Procedure (LSP) (seeSection 3.3.1), and Linear Dynamic Procedure(LDP) (see Section 3.3.2), including:– Response Spectrum Analysis (seeSection 3.3.2.2C), and– Linear Time-History Analysis (seeSection 3.3.2.2D), or• Nonlinear analysis, in accordance with Section 3.3,including Nonlinear Static Procedure (NSP) inSection 3.3.3 and Nonlinear Dynamic Procedure(NDP) in Section 3.3.4, or• Alternative rational analysisLimitations with regard to the use of these proceduresare given in Sections 2.9.1, 2.9.2, and 2.9.3. Criteriaused to determine whether the results of an analysisindicate acceptable performance for the building arediscussed in Section 2.9.4.2.9.1 Linear ProceduresLinear procedures may be used for any of therehabilitation strategies contained in Section 2.10except those strategies incorporating the use ofsupplemental energy dissipation systems and sometypes of seismic isolation systems. For the specificanalysis procedures applicable to these rehabilitationstrategies, refer to Chapter 9.The results of the linear procedures can be veryinaccurate when applied to buildings with highlyirregular structural systems, unless the building iscapable of responding to the design earthquake(s) in anearly elastic manner. Therefore, linear proceduresshould not be used for highly irregular buildings, unlessthe earthquake ductility demands on the building aresuitably low.2.9.1.1 Method to Determine Applicability ofLinear ProceduresThe methodology indicated in this section may be usedto determine whether a building can be analyzed withsufficient accuracy by linear procedures. The basicapproach is to perform a linear analysis using the loadsdefined in either Section 3.3.1 or 3.3.2 and then toexamine the results of this analysis to identify themagnitude and uniformity of distribution of inelasticdemands on the various components of the primarylateral-force-resisting elements. The magnitude anddistribution of inelastic demands are indicated bydemand-capacity ratios (DCRs), as defined below. Notethat these DCRs are not used to determine theacceptability of component behavior. The adequacy ofstructural components and elements must be evaluatedusing the procedures contained in Chapter 3, togetherwith the acceptance criteria provide in Chapters 4through 8. DCRs are used only to determine astructure’s regularity. It should be noted that forcomplex structures, such as buildings with perforatedshear walls, it may be easier to use one of the nonlinearprocedures than to ensure that the building hassufficient regularity to permit use of linear procedures.DCRs for existing and added building components shallbe computed in accordance with the equation:where:Q UDQ CEQDCR = ----------- UDQ CE= Force calculated in accordance withSection 3.4, due to the gravity andearthquake loads of Section 3.3= Expected strength of the component orelement, calculated in accordance withChapters 5 through 8(2-12)DCRs should be calculated for each controlling action(such as axial force, moment, shear) of eachcomponent. If all of the computed controlling DCRs fora component are less than or equal to 1.0, then thecomponent is expected to respond elastically to theearthquake ground shaking being evaluated. If one ormore of the computed DCRs for a component areFEMA 273 Seismic Rehabilitation Guidelines 2-29

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)greater than 1.0, then the component is expected torespond inelastically to the earthquake ground shaking.The largest DCR calculated for a given componentdefines the critical action for the component, i.e., themode in which the component will first yield, or fail.This DCR is termed the critical component DCR. If anelement is composed of multiple components, then thecomponent with the largest computed DCR is thecritical component for the element, i.e., this will be thefirst component in the element to yield, or fail. Thelargest DCR for any component in an element at aparticular story is termed the critical element DCR atthat story.If the DCRs computed for all of the critical actions(axial force, moment, shear) of all of the components(such as beams, columns, wall piers, braces, andconnections) of the primary elements are less than 2.0,then linear procedures are applicable, regardless ofconsiderations of regularity. If some computed DCRsexceed 2.0, then linear procedures should not be used ifany of the following apply:• There is an in-plane discontinuity in any primaryelement of the lateral-force-resisting system. Inplanediscontinuities occur whenever a lateral-forceresistingelement is present in one story, but does notcontinue, or is offset, in the story immediatelybelow. Figure 2-2 depicts such a condition. Thislimitation need not apply if the coefficient J inEquation 3-15 is taken as 1.0.Nonlateral forceresistingbaysystem. An out-of-plane discontinuity exists whenan element in one story is offset relative to thecontinuation of that element in an adjacent story, asdepicted in Figure 2-3. This limitation need notapply if the coefficient J in Equation 3-15 is takenas 1.0.Figure 2-3• There is a severe weak story irregularity present atany story in any direction of the building. A severeweak story irregularity may be deemed to exist if theratio of the average shear DCR for any story to thatfor an adjacent story in the same direction exceeds125%. The average DCR for a story may becalculated by the equation:where:Typical Building with Out-of-Plane OffsetIrregularity∑DCR i V iDCR = -------------------------1n∑1V iShear wall atupper storiesSetback shear wallat first story(2-13)Figure 2-2Lateral force-resistingbayIn-Plane Discontinuity in Lateral SystemDCRDCR iV in= Average DCR for the story= Critical action DCR for element i= Total calculated lateral shear force in anelement i due to earthquake response,assuming that the structure remainselastic= Total number of elements in the story• There is an out-of-plane discontinuity in anyprimary element of the lateral-force-resistingFor buildings with flexible diaphragms, each line offraming should be independently evaluated.2-30 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)• There is a severe torsional strength irregularitypresent in any story. A severe torsional strengthirregularity may be deemed to exist in a story whenthe diaphragm above the story is not flexible and theratio of the critical element DCRs for primaryelements on one side of the center of resistance in agiven direction for a story, to those on the other sideof the center of resistance for the story, exceeds 1.5.If the guidelines above indicate that a linear procedureis applicable, then either the LSP or the LDP may beused, unless one or more of the following apply, inwhich case the LSP should not be used:• The building height exceeds 100 feet.• The ratio of the building’s horizontal dimension atany story to the corresponding dimension at anadjacent story exceeds 1.4 (excluding penthouses).• The building is found to have a severe torsionalstiffness irregularity in any story. A severe torsionalstiffness irregularity may be deemed to exist in astory if the diaphragm above the story is not flexibleand the results of the analysis indicate that the driftalong any side of the structure is more than 150% ofthe average story drift.• The building is found to have a severe vertical massor stiffness irregularity. A severe vertical mass orstiffness irregularity may be deemed to exist whenthe average drift in any story (except penthouses)exceeds that of the story above or below by morethan 150%.• The building has a nonorthogonal lateral-forceresistingsystem.2.9.2 Nonlinear ProceduresNonlinear Analysis Procedures may be used for any ofthe rehabilitation strategies contained in Section 2.10.Nonlinear procedures are especially recommended foranalysis of buildings having irregularities as identifiedin Section 2.9.1.1. The NSP is mainly suitable forbuildings without significant higher-mode response.The NDP is suitable for any structure, subject to thelimitations in Section 2.9.2.2.2.9.2.1 Nonlinear Static Procedure (NSP)The NSP may be used for any structure and anyRehabilitation Objective, with the following exceptionsand limitations.• The NSP should not be used for structures in whichhigher mode effects are significant, unless an LDPevaluation is also performed. To determine if highermodes are significant, a modal response spectrumanalysis should be performed for the structure usingsufficient modes to capture 90% mass participation,and a second response spectrum analysis should beperformed considering only the first modeparticipation. Higher mode effects should beconsidered significant if the shear in any storycalculated from the modal analysis considering allmodes required to obtain 90% mass participationexceeds 130% of the corresponding story shearresulting from the analysis considering only the firstmode response. When an LDP is performed tosupplement an NSP for a structure with significanthigher mode effects, the acceptance criteria valuesfor deformation-controlled actions (m values),provided in Chapters 5 through 9, may be increasedby a factor of 1.33.• The NSP should not be used unless comprehensiveknowledge of the structure has been obtained, asindicated in Section 2.7.2.2.9.2.2 Nonlinear Dynamic Procedure (NDP)The NDP may be used for any structure and anyRehabilitation Objective, with the following exceptionsand limitations.• The NDP is not recommended for use with woodframe structures.• The NDP should not be utilized unlesscomprehensive knowledge of the structure has beenobtained, as indicated in Section 2.7.2.• The analysis and design should be subject to reviewby an independent third-party professional engineerwith substantial experience in seismic design andnonlinear procedures.2.9.3 Alternative Rational AnalysisNothing in the Guidelines should be interpreted aspreventing the use of any alternative analysis procedurethat is rational and based on fundamental principles ofFEMA 273 Seismic Rehabilitation Guidelines 2-31

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)engineering mechanics and dynamics. Such alternativeanalyses should not adopt the acceptance criteriacontained in the Guidelines without careful review as totheir applicability. All projects using alternative rationalanalysis procedures should be subject to review by anindependent third-party professional engineer withsubstantial experience in seismic design.2.9.4 Acceptance CriteriaThe Analysis Procedures indicate the building’sresponse to the design earthquake(s) and the forces anddeformations imposed on the various components, aswell as global drift demands on the structure. WhenLSP or LDP analysis is performed, acceptability ofcomponent behavior is evaluated for each of thecomponent’s various actions using Equation 3-18 forductile (deformation-controlled) actions andEquation 3-19 for nonductile (force-controlled) actions.Figure 2-4 indicates typical idealized force-deformationcurves for various types of component actions.The type 1 curve is representative of typical ductilebehavior. It is characterized by an elastic range (point 0to point 1 on the curve), followed by a plastic range(points 1 to 3) that may include strain hardening orsoftening (points 1 to 2), and a strength-degraded range(points 2 to 3) in which the residual force that can beresisted is significantly less than the peak strength, butstill substantial. Acceptance criteria for primaryelements that exhibit this behavior are typically withinthe elastic or plastic ranges between points 1 and 2,depending on the Performance Level. Acceptancecriteria for secondary elements can be within any of theranges. Primary component actions exhibiting thisbehavior are considered deformation-controlled if thestrain-hardening or strain-softening range is sufficientlylarge e > 2g; otherwise, they are considered forcecontrolled.Secondary component actions exhibitingthis behavior are typically considered to bedeformation-controlled.The type 2 curve is representative of another type ofductile behavior. It is characterized by an elastic rangeand a plastic range, followed by a rapid and completeloss of strength. If the plastic range is sufficiently large(e ≥ 2g), this behavior is categorized as deformationcontrolled.Otherwise it is categorized as forcecontrolled.Acceptance criteria for primary andsecondary components exhibiting this behavior will bewithin the elastic or plastic ranges, depending on theperformance level.The type 3 curve is representative of a brittle ornonductile behavior. It is characterized by an elasticrange, followed by a rapid and complete loss ofstrength. Component actions displaying this behaviorare always categorized as force-controlled. Acceptancecriteria for primary and secondary componentsexhibiting this behavior are always within the elasticrange.Q2Q2Q111aQ yg g gb3aed ∆e ∆Type 1 curve Type 2 curve Type 3 curve0 0 0Figure 2-4 General Component Behavior Curves∆2-32 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Figure 2-5 shows an idealized force versus deformationcurve that is used throughout the Guidelines to specifyacceptance criteria for deformation-controlledcomponent and element actions for any of the four basictypes of materials. Linear response is depicted betweenpoint A (unloaded component) and an effective yieldpoint B. The slope from B to C is typically a smallpercentage (0–10%) of the elastic slope, and is includedto represent phenomena such as strain hardening. C hasan ordinate that represents the strength of thecomponent, and an abscissa value equal to thedeformation at which significant strength degradationbegins (line CD). Beyond point D, the componentresponds with substantially reduced strength to point E.At deformations greater than point E, the componentstrength is essentially zero.QQ CEAB(a) DeformationQQ CEθbaor∆CDEcIn Figure 2-4, Q y represents the yield strength of thecomponent. In a real structure, the yield strength ofindividual elements that appear similar will actuallyhave some variation. This is due to inherent variabilityin the material strength comprising the individualelements as well as differences in workmanship andphysical condition. When evaluating the behavior ofdeformation-controlled components, the expectedstrength, Q CE , rather than the yield strength Q y is used.Q CE is defined as the mean value of resistance at thedeformation level anticipated, and includesconsideration of the variability discussed above as wellas phenomena such as strain hardening and plasticsection development. When evaluating the behavior offorce-controlled components, a lower bound estimate ofthe component strength, Q CL , is considered. Q CL isstatistically defined as the mean minus one standarddeviation of the yield strengths Q y for a population ofsimilar components.For some components it is convenient to prescribeacceptance criteria in terms of deformation (e.g., θor ∆), while for others it is more convenient to givecriteria in terms of deformation ratios. To accommodatethis, two types of idealized force versus deformationcurves are used in the Guidelines as illustrated inFigures 2-5(a) and (b). Figure 2-5(a) shows normalizedforce (Q/Q CE ) versus deformation (θ or ∆) and theparameters a, b, and c. Figure 2-5(b) shows normalizedforce (Q/Q CE ) versus deformation ratio (θ/θ y, ∆ /∆ y, or∆/h) and the parameters d, e, and c. Elastic stiffnessesand values for the parameters a, b, c, d, and e that can beused for modeling components are given in Chapters 5through 8.(b) Deformation ratioNormalized forceFigure 2-5ABBdθθyP,e∆∆yCDor∆hI.O.D EADeformation or deformation ratio(c) Component or element deformation limitsPCIdealized Component Load versusDeformation Curves for DepictingComponent Modeling and AcceptabilityPEL.S.ScC.P.SFEMA 273 Seismic Rehabilitation Guidelines 2-33

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Figure 2-5(c) graphically shows the approximatedeformation or deformation ratio, in relation to theidealized force versus deformation curve, that aredeemed acceptable in the Guidelines for Primary (P)and Secondary (S) components for ImmediateOccupancy (IO), Life Safety (LS), and CollapsePrevention (CP) Performance Levels. Numerical valuesof the acceptable deformations or deformation ratios aregiven in Chapters 5 through 8 for all types ofcomponents and elements.If nonlinear procedures are used, component capacitiesconsist of permissible inelastic deformation demandsfor deformation-controlled components, and ofpermissible strength demands for force-controlledcomponents. If linear procedures are used, capacitiesare defined as the product of factors m and expectedstrengths Q CE for deformation-controlled componentsand as permissible strength demands for forcecontrolledcomponents. Tables 2-16 and 2-17summarize these capacities. In this table, κ is theknowledge-based factor defined in Section 2.7.2, and σis the standard deviation of the material strengths.Detailed guidelines on the calculation of individualcomponent force and deformation capacities may befound in the individual materials chapters as follows:• Foundations—Chapter 4• Elements and components composed of steel or castiron—Chapter 5• Elements and components composed of reinforcedconcrete—Chapter 6• Elements and components composed of reinforcedor unreinforced masonry—Chapter 7• Elements and components composed of timber, lightmetal studs, gypsum, or plaster products—Chapter 8• Seismic isolation systems and energy dissipationsystems—Chapter 9• Nonstructural (architectural, mechanical, andelectrical) components—Chapter 11Table 2-16ParameterExisting MaterialStrengthExisting ActionCapacityNew MaterialStrengthNew ActionCapacityCalculation of Component ActionCapacity—Linear ProceduresDeformation-ControlledExpected meanvalue withallowance forstrain hardeningκ · Q CEExpected materialstrengthQ CEForce-ControlledLower bound value(approximately -1σlevel)κ · Q CESpecified materialstrengthQ CENote: Capacity reduction (φ) factors are typically taken as unity in theevaluation of capacities.Table 2-17ParameterDeformationCapacity—ExistingComponentDeformationCapacity—New ComponentStrengthCapacity—ExistingComponentStrengthCapacity—New ElementCalculation of Component ActionCapacity—Nonlinear ProceduresDeformation-Controlledκ · deformationlimitdeformation limitN/AN/AForce-ControlledN/AN/Aκ · Q CLQ CLNote: Capacity reduction (φ) factors are typically taken as unity in theevaluation of capacities.Acceptance criteria for elements and components forwhich criteria are not presented in the Guidelines shallbe determined by a qualification testing program, inaccordance with the procedures of Section 2.13.• Elements and components comprising combinationsof materials—covered in the chapters associated2-34 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.10 Rehabilitation StrategiesRehabilitation of buildings may be achieved by one ormore of the strategies indicated in this section.Although not specifically required by any of thestrategies, it is very beneficial for the rehabilitatedbuilding’s lateral-force-resisting system to have anappropriate level of redundancy, so that any localizedfailure of a few elements of the system will not result inlocal collapse or an instability. This should beconsidered when developing rehabilitation designs.2.10.1 Local Modification of ComponentsSome existing buildings have substantial strength andstiffness; however, some of their components do nothave adequate strength, toughness, or deformationcapacity to satisfy the Rehabilitation Objectives. Anappropriate strategy for such structures may be toperform local modifications of those components thatare inadequate, while retaining the basic configurationof the building’s lateral-force-resisting system. Localmodifications that can be considered includeimprovement of component connectivity, componentstrength, and/or component deformation capacity. Thisstrategy tends to be the most economical approach torehabilitation when only a few of the building’scomponents are inadequate.Local strengthening allows one or more understrengthelements or connections to resist the strength demandspredicted by the analysis, without affecting the overallresponse of the structure. This could include measuressuch as cover plating steel beams or columns, or addingplywood sheathing to an existing timber diaphragm.Such measures increase the strength of the element orcomponent and allow it to resist more earthquakeinducedforce before the onset of damage.Local corrective measures that improve the deformationcapacity or ductility of a component allow it to resistlarge deformation levels with reduced amounts ofdamage, without necessarily increasing the strength.One such measure is placement of a confinement jacketaround a reinforced concrete column to improve itsability to deform without spalling or degradingreinforcement splices. Another measure is reduction ofthe cross section of selected structural components toincrease their flexibility and response displacementcapacity.2.10.2 Removal or Lessening of ExistingIrregularities and DiscontinuitiesStiffness, mass, and strength irregularities are commoncauses of undesirable earthquake performance. Whenreviewing the results of a linear analysis, theirregularities can be detected by examining thedistribution of structural displacements and DCRs.When reviewing the results of a nonlinear analysis, theirregularities can be detected by examining thedistribution of structural displacements and inelasticdeformation demands. If the values of structuraldisplacements, DCRs, or inelastic deformation demandspredicted by the analysis are unbalanced, with largeconcentrations of high values within one story or at oneside of a building, then an irregularity exists. Suchirregularities are often, but not always, caused by thepresence of a discontinuity in the structure, as forexample, termination of a perimeter shear wall abovethe first story. Simple removal of the irregularity maybe sufficient to reduce demands predicted by theanalysis to acceptable levels. However, removal ofdiscontinuities may be inappropriate in the case ofhistoric buildings, and the effect of such alterations onimportant historic features should be consideredcarefully.Effective corrective measures for removal or reductionof irregularities and discontinuities, such as soft orweak stories, include the addition of braced frames orshear walls within the soft/weak story. Torsionalirregularities can be corrected by the addition ofmoment frames, braced frames, or shear walls tobalance the distribution of stiffness and mass within astory. Discontinuous components such as columns orwalls can be extended through the zone of discontinuity.Partial demolition can also be an effective correctivemeasure for irregularities, although this obviously hassignificant impact on the appearance and utility of thebuilding, and this may not be an appropriate alternativefor historic structures. Portions of the structure thatcreate the irregularity, such as setback towers or sidewings, can be removed. Expansion joints can be createdto transform a single irregular building into multipleregular structures; however, care must be taken to avoidthe potential problems associated with pounding.2.10.3 Global Structural StiffeningSome flexible structures behave poorly in earthquakesbecause critical components and elements do not haveadequate ductility or toughness to resist the large lateralFEMA 273 Seismic Rehabilitation Guidelines 2-35

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)deformations that ground shaking induces in thestructure. For structures comprising many suchelements, an effective way to improve performance is tostiffen the structure so that its response produces lesslateral deformation. Construction of new braced framesor shear walls within an existing structure are effectivemeasures for adding stiffness.2.10.4 Global Structural StrengtheningSome existing buildings have inadequate strength toresist lateral forces. Such structures exhibit inelasticbehavior at very low levels of ground shaking. Analysesof such buildings indicate large DCRs (or inelasticdeformation demands) throughout the structure. Byproviding supplemental strength to such a building’slateral-force-resisting system, it is possible to raise thethreshold of ground motion at which the onset ofdamage occurs. Shear walls and braced frames areeffective elements for this purpose; however, they maybe significantly stiffer than the structure to which theyare added, requiring that they be designed to providenearly all of the structure’s lateral resistance. Momentresistingframes, being more flexible, may be morecompatible with existing elements in some structures;however, such flexible elements may not becomeeffective in the building’s response until existing brittleelements have already been damaged.2.10.5 Mass ReductionTwo of the primary characteristics that control theamount of force and deformation induced in a structureby ground motion are its stiffness and mass. Reductionsin mass result in direct reductions in both the amount offorce and deformation demand produced byearthquakes, and therefore can be used in lieu ofstructural strengthening and stiffening. Mass can bereduced through demolition of upper stories,replacement of heavy cladding and interior partitions,or removal of heavy storage and equipment loads.2.10.6 Seismic IsolationWhen a structure is seismically isolated, compliantbearings are inserted between the superstructure and itsfoundations. This produces a system (structure andisolation bearings) with fundamental response thatconsists of nearly rigid body translation of the structureabove the bearings. Most of the deformation induced inthe isolated system by the ground motion occurs withinthe compliant bearings, which have been specificallydesigned to resist these concentrated displacements.Most bearings also have excellent energy dissipationcharacteristics (damping). Together, this results ingreatly reduced demands on the existing elements of thestructure, including contents and nonstructuralcomponents. For this reason, seismic isolation is oftenan appropriate strategy to achieve EnhancedRehabilitation Objectives that include protection ofhistoric fabric, valuable contents, and equipment, or forbuildings that contain important operations andfunctions. This technique is most effective for relativelystiff buildings with low profiles and large mass. It is lesseffective for light, flexible structures.2.10.7 Supplemental Energy DissipationA number of technologies are available that allow theenergy imparted to a structure by ground motion to bedissipated in a controlled manner through the action ofspecial devices—such as fluid viscous dampers(hydraulic cylinders), yielding plates, or friction pads—resulting in an overall reduction in the displacements ofthe structure. The most common devices dissipateenergy through frictional, hysteretic, or viscoelasticprocesses. In order to dissipate substantial energy,dissipation devices must typically undergo significantdeformation (or stroke) which requires that thestructural experience substantial lateral displacements.Therefore, these systems are most effective in structuresthat are relatively flexible and have some inelasticdeformation capacity. Energy dissipaters are mostcommonly installed in structures as components ofbraced frames. Depending on the characteristics of thedevice, either static or dynamic stiffness is added to thestructure as well as energy dissipation capacity(damping). In some cases, although the structuraldisplacements are reduced, the forces delivered to thestructure can actually be increased.2.11 General Analysis and DesignRequirementsThe detailed guidelines of this section apply to allbuildings rehabilitated to achieve either the BSO or anyEnhanced Rehabilitation Objectives. Thoughcompliance with the guidelines in this section is notrequired for buildings rehabilitated to LimitedRehabilitation Objectives, such compliance should beconsidered. Unless otherwise noted, all numericalvalues apply to the Life Safety Performance Level, andmust be multiplied by 1.25 to apply to ImmediateOccupancy.2-36 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.11.1 Directional EffectsThe lateral-load-resisting system shall be demonstratedto be capable of responding to ground-motionproducinglateral forces in any horizontal direction. Forbuildings with orthogonal primary axes of resistance,this may be satisfied by evaluating the response of thestructure to such forces in each of the two orthogonaldirections. As a minimum, the effects of structuralresponse in each of these orthogonal directions shall beconsidered independently. In addition, the combinedeffect of simultaneous response in both directions shallbe considered, in accordance with the applicableprocedures of Section 3.2.7.2.11.2 P-∆ EffectsThe structure shall be investigated to ensure that lateraldrifts induced by earthquake response do not result in acondition of instability under gravity loads. At eachstory, the quantity θ i shall be calculated for eachdirection of response, as follows:where:θ iP iδ= --------- iV i h i(2-14)P i = Portion of the total weight of the structureincluding dead, permanent live, and 25% oftransient live loads acting on the columns andbearing walls within story level iV i = Total calculated lateral shear force in thedirection under consideration at story i due toearthquake response, assuming that thestructure remains elastich i = Height of story i, which may be taken as thedistance between the centerline of floorframing at each of the levels above and below,the distance between the top of floor slabs ateach of the levels above and below, or similarcommon points of referenceδ i = Lateral drift in story i, in the direction underconsideration, at its center of rigidity, using thesame units as for measuring h iIn any story in which θ i is less than or equal to 0.1, thestructure need not be investigated further for stabilityconcerns. When the quantity θ i in a story exceeds 0.1,the analysis of the structure shall consider P-∆ effects,in accordance with the applicable procedures ofSection 3.2.5. When the value of θ i exceeds 0.33, thestructure should be considered potentially unstable andthe rehabilitation design modified to reduce thecomputed lateral deflections in the story.2.11.3 TorsionAnalytical models used to evaluate the response of thebuilding to earthquake ground motion shall account forthe effects of torsional response resulting fromdifferences in the plan location of the center of massand center of rigidity of the structure at all diaphragmlevels that are not flexible.2.11.4 OverturningThe effects of overturning at each level of the structureshall be evaluated cumulatively from the top of thestructure to its base (See the commentary and furtherguidance in the sidebar, “Overturning Issues andAlternative Methods.”)2.11.4.1 Linear ProceduresWhen a linear procedure is followed, each primaryelement at each level of the structure shall beinvestigated for stability against overturning under theeffects of seismic forces applied at and above the levelunder consideration. Overturning effects may beresisted either through the stabilizing effect of deadloads or through positive connection of the element tostructural components located below.Where dead loads are used to resist the effects ofoverturning, the following shall be satisfied:whereM OTM STM ST> M OT⁄ ( C 1 C 2 C 3 J)(2-15)= Total overturning moment inducedon the element by seismic forcesapplied at and above the levelunder consideration= Stabilizing moment produced bydead loads acting on the element,calculated as the sum of theproducts of each separate dead loadand the horizontal distancebetween its vertical line of actionand the centroid of the resistingFEMA 273 Seismic Rehabilitation Guidelines 2-37

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Overturning Issues and Alternative MethodsResponse to earthquake ground motion results in a tendencyfor structures, and individual vertical elements of structures,to overturn about their bases. Although actual overturning isvery rare, overturning effects can result in significantstresses, which have caused some local and global failures.In new building design, earthquake effects, includingoverturning, are evaluated for lateral forces that aresignificantly reduced (by the R-factor) from those whichmay actually develop in the structure.For elements with positive attachment between levels, thatbehave as single units, such as reinforced concrete walls, theoverturning effects are resolved into component forces (e.g.,flexure and shear at the base of the wall) and the element isthen proportioned with adequate strength to resist theseoverturning effects resulting from the reduced force levels.Some elements, such as wood shear walls and foundations,may not be provided with positive attachment betweenlevels. For them, an overturning stability check isperformed. If the element has sufficient dead load to remainstable under the overturning effects of the design lateralforces and sufficient shear connection to the level below,then the design is deemed adequate. However, if dead loadis inadequate to provide stability, then hold-downs, piles, orother types of uplift anchors are provided to resist theresidual overturning caused by the design forces.In the linear and nonlinear procedures of the Guidelines,lateral forces are not reduced by an R-factor, as they are fornew buildings. Thus, computed overturning effects arelarger than typically calculated for new buildings. Thoughthe procedure used for new buildings is not completelyrational, it has resulted in successful performance.Therefore, it was felt inappropriate to require that structuresand elements of structures remain stable for the full lateralforces used in the linear procedures. Instead, the designermust determine if positive direct attachment will be used toresist overturning effects, or if dead loads will be used. Ifpositive direct attachment is to be used, then this attachmentis treated just as any other element or component action.However, if dead loads alone are used to resist overturning,then overturning is treated as a force-controlled behaviorand the overturning demands are reduced to an estimate ofthe real overturning demands which can be transmitted tothe element, considering the overall limiting strength of thestructure.There is no rational method available, that has been shownto be consistent with observed behavior, to design orevaluate elements for overturning effects. The methoddescribed in the Guidelines is rational, but inconsistent withprocedures used for new buildings. To improve damagecontrol, the Guidelines method is recommended forchecking acceptability for Performance Levels higher thanLife Safety.A simplified alternative, described below, for evaluating theadequacy of dead load to provide stability againstoverturning for Collapse Prevention or Life SafetyPerformance Levels is to use procedures similar to thoseused for the design of new buildings:The load combination represented byQQ = 0.9Q ED + ---------R OTwhere Q D and Q E have opposite signs, and R OT = 7.5 forCollapse Prevention Performance Level, or 6.0 for LifeSafety, is used for evaluating the adequacy of the dead loadalone. In the event that the dead load is inadequate, thedesign of any required hold-downs, piles, or other types ofuplift anchors is performed according to the Guidelines.Acceptability criteria for components shall be taken fromChapters 5 through 8 with m = 1.Additional studies are needed on the parameters that controloverturning in seismic rehabilitation. These alternativemethods are tentative, pending results from this futureresearch.C 1 , C 2 , and C 3Jforce at the toe of the elementabout which the seismic forcestend to cause overturning= Coefficients defined inSection 3.3.1.3= Coefficient defined inEquation 3-17The quantity M OT /J need not exceed the overturningmoment that can be applied to the element, as limitedby the expected strength of the structure respondingwith an acceptable inelastic mechanism. The elementshall be evaluated for the effects of compression on thetoe about which it is being overturned. For this purpose,compression at the toe of the element shall beconsidered a force-controlled action, and shall beevaluated in accordance with the procedures of2-38 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Section 3.4.2.1. Refer to Chapter 4 for specialconsiderations related to overturning effects onfoundations.Where dead loads acting on an element are insufficientto provide stability, positive attachment of the elementto the structure located above and below the level underconsideration shall be provided. These attachmentsshall be evaluated either as force-controlled ordeformation-controlled actions, in accordance with theapplicable guidelines provided in Chapters 5 through 8.2.11.4.2 Nonlinear ProceduresWhen a nonlinear procedure is followed, the effect ofearthquake-induced rocking of elements shall beincluded in the analytical model as a nonlinear degreeof freedom, whenever such rocking can occur. Theadequacy of elements above and below the level atwhich rocking occurs, including the foundations, shallbe evaluated for any redistribution of loads that occursas a result of this rocking in accordance with theprocedures of Section 3.4.3.2.11.5 ContinuityAll elements of the structure shall be thoroughly andintegrally tied together to form a complete path for thelateral inertial forces generated by the building’sresponse to earthquake demands as follows:• Every smaller portion of a structure, such as anoutstanding wing, shall be tied to the structure as awhole with components capable of resistinghorizontal forces equal, at a minimum, to 0.133S XStimes the weight of the smaller portion of thestructure, unless the individual portions of thestructure are self-supporting and are separated by aseismic joint.• Every component shall be connected to the structureto resist a horizontal force in any direction equal, at aminimum, to 0.08S XS times the weight of thecomponent. For connections resisting concentratedloads, a minimum force of 1120 pounds shall beused; for distributed load connections, the minimumforce shall be 280 pounds per lineal foot.• Where a sliding support is provided at the end(s) of acomponent, the bearing length shall be sufficient toaccommodate the expected differential displacementsof the component relative to its support.2.11.6 DiaphragmsDiaphragms shall be provided at each level of thestructure as necessary to connect building masses to theprimary vertical elements of the lateral-force-resistingsystem. The analytical model used to analyze thebuilding shall account for the behavior of thediaphragms, which shall be evaluated for the forces anddisplacements indicated by the Analysis Procedure. Inaddition, the following shall apply:• Diaphragm Chords: Except for diaphragmsevaluated as “unchorded” using Chapter 8 of theGuidelines, a component shall be provided todevelop horizontal shear stresses at each diaphragmedge (either interior or exterior). This componentshall consist of either a continuous diaphragm chord,a continuous wall or frame element, or a continuouscombination of wall, frame, and chord elements. Theforces accumulated in these components andelements due to their action as diaphragmboundaries shall be considered in the evaluation oftheir adequacy. At re-entrant corners in diaphragms,and at the corners of openings in diaphragms,diaphragm chords shall be extended into thediaphragm a sufficient distance beyond the corner todevelop the accumulated diaphragm boundarystresses through the attachment of the extendedportion of the chord to the diaphragm.• Diaphragm Collectors: At each vertical elementto which a diaphragm is attached, a diaphragmcollector shall be provided to transfer to the verticalelement those calculated diaphragm forces thatcannot be transferred directly by the diaphragm inshear. The diaphragm collector shall be extendedinto and attached to the diaphragm sufficiently totransfer the required forces.• Diaphragm Ties: Diaphragms shall be providedwith continuous tension ties between their chords orboundaries. Ties shall be spaced at a distance notexceeding three times the length of the tie. Ties shallbe designed for an axial tensile force equal to 0.4S XStimes the weight tributary to that portion of thediaphragm located halfway between the tie and eachadjacent tie or diaphragm boundary. Wherediaphragms of timber, gypsum, or metal deckconstruction provide lateral support for walls ofmasonry or concrete construction, ties shall bedesigned for the wall anchorage forces specified inFEMA 273 Seismic Rehabilitation Guidelines 2-39

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Section 2.11.7 for the area of wall tributary to thediaphragm tie.2.11.7 WallsWalls shall be anchored to the structure as described inthis section, and evaluated for out-of-plane inertialforces as indicated in Chapters 5 through 8.• Walls shall be positively anchored to all diaphragmsthat provide lateral support for the wall or arevertically supported by the wall. Walls shall beanchored to diaphragms at horizontal distances notexceeding eight feet, unless it can be demonstratedthat the wall has adequate capacity to spanlongitudinally between the supports for greaterdistances. Walls shall be anchored to eachdiaphragm for the larger of 400S XS pounds per footof wall or χS XS times the weight of the wall tributaryto the anchor, where χ shall be taken fromTable 2-18. The anchorage forces shall be developedinto the diaphragm. For flexible diaphragms, theanchorage forces shall be taken as three times thosespecified above and shall be developed into thediaphragm by continuous diaphragm crossties. Forthis purpose, diaphragms may be partitioned into aseries of subdiaphragms. Each subdiaphragm shallbe capable of transmitting the shear forces due towall anchorage to a continuous diaphragm tie.Subdiaphragms shall have length-to-depth ratios ofthree or less. Where wall panels are stiffened for outof-planebehavior by pilasters and similar elements,anchors shall be provided at each such element andthe distribution of out-of-plane forces to wallanchors and diaphragm ties shall consider thestiffening effect. Wall anchor connections should beconsidered force-controlled.Table 2-18Coefficient χ for Calculation ofOut-of-Plane Wall ForcesPerformance LevelχCollapse Prevention 0.3Life Safety 0.4Immediate Occupancy 0.6• A wall shall have a strength adequate to spanbetween locations of out-of-plane support whensubjected to out-of-plane forces equal to 0.4S XStimes the unit weight of the wall, over its area.2.11.8 Nonstructural ComponentsNonstructural components, including architectural,mechanical and electrical components, shall beanchored and braced to the structure in accordance withthe provisions of Chapter 11. Post-earthquakeoperability of these components, as required for somePerformance Levels, shall also be provided for inaccordance with the requirements of Chapter 11 and theproject Rehabilitation Objectives.2.11.9 Structures Sharing Common ElementsWhere two or more buildings share common elements,such as party walls or columns, and either the BSO orEnhanced Rehabilitation Objectives are desired, one ofthe following approaches shall be followed.• The structures shall be thoroughly tied together so asto behave as an integral unit. Ties between thestructures at each level shall be designed for theforces indicated in Section 2.11.5. Analyses of thebuildings’ response to earthquake demands shallaccount for the interconnection of the structures andshall evaluate the structures as integral units.• The buildings shall be completely separated byintroducing seismic joints between the structures.Independent lateral-force-resisting systems shall beprovided for each structure. Independent verticalsupport shall be provided on each side of the seismicjoint, except that slide bearings to support loadsfrom one structure off the other may be used ifadequate bearing length is provided to accommodatethe expected independent lateral movement of eachstructure. It shall be assumed for such purposes thatthe structures may move out of phase with eachother in each direction simultaneously. The originalshared element shall be either completely removedor anchored to one of the structures in accordancewith the applicable requirements of Section 2.11.5.2.11.10 Building Separation2.11.10.1 GeneralBuildings intended to meet either the BSO or EnhancedObjectives shall be adequately separated from adjacentstructures to prevent pounding during response to thedesign earthquakes, except as indicated inSection 2.11.10.2. Pounding may be presumed not tooccur whenever the buildings are separated at any leveli by a distance greater than or equal to s i as given by theequation:2-40 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)where:s i =2∆ i1∆ i1 = Estimated lateral deflection of building 1relative to the ground at level i∆ i2 = Estimated lateral deflection of building 2relative to the ground at level iThe value of s i calculated by Equation 2-16 need notexceed 0.04 times the height of the buildings abovegrade at the zone of potential impacts.2.11.10.2 Special Considerations(2-16)Buildings not meeting the separation requirements ofSection 2.11.10.1 may be rehabilitated to meet the BSO,subject to the following limitations.A properly substantiated analysis shall be conductedthat accounts for the transfer of momentum and energybetween the structures as they impact, and either:• The diaphragms of the structures shall be located atthe same elevations and shall be demonstrated to becapable of transferring the forces resulting fromimpact; or• The structures shall be demonstrated to be capableof resisting all required vertical and lateral forcesindependent of any elements and components thatmay be severely damaged by impact of thestructures.2.11.11 Vertical Earthquake EffectsThe effects of the vertical response of a structure toearthquake ground motion should be considered for anyof the following cases:• Cantilever elements and components of structures• Pre-stressed elements and components of structures• Structural components in which demands due todead and permanent live loads exceed 80% of thenominal capacity of the component+2∆ i22.12 Quality AssuranceQuality assurance of seismic rehabilitation constructionfor all buildings and all Rehabilitation Objectivesshould, as a minimum, conform to the recommendationsof this section. These recommendations supplement therecommended testing and inspection requirementscontained in the reference standards given in Chapters 5through 11. The design professional responsible for theseismic rehabilitation of a specific building may find itappropriate to specify more stringent or more detailedrequirements. Such additional requirements may beparticularly appropriate for those buildings havingEnhanced Rehabilitation Objectives.2.12.1 Construction Quality Assurance PlanThe design professional in responsible charge shouldprepare a Quality Assurance Plan (QAP) for submittalto the regulatory agency as part of the overall submittalof construction documents. The QAP should specify theseismic-force-resisting elements, components, orsystems that are subject to special quality assurancerequirements. The QAP should, as a minimum, includethe following:• Required contractor quality control procedures• Required design professional construction qualityassurance services, including but not limited to thefollowing:– Review of required contractor submittals– Monitoring of required inspection reports andtest results– Construction consultation as required by thecontractor on the intent of the constructiondocuments– Procedures for modification of the constructiondocuments to reflect the demands of unforeseenfield conditions discovered during construction– Construction observation in accordance withSection 2.12.2.1.• Required special inspection and testing requirementsFEMA 273 Seismic Rehabilitation Guidelines 2-41

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.12.2 Construction Quality AssuranceRequirements2.12.2.1 Requirements for the StructuralDesign ProfessionalThe design professional in responsible charge, or adesign professional designated by the designprofessional in responsible charge, should performstructural observation of the rehabilitation measuresshown on the construction documents. Constructionobservation should include visual observation of thestructural system, for general conformance to theconditions assumed during design, and for generalconformance to the approved construction documents.Structural observation should be performed atsignificant construction stages and at completion of thestructural/seismic system. Structural constructionobservation does not include the responsibilities forinspection required by other sections of the Guidelines.Following such structural observations, the structuralconstruction observer should report any observeddeficiencies in writing to the owner’s representative, thespecial inspector, the contractor, and the regulatoryagency. The structural construction observer shouldsubmit to the building official a written statementattesting that the site visits have been made, andidentifying any reported deficiencies that, to the best ofthe structural construction observer’s knowledge, havenot been resolved or rectified.2.12.2.2 Special InspectionThe owner should employ a special inspector to observethe construction of the seismic-force-resisting system inaccordance with the QAP for the following constructionwork:• Items designated in Sections 1.6.2.1 through 1.6.2.9in the 1994 and 1997 NEHRP RecommendedProvisions (BSSC, 1995, 1997)• All other elements and components designated forsuch special inspection by the design professional• All other elements and components required by theregulatory agency2.12.2.3 TestingThe special inspector(s) shall be responsible forverifying that the special test requirements, as describedin the QAP, are performed by an approved testingagency for the types of work in the seismic-forceresistingsystem listed below:• All work described in Sections 1.6.3.1 through1.6.3.6 of the 1994 and 1997 NEHRP RecommendedProvisions (BSSC, 1995, 1997)• Other types of work designated for such testing bythe design professional• Other types of work required by the regulatoryagency2.12.2.4 Reporting and ComplianceProceduresThe special inspector(s) should furnish to the regulatoryagency, the design professional in responsible charge,the owner, the persons preparing the QAP, and thecontractor copies of progress reports of observations,noting therein any uncorrected deficiencies andcorrections of previously reported deficiencies. Allobserved deficiencies should be brought to theimmediate attention of the contractor for correction.At the completion of construction, the specialinspector(s) should submit a final report to theregulatory agency, owner, and design professional inresponsible charge indicating the extent to whichinspected work was completed in accordance withapproved construction documents. Any work not incompliance should be described.2.12.3 Regulatory Agency ResponsibilitiesThe regulatory agency having jurisdiction overconstruction of a building that is to be seismicallyrehabilitated should act to enhance and encourage theprotection of the public that is represented by suchrehabilitation. These actions should include thosedescribed in the following subsections.2.12.3.1 Construction Document Submittals—PermittingAs part of the permitting process, the regulatory agencyshould require that construction documents besubmitted for a permit to construct the proposed seismicrehabilitation measures. The documents should includea statement of the design basis for the rehabilitation,drawings (or adequately detailed sketches), structural/seismic calculations, and a QAP as recommended bySection 2.12.1. Appropriate structural constructionspecifications are also recommended, if structural2-42 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)requirements are not adequately defined by notes ondrawings.The regulatory agency should require that it bedemonstrated (in the design calculations, by third-partyreview, or by other means) that the design of the seismicrehabilitation measures has been performed inconformance with local building regulations, the stateddesign basis, the intent of the Guidelines, and/oraccepted engineering principles. The regulatory agencyshould be aware that compliance with the building codeprovisions for new structures is often not possible nor isit required by the Guidelines. It is not intended that theregulatory agency assure compliance of the submittalswith the structural requirements for new construction.The regulatory agency should maintain a permanentpublic file of the construction documents submitted aspart of the permitting process for construction of theseismic rehabilitation measures.2.12.3.2 Construction Phase RoleThe regulatory agency having jurisdiction over theconstruction of seismic rehabilitation measures shouldmonitor the implementation of the QAP. In particular,the following actions should be taken.• Files of inspection reports should be maintained fora defined length of time following completion ofconstruction and issuance of a certificate ofoccupancy. These files should include both reportssubmitted by special inspectors employed by theowner, as in Section 2.12.2.2, and those submittedby inspectors employed by the regulatory agency.• Prior to issuance of certificates of occupancy, theregulatory agency should ascertain that either allreported noncompliant aspects of construction havebeen rectified, or such noncompliant aspects havebeen accepted by the design professional inresponsible charge as acceptable substitutes andconsistent with the general intent of the constructiondocuments.• Files of test reports prepared in accordance withSection 2.12.2.3 should be maintained for a definedlength of time following completion of constructionand issuance of a certificate of occupancy.2.13 Alternative Materials andMethods of ConstructionWhen an existing building or rehabilitation schemecontains elements and/or components for whichstructural modeling parameters and acceptance criteriaare not provided in these Guidelines, the requiredparameters and acceptance criteria should be based onthe experimentally derived cyclic responsecharacteristics of the assembly, determined inaccordance with this section. Independent third-partyreview of this process, by persons knowledgeable instructural component testing and the derivation ofdesign parameters from such testing, shall be requiredunder this section. The provisions of this section mayalso be applied to new materials and systems to assesstheir suitability for seismic rehabilitation.2.13.1 Experimental SetupWhen relevant data on the inelastic force-deformationbehavior for a structural subassembly (elements orcomponents) are not available, such data should beobtained based on experiments consisting of physicaltests of representative subassemblies. Eachsubassembly should be an identifiable portion of thestructural element or component, the stiffness of whichis to be modeled as part of the structural analysisprocess. The objective of the experiment should be topermit estimation of the lateral-force-displacementrelationships (stiffness) for the subassemblies atdifferent loading increments, together with the strengthand deformation capacities for the desired performancelevels. These properties are to be used in developing ananalytical model of the structure’s response toearthquake ground motions, and in judging theacceptability of this predicted behavior. The limitingstrength and deformation capacities should bedetermined from the experimental program from theaverage values of a minimum of three identical orsimilar tests performed for a unique designconfiguration.The experimental setup should simulate, to the extentpractical, the actual construction details, supportconditions, and loading conditions expected in thebuilding. Specifically, the effects of axial load, moment,and shear, if expected to be significant in the building,should be properly simulated in the experiments. Fullscaletests are recommended. The loading shouldconsist of fully reversed cyclic loading at increasingdisplacement levels. The test protocol for number ofFEMA 273 Seismic Rehabilitation Guidelines 2-43

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)cycles and displacement levels shall conform togenerally accepted procedures. Increments should becontinued until the subassembly exhibits completefailure, characterized by a complete (or near-complete)loss of lateral- and gravity-load-resisting ability.2.13.2 Data Reduction and ReportingA report should be prepared for each experiment. Thereport should include the following:• Description of the subassembly being tested• Description of the experimental setup, including:– Details on fabrication of the subassembly– Location and date of experiment– Description of instrumentation employed– Name of the person in responsible charge of thetest– Photographs of the specimen, taken prior totesting• Description of the loading protocol employed,including:– Increment of loading (or deformation) applied– Rate of loading application– Duration of loading at each stage• Description, including photographic documentation,and limiting deformation value for all importantbehavior states observed during the test, includingthe following, as applicable:– Elastic range with effective stiffness reported– Plastic range– Onset of apparent damage– Loss of lateral-force-resisting capacity– Loss of vertical-load-carrying capacity– Force deformation plot for the subassembly(noting the various behavior states)– Description of limiting behavior states andfailure modes2.13.3 Design Parameters and AcceptanceCriteriaThe following procedure should be followed to developdesign parameters and acceptance criteria forsubassemblies based on experimental data:1. An idealized lateral-force-deformation pushovercurve should be developed from the experimentaldata for each experiment, and for each direction ofloading with unique behavior. The curve should beplotted in a single quadrant (positive force versuspositive deformation, or negative force versusnegative deformation). The curve should beconstructed as follows:a. The appropriate quadrant of data from the lateralforce-deformationplot from the experimentalreport should be taken.b. A smooth “backbone” curve should be drawnthrough the intersection of the first cycle curvefor the (i)th deformation step with the secondcycle curve of the (i-1)th deformation step, for alli steps, as indicated in Figure 2-6.c. The backbone curve so derived shall beapproximated by a series of linear segments,drawn to form a multisegmented curveconforming to one of the types indicated inFigure 2-4.2. The approximate multilinear curves derived for allexperiments involving the subassembly should becompared and an average multilinear representationof the subassembly behavior should be derivedbased on these curves. Each segment of thecomposite curve should be assigned the averagestiffness (either positive or negative) of the similarsegments in the approximate multilinear curves forthe various experiments. Each segment on thecomposite curve shall terminate at the average of thedeformation levels at which the similar segments ofthe approximate multilinear curves for the variousexperiments terminate.2-44 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Test ForceBackbone curveTest DeformationFigure 2-6Backbone Curve for Experimental Data3. The stiffness of the subassembly for use in linearprocedures should be taken as the slope of the firstsegment of the composite curve.4. For the purpose of determining acceptance criteria,assemblies should be classified as being either forcecontrolledor deformation-controlled. Assembliesshould be classified as force-controlled unless any ofthe following apply.– The composite multilinear force-deformationcurve for the assembly, determined in accordancewith (2), above, conforms to either Type 1 orType 2, as indicated in Figure 2-4; and thedeformation parameter e, as indicated inFigure 2-4, is at least twice the deformationparameter g, as also indicated in Figure 2-4.– The composite multilinear force-deformationcurve for the assembly determined in accordancewith (2), above, conforms to Type 1, as indicatedin Figure 2-4, and the deformation parameter e isless than twice the deformation parameter g, butthe deformation parameter d is at least twice thedeformation parameter g. In this case, acceptancecriteria may be determined by redrawing theforce-deformation curve as a Type 2 curve, withthat portion of the original curve between points2 and 3 extended back to intersect the first linearsegment at point 1' as indicated in Figure 2-7.The parameters a' and Q' y shall be taken asindicated in Figure 2-7 and shall be used in placeof a and Q y in Figure 2-4.5. The strength capacity, Q CL , for force-controlledelements evaluated using either the linear ornonlinear procedures shall be taken as follows:– For any Performance Level or Range, the loweststrength Q y determined from the series ofrepresentative assembly tests6. The acceptance criteria for deformation-controlledassemblies used in nonlinear procedures shall be theFEMA 273 Seismic Rehabilitation Guidelines 2-45

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Q12Q y Actual Type 1 curveSubstitute Type 2 curve1'Q' ya3,2'ba’g e d ∆Figure 2-7deformations corresponding with the followingpoints on the curves of Figure 2-4:a. Primary Elements- Immediate Occupancy: the deformation atwhich significant, permanent, visible damageoccurred in the experiments- Life Safety: 0.75 times the deformation atpoint 2 on the curves- Collapse Prevention: 0.75 times thedeformation at point 3 on the Type 1 curve,but not greater than point 2b. Secondary ElementsAlternative Force Deformation Curve- Immediate Occupancy: the deformation atwhich significant, permanent, visible damageoccurred in the experiments- Life Safety: 100% of the deformation at point2 on the Type 1 curve, but not less than 75%of the deformation at point 3- Collapse Prevention: 100% of thedeformation at point 3 on the curve7. The m values used as acceptance criteria fordeformation-controlled assemblies in the linearprocedures shall be taken as 0.75 times the ratio ofthe deformation acceptance criteria, given in (6)above, to the deformation at yield, represented bythe deformation parameter g in the curves shown inFigure 2-4.2.14 DefinitionsAcceptance criteria: Permissible values of suchproperties as drift, component strength demand, andinelastic deformation used to determine theacceptability of a component’s projected behavior at agiven Performance Level.Action: Sometimes called a generalized force, mostcommonly a single force or moment. However, anaction may also be a combination of forces andmoments, a distributed loading, or any combination offorces and moments. Actions always produce or causedisplacements or deformations; for example, a bendingmoment action causes flexural deformation in a beam;an axial force action in a column causes axialdeformation in the column; and a torsional momentaction on a building causes torsional deformations(displacements) in the building.Assembly:Two or more interconnected components.BSE-1: Basic Safety Earthquake-1, which is thelesser of the ground shaking at a site for a 10%/50 yearearthquake or two-thirds of the Maximum ConsideredEarthquake (MCE) at the site.BSE-2: Basic Safety Earthquake-2, which is theground shaking at a site for an MCE.BSO: Basic Safety Objective, a RehabilitationObjective in which the Life Safety Performance Levelis reached for the BSE-1 demand and the CollapsePrevention Performance Level is reached for the BSE-2.Building Performance Level: A limiting damagestate, considering structural and nonstructural buildingcomponents, used in the definition of RehabilitationObjectives.Capacity: The permissible strength or deformationfor a component action.Coefficient of variation: For a sample of data, theratio of the standard deviation for the sample to themean value for the sample.2-46 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Components: The basic structural members thatconstitute the building, such as beams, columns, slabs,braces, piers, coupling beams, and connections.Components, such as columns and beams, are combinedto form elements (e.g., a frame).Corrective measure: Any modification of acomponent or element, or the structure as a whole,intended to reduce building vulnerability.Critical action: That component action that reachesits elastic limit at the lowest level of lateral deflection,or loading, for the structure.Demand: The amount of force or deformationimposed on an element or component.Diaphragm: A horizontal (or nearly horizontal)structural element used to distribute inertial lateralforces to vertical elements of the lateral-force-resistingsystem.Diaphragm chord: A diaphragm componentprovided to develop shears at the edge of thediaphragm, resisted either in tension or compression.Diaphragm collector: A diaphragm componentprovided to transfer lateral force from the diaphragm tovertical elements of the lateral-force-resisting system orto other portions of the diaphragm.Element: An assembly of structural components thatact together in resisting lateral forces, such as momentresistingframes, braced frames, shear walls, anddiaphragms.Flexible diaphragm: A diaphragm with stiffnesscharacteristics indicated in Section 3.2.4.Hazard level: Earthquake shaking demands ofspecified severity, determined on either a probabilisticor deterministic basis.Lateral-force-resisting system: Those elements ofthe structure that provide its basic lateral strength andstiffness, and without which the structure would belaterally unstable.Maximum Considered Earthquake (MCE): Anextreme earthquake hazard level used in the formationof Rehabilitation Objectives. (See BSE-2.)Mean return period: The average period of time, inyears, between the expected occurrences of anearthquake of specified severity.Nonstructural Performance Level: A limitingdamage state for nonstructural building componentsused to define Rehabilitation Objectives.Primary component: Those components that arerequired as part of the building’s lateral-force-resistingsystem (as contrasted to secondary components).Primary element: An element that is essential to theability of the structure to resist earthquake-induceddeformations.Rehabilitation Method: A procedural methodologyfor the reduction of building earthquake vulnerability.Rehabilitation Objective: A statement of the desiredlimits of damage or loss for a given seismic demand,usually selected by the owner, engineer, and/or relevantpublic agencies.Rehabilitation strategy: A technical approach fordeveloping rehabilitation measures for a building toreduce its earthquake vulnerability.Secondary component: Those components that arenot required for lateral force resistance (contrasted toprimary components). They may or may not actuallyresist some lateral forces.Secondary element: An element that does not affectthe ability of the structure to resist earthquake-induceddeformations.Seismic demand: Seismic hazard level commonlyexpressed in the form of a ground shaking responsespectrum. It may also include an estimate of permanentground deformation.Simplified Rehabilitation Method: An approach,applicable to some types of buildings and RehabilitationObjectives, in which analyses of the entire building’sresponse to earthquake hazards are not required.Strength: The maximum axial force, shear force, ormoment that can be resisted by a component.FEMA 273 Seismic Rehabilitation Guidelines 2-47

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Stress resultant: The net axial force, shear, orbending moment imposed on a cross section of astructural component.Structural Performance Level: A limiting structuraldamage state, used in the definition of RehabilitationObjectives.Structural Performance Range: A range ofstructural damage states, used in the definition ofRehabilitation Objectives.Subassembly:A portion of an assembly.Systematic Rehabilitation Method: An approach torehabilitation in which complete analysis of thebuilding’s response to earthquake shaking is performed.2.15 SymbolsB SB 1C 1C 2C 3DCRDCRF aF vHCoefficient used to adjust short period spectralresponse for the effect of viscous dampingCoefficient used to adjust one-second periodspectral response for the effect of viscousdampingModification factor to relate expectedmaximum inelastic displacements todisplacements calculated for linear elasticresponse, calculated in accordance withSection 3.3.1.3.Modification factor to represent the effect ofhysteresis shape on the maximum displacementresponse, calculated in accordance withSection 3.3.1.3.Modification factor to represent increaseddisplacements due to second-order effects,calculated in accordance with Section 3.3.1.3.Demand-capacity ratio, computed inaccordance with Equation 2-12 or required inEquation 2-13Average demand-capacity ratio for a story,computed in accordance with Equation 2-13Factor to adjust spectral acceleration in theshort period range for site classFactor to adjust spectral acceleration at onesecond for site classThickness of a soil layer in feetJLDPLSPM STM OTNNNDPNSPP E50PIP iP RQ CEQ CLQ DCoefficient used in linear procedures toestimate the maximum earthquake forces that acomponent can sustain and correspondinglydeliver to other components. The use of Jrecognizes that the framing system cannotlikely deliver the force Q E because ofnonlinear response in the framing systemLinear Dynamic Procedure—a method oflateral response analysisLinear Static Procedure—a method of lateralresponse analysisThe stabilizing moment for an element,calculated as the sum of the dead loads actingon the element times the distance between thelines of action of these dead loads and the toeof the element.The overting moment on an element, calculatedas the sum of the lateral forces applied on theelement times the distance between the lines ofaction of these lateral forces and the toe of theelement.Blow count in soil obtained from a standardpenetration test (SPT)Average blow count in soil within the upper100 feet of soil, calculated in accordance withEquation 2-6Nonlinear Dynamic Procedure—a method oflateral response analysisNonlinear Static Procedure—a method oflateral response analysisProbability of exceedance in 50 yearsPlasticity Index for soil, determined as thedifference in water content of soil at the liquidlimit and plastic limitThe total weight of the structure, includingdead, permanent live, and 25% of transient liveloads acting on the columns and bearing wallswithin story level iMean return periodExpected strength of a component or element atthe deformation level under consideration fordeformation-controlled actionsLower-bound estimate of the strength of acomponent or element at the deformation levelunder consideration for force-controlled actionsCalculated stress resultant in a component dueto dead load effects2-48 Seismic Rehabilitation Guidelines FEMA 273

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)Q E Calculated earthquake stress resultant in acomponentQ UD The dead load force on a component.Q y Yield strength of a componentS S Spectral response acceleration at short periods,obtained from response acceleration maps, gS XS Spectral response acceleration at short periodsfor any hazard level and any damping, gS X1 Spectral response acceleration at a one-secondperiod for any hazard level and any damping, gS a Spectral acceleration, gS aD Design BSE-1 spectral response acceleration atany period T, gS aM Design BSE-2 spectral response acceleration atany period T, gS 1 Spectral response acceleration at a one-secondperiod, obtained from response accelerationmaps, gT Fundamental period of the building in thedirection under considerationT 0 Period at which the constant acceleration andconstant velocity regions of the designspectrum intersectV i Total calculated lateral shear force in story idue to earthquake response, assuming that thestructure remains elasticd iDepth, in feet, of a layer of soils having similarproperties, and located within 100 feet of thesurfaceh i Height, in feet, of story i; this may be taken asthe distance between the centerline of floorframing at each of the levels above and below,the distance between the top of floor slabs ateach of the levels above and below, or similarcommon points of referencem Modification factor used in the acceptancecriteria of deformation-controlled componentsor elements, indicating the available ductility ofa component actions i Horizontal distance, in feet, between adjacentbuildings at the height above ground at whichpounding may occurs u Undrained shear strength of soil, pounds/ft2s uv sv swβAverage value of the undrained soil shearstrength in the upper 100 feet of soil, calculatedin accordance with Equation 2-6, pounds/ft 2Shear wave velocity in soil, in feet/secAverage value of the soil shear wave velocity inthe upper 100 feet of soil, calculated inaccordance with Equation 2-6, feet/secWater content of soil, calculated as the ratio ofthe weight of water in a unit volume of soil tothe weight of soil in the unit volume, expressedas a percentageModal damping ratio∆ i1 Estimated lateral deflection of building 1relative to the ground at level i∆ i2 Estimated lateral deflection of building 2relative to the ground at level iδ iθ iκσφχThe lateral drift in story i, at its center ofrigidityA parameter indicative of the stability of astructure under gravity loads and earthquakeinducedlateral deflectionA reliability coefficient used to reducecomponent strength values for existingcomponents based on the quality of knowledgeabout the components’ properties. (SeeSection 2.7.2.)Standard deviation of the variation of thematerial strengthsA capacity reduction coefficient used to reducethe design strength of new components toaccount for variations in material strength,cross-section dimension, and constructionqualityA coefficient used to determine the out-ofplaneforces required for anchorage ofstructural walls to diaphragmsFEMA 273 Seismic Rehabilitation Guidelines 2-49

Chapter 2: General Requirements(Simplified and Systematic Rehabilitation)2.16 ReferencesATC, 1988, Rapid Visual Screening of Buildings forPotential Seismic Hazards: A Handbook, prepared bythe Applied Technology Council (Report No. ATC-21)for the Federal Emergency Management Agency(Report No. FEMA 154), Washington, D.C.BOCA, 1993, National Building Code, BuildingOfficials and Code Administrators International,Country Club Hill, Illinois.BSSC, 1992, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 178), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos.FEMA 222Aand 223A), Washington, D.C.BSSC, 1997, NEHRP Recommended Provisions forSeismic Regulations for New Buildings and OtherStructures, 1997 Edition, Part 1: Provisions and Part 2:Commentary, prepared by the Building Seismic SafetyCouncil for the Federal Emergency ManagementAgency (Report Nos. FEMA 302 and 303),Washington, D.C.ICBO, 1994, Uniform Building Code, InternationalConference of Building Officials, Whittier, California.Kariotis, J. C., Hart, G., Youssef, N., Guh, J., Hill, J.,and Nglem, D., 1994, Simulation of Recorded Responseof Unreinforced Masonry (URM) Infill FrameBuildings, SMIP 94-05, California Division of Minesand Geology, Sacramento, California.SBCC, 1994, Standard Building Code, SouthernBuilding Code Congress International, Birmingham,Alabama.2-50 Seismic Rehabilitation Guidelines FEMA 273

3.Modeling and Analysis(Systematic Rehabilitation)3.1 ScopeThis chapter presents Analysis Procedures and designrequirements for seismic rehabilitation of existingbuildings. Section 3.2 presents general requirements foranalysis and design that are relevant to all four AnalysisProcedures presented in this chapter. The four AnalysisProcedures for seismic rehabilitation are presented inSection 3.3, namely: Linear Static Procedure, LinearDynamic Procedure, Nonlinear Static Procedure, andNonlinear Dynamic Procedure. Modeling and analysisassumptions, and procedures for determination ofdesign actions and design deformations, are alsopresented in Section 3.3. Acceptance criteria forelements and components analyzed using any one of thefour procedures presented in Section 3.3 are provided inSection 3.4. Section 3.5 provides definitions for keyterms used in this chapter, and Section 3.6 defines thesymbols used in this chapter. Section 3.7 contains a listof references.The relationship of the Analysis Procedures describedin this chapter with specifications in other chapters inthe Guidelines is as follows.• Information on Rehabilitation Objectives to be usedfor design, including hazard levels (that is,earthquake shaking) and on Performance Levels, isprovided in Chapter 2.• The provisions set forth in this chapter are intendedfor Systematic Rehabilitation only. Provisions forSimplified Rehabilitation are presented inChapter 10.• Guidelines for selecting an appropriate AnalysisProcedure are provided in Chapter 2. Chapter 3describes the loading requirements, mathematicalmodel, and detailed analytical procedures requiredto estimate seismic force and deformation demandson elements and components of a building.Information on the calculation of appropriatestiffness and strength characteristics for componentsand elements is provided in Chapters 4 through 9.• General requirements for analysis and design,including requirements for multidirectionalexcitation effects, P-∆ effects, torsion, andoverturning; basic analysis requirements for thelinear and nonlinear procedures; and basic designrequirements for diaphragms, walls, continuity ofthe framing system, building separation, structuressharing common components, and nonstructuralcomponents are given in Section 2.11.• Component strength and deformation demandsobtained from analysis using procedures describedin this chapter, based on component acceptancecriteria outlined in this chapter, are compared withpermissible values provided in Chapters 4 through 9for the desired Performance Level.• Design methods for walls subjected to out-of-planeseismic forces are addressed in Chapter 2. Analysisand design methods for nonstructural components,and mechanical and electrical equipment, arepresented in Chapter 11.• Specific analysis and design requirements forbuildings incorporating seismic isolation and/orsupplemental damping hardware are given inChapter 9.3.2 General RequirementsModeling, analysis, and evaluation for SystematicRehabilitation shall follow the guidelines of thischapter.3.2.1 Analysis Procedure SelectionFour procedures are presented for seismic analysis ofbuildings: two linear procedures, and two nonlinearprocedures. The two linear procedures are termed theLinear Static Procedure (LSP) and the Linear DynamicProcedure (LDP). The two nonlinear procedures aretermed the Nonlinear Static Procedure (NSP) andNonlinear Dynamic Procedure (NDP).Either the linear procedures of Section 3.3.1 andSection 3.3.2, or the nonlinear procedures ofSections 3.3.3 and 3.3.4, may be used to analyze abuilding, subject to the limitations set forth inSection 2.9.FEMA 273 Seismic Rehabilitation Guidelines 3-1

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)3.2.2 Mathematical Modeling3.2.2.1 Basic AssumptionsIn general, a building should be modeled, analyzed, andevaluated as a three-dimensional assembly of elementsand components. Three-dimensional mathematicalmodels shall be used for analysis and evaluation ofbuildings with plan irregularity (see Section 3.2.3).Two-dimensional modeling, analysis, and evaluation ofbuildings with stiff or rigid diaphragms (seeSection 3.2.4) is acceptable if torsional effects are eithersufficiently small to be ignored, or indirectly captured(see Section 3.2.2.2).Vertical lines of seismic framing in buildings withflexible diaphragms (see Section 3.2.4) may beindividually modeled, analyzed, and evaluated as twodimensionalassemblies of components and elements, ora three-dimensional model may be used with thediaphragms modeled as flexible elements.Explicit modeling of a connection is required fornonlinear procedures if the connection is weaker thanthe connected components, and/or the flexibility of theconnection results in a significant increase in therelative deformation between the connectedcomponents.3.2.2.2 Horizontal TorsionThe effects of horizontal torsion must be considered.The total torsional moment at a given floor level shallbe set equal to the sum of the following two torsionalmoments:• The actual torsion; that is, the moment resultingfrom the eccentricity between the centers of mass atall floors above and including the given floor, andthe center of rigidity of the vertical seismic elementsin the story below the given floor, and• The accidental torsion; that is, an accidentaltorsional moment produced by horizontal offset inthe centers of mass, at all floors above and includingthe given floor, equal to a minimum of 5% of thehorizontal dimension at the given floor levelmeasured perpendicular to the direction of theapplied load.In buildings with rigid diaphragms the effect of actualtorsion shall be considered if the maximum lateraldisplacement from this effect at any point on any floordiaphragm exceeds the average displacement by morethan 10%. The effect of accidental torsion shall beconsidered if the maximum lateral displacement due tothis effect at any point on any floor diaphragm exceedsthe average displacement by more than 10%. This effectshall be calculated independent of the effect of actualtorsion.If the effects of torsion are required to be investigated,the increased forces and displacements resulting fromhorizontal torsion shall be evaluated and considered fordesign. The effects of torsion cannot be used to reduceforce and deformation demands on components andelements.For linear analysis of buildings with rigid diaphragms,when the ratio δ max / δ avg due to total torsional momentexceeds 1.2, the effect of accidental torsion shall beamplified by a factor, A x :where:δ maxδ avg= Maximum displacement at any point of thediaphragm at level x= Average of displacements at the extremepoints of the diaphragm at level xA x need not exceed 3.0.⎛ δ max ⎞ 2A x = ⎜-----------------⎟⎝1.2δ avg ⎠(3-1)If the ratio η of (1) the maximum displacement at anypoint on any floor diaphragm (including torsionalamplification), to (2) the average displacement,calculated by rational analysis methods, exceeds 1.50,three-dimensional models that account for the spatialdistribution of mass and stiffness shall be used foranalysis and evaluation. Subject to this limitation, theeffects of torsion may be indirectly captured foranalysis of two-dimensional models as follows.• For the LSP (Section 3.3.1) and the LDP(Section 3.3.2), the design forces and displacementsshall be increased by multiplying by the maximumvalue of η calculated for the building.3-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)• For the NSP (Section 3.3.3), the target displacementshall be increased by multiplying by the maximumvalue of η calculated for the building.• For the NDP (Section 3.3.4), the amplitude of theground acceleration record shall be increased bymultiplying by the maximum value of η calculatedfor the building.3.2.2.3 Primary and Secondary Actions,Components, and ElementsComponents, elements, and component actions shall beclassified as either primary or secondary. Primaryactions, components, and elements are key parts of theseismic framing system required in the design to resistearthquake effects. These shall be evaluated, andrehabilitated as necessary, to sustain earthquakeinducedforces and deformations while simultaneouslysupporting gravity loads. Secondary actions,components, and elements are not designated as part ofthe lateral-force-resisting system, but nevertheless shallbe evaluated, and rehabilitated as necessary, to ensurethat such actions, components, and elements cansimultaneously sustain earthquake-induceddeformations and gravity loads. (See the Commentaryon this section.)For linear procedures (Sections 3.3.1 and 3.3.2), onlythe stiffness of primary components and elements shallbe included in the mathematical model. Secondarycomponents and elements shall be checked for thedisplacements estimated by such analysis. For linearprocedures, the total lateral stiffness of the secondarycomponents and elements shall be no greater than 25%of the total stiffness of the primary components andelements, calculated at each level of the building. If thislimit is exceeded, some secondary components shall bereclassified as primary components.For nonlinear procedures (Sections 3.3.3 and 3.3.4), thestiffness and resistance of all primary and secondarycomponents (including strength loss of secondarycomponents) shall be included in the mathematicalmodel. Additionally, if the total stiffness of thenonstructural components—such as precast exteriorpanels—exceeds 10% of the total lateral stiffness of astory, the nonstructural components shall be included inthe mathematical model.The classification of components and elements shall notresult in a change in the classification of a building’sconfiguration (see Section 3.2.3); that is, componentsand elements shall not be selectively assigned as eitherprimary or secondary to change the configuration of abuilding from irregular to regular.3.2.2.4 Deformation- and Force-ControlledActionsActions shall be classified as either deformationcontrolledor force-controlled. A deformationcontrolledaction is one that has an associateddeformation that is allowed to exceed the yield value;the maximum associated deformation is limited by theductility capacity of the component. A force-controlledaction is one that has an associated deformation that isnot allowed to exceed the yield value. Actions withlimited ductility (such as allowing a < g in Figure 2-4)may also be considered force-controlled. Guidance onthese classifications may be found in Chapters 5through 8.3.2.2.5 Stiffness and Strength AssumptionsElement and component stiffness properties andstrength estimates for both linear and nonlinearprocedures shall be determined from information givenin Chapters 4 through 9, and 11. Guidelines formodeling structural components are given in Chapters 5through 8. Similar guidelines for modeling foundationsand nonstructural components are given in Chapters 4and 11, respectively.3.2.2.6 Foundation ModelingThe foundation system may be included in themathematical model for analysis with stiffness anddamping properties as defined in Chapter 4. Otherwise,unless specifically prohibited, the foundation may beassumed to be rigid and not included in themathematical model.3.2.3 ConfigurationBuilding irregularities are discussed in Section 2.9.Such classification shall be based on the plan andvertical configuration of the framing system, using amathematical model that considers both primary andsecondary components.One objective of seismic rehabilitation should be theimprovement of the regularity of a building through thejudicious placement of new framing elements.FEMA 273 Seismic Rehabilitation Guidelines 3-3

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)3.2.4 Floor DiaphragmsFloor diaphragms transfer earthquake-induced inertialforces to vertical elements of the seismic framingsystem. Roof diaphragms are considered to be floordiaphragms. Connections between floor diaphragmsand vertical seismic framing elements must havesufficient strength to transfer the maximum calculateddiaphragm shear forces to the vertical framingelements. Requirements for design and detailing ofdiaphragm components are given in Section 2.11.6.Floor diaphragms shall be classified as either flexible,stiff, or rigid. (See Chapter 10 for classification ofdiaphragms to be used for determining whetherSimplified Rehabilitation Methods are applicable.)Diaphragms shall be considered flexible when themaximum lateral deformation of the diaphragm alongits length is more than twice the average interstory driftof the story immediately below the diaphragm. Fordiaphragms supported by basement walls, the averageinterstory drift of the story above the diaphragm may beused in lieu of the basement story. Diaphragms shall beconsidered rigid when the maximum lateraldeformation of the diaphragm is less than half theaverage interstory drift of the associated story.Diaphragms that are neither flexible nor rigid shall beclassified as stiff. The interstory drift and diaphragmdeformations shall be estimated using the seismiclateral forces (Equation 3-6). The in-plane deflection ofthe floor diaphragm shall be calculated for an in-planedistribution of lateral force consistent with thedistribution of mass, as well as all in-plane lateral forcesassociated with offsets in the vertical seismic framing atthat floor.Mathematical models of buildings with stiff or flexiblediaphragms should be developed considering the effectsof diaphragm flexibility. For buildings with flexiblediaphragms at each floor level, the vertical lines ofseismic framing may be designed independently, withseismic masses assigned on the basis of tributary area.3.2.5 P-∆ EffectsTwo types of P-∆ (second-order) effects are addressedin the Guidelines: (1) static P-∆ and (2) dynamic P-∆.3.2.5.1 Static P-∆ EffectsFor linear procedures, the stability coefficient θ shouldbe evaluated for each story in the building usingEquation 2-14. This process is iterative. The story driftscalculated by linear analysis, δ i in Equation 2-14, shallbe increased by 1/(1 – θ i ) for evaluation of the stabilitycoefficient. If the coefficient is less than 0.1 in allstories, static P-∆ effects will be small and may beignored. If the coefficient exceeds 0.33, the buildingmay be unstable and redesign is necessary(Section 2.11.2). If the coefficient lies between 0.1 and0.33, the seismic force effects in story i shall beincreased by the factor 1/(1 –θ i ).For nonlinear procedures, second-order effects shall beconsidered directly in the analysis; the geometricstiffness of all elements and components subjected toaxial forces shall be included in the mathematicalmodel.3.2.5.2 Dynamic P-∆ EffectsDynamic P-∆ effects may increase component actionsand deformations, and story drifts. Such effects areindirectly evaluated for the linear procedures and theNSP using the coefficient C 3 . Refer toSections 3.3.1.3A and 3.3.3.3A for additionalinformation.Second-order effects shall be considered directly fornonlinear procedures; the geometric stiffness of allelements and components subjected to axial forces shallbe included in the mathematical model.3.2.6 Soil-Structure InteractionSoil-structure interaction (SSI) may modify the seismicdemand on a building. Two procedures for computingthe effects of SSI are provided below. Other rationalmethods of modeling SSI may also be used.For those rare cases (such as for near-field and soft soilsites) in which the increase in fundamental period dueto SSI increases spectral accelerations, the effects ofSSI on building response must be evaluated; theincrease in fundamental period may be calculated usingthe simplified procedures referred to in Section 3.2.6.1.Otherwise, the effects of SSI may be ignored. Inaddition, SSI effects need not be considered for anybuilding permitted to be rehabilitated using theSimplified Rehabilitation Method (Table 10-1).The simplified procedures referred to in Section 3.2.6.1can be used with the LSP of Section 3.3.1.Consideration of SSI effects with the LDP ofSection 3.3.2, the NSP of Section 3.3.3, and the NDP of3-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)Section 3.3.4 shall include explicit modeling offoundation stiffness as in Section 3.2.6.2. Modaldamping ratios may be calculated using the methodreferred to in Section 3.2.6.1.Soil-structure interaction effects shall not be used toreduce component and element actions by morethan 25%.3.2.6.1 Procedures for Period and DampingThe simplified procedures presented in Chapter 2 of the1997 NEHRP Recommended Provisions for SeismicRegulations for New Buildings and Other Structures(BSSC, 1997) may be used to calculate seismicdemands using the effective fundamental period T˜ andeffective fundamental damping ratio β˜ of thefoundation-structure system.3.2.6.2 Explicit Modeling of SSISoil-structure interaction may be modeled explicitly bymodeling the stiffness and damping for individualfoundation elements. Guidance on the selection ofspring characteristics to represent foundation stiffness ispresented in Section 4.4.2. Unless otherwisedetermined, the damping ratio for individual foundationelements shall be set equal to that value of the dampingratio used for the elastic superstructure. For the NSP, thedamping ratio of the foundation-structure system β˜shall be used to calculate the spectral demands.3.2.7 Multidirectional Excitation EffectsBuildings shall be designed for seismic forces in anyhorizontal direction. For regular buildings, seismicdisplacements and forces may be assumed to actnonconcurrently in the direction of each principal axisof a building. For buildings with plan irregularity(Section 3.2.3) and buildings in which one or morecomponents form part of two or more intersectingelements, multidirectional excitation effects shall beconsidered. Multidirectional effects on componentsshall include both torsional and translational effects.The requirement that multidirectional (orthogonal)excitation effects be considered may be satisfied bydesigning elements or components for the forces anddeformations associated with 100% of the seismicdisplacements in one horizontal direction plus theforces associated with 30% of the seismicdisplacements in the perpendicular horizontal direction.Alternatively, it is acceptable to use SRSS to combinemultidirectional effects where appropriate.The effects of vertical excitation on horizontalcantilevers and prestressed elements shall be consideredby static or dynamic response methods. Verticalearthquake shaking may be characterized by a spectrumwith ordinates equal to 67% of those of the horizontalspectrum (Section 2.6.1.5) unless alternative verticalresponse spectra are developed using site-specificanalysis.3.2.8 Component Gravity Loads and LoadCombinationsThe following component gravity forces, Q G , shall beconsidered for combination with seismic loads.When the effects of gravity and seismic loads areadditive,When the effects of gravity counteract seismic loads,where:Q DQ LQ SQ G = 1.1( Q D + Q L + Q S )Q G = 0.9Q D(3-2)(3-3)= Dead load effect (action)= Effective live load effect (action), equal to25% of the unreduced design live load but notless than the measured live load= Effective snow load effect (action), equal toeither 70% of the full design snow load or,where conditions warrant and approved by theregulatory agency, not less than 20% of thefull design snow load, except that where thedesign snow load is 30 pounds per square footor less, Q S = 0.0Evaluation of components for gravity and wind forces,in the absence of earthquake forces, is beyond the scopeof this document.3.2.9 Verification of Design AssumptionsEach component shall be evaluated to determine thatassumed locations of inelastic deformations areconsistent with strength and equilibrium requirementsFEMA 273 Seismic Rehabilitation Guidelines 3-5

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)at all locations along the component length. Further,each component should be evaluated by rationalanalysis for adequate post-earthquake residual gravityload capacity, considering reduction of stiffness causedby earthquake damage to the structure.Where moments in horizontally-spanning primarycomponents, due to the gravity load combinations ofEquations 3-2 and 3-3, exceed 50% of the expectedmoment strength at any location, the possibility forinelastic flexural action at locations other thancomponent ends shall be specifically investigated bycomparing flexural actions with expected componentstrengths, and the post-earthquake gravity load capacityshould be investigated. Sample checking procedures arepresented in the Commentary. Formation of flexuralplastic hinges away from component ends is notpermitted unless it is explicitly accounted for inmodeling and analysis.3.3 Analysis Procedures3.3.1 Linear Static Procedure (LSP)3.3.1.1 Basis of the ProcedureUnder the Linear Static Procedure (LSP), designseismic forces, their distribution over the height of thebuilding, and the corresponding internal forces andsystem displacements are determined using a linearlyelastic,static analysis. Restrictions on the applicabilityof this procedure are given in Section 2.9.In the LSP, the building is modeled with linearly-elasticstiffness and equivalent viscous damping thatapproximate values expected for loading to near theyield point. Design earthquake demands for the LSP arerepresented by static lateral forces whose sum is equalto the pseudo lateral load defined by Equation 3-6. Themagnitude of the pseudo lateral load has been selectedwith the intention that when it is applied to the linearlyelastic model of the building it will result in designdisplacement amplitudes approximating maximumdisplacements that are expected during the designearthquake. If the building responds essentiallyelastically to the design earthquake, the calculatedinternal forces will be reasonable approximations ofthose expected during the design earthquake. If thebuilding responds inelastically to the design earthquake,as will commonly be the case, the internal forces thatwould develop in the yielding building will be less thanthe internal forces calculated on an elastic basis.Results of the LSP are to be checked using the applicableacceptance criteria of Section 3.4. Calculated internalforces typically will exceed those that the building candevelop, because of anticipated inelastic response ofcomponents and elements. These obtained design forcesare evaluated through the acceptance criteria ofSection 3.4.2, which include modification factors andalternative Analysis Procedures to account foranticipated inelastic response demands and capacities.3.3.1.2 Modeling and AnalysisConsiderationsPeriod Determination. The fundamental period of abuilding, in the direction under consideration, shall becalculated by one of the following three methods.(Method 1 is preferred.)Method 1. Eigenvalue (dynamic) analysis of themathematical model of the building. The model forbuildings with flexible diaphragms shall considerrepresentation of diaphragm flexibility unless it can beshown that the effects of omission will not besignificant.Method 2. Evaluation of the following equation:where:TC th nT = C t h3 ⁄ 4nMethod 2 is not applicable to unreinforced masonrybuildings with flexible diaphragms.(3-4)= Fundamental period (in seconds) in thedirection under consideration= 0.035 for moment-resisting frame systems ofsteel= 0.030 for moment-resisting frames ofreinforced concrete= 0.030 for eccentrically-braced steel frames= 0.020 for all other framing systems= 0.060 for wood buildings (types 1 and 2 inTable 10-2)= Height (in feet) above the base to the roof levelMethod 3. The fundamental period of a one-storybuilding with a single span flexible diaphragm may becalculated as:3-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)(3-5)where ∆ w and ∆ d are in-plane wall and diaphragmdisplacements in inches, due to a lateral load, in thedirection under consideration, equal to the weighttributary to the diaphragm (see Commentary,Figure C3-2). For multiple-span diaphragms, a lateralload equal to the gravity weight tributary to thediaphragm span under consideration shall be applied toeach diaphragm span to calculate a separate period foreach diaphragm span. The period so calculated thatmaximizes the pseudo lateral load (see Equation 3-6)shall be used for design of all walls and diaphragmspans in the building.3.3.1.3 Determination of Actions andDeformationsA. Pseudo Lateral LoadThe pseudo lateral load in a given horizontal directionof a building is determined using Equation 3-6. Thisload, increased as necessary to account for the effects oftorsion (see Section 3.2.2.2), shall be used for thedesign of the vertical seismic framing system.where:VT = ( 0.1∆ w + 0.078∆ d ) 0.5V=C 1 C 2 C 3 S a W= Pseudo lateral load(3-6)This force, when distributed over the heightof the linearly-elastic analysis model of thestructure, is intended to produce calculatedlateral displacements approximately equal tothose that are expected in the real structureduring the design event. If it is expected thatthe actual structure will yield during thedesign event, the force given by Equation 3-6may be significantly larger than the actualstrength of the structure to resist this force.The acceptance criteria in Section 3.4.2 aredeveloped to take this aspect into account.C 1C 1= Modification factor to relate expectedmaximum inelastic displacements todisplacements calculated for linear elasticresponse. may be calculated using theprocedure indicated in Section 3.3.3.3. withthe elastic base shear capacity substituted forV y . Alternatively, C 1 may be calculated asfollows:C 1 = 1.5 for T < 0.10 secondC 1 = 1.0 for T ≥ T 0 secondLinear interpolation shall be used to calculateC 1 for intermediate values of T .T = Fundamental period of the building inthe direction under consideration. If soilstructureinteraction is considered, theeffective fundamental period T˜ shall besubstituted for T .T 0 = Characteristic period of the responsespectrum, defined as the period associatedwith the transition from the constantacceleration segment of the spectrum to theconstant velocity segment of the spectrum.(See Sections 2.6.1.5 and 2.6.2.1.)C = Modification factor to represent the effect of2stiffness degradation and strengthdeterioration on maximum displacementresponse. Values of C 2 for different framingsystems and Performance Levels are listed inTable 3-1. Linear interpolation shall be usedto estimate values for C 2 for intermediatevalues of T .C = Modification factor to represent increased3displacements due to dynamic P-∆ effects.This effect is in addition to the considerationof static P-D effects as defined inSection 3.2.5.1. For values of the stabilitycoefficient θ (see Equation 2-14) less than0.1, C 3 may be set equal to 1.0. For values ofθ greater than 0.1, C 3 shall be calculated as1 + 5( θ – 0.1) ⁄ T . The maximum value of θfor all stories in the building shall be used tocalculate C 3 .FEMA 273 Seismic Rehabilitation Guidelines 3-7

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)S = Response spectrum acceleration, at theafundamental period and damping ratio of thebuilding in the direction under consideration.The value of S a shall be obtained from theprocedure in Section 2.6.1.5.W = Total dead load and anticipated live load asindicated below:• In storage and warehouse occupancies, aminimum of 25% of the floor live load• The actual partition weight or minimumweight of 10 psf of floor area, whicheveris greater• The applicable snow load—see theNEHRP Recommended Provisions(BSSC, 1995)• The total weight of permanent equipmentand furnishingsB. Vertical Distribution of Seismic ForcesThe lateral load F x applied at any floor level x shall bedetermined from the following equations:F x = C vx V(3-7)C. Horizontal Distribution of Seismic ForcesThe seismic forces at each floor level of the buildingshall be distributed according to the distribution of massat that floor level.D. Floor DiaphragmsFloor diaphragms shall be designed to resist the effectsof (1) the inertia forces developed at the level underconsideration (equal to F px in Equation 3-9), and (2)the horizontal forces resulting from offsets in, orchanges in stiffness of, the vertical seismic framingelements above and below the diaphragm. Forcesresulting from offsets in, or changes in stiffness of, thevertical seismic framing elements shall be taken to beequal to the elastic forces (Equation 3-6) withoutreduction, unless smaller forces can be justified byrational analysis.where:n1wF px------------------- x= ∑ F i--------------nC 1 C 2 C 3i = x∑ w ii = x(3-9)kF = Total diaphragm force at level xwC x h pxvx = --------------------- x(3-8)nF = Lateral load applied at floor level i given byikEquation 3-7∑ w i h i w = Portion of the total building weight Wii = 1located on or assigned to floor level iwhere:w = Portion of the total building weight Wxlocated on or assigned to floor level xk = 1.0 for T ≤ 0.5 second= 2.0 for T ≥ 2.5 secondsCoefficients C 1 , C 2 , and C 3 are described above inLinear interpolation shall be used to estimate Section 3.3.1.3A.values of k for intermediate values of T.The lateral seismic load on each flexible diaphragmC = Vertical distribution factorvxshall be distributed along the span of that diaphragm,considering its displaced shape.V = Pseudo lateral load from Equation 3-6w = Portion of the total building weight WE. Determination of Deformationsilocated on or assigned to floor level iStructural deformations and story drifts shall bew = Portion of the total building weight Wcalculated using lateral loads in accordance withxlocated on or assigned to floor level xEquations 3-6, 3-7, and 3-9 and stiffnesses obtainedh = Height (in ft) from the base to floor level i from Chapters 5, 6, 7, and 8.ih = Height (in ft) from the base to floor level xx3-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)Table 3-1Values for Modification Factor C 2T = 0.1 second T ≥ T 0secondPerformance LevelFramingType 1 1FramingType 2 2FramingType 1 1FramingType 2 2Immediate Occupancy 1.0 1.0 1.0 1.0Life Safety 1.3 1.0 1.1 1.0Collapse Prevention 1.5 1.0 1.2 1.01. Structures in which more than 30% of the story shear at any level is resisted by components or elements whose strength and stiffness may deteriorateduring the design earthquake. Such elements and components include: ordinary moment-resisting frames, concentrically-braced frames, frames withpartially-restrained connections, tension-only braced frames, unreinforced masonry walls, shear-critical walls and piers, or any combination of the above.2. All frames not assigned to Framing Type 1.3.3.2 Linear Dynamic Procedure (LDP)3.3.2.1 Basis of the ProcedureUnder the Linear Dynamic Procedure (LDP), designseismic forces, their distribution over the height of thebuilding, and the corresponding internal forces andsystem displacements are determined using a linearlyelastic,dynamic analysis. Restrictions on theapplicability of this procedure are given in Section 2.9.The basis, modeling approaches, and acceptance criteriaof the LDP are similar to those for the LSP. The mainexception is that the response calculations are carriedout using either modal spectral analysis or Time-History Analysis. Modal spectral analysis is carried outusing linearly-elastic response spectra that are notmodified to account for anticipated nonlinear response.As with the LSP, it is expected that the LDP willproduce displacements that are approximately correct,but will produce internal forces that exceed those thatwould be obtained in a yielding building.Results of the LDP are to be checked using theapplicable acceptance criteria of Section 3.4. Calculateddisplacements are compared directly with allowablevalues. Calculated internal forces typically will exceedthose that the building can sustain because ofanticipated inelastic response of components andelements. These obtained design forces are evaluatedthrough the acceptance criteria of Section 3.4.2, whichinclude modification factors and alternative analysisprocedures to account for anticipated inelastic responsedemands and capacities.3.3.2.2 Modeling and AnalysisConsiderationsA. GeneralThe LDP shall conform to the criteria of this section.The analysis shall be based on appropriatecharacterization of the ground motion (Section 2.6.1).The modeling and analysis considerations set forth inSection 3.3.1.2 shall apply to the LDP but alternativeconsiderations are presented below.The LDP includes two analysis methods, namely, theResponse Spectrum and Time-History AnalysisMethods. The Response Spectrum Method uses peakmodal responses calculated from dynamic analysis of amathematical model. Only those modes contributingsignificantly to the response need to be considered.Modal responses are combined using rational methodsto estimate total building response quantities. TheTime-History Method (also termed Response-HistoryAnalysis) involves a time-step-by-time-step evaluationof building response, using discretized recorded orsynthetic earthquake records as base motion input.Requirements for the two analysis methods are outlinedin C and D below.B. Ground Motion CharacterizationThe horizontal ground motion shall be characterized fordesign by the requirements of Section 2.6 and shall beone of the following:• A response spectrum (Section 2.6.1.5)• A site-specific response spectrum (Section 2.6.2.1)• Ground acceleration time histories (Section 2.6.2.2)FEMA 273 Seismic Rehabilitation Guidelines 3-9

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)C. Response Spectrum MethodThe requirement that all significant modes be includedin the response analysis may be satisfied by includingsufficient modes to capture at least 90% of theparticipating mass of the building in each of thebuilding’s principal horizontal directions. Modaldamping ratios shall reflect the damping inherent in thebuilding at deformation levels less than the yielddeformation.The peak member forces, displacements, story forces,story shears, and base reactions for each mode ofresponse shall be combined by recognized methods toestimate total response. Modal combination by eitherthe SRSS (square root sum of squares) rule or the CQC(complete quadratic combination) rule is acceptable.Multidirectional excitation effects shall be accountedfor by the requirements of Section 3.2.7.D. Time-History MethodThe requirements for the mathematical model for Time-History Analysis are identical to those developed forResponse Spectrum Analysis. The damping matrixassociated with the mathematical model shall reflect thedamping inherent in the building at deformation levelsless than the yield deformation.Time-History Analysis shall be performed using timehistories prepared according to the requirements ofSection 2.6.2.2.Response parameters shall be calculated for each Time-History Analysis. If three Time-History Analyses areperformed, the maximum response of the parameter ofinterest shall be used for design. If seven or more pairsof horizontal ground motion records are used for Time-History Analysis, the average response of the parameterof interest may be used for design.Multidirectional excitation effects shall be accountedfor in accordance with the requirements ofSection 3.2.7. These requirements may be satisfied byanalysis of a three-dimensional mathematical modelusing simultaneously imposed pairs of earthquakeground motion records along each of the horizontal axesof the building.3.3.2.3 Determination of Actions andDeformationsA. Modification of DemandsAll actions and deformations calculated using either ofthe LDP analysis methods—Response Spectrum orTime-History Analysis—shall be multiplied by theproduct of the modification factors C 1 , C 2 , and C 3defined in Section 3.3.1.3, and further increased asnecessary to account for the effects of torsion (seeSection 3.2.2.2). However, floor diaphragm actionsneed not be increased by the product of the modificationfactors.B. Floor DiaphragmsFloor diaphragms shall be designed to resistsimultaneously (1) the seismic forces calculated by theLDP, and (2) the horizontal forces resulting from offsetsin, or changes in stiffness of, the vertical seismicframing elements above and below the diaphragm. Theseismic forces calculated by the LDP shall be taken asnot less than 85% of the forces calculated usingEquation 3-9. Forces resulting from offsets in, orchanges in stiffness of, the vertical seismic framingelements shall be taken to be equal to the elastic forceswithout reduction, unless smaller forces can be justifiedby rational analysis.3.3.3 Nonlinear Static Procedure (NSP)3.3.3.1 Basis of the ProcedureUnder the Nonlinear Static Procedure (NSP), a modeldirectly incorporating inelastic material response isdisplaced to a target displacement, and resulting internaldeformations and forces are determined. The nonlinearload-deformation characteristics of individualcomponents and elements of the building are modeleddirectly. The mathematical model of the building issubjected to monotonically increasing lateral forces ordisplacements until either a target displacement isexceeded or the building collapses. The targetdisplacement is intended to represent the maximumdisplacement likely to be experienced during the designearthquake. The target displacement may be calculatedby any procedure that accounts for the effects ofnonlinear response on displacement amplitude; onerational procedure is presented in Section 3.3.3.3.Because the mathematical model accounts directly foreffects of material inelastic response, the calculatedinternal forces will be reasonable approximations ofthose expected during the design earthquake.3-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)Results of the NSP are to be checked using theapplicable acceptance criteria of Section 3.4.3.Calculated displacements and internal forces arecompared directly with allowable values.3.3.3.2 Modeling and AnalysisConsiderationsA. GeneralIn the context of these Guidelines, the NSP involves themonotonic application of lateral forces or displacementsto a nonlinear mathematical model of a building untilthe displacement of the control node in themathematical model exceeds a target displacement. Forbuildings that are not symmetric about a planeperpendicular to the applied lateral loads, the lateralloads must be applied in both the positive and negativedirections, and the maximum forces and deformationsused for design.The relation between base shear force and lateraldisplacement of the control node shall be established forcontrol node displacements ranging between zero and150% of the target displacement, , given byEquation 3-11. Acceptance criteria shall be based onthose forces and deformations (in components andelements) corresponding to a minimum horizontaldisplacement of the control node equal to .C. Lateral Load PatternsLateral loads shall be applied to the building in profilesthat approximately bound the likely distribution ofinertia forces in an earthquake. For three-dimensionalanalysis, the horizontal distribution should simulate thedistribution of inertia forces in the plane of each floordiaphragm. For both two- and three-dimensionalanalysis, at least two vertical distributions of lateralload shall be considered. The first pattern, often termedthe uniform pattern, shall be based on lateral forces thatare proportional to the total mass at each floor level.The second pattern, termed the modal pattern in theseGuidelines, should be selected from one of thefollowing two options:• a lateral load pattern represented by values of C vxgiven in Equation 3-8, which may be used if morethan 75% of the total mass participates in thefundamental mode in the direction underconsideration; or• a lateral load pattern proportional to the story inertiaforces consistent with the story shear distributioncalculated by combination of modal responses using(1) Response Spectrum Analysis of the buildingincluding a sufficient number of modes to capture90% of the total mass, and (2) the appropriateground motion spectrum.δ tδ tT eK eGravity loads shall be applied to appropriate elementsand components of the mathematical model during theNSP. The loads and load combination presented inEquation 3-2 (and Equation 3-3 as appropriate) shall beused to represent such gravity loads.The analysis model shall be discretized in sufficientdetail to represent adequately the load-deformationresponse of each component along its length. Particularattention should be paid to identifying locations ofinelastic action along the length of a component, as wellas at its ends.B. Control NodeThe NSP requires definition of the control node in abuilding. These Guidelines consider the control node tobe the center of mass at the roof of a building; the top ofa penthouse should not be considered as the roof. Thedisplacement of the control node is compared with thetarget displacement—a displacement that characterizesthe effects of earthquake shaking.D. Period DeterminationThe effective fundamental period in the directionunder consideration shall be calculated using theforce-displacement relationship of the NSP. Thenonlinear relation between base shear and displacementof the target node shall be replaced with a bilinearrelation to estimate the effective lateral stiffness, ,and the yield strength, V y , of the building. Theeffective lateral stiffness shall be taken as the secantstiffness calculated at a base shear force equal to 60% ofthe yield strength. The effective fundamental period T eshall be calculated as:KT e = T ii -----K e(3-10)FEMA 273 Seismic Rehabilitation Guidelines 3-11

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)where:T iK iK eSee Figure 3-1 for further information.Base shear= Elastic fundamental period (in seconds) inthe direction under consideration calculatedby elastic dynamic analysis= Elastic lateral stiffness of the building in thedirection under consideration= Effective lateral stiffness of the building inthe direction under considerationV yαK eIf multidirectional excitation effects are to beconsidered, component deformation demands andactions shall be computed for the following cases:100% of the target displacement along axis 1 and 30%of the target displacement along axis 2; and 30% of thetarget displacement along axis 1 and 100% of the targetdisplacement along axis 2.3.3.3.3 Determination of Actions andDeformationsA. Target DisplacementThe target displacement δ t for a building with rigiddiaphragms (Section 3.2.4) at each floor level shall beestimated using an established procedure that accountsfor the likely nonlinear response of the building.Actions and deformations corresponding to the controlnode displacement equaling or exceeding the targetdisplacement shall be used for component checking inSection 3.4.K iδ y δ t0.6V yK eOne procedure for evaluating the target displacement isgiven by the following equation:2T eδ t = C 0 C 1 C C g (3-11)2 3 S a--------4π 2Figure 3-1Roof displacementCalculation of Effective Stiffness, K eE. Analysis of Three-Dimensional ModelsStatic lateral forces shall be imposed on the threedimensionalmathematical model corresponding to themass distribution at each floor level. The effects ofaccidental torsion shall be considered (Section 3.2.2.2).Independent analysis along each principal axis of thethree-dimensional mathematical model is permittedunless multidirectional evaluation is required(Section 3.2.7).F. Analysis of Two-Dimensional ModelsMathematical models describing the framing along eachaxis (axis 1 and axis 2) of the building shall bedeveloped for two-dimensional analysis. The effects ofhorizontal torsion shall be considered (Section 3.2.2.2).where:T e= Effective fundamental period of the buildingin the direction under consideration, sec= Modification factor to relate spectraldisplacement and likely building roofdisplacementC 0C 0Estimates for can be calculated using oneof the following:• the first modal participation factor at thelevel of the control node• the modal participation factor at the levelof the control node calculated using ashape vector corresponding to the deflectedshape of the building at the targetdisplacement• the appropriate value from Table 3-23-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)C = Modification factor to relate expected1maximum inelastic displacements todisplacements calculated for linear elasticresponse= 1.0 for T e ≥ T 0= [ 1.0 + ( R – 1)T 0 ⁄ T e ] ⁄ R for T e < T 0Values for C 1 need not exceed those valuesgiven in Section 3.3.1.3.In no case may C 1 be taken as less than 1.0.T = Characteristic period of the response0spectrum, defined as the period associatedwith the transition from the constantacceleration segment of the spectrum to theconstant velocity segment of the spectrum.(See Sections 2.6.1.5 and 2.6.2.1.)R = Ratio of elastic strength demand to calculatedyield strength coefficient. See below foradditional information.C = Modification factor to represent the effect of2hysteresis shape on the maximumdisplacement response. Values for C 2 areestablished in Section 3.3.1.3.C = Modification factor to represent increased3displacements due to dynamic P-∆ effects. Forbuildings with positive post-yield stiffness, C 3shall be set equal to 1.0. For buildings withnegative post-yield stiffness, values of C 3shall be calculated using Equation 3-13.Values for C 3 need not exceed the values setforth in Section 3.3.1.3.S = Response spectrum acceleration, at theaeffective fundamental period and dampingratio of the building in the direction underconsideration, g. The value of S a is calculatedin Sections 2.6.1.5 and 2.6.2.1.The strength ratio R shall be calculated as:R=S a 1-------------- ⋅ -----V y ⁄ W C 0(3-12)Table 3-2Values for Modification Factor C 0Number of Stories Modification Factor 11 1.02 1.23 1.35 1.410+ 1.51. Linear interpolation should be used to calculate intermediate values.where S a and C 0 are as defined above, and:V yW= Yield strength calculated using results of NSP,where the nonlinear force-displacement (i.e.,base shear force versus control nodedisplacement) curve of the building ischaracterized by a bilinear relation(Figure 3-1)= Total dead load and anticipated live load, ascalculated in Section 3.3.1.3Coefficient C 3 shall be calculated as follows if therelation between base shear force and control nodedisplacement exhibits negative post-yield stiffness.where R and T e are as defined above, and:αα ( R – 1) C 3 = 1.0 + -------------------------------32 /T e= Ratio of post-yield stiffness to effectiveelastic stiffness, where the nonlinear forcedisplacementrelation is characterized by abilinear relation (Figure 3-1)(3-13)For a building with flexible diaphragms (Section 3.2.4)at each floor level, a target displacement shall beestimated for each line of vertical seismic framing. Thetarget displacements shall be estimated using anestablished procedure that accounts for the likelynonlinear response of the seismic framing. Oneprocedure for evaluating the target displacement for anindividual line of vertical seismic framing is given byEquation 3-11. The fundamental period of each verticalline of seismic framing, for calculation of the targetdisplacement, shall follow the general proceduresFEMA 273 Seismic Rehabilitation Guidelines 3-13

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)described for the NSP; masses shall be assigned to eachlevel of the mathematical model on the basis oftributary area.For a building with neither rigid nor flexiblediaphragms at each floor level, the target displacementshall be calculated using rational procedures. Oneacceptable procedure for including the effects ofdiaphragm flexibility is to multiply the displacementcalculated using Equation 3-11 by the ratio of themaximum displacement at any point on the roof and thedisplacement of the center of mass of the roof, bothcalculated by modal analysis of a three-dimensionalmodel of the building using the design responsespectrum. The target displacement so calculated shall beno less than that displacement given by Equation 3-11,assuming rigid diaphragms at each floor level. Novertical line of seismic framing shall be evaluated fordisplacements smaller than the target displacement. Thetarget displacement should be modified according toSection 3.2.2.2 to account for system torsion.B. Floor DiaphragmsFloor diaphragms may be designed to resistsimultaneously both the seismic forces determinedusing either Section 3.3.1.3D or Section 3.3.2.3B, andthe horizontal forces resulting from offsets in, orchanges in stiffness of, the vertical seismic framingelements above and below the diaphragm.3.3.4 Nonlinear Dynamic Procedure (NDP)3.3.4.1 Basis of the ProcedureUnder the Nonlinear Dynamic Procedure (NDP), designseismic forces, their distribution over the height of thebuilding, and the corresponding internal forces andsystem displacements are determined using an inelasticresponse history dynamic analysis.The basis, modeling approaches, and acceptance criteriaof the NDP are similar to those for the NSP. The mainexception is that the response calculations are carriedout using Time-History Analysis. With the NDP, thedesign displacements are not established using a targetdisplacement, but instead are determined directlythrough dynamic analysis using ground motionhistories. Calculated response can be highly sensitive tocharacteristics of individual ground motions; therefore,it is recommended to carry out the analysis with morethan one ground motion record. Because the numericalmodel accounts directly for effects of material inelasticresponse, the calculated internal forces will bereasonable approximations of those expected during thedesign earthquake.Results of the NDP are to be checked using theapplicable acceptance criteria of Section 3.4. Calculateddisplacements and internal forces are compared directlywith allowable values.3.3.4.2 Modeling and Analysis AssumptionsA. GeneralThe NDP shall conform to the criteria of this section.The analysis shall be based on characterization of theseismic hazard in the form of ground motion records(Section 2.6.2). The modeling and analysisconsiderations set forth in Section 3.3.3.2 shall apply tothe NDP unless the alternative considerations presentedbelow are applied.The NDP requires Time-History Analysis of a nonlinearmathematical model of the building, involving a timestep-by-time-stepevaluation of building response,using discretized recorded or synthetic earthquakerecords as base motion input.B. Ground Motion CharacterizationThe earthquake shaking shall be characterized byground motion time histories meeting the requirementsof Section 2.6.2.C. Time-History MethodTime-History Analysis shall be performed usinghorizontal ground motion time histories preparedaccording to the requirements of Section 2.6.2.2.Multidirectional excitation effects shall be accountedfor by meeting the requirements of Section 3.2.7. Therequirements of Section 3.2.7 may be satisfied byanalysis of a three-dimensional mathematical modelusing simultaneously imposed pairs of earthquakeground motion records along each of the horizontal axesof the building.3.3.4.3 Determination of Actions andDeformationsA. Modification of DemandsThe effects of torsion shall be considered according toSection 3.2.2.2.3-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)B. Floor DiaphragmsFloor diaphragms shall be designed to resistsimultaneously both the seismic forces calculated bydynamic analysis and the horizontal forces resultingfrom offsets in, or changes in stiffness of, the verticalseismic framing elements above and below thediaphragm.3.4 Acceptance Criteria3.4.1 General RequirementsComponents and elements analyzed using the linearprocedures of Sections 3.3.1 and 3.3.2 shall satisfy therequirements of this section and Section 3.4.2.Components and elements analyzed using the nonlinearprocedures of Sections 3.3.3 and 3.3.4 shall satisfy therequirements of this section and Section 3.4.3.For the purpose of evaluating acceptability, actionsshall be categorized as being either deformationcontrolledor force-controlled, as defined inSection 3.2.2.4.Foundations shall satisfy the criteria set forth inChapter 4.3.4.2 Linear Procedures3.4.2.1 Design ActionsA. Deformation-Controlled ActionsDesign actions Q UD shall be calculated according toEquation 3-14.B. Force-Controlled ActionsThe value of a force-controlled design action Q UF neednot exceed the maximum action that can be developedin a component considering the nonlinear behavior ofthe building. It is recommended that this value be basedon limit analysis. In lieu of more rational analysis,design actions may be calculated according toEquation 3-15 or Equation 3-16.where:Q UFJQ EQ UF = Q G ± ----------------------C 1 C 2 C 3 JQ EQ UF = Q G ± -----------------------C 1 C 2 C 3= Design actions due to gravity loads andearthquake loads= Force-delivery reduction factor given byEquation 3-17(3-15)(3-16)Equation 3-16 can be used in all cases. Equation 3-15can only be used if the forces contributing to Q UF aredelivered by yielding components of the seismicframing system.The coefficient J shall be established usingEquation 3-17.J = 1.0 + S XS, not to exceed 2 (3-17)Q UD = Q G ± Q E(3-14)where:where:Q EQ GQ UD= Action due to design earthquake loadscalculated using forces and analysis modelsdescribed in either Section 3.3.1 orSection 3.3.2= Action due to design gravity loads asdefined in Section 3.2.8= Design action due to gravity loads andearthquake loadsS XS= Spectral acceleration, calculated inSection 2.6.1.4Alternatively, J may be taken as equal to the smallestDCR of the components in the load path deliveringforce to the component in question.3.4.2.2 Acceptance Criteria for LinearProceduresA. Deformation-Controlled ActionsDeformation-controlled actions in primary andsecondary components and elements shall satisfyEquation 3-18.FEMA 273 Seismic Rehabilitation Guidelines 3-15

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)where:where:mκQ CE ≥ Q UD(3-18)m = Component or element demand modifier toaccount for expected ductility of thedeformation associated with this action atselected Performance Level (see Chapters 4through 8)Q = Expected strength of the component orCEelement at the deformation level underconsideration for deformation-controlledactionsκ = Knowledge factor (Section 2.7.2)For Q CE , the expected strength shall be determinedconsidering all coexisting actions acting on thecomponent under the design loading condition.Procedures to determine the expected strength are givenin Chapters 4 through 8.B. Force-Controlled ActionsForce-controlled actions in primary and secondarycomponents and elements shall satisfy Equation 3-19.κQ CL ≥ Q UF(3-19)Q CL= Lower-bound strength of a component orelement at the deformation level underconsideration for force-controlled actionsFor Q CL , the lower-bound strength shall be determinedconsidering all coexisting actions acting on thecomponent under the design loading condition.Procedures to determine the lower-bound strength arespecified in Chapters 5 through 8.C. Verification of Design AssumptionsEach component shall be evaluated to determine thatassumed locations of inelastic deformations areconsistent with strength and equilibrium requirementsat all locations along the component length.Where moments due to gravity loads in horizontallyspanningprimary components exceed 75% of theexpected moment strength at any location, thepossibility for inelastic flexural action at locations otherthan member ends shall be specifically investigated bycomparing flexural actions with expected memberstrengths. Formation of flexural plastic hinges awayfrom member ends shall not be permitted where designis based on the LSP or the LDP.3.4.3 Nonlinear Procedures3.4.3.1 Design Actions and DeformationsDesign actions (forces and moments) and deformationsshall be the maximum values determined from the NSPor the NDP, whichever is applied.3.4.3.2 Acceptance Criteria for NonlinearProceduresA. Deformation-Controlled ActionsPrimary and secondary components shall have expecteddeformation capacities not less than the maximumdeformations. Expected deformation capacities shall bedetermined considering all coexisting forces anddeformations. Procedures for determining expecteddeformation capacities are specified in Chapters 5through 8.B. Force-Controlled ActionsPrimary and secondary components shall have lowerboundstrengths Q CL not less than the maximumdesign actions. Lower-bound strength shall bedetermined considering all coexisting forces anddeformations. Procedures for determining lower-boundstrengths are specified in Chapters 5 through 8.3.5 DefinitionsThis section provides definitions for all key terms usedin this chapter and not previously defined.Action: Sometimes called a generalized force, mostcommonly a single force or moment. However, anaction may also be a combination of forces andmoments, a distributed loading, or any combination offorces and moments. Actions always produce or causedisplacements or deformations. For example, a bendingmoment action causes flexural deformation in a beam;an axial force action in a column causes axialdeformation in the column; and a torsional momentaction on a building causes torsional deformations(displacements) in the building.3-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)Base: The level at which earthquake effects areconsidered to be imparted to the building.Components: The basic structural members thatconstitute the building, such as beams, columns, slabs,braces, piers, coupling beams, and connections.Components, such as columns and beams, are combinedto form elements (e.g., a frame).Control node: The node in the mathematical modelof a building used to characterize mass and earthquakedisplacement.Deformation: Relative displacement or rotation ofthe ends of a component or element.Displacement: The total movement, typicallyhorizontal, of a component or element or node.Flexible diaphragm: A diaphragm that meetsrequirements of Section 3.2.4.Framing type:Type of seismic resisting system.Element: An assembly of structural components thatact together in resisting lateral forces, such as momentresistingframes, braced frames, shear walls, anddiaphragms.Fundamental period: The first mode period of thebuilding in the direction under consideration.Inter-story drift: The relative horizontaldisplacement of two adjacent floors in a building.Inter-story drift can also be expressed as a percentage ofthe story height separating the two adjacent floors.Primary component: Those components that arerequired as part of the building’s lateral-force-resistingsystem (as contrasted to secondary components).Rigid diaphragm: A diaphragm that meetsrequirements of Section 3.2.4Secondary component: Those components that arenot required for lateral force resistance (contrasted toprimary components). They may or may not actuallyresist some lateral forces.Stiff diaphragm: A diaphragm that meetsrequirements of Section 3.2.4.Target displacement: An estimate of the likelybuilding roof displacement in the design earthquake.3.6 SymbolsThis section provides symbols for all key variables usedin this chapter and not defined previously.C 0C 1C 2C 3C tC vxF dF i and F xF pxJK eK iL dQ CEModification factor to relate spectraldisplacement and likely building roofdisplacementModification factor to relate expectedmaximum inelastic displacements todisplacements calculated for linearelastic responseModification factor to represent theeffect of hysteresis shape on themaximum displacement responseModification factor to representincreased displacements due to secondordereffectsNumerical values followingEquation 3-4Vertical distribution factor for thepseudo lateral loadTotal lateral load applied to a single bayof a diaphragmLateral load applied at floor levels i andx, respectivelyDiaphragm lateral force at floor level xA coefficient used in linear procedures toestimate the actual forces delivered toforce-controlled components by other(yielding) components.Effective stiffness of the building in thedirection under consideration, for usewith the NSPElastic stiffness of the building in thedirection under consideration, for usewith the NSPSingle-bay diaphragm spanExpected strength of a component orelement at the deformation level underconsideration in a deformationcontrolledactionFEMA 273 Seismic Rehabilitation Guidelines 3-17

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)Lower-bound estimate of the strength ofa component or element at thedeformation level under considerationfor force-controlled actionsDead load force (action)toQ h CLi and h x Height from the base of a buildingfloor levels i and x, respectivelyf dLateral load per foot of diaphragm spanh n Height to roof level, ftQ DkExponent used for determining thevertical distribution of lateral forcesQ Earthquake force (action) calculated m A modification factor used in theEusing procedures of Section 3.3.1 oracceptance criteria of deformationcontrolledcomponents or elements,3.3.2Gravity load force (action)indicating the available ductility of aQ Gcomponent actionQ Effective live load force (action)Lw i and w x Portion of the total building weightcorresponding to floor levels i and x,Q Effective snow load force (action)SrespectivelyQ UDDeformation-controlled design action xQ UFForce-controlled design action∆ dDiaphragm deformationR Ratio of the elastic strength demand tothe yield strength coefficient∆ wAverage in-plane wall displacementS Response spectrum acceleration at theafundamental period and damping ratio ofαS XSthe building, gδ tSpectral response acceleration at shortperiods for any hazard level andδ Yield displacement of buildingydamping, gT Fundamental period of the building in ηthe direction under considerationT Effective fundamental period of the θebuilding in the direction underconsideration, for use with the NSPT Elastic fundamental period of theibuilding in the direction underκconsideration, for use with the NSPT Period at which the constant acceleration0and constant velocity regions of thedesign spectrum intersectV Pseudo lateral loadV Yield strength of the building in theydirection under consideration, for use3.7 Referenceswith the NSPW Total dead load and anticipated live loadW i and W xWeight of floors i and x, respectivelyg Acceleration of gravity (386.1 in./sec 2 ,or 9,807 mm/sec 2 for SI units)Distance from the diaphragm center lineRatio of post-yield stiffness to effectivestiffnessTarget roof displacement(Figure 3-1)Displacement multiplier, greater than1.0, to account for the effects of torsionStability coefficient (Equation 2-14)—aparameter indicative of the stability of astructure under gravity loads andearthquake-induced deflectionReliability coefficient used to reducecomponent strength values for existingcomponents based on the quality ofknowledge about the components’properties. (See Section 2.7.2.)BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council, for the FederalEmergency Management Agency (Report Nos.FEMA 222A and 223A), Washington, D.C.BSSC, 1997, NEHRP Recommended Provisions forSeismic Regulations for New Buildings and OtherStructures, 1997 Edition, Part 1: Provisions and Part 2:3-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 3: Modeling and Analysis(Systematic Rehabilitation)Commentary, prepared by the Building Seismic SafetyCouncil for the Federal Emergency ManagementAgency (Report Nos. FEMA 302 and 303),Washington, D.C.FEMA 273 Seismic Rehabilitation Guidelines 3-19

4.Foundations and Geotechnical Hazards(Systematic Rehabilitation)4.1 ScopeThis chapter provides geotechnical engineeringguidance regarding building foundations and seismicgeologicsite hazards. Acceptability of the behavior ofthe foundation system and foundation soils for a givenPerformance Level cannot be determined apart from thecontext of the behavior of the superstructure.Geotechnical requirements for buildings that aresuitable for Simplified Rehabilitation are included inChapter 10.Structural engineering issues of foundation systems arediscussed in the chapters on Steel (Chapter 5), Concrete(Chapter 6), Masonry (Chapter 7), and Wood(Chapter 8).This chapter describes rehabilitation measures forfoundations and geotechnical site hazards. Section 4.2provides guidelines for establishing site soilcharacteristics and identifying geotechnical sitehazards, including fault rupture, liquefaction,differential compaction, landslide and rock fall, andflooding. Techniques for mitigating these geotechnicalsite hazards are described in Section 4.3. Section 4.4presents criteria for establishing soil strength capacity,stiffness, and soil-structure interaction (SSI) parametersfor making foundation design evaluations. Retainingwalls are discussed in Section 4.5. Section 4.6 containsguidelines for improving or strengthening foundations.4.2 Site CharacterizationThe geotechnical requirements for buildings suitable forSimplified Rehabilitation are described in Chapter 10.For all other buildings, specific geotechnical sitecharacterization consistent with the selected method ofSystematic Rehabilitation is required. Sitecharacterization consists of the compilation ofinformation on site subsurface soil conditions,configuration and loading of existing buildingfoundations, and seismic-geologic site hazards.In the case of historic buildings, the guidance of theState Historic Preservation Officer should be obtained ifhistoric or archeological resources are present at thesite.4.2.1 Foundation Soil InformationSpecific information describing the foundationconditions of the building to be rehabilitated is required.Useful information also can be gained from knowledgeof the foundations of adjacent or nearby buildings.Foundation information may include subsurface soiland ground water data, configuration of the foundationsystem, design foundation loads, and load-deformationcharacteristics of the foundation soils.4.2.1.1 Site Foundation ConditionsSubsurface soil conditions must be defined in sufficientdetail to assess the ultimate capacity of the foundationand to determine if the site is susceptible to seismicgeologichazards.Information regarding the structural foundation type,dimensions, and material are required irrespective ofthe subsurface soil conditions. This informationincludes:• Foundation type—spread footings, mat foundation,piles, drilled shafts.• Foundation dimensions—plan dimensions andlocations. For piles, tip elevations, vertical variations(tapered sections of piles or belled caissons).• Material composition/construction. For piles, type(concrete/steel/wood), and installation method (castin-place,open/closed-end driving).Subsurface conditions shall be determined for theselected Performance Level as follows.A. Collapse Prevention and Life Safety PerformanceLevelsDetermine type, composition, consistency, relativedensity, and layering of soils to a depth at which thestress imposed by the building is approximately 10% ofthe building weight divided by the total foundation area.Determine the location of the water table and itsseasonal fluctuations beneath the building.FEMA 273 Seismic Rehabilitation Guidelines 4-1

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)B. Enhanced Rehabilitation Objectives and/or DeepFoundationsFor each soil type, determine soil unit weight γ, soilshear strength c, soil friction angle φ, soilcompressibility characteristics, soil shear modulus G,and Poisson’s ratio ν.4.2.1.2 Nearby Foundation ConditionsSpecific foundation information developed for anadjacent or nearby building may be useful if subsurfacesoils and ground water conditions in the site region areknown to be uniform. However, less confidence willresult if subsurface data are developed from anywherebut the site being rehabilitated. Adjacent sites whereconstruction has been done recently may provide aguide for evaluation of subsurface conditions at the sitebeing considered.4.2.1.3 Design Foundation LoadsInformation on the design foundation loads is required,as well as actual dead loads and realistic estimates oflive loads.4.2.1.4 Load-Deformation CharacteristicsUnder Seismic LoadingTraditional geotechnical engineering treats loaddeformationcharacteristics for long-term dead loadsplus frequently applied live loads only. In most cases,long-term settlement governs foundation design. Shortterm(earthquake) load-deformation characteristics havenot traditionally been used for design; consequently,such relationships are not generally found in the soilsand foundation reports for existing buildings. Loaddeformationrelationships are discussed in detail inSection 4.4.4.2.2 Seismic Site HazardsIn addition to ground shaking, seismic hazards includesurface fault rupture, liquefaction, differentialcompaction, landsliding, and flooding. The potential forground displacement hazards at a site should beevaluated. The evaluation should include an assessmentof the hazards in terms of ground movement. Ifconsequences are unacceptable for the desiredPerformance Level, then the hazards should bemitigated as described in Section 4.3.4.2.2.1 Fault RuptureGeologic site conditions must be defined in sufficientdetail to assess the potential for the trace of an activefault to be present in the building foundation soils. If thetrace of a fault is known or suspected to be present, thefollowing information may be required:• The degree of activity—that is, the age of mostrecent movement (e.g., historic, Holocene, lateQuaternary)—must be determined.• The fault type must be identified, whether strikeslip,normal-slip, reverse-slip, or thrust fault.• The sense of slip with respect to building geometrymust be determined, particularly for normal-slip andreverse-slip faults.• Magnitudes of vertical and/or horizontaldisplacements with recurrence intervals consistentwith Rehabilitation Objectives must be determined.• The width of the fault-rupture zone (concentrated ina narrow zone or distributed) must be identified.4.2.2.2 LiquefactionSubsurface soil and ground water conditions must bedefined in sufficient detail to assess the potential forliquefiable materials to be present in the buildingfoundation soils. If liquefiable soils are suspected to bepresent, the following information must be developed.• Soil type: Liquefiable soils typically are granular(sand, silty sand, nonplastic silt).• Soil density: Liquefiable soils are loose to mediumdense.• Depth to water table: Liquefiable soils must besaturated, but seasonal fluctuations of the water tablemust be estimated.• Ground surface slope and proximity of free-faceconditions: Lateral-spread landslides can occur ongently sloping sites, particularly if a free-facecondition—such as a canal or stream channel—ispresent nearby.• Lateral and vertical differential displacement:Amount and direction at the building foundationmust be calculated.4-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)The hazard of liquefaction should be evaluated initiallyto ascertain whether the site is clearly free of ahazardous condition or whether a more detailedevaluation is required. It can be assumed generally thata significant hazard due to liquefaction does not exist ata site if the site soils or similar soils in the site vicinityhave not experienced historical liquefaction and if anyof the following criteria are met:• The geologic materials underlying the site are eitherbedrock or have a very low liquefactionsusceptibility, according to the relative susceptibilityratings based upon general depositional environmentand geologic age of the deposit, as shown inTable 4-1.Table 4-1Estimated Susceptibility to Liquefaction of Surficial Deposits During Strong Ground Shaking(after Youd and Perkins, 1978)Type of DepositGeneral Distributionof CohesionlessSediments inDepositsLikelihood that Cohesionless Sediments, When Saturated,Would be Susceptible to Liquefaction (by Age of Deposit)Modern< 500 yr.Holocene< 11,000 yr.Pleistocene< 2 million yr.Pre-Pleistocene> 2 million yr.(a) Continental DepositsRiver channelFlood plainLocally variableLocally variableVery highHighHighModerateLowLowVery lowVery lowAlluvial fan, plainMarine terraceWidespreadWidespreadModerate—LowLowLowVery lowVery lowVery lowDelta, fan deltaLacustrine, playaColluviumWidespreadVariableVariableHighHighHighModerateModerateModerateLowLowLowVery lowVery lowVery lowTalusDuneWidespreadWidespreadLowHighLowModerateVery lowLowVery lowVery lowLoessGlacial tillVariableVariableHighLowHighLowHighVery lowUnknownVery lowTuffTephraResidual soilsRareWidespreadRareLowHighLowLowLowHighVery low?Very lowVery low?Very lowSebkaLocally variableHighModerateLowVery low(b) Coastal Zone DepositsDeltaEsturineWidespreadLocally variableVery highHighHighModerateLowLowVery lowVery lowBeach, high energyBeach, low energyWidespreadWidespreadModerateHighLowModerateVery lowLowVery lowVery lowLagoonForeshoreLocally variableLocally variableHighHighModerateModerateLowLowVery lowVery low(c) Fill MaterialsUncompacted fillCompacted fillVariableVariableVery highLow——————FEMA 273 Seismic Rehabilitation Guidelines 4-3

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)• The soils underlying the site are stiff clays or clayeysilts, unless the soils are highly sensitive, based onlocal experience; or, the soils are cohesionless (i.e.,sand, silts, or gravels) with a minimum normalizedStandard Penetration Test (SPT) resistance, (N 1 ) 60 ,value of 30 blows/foot for depths below thegroundwater table, or with clay content greater than20%. The parameter (N 1 ) 60 is defined as the SPTblow count normalized to an effective overburdenpressure of 2 ksf. Clay has soil particles withnominal diameters ≤ 0.005 mm.• The groundwater table is at least 35 feet below thedeepest foundation depth, or 50 feet below theground surface, whichever is shallower, includingconsiderations for seasonal and historic groundwaterlevel rises, and any slopes or free-faceconditions in the site vicinity do not extend belowthe ground-water elevation at the site.If, by applying the above criteria, a possibleliquefaction hazard at the site cannot be eliminated,then a more detailed evaluation is required. Guidancefor detailed evaluations is presented in the Commentary.4.2.2.3 Differential CompactionSubsurface soil conditions must be defined in sufficientdetail to assess the potential for differential compactionto occur in the building foundation soils.Differential compaction or densification of soils mayaccompany strong ground shaking. The resultingdifferential settlements can be damaging to structures.Types of soil that are susceptible to liquefaction (that is,relatively loose natural soils, or uncompacted or poorlycompacted fill soils) are also susceptible to compaction.Compaction can occur in soils above and below thegroundwater table.It can generally be assumed that a significant hazarddue to differential compaction does not exist if the soilconditions meet both of the following criteria:• The geologic materials underlying foundations andbelow the groundwater table do not pose asignificant liquefaction hazard, based on the criteriain Section 4.2.2.2.• The geologic materials underlying foundations andabove the groundwater table are either Pleistocene ingeologic age (older than 11,000 years), stiff clays orclayey silts, or cohesionless sands, silts, and gravelswith a minimum (N 1 ) 60 of 20 blows/0.3 m (20blows/foot).If a possible differential compaction hazard at the sitecannot be eliminated by applying the above criteria,then a more detailed evaluation is required. Guidancefor a detailed evaluation is presented in theCommentary.4.2.2.4 LandslidingSubsurface soil conditions must be defined in sufficientdetail to assess the potential for a landslide to causedifferential movement of the building foundation soils.Hillside stability shall be evaluated at sites with:• Existing slopes exceeding approximately 18 degrees(three horizontal to one vertical)• Prior histories of instability (rotational ortranslational slides, or rock fall)Pseudo-static analyses shall be used to determine sitestability, provided the soils are not liquefiable orotherwise expected to lose shear strength duringdeformation. Pseudo-static analyses shall use a seismiccoefficient equal to one-half the peak groundacceleration (calculated as S XS /2.5) at the site associatedwith the desired Rehabilitation Objective. Sites with astatic factor of safety equal to or greater than 1.0 shallbe judged to have adequate stability, and require nofurther stability analysis.Sites with a static factor of safety of less than 1.0 willrequire a sliding-block displacement analysis(Newmark, 1965). The displacement analysis shalldetermine the magnitude of potential ground movementfor use by the structural engineer in determining itseffect upon the performance of the structure and thestructure’s ability to meet the desired PerformanceLevel. Where the structural performance cannotaccommodate the computed ground displacements,appropriate mitigation schemes shall be employed asdescribed in Section 4.3.4.In addition to potential effects of landslides onfoundation soils, the possible effects of rock fall or slidedebris from adjacent slopes should be considered.4-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)4.2.2.5 Flooding or InundationFor Performance Levels exceeding Life Safety, siteconditions should be defined in sufficient detail toassess the potential for earthquake-induced flooding orinundation to prevent the rehabilitated building frommeeting the desired Performance Level. Sources ofearthquake-induced flooding or inundation include:• Dams located upstream damaged by earthquakeshaking or fault rupture• Pipelines, aqueducts, and water-storage tankslocated upstream damaged by fault rupture,earthquake-induced landslides, or strong shaking• Low-lying coastal areas within tsunami zones orareas adjacent to bays or lakes that may be subject toseiche waves• Low-lying areas with shallow ground water whereregional subsidence could cause surface ponding ofwater, resulting in inundation of the sitePotential damage to buildings from flooding orinundation must be evaluated on a site-specific basis.Consideration must be given to potential scour ofbuilding foundation soils from swiftly flowing water.4.3 Mitigation of Seismic SiteHazardsOpportunities exist to improve seismic performanceunder the influence of some site hazards at reasonablecost; however, some site hazards may be so severe thatthey are economically impractical to include in riskreductionmeasures. The discussions presented beloware based on the concept that the extent of site hazardsis discovered after the decision for seismicrehabilitation of a building has been made; however, thedecision to rehabilitate a building and the selection of aRehabilitation Objective may have been made with fullknowledge that significant site hazards exist and mustbe mitigated as part of the rehabilitation.4.3.1 Fault RuptureLarge movements caused by fault rupture generallycannot be mitigated economically. If the structuralconsequences of the estimated horizontal and verticaldisplacements are unacceptable for any PerformanceLevel, either the structure, its foundation, or both, mightbe stiffened or strengthened to reach acceptableperformance. Measures are highly dependent onspecific structural characteristics and inadequacies.Grade beams and reinforced slabs are effective inincreasing resistance to horizontal displacement.Horizontal forces are sometimes limited by slidingfriction capacity of spread footings or mats. Verticaldisplacements are similar in nature to those caused bylong-term differential settlement. Mitigative techniquesinclude modifications to the structure or its foundationto distribute the effects of differential verticalmovement over a greater horizontal distance to reduceangular distortion.4.3.2 LiquefactionThe effectiveness of mitigating liquefaction hazardsmust be evaluated by the structural engineer in thecontext of the global building system performance. If ithas been determined that liquefaction is likely to occurand the consequences in terms of estimated horizontaland vertical displacements are unacceptable for thedesired Performance Level, then three general types ofmitigating measures can be considered alone or incombination.Modify the structure: The structure can bestrengthened to improve resistance against the predictedliquefaction-induced ground deformation. This solutionmay be feasible for small ground deformations.Modify the foundation: The foundation system can bemodified to reduce or eliminate the potential for largefoundation displacements; for example, byunderpinning existing shallow foundations to achievebearing on deeper, nonliquefiable strata. Alternatively(or in concert with the use of deep foundations), ashallow foundation system can be made more rigid (forexample, by a system of grade beams between isolatedfootings) in order to reduce the differential groundmovements transmitted to the structure.Modify the soil conditions: A number of types ofground improvement can be considered to reduce oreliminate the potential for liquefaction and its effects.Techniques that generally are potentially applicable toexisting buildings include soil grouting, eitherthroughout the entire liquefiable strata beneath abuilding, or locally beneath foundation elements (e.g.,grouted soil columns); installation of drains (e.g., stonecolumns); and installation of permanent dewateringsystems. Other types of ground improvement that arewidely used for new construction are less applicable toFEMA 273 Seismic Rehabilitation Guidelines 4-5

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)existing buildings because of the effects of theprocedures on the building. Thus, removal andreplacement of liquefiable soil or in-place densificationof liquefiable soil by various techniques are notapplicable beneath an existing building.If potential for significant liquefaction-induced lateralspreading movements exists at a site, then theremediation of the liquefaction hazard may be moredifficult. This is because the potential for lateralspreading movements beneath a building may dependon the behavior of the soil mass at distances wellbeyond the building as well as immediately beneath it.Thus, measures to prevent lateral spreading may, insome cases, require stabilizing large soil volumes and/or constructing buttressing structures that can reducethe potential for, or the amount of, lateral movements.4.3.3 Differential CompactionThe effectiveness of mitigating differential compactionhazards must be evaluated by the structural engineer inthe context of the global building system performance.For cases of predicted significant differentialsettlements of a building foundation, mitigation optionsare similar to those described above to mitigateliquefaction hazards. There are three options: designingfor the ground movements, strengthening thefoundation system, and improving the soil conditions.4.3.4 LandslideThe effectiveness of mitigating landslide hazards mustbe evaluated by the structural engineer in the context ofthe global building system performance. A number ofschemes are available for reducing potential impacts forearthquake-induced landslides, including:• Regrading• Drainage• Buttressing• Structural Improvements– Gravity walls– Tieback/soil nail walls– Mechanically stabilized earth walls– Barriers for debris torrents or rock fall– Building strengthening to resist deformation- Grade beams- Shear walls• Soil Modification/Replacement– Grouting– DensificationThe effectiveness of any of these schemes must beconsidered based upon the amount of ground movementthat the building can tolerate and still meet the desiredPerformance Level.4.3.5 Flooding or InundationThe effectiveness of mitigating flooding or inundationhazards must be evaluated by the structural engineer inthe context of the global building system performance.Potential damage caused by earthquake-inducedflooding or inundation may be mitigated by a number ofschemes, as follows:• Improvement of nearby dam, pipeline, or aqueductfacilities independent of the rehabilitated building• Diversion of anticipated peak flood flows• Installation of pavement around the building tominimize scour• Construction of sea wall or breakwater for tsunamior seiche protection4.4 Foundation Strength andStiffnessIt is assumed in this section that the foundation soils arenot susceptible to significant strength loss due toearthquake loading. With this assumption, the followingparagraphs provide an overview of the requirementsand procedures for evaluating the ability of foundationsto withstand the imposed seismic loads withoutexcessive deformations. If soils are susceptible tosignificant strength loss, due to either the direct effectsof the earthquake shaking on the soil or the foundationloading on the soil induced by the earthquake, theneither improvement of the soil foundation conditionshould be considered or special analyses should be4-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)carried out to demonstrate that the effects of soilstrength loss do not result in excessive structuraldeformations.Consideration of foundation behavior is only one part ofseismic rehabilitation of buildings. Selection of thedesired Rehabilitation Objective probably will be donewithout regard to specific details of the building,including the foundation. The structural engineer willchoose the appropriate type of analysis procedures forthe selected Performance Level (e.g., SystematicRehabilitation, with Linear Static or DynamicProcedures, or Nonlinear Static or DynamicProcedures). As stated previously, foundationrequirements for buildings that qualify for SimplifiedRehabilitation are included in Chapter 10.4.4.1 Ultimate Bearing Capacities and LoadCapacitiesThe ultimate load capacity of foundation componentsmay be determined by one of the three methodsspecified below. The choice of method depends on thecompleteness of available information on foundationproperties (see Section 4.2.1.1) and the requirements ofthe selected Performance Level.4.4.1.1 Presumptive Ultimate CapacitiesPresumptive capacities are to be used when the amountof information on foundation soil properties is limitedand relatively simple analysis procedures are used.Presumptive ultimate load parameters for spreadfootings and mats are presented in Table 4-2.4.4.1.2 Prescriptive Ultimate CapacitiesPrescriptive capacities may be used when eitherconstruction documents for the existing building orprevious geotechnical reports provide information onfoundation soils design parameters.The ultimate prescriptive bearing pressure for a spreadfooting may be assumed to be twice the allowable deadplus live load bearing pressure specified for design.q c = 2q allow.D + L(4-1)Table 4-2Presumptive Ultimate Foundation PressuresLateral BearingLateral Sliding 1PressureVertical FoundationClass of Materials 2Pressure 3Lbs./Sq. Ft./Ft. ofDepth BelowLbs./Sq. Ft. (q c )Natural Grade 4 Coefficient 5 Resistance 6Lbs./Sq. Ft.Massive Crystalline Bedrock 8000 2400 0.80 —Sedimentary and Foliated Rock 4000 800 0.70 —Sandy Gravel and/or Gravel (GW 4000 400 0.70 —and GP)Sand, Silty Sand, Clayey Sand, SiltyGravel, and Clayey Gravel (SW, SP,SM, SC, GM, and GC)3000 300 0.50 —Clay, Sandy Clay, Silty Clay, andClayey Silt (CL, ML, MH, and CH)20007 200 — 2601. Lateral bearing and lateral sliding resistance may be combined.2. For soil classifications OL, OH, and PT (i.e., organic clays and peat), a foundation investigation shall be required.3. All values of ultimate foundation pressure are for footings having a minimum width of 12 inches and a minimum depth of 12 inches into natural grade.Except where Footnote 7 below applies, increase of 20% allowed for each additional foot of width or depth to a maximum value of three times thedesignated value.4. May be increased by the amount of the designated value for each additional foot of depth to a maximum of 15 times the designated value.5. Coefficient applied to the dead load.6. Lateral sliding resistance value to be multiplied by the contact area. In no case shall the lateral sliding resistance exceed one-half the dead load.7. No increase for width is allowed.FEMA 273 Seismic Rehabilitation Guidelines 4-7

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)For deep foundations, the ultimate prescriptive verticalcapacity of individual piles or piers may be assumed tobe 50% greater than the allowable dead plus live loadsspecified for design.Q c = 1.5Q allow.D + L(4-2)As an alternative, the prescriptive ultimate capacity ofany footing component may be assumed to be 50%greater than the total working load acting on thecomponent, based on analyses using the original designrequirements.the likely variability of soils supporting foundations, anequivalent elasto-plastic representation ofload-deformation behavior is recommended. Inaddition, to allow for such variability or uncertainty, anupper and lower bound approach to defining stiffnessand capacity is recommended (as shown in Figure 4-1a)to permit evaluation of structural response sensitivity.The selection of uncertainty represented by the upperand lower bounds should be determined jointly by thegeotechnical and structural engineers.Q c=1.5Q max.(4-3)Upper boundwhere Q max. = Q D + Q L + Q S4.4.1.3 Site-Specific CapacitiesA detailed analysis may be conducted by a qualifiedgeotechnical engineer to determine ultimate foundationcapacities based on the specific characteristics of thebuilding site.LoadLower bound4.4.2 Load-Deformation Characteristics forFoundationsLoad-deformation characteristics are required where theeffects of foundations are to be taken into account inLinear Static or Dynamic Procedures (LSP or LDP),Nonlinear Static (pushover) Procedures (NSP), orNonlinear Dynamic (time-history) Procedures (NDP).Foundation load-deformation parameters characterizedby both stiffness and capacity can have a significanteffect on both structural response and load distributionamong structural elements.Foundation systems for buildings can in some cases becomplex, but for the purpose of simplicity, threefoundation types are considered in these Guidelines:• shallow bearing foundations• pile foundations• drilled shaftsWhile it is recognized that the load-deformationbehavior of foundations is nonlinear, because of thedifficulties in determining soil properties and staticfoundation loads for existing buildings, together withFigure 4-1PDeformation(a)MHFoundation load(b)k shk svUncoupled spring model(a) Idealized Elasto-Plastic Load-Deformation Behavior for Soils(b) Uncoupled Spring Model for RigidFootings4.4.2.1 Shallow Bearing FoundationsA. Stiffness ParametersThe shear modulus, G, for a soil is related to themodulus of elasticity, E, and Poisson’s ratio, ν, by therelationshipk sr4-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)GE= -------------------21 ( + ν)(4-4)Most soils are intrinsically nonlinear and the shear wavemodulus decreases with increasing shear strain. TheTable 4-3 Effective Shear Modulus and Shearlarge-strain shear wave velocity, v s ′ , and the effectivePoisson’s ratio may be assumed as 0.35 for unsaturated shear modulus, G, can be estimated based on thesoils and 0.50 for saturated soils.Effective Peak Acceleration coefficient for theThe initial shear modulus, G o , is related to the shearearthquake under consideration, in accordance withTable 4-3.wave velocity at low strains, v s , and the mass density ofthe soil, ρ, by the relationshipexpressed in pounds per square foot, as is σ o ′ .plates in terms of an equivalent circular radius.G o =2ρv s (4-5)Wave VelocityEffective Peak(In the fonts currently in use in the Guidelines, theAcceleration, S XS /2.5italicized v is similar to the Greek ν.) Converting mass0.10 0.70density to unit weight, γ, gives an alternative expressionRatio of effective to initial shear 0.50 0.20modulus (G/G o )2γvG s Ratio of effective to initial shear 0.71 0.45o = -------(4-6) wave velocity (ν' /νgs ) swhere g is acceleration due to gravity.Notes:1. Site-specific values may be substituted if documented in a detailedgeotechnical site investigation.The initial shear modulus also has been related to2. Linear interpolation may be used for intermediate values.normalized and corrected blow count, (N ) , and1 60effective vertical stress, σ , as follows (from Seed eto ′al., 1986):To reflect the upper and lower bound concept illustratedin Figure 4-1a in the absence of a detailed geotechnicalsite study, the upper bound stiffness of rectangular1 ⁄ 3footings should be based on twice the effective shearG o ≅ 20,000 ( N 1 ) σ (4-7)60 o ′modulus, G, determined in accordance with the aboveprocedure. The lower bound stiffness should be basedwhere:on one-half the effective shear modulus. Thus the rangeof stiffness should incorporate a factor of four from( N = Blow count normalized for 1.0 ton per lower to upper bound.1 ) 60square foot confining pressure and 60%energy efficiency of hammerMost shallow bearing footings are stiff relative to theσ′ = Effective vertical stress in psfsoil upon which they rest. For simplified analyses, anouncoupled spring model, as shown in Figure 4-1b, mayandbe sufficient. The three equivalent spring constants mayσ′ o= γbe determined using conventional theoretical solutionst d – γ w ( d – d w )for rigid plates resting on a semi-infinite elasticγ t = Total unit weight of soilmedium. Although frequency-dependent solutions areavailable, results are reasonably insensitive to loadingγ w = Unit weight of waterfrequencies within the range of parameters of interestd = Depth to samplefor buildings subjected to earthquakes. It is sufficient tod w = Depth to water leveluse static stiffnesses as representative of repeatedloading conditions.It should be noted that the G o in Equation 4-7 isFigure 4-2 presents stiffness solutions for rectangularFEMA 273 Seismic Rehabilitation Guidelines 4-9

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)Stiffnesses are adjusted for shape and depth usingfactors similar to those in Figure 4-3. Otherformulations incorporating a wider range of variablesmay be found in Gazetas (1991). For the case ofhorizontal translation, the solution representsmobilization of base traction (friction) only. If the sidesof the footing are in close contact with adjacent in situfoundation soil or well-compacted fill, significantadditional stiffness may be assumed from passivepressure. A solution for passive pressure stiffness ispresented in Figure 4-4.For more complex analyses, a finite elementrepresentation of linear or nonlinear foundationbehavior may be accomplished using Winklercomponent models. Distributed vertical stiffnessproperties may be calculated by dividing the totalvertical stiffness by the area. Similarly, the uniformlydistributed rotational stiffness can be calculated bydividing the total rotational stiffness of the footing bythe moment of inertia of the footing in the direction ofloading. In general, however, the uniformly distributedvertical and rotational stiffnesses are not equal. The twomay be effectively decoupled for a Winkler model usinga procedure similar to that illustrated in Figure 4-5. Theends of the rectangular footing are represented by endzones of relatively high stiffness over a length ofapproximately one-sixth of the footing width. Thestiffness per unit length in these end zones is based onthe vertical stiffness of a B x B/6 isolated footing. Thestiffness per unit length in the middle zone is equivalentto that of an infinitely long strip footing.In some instances, the stiffness of the structuralcomponents of the footing may be relatively flexiblecompared to the soil material; for example, a slendergrade beam resting on stiff soil. Classical solutions forbeams on elastic supports can provide guidance onwhen such effects are important. For example, a gradebeam supporting point loads spaced at a distance of Lmight be considered flexible if:where, for the grade beam,EI= Effective modulus of elasticity= Moment of inertiaEI-----L 4 < 10k sv B(4-8)B = WidthFor most flexible foundation systems, the unit subgradespring coefficient, k sv, may be taken asB. Capacity Parameters(4-9)The specific capacity of shallow bearing foundationsshould be determined using fully plastic concepts andthe generalized capacities of Section 4.4.1. Upper andlower bounds of capacities, as illustrated in Figure 4-1a,should be determined by multiplying the best estimatevalues by 2.0 and 0.5, respectively.In the absence of moment loading, the vertical loadcapacity of a rectangular footing of width B and lengthL is(4-10)For rigid footings subject to moment and vertical load,contact stresses become concentrated at footing edges,particularly as uplift occurs. The ultimate momentcapacity, M c , is dependent upon the ratio of the verticalload stress, q, to the vertical stress capacity, q c .Assuming that contact stresses are proportional tovertical displacement and remain elastic up to thevertical stress capacity, q c , it can be shown that upliftwill occur prior to plastic yielding of the soil when q/q cis less than 0.5. If q/q c is greater than 0.5, then the soilat the toe will yield prior to uplift. This is illustrated inFigure 4-6. In general the moment capacity of arectangular footing may be expressed as:wherePq =BLM c= Vertical loadP------BL= Footing width1.3Gk sv = --------------------B( 1 – ν)Q c==q c BLLP------ ⎛2 ⎝1q– ---- ⎞q ⎠ c= Footing length in direction of bending(4-11)4-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)Radii of circular footings equivalent to rectangular footingsRectangular footingEquivalent circular footingzxyBRBLLDegree of freedomEquivalentradius, RRockingTorsionTranslationAbout x-axis About y-axis About z-axis( BLBL 33 ππ )1/2 ( ) 1/4B( 3 L) 1/4BL(B[ 2 + L 2 )] 1/43 π6 πSpring constants for embedded rectangular footingsSpring constants for shallow rectangular footings are obtainedby modifying the solution for a circular footing, bonded tothe surface of an elastic half-space, i.e., k = αβk owherek o = Stiffness coefficient for the equivalent circular footingα = Foundation shape correction factor (Figure 4-3a)β = Embedment factor (Figure 4-3b)To use the equation, the radius of an equivalent circular footingis first calculated according to the degree of freedom beingconsidered. The figure above summarizes the appropriate radii.k o is calculated using the table below:Displacement degree of freedomVertical translationHorizontal translationTorsional rotationRocking rotationk o4 GR1-ν8 GR2 -ν16 GR 338 GR 33(1- )νNote:G and ν are theshear modulus andPoisson's ratio forthe elastic half-space.G is related to Young'smodulus, E, as follows:E = 2 (1 + ν ) GR = Equivalent radiusFigure 4-2 Elastic Solutions for Rigid Footing Spring Constants (based on Gazetas, 1991 and Lam et al., 1991)FEMA 273 Seismic Rehabilitation Guidelines 4-11

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)1.20Shape factor, α1.151.101.05LzxyBRocking (z-axis)Horizontal translation (x-direction)RotationsRocking (x-axis)Vertical translation (z-direction)Horizontal translation (y-direction)Rocking (y-axis)(a)1.001.0 1.5 2.0 2.5 3.0 3.5 4.0L/B2.5Embedment factor, β2.01.5RDFrom Figure 4-2Torsional rotationRocking rotationHorizontal translationVertical rotation(b)Figure 4-31.00 0.1 0.2 0.3 0.4 0.5D/R(a) Foundation Shape Correction Factors (b) Embedment Correction Factors4-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)4l = Dimension of long side of contact aread = Dimension of short side of contact areaλShape factor,32101 10 100l/dK L=2G (1+v) lλ (1-v 2 )Figure 4-4 Lateral Foundation-to-Soil Stiffness for Passive Pressure (after Wilson, 1988)The lateral capacity of a footing should assumed to beattained when the displacement, considering both basetraction and passive pressure stiffnesses, reaches 2% ofthe thickness of the footing. Upper and lower bounds oftwice and one-half of this value, respectively, alsoapply.4.4.2.2 Pile FoundationsPile foundations, in the context of this subsection, referto those foundation systems that are composed of a pilecap and associated driven or cast-in-place piles, whichtogether form a pile group. A single pile group maysupport a load-bearing column, or a linear sequence ofpile groups may support a shear wall.Generally, individual piles in a group could be expectedto be less than two feet in diameter. The stiffnesscharacteristics of single large-diameter piles or drilledshafts are described in Section 4.4.2.3.A. Stiffness ParametersFor the purpose of simplified analyses, the uncoupledspring model as shown in Figure 4-1b may be usedwhere the footing in the figure represents the pile cap.In the case of the vertical and rocking springs, it can beassumed that the contribution of the pile cap isrelatively small compared to the contribution of thepiles. In general, mobilization of passive pressures byeither the pile caps or basement walls will controllateral spring stiffness. Hence, estimates of lateralspring stiffness can be computed using elastic solutionsas described in Section 4.4.2.1A. In instances wherepiles may contribute significantly to lateral stiffness(i.e., very soft soils, battered piles), solutions usingbeam-column pile models are recommended.Axial pile group stiffness spring values, k sv , may beassumed to be in an upper and lower bound range,respectively, given by:k sv =N∑0.5 A E------------------Lton = 1N∑n = 12 A E-------------L(4-12)FEMA 273 Seismic Rehabilitation Guidelines 4-13

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)L(length)yB/6 End zone each sideB(width)xxyPlanStiffness per unit length:6.83 Gk end= for B/6 end zones1-νk mid=0.73 G1-νfor middle zonek endk midSectionk endEnd zone Middle zone End zonel 1 l 3l 4l 5Component stiffnesses:K i= l ikK 2K 3l 2K 5K 4where k is the appropriatestiffness per unit length forthe end zone or middle zoneFigure 4-5K 1Soil componentsVertical Stiffness Modeling for Shallow Bearing Footings4-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)PPPMM cqqc< 0.5qq cq cPPPMM cqq cq cqqc> 0.5Figure 4-6Idealized Concentration of Stress at Edge of Rigid Footings Subjected to Overturning MomentwhereA = Cross-sectional area of a pileE = Modulus of elasticity of pilesL = Length of pilesN = Number of piles in groupThe rocking spring stiffness values about eachhorizontal pile cap axis may be computed by assumingeach axial pile spring acts as a discrete Winkler spring.The rotational spring constant (moment per unitrotation) is then given by:where(4-13)k vn = Axial stiffness of the nth pile= Distance between nth pile and axis of rotationS nNk sr =2∑ k vn S nn = 1Whereas the effects of group action and the influence ofpile batter are not directly accounted for in the form ofthe above equations, it can be reasonably assumed thatthe latter effects are accounted for in the range ofuncertainties expressed for axial pile stiffness.B. Capacity ParametersBest-estimate vertical load capacity of piles (for bothaxial compression and axial tensile loading) should bedetermined using accepted foundation engineeringpractice, using best estimates of soil properties.Consideration should be given to the capability of pilecap and splice connections to take tensile loads whenevaluating axial tensile load capacity. Upper and lowerbound axial load capacities should be determined bymultiplying best-estimate values by factors of 2.0 and0.5, respectively.The upper and lower bound moment capacity of a pilegroup should be determined assuming a rigid pile cap,leading to an initial triangular distribution of axial pileloading from applied seismic moments. However, fullaxial capacity of piles may be mobilized whencomputing ultimate moment capacity, leading to arectangular distribution of resisting moment in aFEMA 273 Seismic Rehabilitation Guidelines 4-15

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)manner analogous to that described for a footing inFigure 4-6.The lateral capacity of a pile group is largely dependenton that of the cap as it is restrained by passive resistanceof the adjacent soil material. The capacity may beassumed to be reached when the displacement reaches2% of the depth of the cap in a manner similar to thatfor a shallow bearing foundation.4.4.2.3 Drilled ShaftsIn general, drilled shaft foundations or piers may betreated similarly to pile foundations. When the diameterof the shaft becomes large (> 24 inches), the bendingand the lateral stiffness and strength of the shaft itselfmay contribute to the overall capacity. This is obviouslynecessary for the case of individual shafts supportingisolated columns. In these instances, the interaction ofthe soil and shaft may be represented using Winklertype models (Pender, 1993; Reese et al., 1994).4.4.3 Foundation Acceptability CriteriaThis section contains acceptability criteria for thegeotechnical components of building foundations.Structural components of foundations shall meet theappropriate requirements of Chapters 5 through 8.Geotechnical components include the soil parts ofshallow spread footings and mats, and friction- and endbearingpiles and piers. These criteria, summarized inTable 4-4, apply to all actions including vertical loads,moments, and lateral forces applied to the soil.4.4.3.1 Simplified RehabilitationThe geotechnical components of buildings qualified forand subject to Simplified Rehabilitation may beconsidered acceptable if they comply with therequirements of Chapter 10.4.4.3.2 Linear ProceduresThe acceptability of geotechnical components subject tolinear procedures depends upon the basic modelingassumptions utilized in the analysis, as follows.Fixed Base Assumption. If the base of the structure hasbeen assumed to be completely rigid, actions ongeotechnical components shall be as on force-controlledcomponents governed by Equation 3-15 and componentcapacities may be assumed as upper-bound values. Afixed base assumption is not recommended for theImmediate Occupancy Performance Level for buildingssensitive to base rotations or other types of foundationmovement.Table 4-4Soil Foundation Acceptability SummaryAnalysis ProcedureSimplifiedRehabilitationLinear Static orDynamicNonlinear Static orDynamicFoundationAssumptionFixedCollapse Prevention and Life SafetySee Chapter 10Performance LevelActions on geotechnical components shallbe assumed as on force-controlledcomponents governed by Equation 3-15and component capacities may be assumedas upper bound values.Immediate OccupancyNot applicable.Not recommended for buildingssensitive to base rotation or otherfoundation movements.Flexible m = ∞ for use in Equation 3-18 m = 2.0 for use in Equation 3-18FixedFlexibleBase reactions limited to upper boundultimate capacity.Geotechnical component displacementsneed not be limited, provided that structurecan accommodate the displacements.Not recommended for buildingssensitive to base rotation or otherfoundation movements.Estimate and accommodate possiblepermanent soil movements.Flexible Base Assumption. If the base of the structureis modeled using linear geotechnical components, thenthe value of m, for use in Equation 3-18, for Life Safetyand Collapse Prevention Performance Levels may beassumed as infinite, provided the resultingdisplacements may be accommodated within theacceptability criteria for the rest of the structure. For the4-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)Immediate Occupancy Performance Levels, m valuesfor geotechnical components shall be limited to 2.0.4.4.3.3 Nonlinear ProceduresThe acceptability of geotechnical components subject tononlinear procedures depends upon the basic modelingassumptions utilized in the analysis, as follows.Fixed Base Assumption. If the base of the structure hasbeen assumed to be completely rigid, then the basereactions for all geotechnical components shall notexceed their upper-bound capacity to meet CollapsePrevention and Life Safety Performance Levels. A rigidbase assumption is not recommended for the ImmediateOccupancy Performance Level for buildings sensitiveto base rotations or other types of foundationmovement.Flexible Base Assumption. If the base of the structureis modeled using flexible nonlinear geotechnicalcomponents, then the resulting componentdisplacements need not be limited to meet Life Safetyand Collapse Prevention Performance Levels, providedthe resulting displacements may be accommodatedwithin the acceptability criteria for the rest of thestructure. For the Immediate Occupancy PerformanceLevel, an estimate of the permanent nonrecoverabledisplacement of the geotechnical components shall bemade based upon the maximum total displacement,foundation and soil type, soil layer thicknesses, andother pertinent factors. The acceptability of thesedisplacements shall be based upon their effects on thecontinuing function and safety of the building.4.5 Retaining WallsPast earthquakes have not caused extensive damage tobuilding walls below grade. In some cases, however, itmay be advisable to verify the adequacy of retainingwalls to resist increased pressure due to seismicloading. These situations might be for walls of poorconstruction quality, unreinforced or lightly reinforcedwalls, walls of archaic materials, unusually tall or thinwalls, damaged walls, or other conditions implying asensitivity to increased loads. The seismic earthpressure acting on a building wall retainingnonsaturated, level soil above the ground-water tablemay be approximated as:∆p=0.4k h γ t H rw(4-14)where∆pk hγ tH rw= Additional earth pressure due to seismicshaking, which is assumed to be a uniformpressure= Horizontal seismic coefficient in the soil,which may be assumed equal to S XS /2.5= The total unit weight of soil= The height of the retaining wallThe seismic earth pressure given above should be addedto the unfactored static earth pressure to obtain the totalearth pressure on the wall. The expression inEquation 4-14 is a conservative approximation of theMononabe-Okabe formulation. The pressure on wallsduring earthquakes is a complex action. If walls do nothave the apparent capacity to resist the pressuresestimated from the above approximate procedures,detailed investigation by a qualified geotechnicalengineer is recommended.4.6 Soil Foundation RehabilitationThis section provides guidelines for modification tofoundations to improve anticipated seismicperformance. Specifically, the scope of this sectionincludes suggested approaches to foundationmodification and behavioral characteristics offoundation elements from a geotechnical perspective.These must be used in conjunction with appropriatestructural material provisions from other chapters.Additionally, the acceptability of a modified structure isdetermined in accordance with Chapter 2 of theGuidelines.4.6.1 Soil Material ImprovementsSoil improvement options to increase the verticalbearing capacity of footing foundations are limited. Soilremoval and replacement and soil vibratorydensification usually are not feasible because theywould induce settlements beneath the footings or beexpensive to implement without causing settlement.Grouting may be considered to increase bearingcapacity. Different grouting techniques are discussed inthe Commentary Section C4.3.2. Compaction groutingcan achieve densification and strengthening of a varietyof soil types and/or extend foundation loads to deeper,stronger soils. The technique requires careful control toavoid causing uplift of foundation elements or adjacentfloor slabs during the grouting process. PermeationFEMA 273 Seismic Rehabilitation Guidelines 4-17

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)grouting with chemical grouts can achieve substantialstrengthening of sandy soils, but the more fine-grainedor silty the sand, the less effective the techniquebecomes. Jet grouting could also be considered. Thesesame techniques also may be considered to increase thelateral frictional resistance at the base of footings.Options that can be considered to increase the passiveresistance of soils adjacent to foundations or gradebeams include removal and replacement of soils withstronger, well-compacted soils or with treated (e.g.,cement-stabilized) soils; in-place mixing of soils withstrengthening materials (e.g., cement); grouting,including permeation grouting and jet grouting; and inplacedensification by impact or vibratory compaction(if the soil layers to be compacted are not too thick andvibration effects on the structure are tolerable).4.6.2 Spread Footings and MatsNew isolated or spread footings may be added toexisting structures to support new structural elementssuch as shear walls or frames. In these instances,capacities and stiffness may be determined inaccordance with the procedures of Section 4.4.Existing isolated or spread footings may be enlarged toincrease bearing or uplift capacity. Generally, capacitiesand stiffness may be determined in accordance with theprocedures of Section 4.4; however, consideration ofexisting contact pressures on the strength and stiffnessof the modified footing may be required, unless auniform distribution is achieved by shoring and/orjacking.Existing isolated or spread footings may beunderpinned to increase bearing or uplift capacity. Thistechnique improves bearing capacity by lowering thecontact horizon of the footing. Uplift capacity isimproved by increasing the resisting soil mass abovethe footing. Generally, capacities and stiffness may bedetermined in accordance with the procedures ofSection 4.4. Considerations of the effects of jacking andload transfer may be required.Where potential for differential lateral displacement ofbuilding foundations exists, provision ofinterconnection with grade beams or a well-reinforcedgrade slab can provide good mitigation of these effects.Ties provided to withstand differential lateraldisplacement should have a strength based on rationalanalysis, with the advice of a geotechnical engineerwhen appropriate.4.6.3 Piers and PilesPiles and pile caps shall have the capacity to resistadditional axial and shear loads caused by overturningforces. Wood piles cannot resist uplift unless a positiveconnection is provided for the loads. Piles must bereviewed for deterioration caused by decay, insectinfestation, or other signs of distress.Driven piles made of steel, concrete, or wood, or castin-placeconcrete piers may be used to support newstructural elements such as shear walls or frames.Capacities and stiffnesses may be determined inaccordance with the procedures of Section 4.4. Whenused in conjunction with existing spread footingfoundations, the effects of differential foundationstiffness should be considered in the analysis of themodified structure.Driven piles made of steel, concrete, or wood, or castin-placeconcrete piers may be used to supplement thevertical and lateral capacities of existing pile and pierfoundation groups and of existing isolated andcontinuous spread footings. Capacities and stiffnessesmay be determined in accordance with the proceduresof Section 4.4. If existing loads are not redistributed byshoring and/or jacking, the potential for differentialstrengths and stiffnesses among individual piles or piersshould be included.4.7 DefinitionsAllowable bearing capacity: Foundation load orstress commonly used in working-stress design (oftencontrolled by long-term settlement rather than soilstrength).Deep foundation:Piles or piers.Differential compaction: An earthquake-inducedprocess in which loose or soft soils become morecompact and settle in a nonuniform manner across asite.Fault: Plane or zone along which earth materials onopposite sides have moved differentially in response totectonic forces.Footing: A structural component transferring theweight of a building to the foundation soils and resistinglateral loads.4-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)Foundation soils: Soils supporting the foundationsystem and resisting vertical and lateral loads.Foundation springs: Method of modeling toincorporate load-deformation characteristics offoundation soils.Foundation system:(footings, piles).Structural componentsLandslide: A down-slope mass movement of earthresulting from any cause.Liquefaction: An earthquake-induced process inwhich saturated, loose, granular soils lose a substantialamount of shear strength as a result of increase in porewaterpressure during earthquake shaking.Pier: Similar to pile; usually constructed of concreteand cast in place.Pile: A deep structural component transferring theweight of a building to the foundation soils and resistingvertical and lateral loads; constructed of concrete, steel,or wood; usually driven into soft or loose soils.Prescriptive ultimate bearing capacity:Assumption of ultimate bearing capacity based onproperties prescribed in Section 4.4.1.2.Presumptive ultimate bearing capacity:Assumption of ultimate bearing capacity based onallowable loads from original design.Retaining wall:one side.Shallow foundation:footings or mats.A free-standing wall that has soil onIsolated or continuous spreadSPT N-Values: Using a standard penetration test(ASTM Test D1586), the number of blows of a 140-pound hammer falling 30 inches required to drive astandard 2-inch-diameter sampler a distance of12 inches.Ultimate bearing capacity: Maximum possiblefoundation load or stress (strength); increase indeformation or strain results in no increase in load orstress.4.8 SymbolsABDEGG oFooting area; also cross-section area ofpileWidth of footingDepth of footing bearing surfaceYoung’s modulus of elasticityShear modulusInitial or maximum shear modulusH Horizontal load on footingH rw Height of retaining wallI Moment of inertiaK L Passive pressure stiffnessL Length of footing in plan dimensionL Length of pile in vertical dimensionM Moment on footingM c Ultimate moment capacity of footingN Number of piles in a pile group(N 1 ) 60 Standard Penetration Test blow countnormalized for an effective stress of 1 tonper square foot and corrected to anequivalent hammer energy efficiency of60%P Vertical load on footingDead (static) loadQ DQ EQ LQ allow.D+LQ cQ SRS XSS nS ScEarthquake loadLive (frequently applied) loadAllowable working dead plus live load fora pile as specified in original designdocumentsUltimate bearing capacitySnow loadRadius of equivalent circular footingSpectral response acceleration at shortperiods for any hazard level or damping, gDistance between nth pile and axis ofrotation of a pile groupSpectral response acceleration at shortperiods, obtained from responseacceleration maps, gCohesive strength of soil, expressed inforce/unit area (pounds/ft 2 or Pa)FEMA 273 Seismic Rehabilitation Guidelines 4-19

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)dShort side of footing lateral contact areav′ sShear wave velocity at high strainδ c Pile complianced Depth to sampleλd w Depth of ground-water levelν Poisson’s ratiog Acceleration of gravity (386.1 in/sec. 2 , or ρk h9,800 mm/sec. 2 for SI units)σ o ′Horizontal seismic coefficient in soilφacting on retaining wallk o Stiffness coefficient for equivalentcircular footing4.9 Referencesk sh Winkler spring coefficient in horizontaldirection, expressed as force/unitdisplacement/unit areak sr Winkler spring coefficient in overturning(rotation), expressed as force/unitdisplacement/unit areak sv Winkler spring coefficient in verticaldirection, expressed as force/unitdisplacement/unit areak vn Axial stiffness of nth pile in a pile groupl Long side of footing lateral contact aream A modification factor used in theacceptance criteria of deformationcontrolledcomponents or elements,indicating the available ductility of acomponent action, and a multiplier ofultimate foundation capacity for checkingimposed foundation loads in Linear Staticor Dynamic Proceduresq Vertical bearing pressureq allow.D+L Allowable working dead plus live loadpressure for a spread footing as specifiedin original design documentsq c Ultimate bearing capacityv s Shear wave velocity at low strain∆pαβγγ tγ wAdditional earth pressure on retainingwall due to seismic shakingFoundation shape correction factorEmbedment factorUnit weight, weight/unit volume (pounds/ft 3 or N/m 3 )Total unit weight of soilUnit weight of waterShape factor for lateral stiffnessSoil mass densityEffective vertical stressAngle of internal friction, degreesASTM, 1994, Standard Test Method for PenetrationTest and Split-Barrel Sampling of Soils: TestDesignation D1586-84, ASTM Standards, AmericanSociety for Testing Materials, Philadelphia,Pennsylvania.BSSC, 1992, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No. FEMA178), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos. FEMA222A and 223A), Washington, D.C.Gazetas, G., 1991, “Foundation Vibrations,”Foundation Engineering Handbook, edited by Fang,H. Y., Van Nostrand Reinhold, New York, New York,pp. 553–593.ICBO, 1994, Uniform Building Code, InternationalConference of Building Officials, Whittier, California.Lam, I. P., Martin, G. R., and Imbsen, R., 1991,“Modeling Bridge Foundations for Seismic Design andRetrofitting,” Transportation Research Record,Washington, D.C., No. 1290.NAVFAC, 1982a, Soil Mechanics: Naval FacilitiesEngineering Command Design Manual, NAVFACDM-7.1, U.S. Department of the Navy, Alexandria,Virginia.NAVFAC, 1982b, Foundation and Earth Structures:Naval Facilities Engineering Command DesignManual, NAVFAC DM-7.2, U.S. Department of theNavy, Alexandria, Virginia.4-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 4: Foundations and Geotechnical Hazards(Systematic Rehabilitation)NAVFAC, 1983, Soil Dynamics, Deep Stabilization,and Special Geotechnical Construction, NAVFACDM-7.3, U.S. Department of the Navy, Alexandria,Virginia.Newmark, N. M., 1965, “Effect of Earthquake on Damsand Embankments,” Geotechnique, Vol. 15,pp. 139–160.Pender, M. J., 1993, “Aseismic Pile Foundation DesignAnalysis,” Bulletin of the New Zealand NationalSociety for Earthquake Engineering, Vol. 26, No. 1,pp. 49–161.Reese, L. C., Wand, S. T., Awashika, K., andLam, P.H.F., 1994, GROUP—a Computer Program forAnalysis of a Group of Piles Subjected to Axial andLateral Loading, Ensoft, Inc., Austin, Texas.SBCCI, 1993, Standard Building Code, SouthernBuilding Code Congress International, Birmingham,Alabama.Secretary of the Interior, 1993, Standards andGuidelines for Archeology and Historic Preservation,published in the Federal Register, Vol. 48, No. 190,pp. 44716–44742.Seed, H. B., Wong, R. T., and Idriss, I. M., 1986,“Moduli and Damping Factors for Dynamic Analysesof Cohesionless Soils,” Journal of the GeotechnicalEngineering Division, American Society of CivilEngineers, New York, New York, Vol. 112, pp. 1016–1032.Wilson, J. C., 1988, “Stiffness of Non-skewedMonolithic Bridge Abutments for Seismic Analysis,”Earthquake Engineering and Structural Dynamics,Vol. 16, pp. 867–883.Youd, T. L., and Perkins, D. M., 1978, “MappingLiquefaction Induced Ground Failure Potential,”Journal of the Geotechnical Engineering Division,American Society of Civil Engineers, New York, NewYork, Vol. 104, No. GT4, pp. 433–446.FEMA 273 Seismic Rehabilitation Guidelines 4-21

5.Steel and Cast Iron(Systematic Rehabilitation)5.1 ScopeRehabilitation measures for steel components andelements are described in this chapter. Informationneeded for systematic rehabilitation of steel buildings,as depicted in Step 4B of the Process Flow chart shownin Figure 1-1, is presented herein. A brief historicalperspective is given in Section 5.2, with a moreexpanded version given in the Commentary.Section 5.3 discusses material properties for new andexisting construction, and describes material testingrequirements for using the nonlinear procedures. Afactor measuring the reliability of assumptions of inplacematerial properties is included in a kappa (κ)factor, used to account for accuracy of knowledge of theexisting conditions. Evaluation methods for in-placematerials are also described.Sections 5.4 and 5.5 provide the attributes of steelmoment frames and braced frames. The stiffness andstrength properties of each steel component required forthe linear and nonlinear procedures described inChapter 3 are given. Stiffness and strength acceptancecriteria are also given and are discussed within thecontext of Tables 2-1, 2-3, and 2-4, given in Chapter 2.These sections also provide guidance on choosing anappropriate rehabilitation strategy.The appropriate procedures for evaluating systems withold and new components are discussed. Steel frameswith concrete or masonry infills are briefly discussed,but the behavior of these systems and procedures forestimating the forces in the steel components are givenin Chapters 6 (concrete) and 7 (masonry). Steel frameswith attached masonry walls are discussed in thischapter and in Chapter 7.Section 5.8 describes engineering properties for typicaldiaphragms found in steel buildings. These include baremetal deck, metal deck with composite concretetopping, noncomposite steel deck with concretetopping, horizontal steel bracing, and archaicdiaphragms. The properties and behavior of wooddiaphragms in steel buildings are presented inChapter 8.Engineering properties, and stiffness and strengthacceptance criteria for steel piles are given inSection 5.9. Methods for calculating the forces in thepiles are described in Chapter 4 and in the Commentaryto Chapter 5.5.2 Historical PerspectiveThe components of steel elements are columns, beams,braces, connections, link beams, and diaphragms. Thecolumns, beams, and braces may be built up with plates,angles, and/or channels connected together with rivets,bolts, or welds. The material used in older constructionis likely to be mild steel with a specified yield strengthbetween 30 ksi and 36 ksi. Cast iron was often used forcolumns in much older construction (before 1900). Castiron was gradually replaced by wrought iron and thensteel. The connectors in older construction were usuallymild steel rivets or bolts. These were later replaced byhigh-strength bolts and welds. The seismic performanceof these components will depend heavily on thecondition of the in-place material. A more detailedhistorical perspective is given in Section C5.2 of theCommentary.As indicated in Chapter 1, great care should beexercised in selecting the appropriate rehabilitationapproaches and techniques for application to historicbuildings in order to preserve their uniquecharacteristics.5.3 Material Properties andCondition Assessment5.3.1 GeneralQuantification of in-place material properties andverification of the existing system configuration andcondition are necessary to analyze or evaluate abuilding. This section identifies properties requiringconsideration and provides guidelines for theiracquisition. Condition assessment is an importantaspect of planning and executing seismic rehabilitationof an existing building. One of the most important stepsin condition assessment is a visit to the building forvisual inspection.The extent of in-place materials testing and conditionassessment that must be accomplished is related toavailability and accuracy of construction and as-builtFEMA 273 Seismic Rehabilitation Guidelines 5-1

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)records, the quality of materials used and constructionperformed, and the physical condition of the structure.Data such as the properties and grades of material usedin component and connection fabrication may beeffectively used to reduce the amount of in-place testingrequired. The design professional is encouraged toresearch and acquire all available records from originalconstruction. The requirements given here aresupplemental to those given in Section 2.7.5.3.2 Properties of In-Place Materials andComponents5.3.2.1 Material PropertiesMechanical properties of component and connectionmaterial dictate the structural behavior of thecomponent under load. Mechanical properties ofgreatest interest include the expected yield (F ye ) andtensile (F te ) strengths of base and connection material,modulus of elasticity, ductility, toughness, elogationalcharacteristics, and weldability. The term “expectedstrength” is used throughout this document in place of“nominal strength” since expected yield and tensilestresses are used in place of nominal values specified inAISC (1994a and b).The effort required to determine these properties isrelated to the availability of original and updatedconstruction documents, original quality ofconstruction, accessibility, and condition of materials.The determination of material properties is bestaccomplished through removal of samples andlaboratory testing. Sampling may take place in regionsof reduced stress—such as flange tips at beam ends andexternal plate edges—to minimize the effects ofreduced area. Types and sizes of specimens should be inaccordance with ASTM standards. Mechanical andmetallurgical properties usually can be established fromlaboratory testing on the same sample. If a connectorsuch as a bolt or rivet is removed for testing, acomparable bolt should be reinstalled at the time ofsampling. Destructive removal of a welded connectionsample must be accompanied by repair of theconnection.5.3.2.2 Component PropertiesBehavior of components, including beams, columns,and braces, is dictated by such properties as area, widthto-thicknessand slenderness ratios, lateral torsionalbuckling resistance, and connection details. Componentproperties of interest are:• Original cross-sectional shape and physicaldimensions• Size and thickness of additional connected materials,including cover plates, bracing, and stiffeners• Existing cross-sectional area, section moduli,moments of inertia, and torsional properties atcritical sections• As-built configuration of intermediate, splice, andend connections• Current physical condition of base metal andconnector materials, including presence ofdeformation.Each of these properties is needed to characterizebuilding performance in the seismic analysis. Thestarting point for establishing component propertiesshould be construction documents. Preliminary reviewof these documents shall be performed to identifyprimary vertical- and lateral-load-carrying elements andsystems, and their critical components and connections.In the absence of a complete set of building drawings,the design professional must direct a testing agency toperform a thorough inspection of the building toidentify these elements and components as indicated inSection 5.3.3.In the absence of degradation, statistical analysis hasshown that mean component cross-sectional dimensionsare comparable to the nominal published values byAISC, AISI, and other organizations. Variance in thesedimensions is also small.5.3.2.3 Test Methods to Quantify PropertiesTo obtain the desired in-place mechanical properties ofmaterials and components, it is necessary to utilizeproven destructive and nondestructive testing methods.To achieve the desired accuracy, mechanical propertiesshould be determined in the laboratory. Particularlaboratory test information that may be sought includesyield and tensile strength, elongation, and charpy notchtoughness. For each test, industry standards publishedby the ASTM exist and shall be followed. TheCommentary provides applicability information andreferences for these particular tests.5-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Of greatest interest to metal building systemperformance are the expected yield and tensile strengthof the installed materials. Notch toughness of structuralsteel and weld material is also important forconnections that undergo cyclic loadings anddeformations during earthquakes. Chemical andmetallurgical properties can provide information onproperties such as compatibility of welds with parentmetal and potential lamellar tearing due to throughthicknessstresses. Virtually all steel component elasticand inelastic limit states are related to yield and tensilestrengths. Past research and accumulation of data byindustry groups have resulted in published materialmechanical properties for most primary metals and theirdate of fabrication. Section 5.3.2.5 provides thisstrength data. This information may be used, togetherwith tests from recovered samples, to rapidly establishexpected strength properties for use in componentstrength and deformation analyses.Review of other properties derived from laboratorytests—such as hardness, impact, fracture, and fatigue—is generally not needed for steel component capacitydetermination, but is required for archaic materials andconnection evaluation. These properties may not beneeded in the analysis phase if significant rehabilitativemeasures are already known to be required.To quantify material properties and analyze theperformance of welded moment connections, moreextensive sampling and testing may be necessary. Thistesting may include base and weld material chemicaland metallurgical evaluation, expected strengthdetermination, hardness, and charpy V-notch testing ofthe heat-affected zone and neighboring base metal, andother tests depending on connection configuration.If any rehabilitative measures are needed and weldedconnection to existing components is required, thecarbon equivalent of the existing component(s) shall bedetermined. Appropriate welding procedures aredependent upon the chemistry of base metal and fillermaterial (for example, the elements in the IIW CarbonEquivalent formula). Consult Section 8 and itsassociated Commentary in the latest edition ofANSI/AWS D1.1 Structural Welding Code.Recommendations given in FEMA 267 (SAC, 1995)may also be followed.5.3.2.4 Minimum Number of TestsIn order to quantify expected strength and other in-placeproperties accurately, it will sometimes be required thata minimum number of tests be conducted onrepresentative components. As stated previously, theminimum number of tests is dictated by available datafrom original construction, the type of structural systememployed, desired accuracy, and quality/condition ofin-place materials. Access to the structural system willalso be a factor in defining the testing program. As analternative, the design professional may elect to utilizethe default strength properties contained inSection 5.3.2.5 instead of the specified testing.However, in some cases these default values may onlybe used for a Linear Static Procedure (LSP).Material properties of structural steel vary much lessthan those of other construction materials. In fact, theexpected yield and tensile stresses are usuallyconsiderably higher than the nominal specified values.As a result, testing for material properties may not berequired. The properties of wrought iron are morevariable than those of steel. The strength of cast ironcomponents cannot be determined from small sampletests, since component behavior is usually governed byinclusions and other imperfections. It is recommendedthat the lower-bound default value for compressivestrength of cast iron given in Table 5-1 be used.The guidelines for determining the expected yield (F ye )and tensile (F te ) strengths are given below.• If original construction documents definingproperties—including material test records ormaterial test reports (MTR)—exist, material testsneed not be carried out, at the discretion of thedesign professional. Default values from Table 5-2may be used. Larger values may be used, at thediscretion of the design professional, if availablehistorical data substantiates them. Larger valuesshould be used if the assumptions produce a largerdemand on associated connections.• If original construction documents definingproperties are limited or do not exist, but the date ofconstruction is known and the single material used isconfirmed to be carbon steel, at least three strengthcoupons shall be randomly removed from eachcomponent type. Conservative material propertiessuch as those given in Table 5-2 may be used in lieuof testing, at the discretion of the designprofessional.• If no knowledge exists of the structural system andmaterials used, at least two strength tensile couponsFEMA 273 Seismic Rehabilitation Guidelines 5-3

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)should be removed from each component type forevery four floors. If it is determined from testing thatmore than one material grade exists, additionaltesting should be performed until the extent of usefor each grade in component fabrication has beenestablished. If it is determined that all componentsare made from steel, the requirements immediatelypreceding this may be followed.• In the absence of construction records definingwelding filler metals and processes used, at least oneweld metal sample for each construction type shouldbe obtained for laboratory testing. The sample shallconsist of both local base and weld metal, such thatcomposite strength of the connection can be derived.Steel and weld filler material properties discussed inSection 5.3.2.3 should also be obtained. Because ofthe destructive nature and necessary repairs thatfollow, default strength properties may besubstituted if original records on welding exist,unless the design professional requires moreaccurate data. If ductility and toughness are requiredat or near the weld, the design professional mayconservatively assume that no ductility is available,in lieu of testing. In this case the joint would have tobe modified. Special requirements for weldedmoment frames are given in FEMA 267 (SAC,1995) and the latest edition of ANSI/AWS D1.1Structural Welding Code.• Testing requirements for bolts and rivets are thesame as for other steel components as given above.In lieu of testing, default values from Table 5-2 maybe used.• For archaic materials, including wrought iron butexcluding cast iron, at least three strength couponsshall be extracted for each component type for everyfour floors of construction. Should significantvariability be observed, in the judgment of thedesign professional, additional tests shall beperformed until an acceptable strength value isobtained. If initial tests provide material propertiesthat are consistent with properties given inTable 5-1, tests are required only for every six floorsof construction.For all laboratory test results, the mean yield and tensilestrengths may be interpreted as the expected strengthfor component strength calculations.For other material properties, the design professionalshall determine the particular need for this type oftesting and establish an adequate protocol consistentwith that given above. In general, it is recommendedthat a minimum of three tests be conducted.If a higher degree of confidence in results is desired, thesample size shall be determined using ASTM StandardE22 guidelines. Alternatively, the prior knowledge ofmaterial grades from Section 5.3.2.5 may be used inconjunction with Bayesian statistics to gain greaterconfidence with the reduced sample sizes noted above.The design professional is encouraged to use theprocedures contained in the Commentary in this regard.5.3.2.5 Default PropertiesThe default expected strength values for key metallicmaterial properties are contained in Tables 5-1 and 5-2.These values are conservative, representing meanvalues from previous research less two standarddeviations. It is recommended that the results of anymaterial testing performed be compared to values inthese tables for the particular era of buildingconstruction. Additional testing is recommended if theexpected yield and tensile strengths determined fromtesting are lower than the default values.Default material strength properties may only be used inconjunction with Linear Static and DynamicProcedures. For the nonlinear procedures, expectedstrengths determined from the test program given aboveshall be used. Nonlinear procedures may be used withthe reduced testing requirements described inCommentary Section C5.3.2.5.5.3.3 Condition Assessment5.3.3.1 GeneralA condition assessment of the existing building and siteconditions shall be performed as part of the seismicrehabilitation process. The goals of this assessment are:• To examine the physical condition of primary andsecondary components and the presence of anydegradation• To verify or determine the presence andconfiguration of components and their connections,and the continuity of load paths betweencomponents, elements, and systems5-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-1 Default Material Properties 1Early unit stresses used in tables of allowable loads as published in catalogs of the following millsFOR CAST IRON 1YearRolling MillExpected YieldStrength, ksi1873 Carnegie Kloman & Co. (“Factor of Safety 3”) 211874 New Jersey Steel & Iron Co. 181881–1884 Carnegie Brothers & Co., Ltd. 18151884 The Passaic Rolling Mill Co. 18151885 The Phoenix Iron Company 181885–1887 Pottsville Iron & Steel Co. 181889 Carnegie Phipps & Co., Ltd. 1815FOR STEEL 11887 Pottsville Iron & Steel Co. 231889–1893 Carnegie Phipps & Co., Ltd. 241893–1908 Jones & Laughlins Ltd.Jones & Laughlins Steel Co.1896 Carnegie Steel Co., Ltd. 241897–1903 The Passaic Rolling Mills Co. 24181898–1919 Cambria Steel Co. 24181900–1903 Carnegie Steel Company 241907–1911 Bethlehem Steel Co. 241915 Lackawanna Steel Co. 24181. Modified from unit stress values in AISC “Iron and Steel Beams from 1873 to 1952.”2418• To review other conditions—such as neighboringparty walls and buildings, the presence ofnonstructural components, and limitations forrehabilitation—that may influence buildingperformance• To formulate a basis for selecting a knowledgefactor (see Section 5.3.4).The physical condition of existing components andelements, and their connections, must be examined forpresence of degradation. Degradation may includeenvironmental effects (e.g., corrosion, fire damage,chemical attack) or past/current loading effects (e.g.,overload, damage from past earthquakes, fatigue,fracture). The condition assessment shall also examinefor configurational problems observed in recentearthquakes, including effects of discontinuouscomponents, improper welding, and poor fit-up.Component orientation, plumbness, and physicaldimensions should be confirmed during an assessment.Connections in steel components, elements, andsystems require special consideration and evaluation.The load path for the system must be determined, andeach connection in the load path(s) must be evaluated.This includes diaphragm-to-component andcomponent-to-component connections. FEMA 267FEMA 273 Seismic Rehabilitation Guidelines 5-5

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-2 Default Expected Material Strengths 1History of ASTM and AISC Structural Steel Specification StressesDate Specification Remarks1900 ASTM, A9Rivet SteelBuildingsMedium Steel1901–1908 ASTM, A9Rivet SteelBuildingsMedium Steel1909–1923 ASTM, A9Structural SteelBuildingsRivet SteelExpected TensileStrength 2 , F te , ksi506050605548ASTM RequirementExpected Yield Strength 2, 3F ye , ksi30351/2 T.S.1/2 T.S.1/2 T.S.1/2 T.S.1924–1931 ASTM, A7 Structural Steel 55 1/2 T.S.or not less than 30Rivet Steel 46 1/2 T.S.or not less than 25ASTM, A9 Structural Steel 55 1/2 T.S.or not less than 30Rivet Steel 46 1/2 T.S.or not less than 251932 ASTM, A140-32T issuedas a tentative revision toASTM, A9 (Buildings)1933 ASTM, A140-32Tdiscontinued and ASTM,A9 (Buildings) revisedOct. 30, 1933ASTM, A9 tentativelyrevised to ASTM, A9-33T(Buildings)ASTM, A141-32T adoptedas a standardPlates, Shapes, Bars 60 1/2 T.S.or not less than 33Eyebar flatsunannealed67 1/2 T.S.or not less than 36Structural Steel 55 1/2 T.S.or not less than 30Structural Steel 60 1/2 T.S.or not less than 33Rivet Steel 52 1/2 T.S.or not less than 281934 on ASTM, A9 Structural Steel 60 1/2 T.S.or not less than 33ASTM, A141 Rivet Steel 52 1/2 T.S.or not less than 281. Duplicated from AISC “Iron and Steel Beams 1873 to 1952.”2. Values shown in this table are based on mean minus two standard deviations and duplicated from “Statistical Analysis of Tensile Data for Wide-FlangeStructural Shapes.” The values have been reduced by 10%, since originals are from mill tests.3. T.S. = Tensile strength5-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-2Default Expected Material Strengths 1 (continued)Additional default assumptionsDate Specification Remarks1961 on ASTM, A36Structural SteelGroup 1Group 2Group 3Group 4Group 5ASTM, A572, Grade 50 Structural SteelGroup 1Group 2Group 3Group 4Group 5Dual GradeStructural SteelGroup 1Group 2Group 3Group 4Expected TensileStrength 2 , F te , ksi5452525361565760627159606464Expected Yield Strength 2, 3F ye , ksi37353230354142444344434346441. Duplicated from AISC “Iron and Steel Beams 1873 to 1952.”2. Values shown in this table are based on mean minus two standard deviations and duplicated from “Statistical Analysis of Tensile Data for Wide-FlangeStructural Shapes.” The values have been reduced by 10%, since originals are from mill tests.3. T.S. = Tensile strength(SAC, 1995) provides recommendations for inspectionof welded steel moment frames.The condition assessment also affords an opportunity toreview other conditions that may influence steelelements and systems and overall buildingperformance. Of particular importance is theidentification of other elements and components thatmay contribute to or impair the performance of the steelsystem in question, including infills, neighboringbuildings, and equipment attachments. Limitationsposed by existing coverings, wall and ceiling space,infills, and other conditions shall also be defined suchthat prudent rehabilitation measures may be planned.5.3.3.2 Scope and ProceduresThe scope of a condition assessment shall include allprimary structural elements and components involvedin gravity and lateral load resistance. The degree ofassessment performed also affects the κ factor that isused (see Section 5.3.4).If coverings or other obstructions exist, indirect visualinspection through use of drilled holes and a fiberscopemay be utilized. If this method is not appropriate, thenlocal removal of covering materials will be necessary.The following guidelines shall be used.• If detailed design drawings exist, exposure of at leastone different primary connection shall occur foreach connection type. If no deviations from thedrawings exist, the sample may be consideredrepresentative. If deviations are noted, then removalof additional coverings from primary connections ofthat type must be done until the design professionalhas adequate knowledge to continue with theevaluation and rehabilitation.FEMA 273 Seismic Rehabilitation Guidelines 5-7

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)• In the absence of construction drawings, the designprofessional shall establish inspection protocol thatwill provide adequate knowledge of the buildingneeded for reliable evaluation and rehabilitation. Forsteel elements encased in concrete, it may be morecost effective to provide an entirely new lateral-loadresistingsystem.Physical condition of components and connectors mayalso dictate the use of certain destructive andnondestructive test methods. If steel elements arecovered by well-bonded fireproofing materials orencased in durable concrete, it is likely that theircondition will be suitable. However, local removal ofthese materials at connections shall be performed aspart of the assessment. The scope of this removal effortis dictated by the component and element design. Forexample, in a braced frame, exposure of several keyconnections may suffice if the physical condition isacceptable and configuration matches the designdrawings. However, for moment frames it may benecessary to expose more connection points because ofvarying designs and the critical nature of theconnections. See FEMA 267 (SAC, 1995) forinspection of welded moment frames.5.3.3.3 Quantifying ResultsThe results of the condition assessment shall be used inthe preparation of building system models in theevaluation of seismic performance. To aid in this effort,the results shall be quantified and reduced, with thefollowing specific topics addressed:• Component section properties and dimensions• Connection configuration and presence of anyeccentricities• Type and location of column splices• Interaction of nonstructural components and theirinvolvement in lateral load resistanceThe acceptance criteria for existing componentsdepends on the design professional’s knowledge of thecondition of the structural system and materialproperties (as previously noted). All deviations notedbetween available construction records and as-builtconditions shall be accounted for and considered in thestructural analysis.5.3.4 Knowledge (κ) FactorAs described in Section 2.7 and Tables 2-16 and 2-17,computation of component capacities and allowabledeformations shall involve the use of a knowledge (κ)factor. For cases where a linear procedure will be usedin the analysis, two categories of κ exist. This sectionfurther describes the requirements specific to metallicstructural elements that must be accomplished in theselection of a κ factor.A κ factor of 1.0 can be utilized when a thoroughassessment is performed on the primary and secondarycomponents and load path, and the requirements ofSection 2.7 are met. The additional requirement for a κfactor of 1.0 is that the condition assessment be done inaccordance with Section 5.3.3. In general, a κ factor of1.0 may be used if the construction documents areavailable.If the configuration and condition of an as-builtcomponent or connection are not adequately known (inthe judgement of the design professional, becausedesign documents are unavailable and it is deemed toocostly to do a thorough condition assessment inaccordance with Section 5.3.3), the κ factor used in thefinal component evaluation shall be reduced to 0.75. Aκ factor of 0.75 shall be used for all cast and wroughtiron components and their connectors. For encasedcomponents where construction documents are limitedand knowledge of configuration and condition isincomplete, a factor of 0.75 shall be used. In addition,for steel moment and braced frames, the use of a κfactor of 0.75 shall occur when knowledge ofconnection details is incomplete. See alsoSection C2.7.2 in the Commentary.5.4 Steel Moment Frames5.4.1 GeneralSteel moment frames are those frames that develop theirseismic resistance through bending of beams andcolumns and shearing of panel zones. Moment-resistingconnections with calculable resistance are requiredbetween the members. The frames are categorized bythe types of connection used and by the local and globalstability of the members. Moment frames may act aloneto resist seismic loads, or they may act in conjunctionwith concrete or masonry shear walls or braced steelframes to form a dual system. Special rules for design5-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)of new dual systems are included in AISC (1994a) andBSSC (1995).Columns, beams, and connections are the componentsof moment frames. Beams and columns may be built-upmembers from plates, angles, and channels, cast orwrought iron segments, hot-rolled members, or coldformedsteel sections. Built-up members may beassembled by riveting, bolting, or welding. Connectionsbetween the members may be fully restrained (FR),partially restrained (PR), or nominally unrestrained(simple shear or pinned). The components may be baresteel, steel with a nonstructural coating for fireprotection, or steel with either concrete or masonryencasement for fire protection.Two types of frames are categorized in this document.Fully restrained (FR) moment frames are those framesfor which no more than 5% of the lateral deflectionsarise from connection deformation. Partially restrained(PR) moment frames are those frames for which morethan 5% of the lateral deflections result from connectiondeformation. In each case, the 5% value refers only todeflection due to beam-column deformation and not toframe deflections that result from column panel zonedeformation.5.4.2 Fully Restrained Moment Frames5.4.2.1 GeneralFully restrained (FR) moment frames are those momentframes with rigid connections. The connection shall beat least as strong as the weaker of the two membersbeing joined. Connection deformation may contributeno more than 5% (not including panel zonedeformation) to the total lateral deflection of the frame.If either of these conditions is not satisfied, the frameshall be characterized as partially restrained. The mostcommon beam-to-column connection used in steel FRmoment frames since the late 1950s required the beamflange to be welded to the column flange usingcomplete joint penetration groove welds. Many of theseconnections have fractured during recent earthquakes.The design professional is referred to the Commentaryand to FEMA 267 (SAC, 1995).Fully restrained moment frames encompass bothSpecial Moment Frames and Ordinary Moment Frames,defined in the Seismic Provisions for Structural SteelBuildings in Part 6 of AISC (1994a). These terms arenot used in the Guidelines, but most of the requirementsfor these systems are reflected in AISC (1994a).Requirements for general or seismic design of steelcomponents given in AISC (1994a) or BSSC (1995) areto be followed unless superseded by provisions in theseGuidelines. In all cases, the expected strength will beused in place of the nominal design strength byreplacing F y with F ye .5.4.2.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresAxial area. This is the complete area of rolled or builtupshapes. For built-up sections, the effective areashould be reduced if adequate load transfer mechanismsare not available. For elements fully encased inconcrete, the stiffness may be calculated assuming fullcomposite action if most of the concrete may beexpected to remain after the earthquake. Compositeaction may not be assumed for strength unless adequateload transfer and ductility of the concrete can beassured.Shear area. This is based on standard engineeringprocedures. The above comments, related to built-upsections, concrete encased elements, and compositeaction of floor beam and slab, apply.Moment of inertia. The calculation of rotationalstiffness of steel beams and columns in bare steelframes shall follow standard engineering procedures.For components encased in concrete, the stiffness shallinclude composite action, but the width of thecomposite section shall be taken as equal to the width ofthe flanges of the steel member and shall not includeparts of the adjoining floor slab, unless there is anadequate and identifiable shear transfer mechanismbetween the concrete and the steel.Joint Modeling. Panel zone stiffness may be consideredin a frame analysis by adding a panel zone element tothe program. The beam flexural stiffness may also beadjusted to account for panel zone stiffness or flexibilityand the stiffness of the concrete encasement. Use centerline analysis for other cases. Strengthened membersshall be modeled similarly to existing members. Theapproximate procedure suggested for calculation ofstiffness of PR moment frames given below may beused to model panel zone effects, if available computerprograms cannot explicitly model panel zones.Connections. The modeling of stiffness for connectionsfor FR moment frames is not required since, bydefinition, the frame displacements are not significantlyFEMA 273 Seismic Rehabilitation Guidelines 5-9

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)(

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Figure 5-2Q and Q CE are the generalized component load andgeneralized component expected strength, respectively.For beams and columns, these refer to the plasticmoment capacity, which is for:Beams: Q CE = M CE = ZF ye(5-3)Columns:(5-4)Panel Zones: Q CE = V CE = 0.55F ye d c t p (5-5)whered cDefinition of Chord Rotation= Column depth, in.E = Modulus of elasticity, ksiF ye = Expected yield strength of the material, ksiI = Moment of inertia, in. 4l bθ = ∆LθChordθ yL(a) Cantilever example=∆ yLChord Rotation:θ = ∆Lθ(b) Frame examplePQ CE = M CE = 1.18ZF ⎛ ye 1 – ------- ⎞ ≤ ZF⎝ P ⎠ yeye= Beam length, in.∆∆ y∆l c = Column length, in.M CE = Expected moment strengthP = Axial force in the member, kipsP ye = Expected axial yield force of the member =A g F ye , kipsQ = Generalized component loadQ CE = Generalized component expected strengtht p = Total panel zone thickness including doublerplates, in.θ = Chord rotationθ y = Yield rotationV CE = Expected shear strength, kipsZ = Plastic section modulus, in. 3C. Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be properly modeled. This behavior must beverified by experiment. This procedure is notrecommended in most cases.5.4.2.3 Strength and DeformationAcceptance CriteriaA. Linear Static and Dynamic ProceduresThe strength and deformation acceptance criteria forthese methods require that the load and resistancerelationships given in Equations 3-18 and 3-19 inChapter 3 be satisfied. The design strength ofcomponents in existing FR moment frames shall bedetermined using the appropriate equations for designstrength given in Section 5.4.2.2 or in Part 6 of AISC(1994a), except that φ shall be taken as 1.0. Designrestrictions given in AISC (1994a) shall be followedunless specifically superseded by provisions in theseGuidelines.Evaluation of component acceptability requiresknowledge of the component expected strength, Q CE ,for Equation 3-18 and the component lower-boundstrength, Q CL , for Equation 3-19, and the componentdemand modifier, m, as given in Table 5-3 forEquation 3-18. Values for Q CE and Q CL for FR momentframe components are given in this section. Q CE andQ CL are used for deformation- and force-controlledcomponents, respectively. Values for m are given inTable 5-3 for the Immediate Occupancy, Life Safety,and Collapse Prevention Performance Levels.FEMA 273 Seismic Rehabilitation Guidelines 5-11

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Beams. The design strength of beams and other flexuralmembers is the lowest value obtained according to thelimit state of yielding, lateral-torsional buckling, localflange buckling, or shear yielding of the web. For fullyconcrete-encased beams where the concrete is expectedto remain in place, because of confining reinforcement,during the earthquake, assume b f = 0 and L p = 0 for thepurpose of determining m. For bare beams bent abouttheir major axes and symmetric about both axes,b f 52with ----- < ----------- (compact section) and l b < L p , the2t f F yevalues for m are given in Table 5-3, and:whereb ft fl bL pM CEM pCEF yeQ CE = M CE = M pCE = ZF ye(5-6)= Width of the compression flange, in.= Thickness of the compression flange, in.= Length of beam, in.= Limiting lateral unbraced length for fullplastic bending capacity for uniformbending from AISC (1994a), in.= Expected flexural strength, kip-in.= Expected plastic moment capacity, kip-in.= Expected mean yield strength determined bythe tests or given in Tables 5-1 or 5-2bIf ----- f 52> --------- and l , values for m are given in2t b > L pf F yTable 5-3. For cases where the moment diagram isnonuniform and L p < L b < L r , but the nominal bendingstrength is still M pCE , the value of m is obtained fromTable 5-3. If M CE < M pCE due to lateral torsionalbuckling, then the value of m shall be m e , where(m e C b m ( m – 1) L b – L p )= – ---------------------- ≤ 8L r – L p(5-7)whereL b = Distance between points braced against lateraldisplacement of the compression flange, orbetween points braced to prevent twist of thecross section (see AISC, 1994a)L p = Limiting unbraced length between points oflateral restraint for the full plastic momentcapacity to be effective (see AISC, 1994a)L r = Limiting unbraced length between points oflateral support beyond which elastic lateraltorsional buckling of the beam is the failuremode (see AISC, 1994a)m = Value of m given in Table 5-3m e = Effective m from Equation 5-7C b = Coefficient to account for effect of nonuniformmoment (see AISC, 1994a)If the beam strength is governed by shear strength of theh 418unstiffened web and ---- ≤ --------- , then:t w F ywhereQ CE = V CE = 0.6F ye A wV CE = Expected shear strength, kipsA w = Nominal area of the web = d b t w , in. 2(5-8)t w = Web thickness, in.h = Distance from inside of compression flange toinside of tension flange, in.For this case, use tabulated values for beams, row a, inTable 5-3. If ----h 418> --------- , the value of V CE should bet w F ycalculated from provisions in Part 6 of AISC (1994a)and the value of m should be chosen using engineeringjudgment, but should be less than 8.The limit state of local flange and lateral torsionalbuckling are not applicable to components eithersubjected to bending about their minor axes or fullyencased in concrete, with confining reinforcement.For built-up shapes, the strength may be governed bythe strength of the lacing plates that carry component5-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)shear. For this case, the lacing plates are not as ductileas the component and should be designed for 0.5 timesthe m value in Table 5-3, unless larger values can bejustified by tests or analysis. For built-up laced beamsand columns fully encased in concrete, local bucklingof the lacing is not a problem if most of the encasementcan be expected to be in place after the earthquake.Columns. The lower-bound strength, Q CL , of steelcolumns under compression only is the lowest valueobtained by the limit stress of buckling, local flangebuckling, or local web buckling. The effective designstrength should be calculated in accordance withprovisions in Part 6 of AISC (1994a), but φ = 1.0 andF ye shall be used for existing components. Acceptanceshall be governed by Equation 3-19 of these Guidelines,since this is a force-controlled member.The lower-bound strength of cast iron columns shall becalculated as:whereCast iron columns can only carry axial compression.(5-9)For steel columns under combined axial and bendingstress, the column shall be considered to bedeformation-controlled and the lower-bound strengthshall be calculated by Equation 5-10 or 5-11.ForForP--------- ≥ 0.2P CLP---------P CLP CL=A g F crF cr = 12 ksi for l c /r ≤ 1081.40 × 10 5F cr = ------------------------ksi( l c /r) 2 for l c /r > 108+8--9P--------- < 0.2P CLM x M-------------------- + -------------------- y≤ 1.0m x M CEx m y M CEy(5-10)whereP M------------ xM y+ -------------------- + -------------------- ≤ 1.02P CL m x M CExm y M CEy(5-11)P = Axial force in the column, kipsP CL = Expected compression strength of thecolumn, kipsM x = Bending moment in the member for the x-axis, kip-inM CEx = Expected bending strength of the column forthe x-axis, kip-inM CEy = Expected bending strength of the column forthe y-axisM y = Bending moment in the member for the y-axis, kip-inm x = Value of m for the column bending about thex-axism y = Value of m for the column bending about they-axisFor columns under combined compression and bending,lateral bracing to prevent torsional buckling shall beprovided as required by AISC (1994a).Panel Zone. The strength of the panel zone shall becalculated as given in Equation 5-5.Connections. By definition, the strength of FRconnections shall be at least equal to, or preferablygreater than, the strength of the members being joined.Some special considerations should be given to FRconnections.Full Penetration Welded Connections (Full-Pen). Fullpenconnections (see Figure 5-3) have the beam flangeswelded to the column flanges with complete penetrationgroove welds. A bolted or welded shear tab is alsoincluded to connect the beam web to the column. Thestrength and ductility of full-pen connections are notfully understood at this time. They are functions of thequality of construction, the l b /d b ratio of the beam(where l b = beam length and d b = beam depth), the weldmaterial, the thickness of the beam and column flanges,the stiffness and strength of the panel zones, jointconfinement, triaxial stresses, and other factors (seeSAC, 1995). In lieu of further study, the value of m forLife Safety for beams with full-pen connections shall benot larger thanFEMA 273 Seismic Rehabilitation Guidelines 5-13

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-3Acceptance Criteria for Linear Procedures—Fully Restrained (FR) Moment Framesm Values for Linear Procedures 8Primary SecondaryComponent/ActionIOmLSmCPmLSmCPmMoment FramesBeams:a.b 52----- < -----------2t f F ye2 6 8 10 12b.b 95----- > -----------2t f F ye1 2 3 3 452c. For -----------b 95≤ ----- ≤ -----------F 2t ye f F yeuse linear interpolationColumns:For P/P ye < 0.20a.b 52----- < -----------2t f F ye2 6 8 10 12b 95b. ----- > -----------2t1 1 2 2 3f F ye52c. For -----------b 95≤ ----- ≤ ----------- use linear interpolationF 2t ye f F yeFor 0.2 ð P/P ye ð 0.50 9b 52a. ----- < -----------2t1 — 1 — 2 — 3 — 4f F yeb 95b. ----- > -----------2t1 1 1.5 2 2f F ye52c. For -----------b 95≤ ----- ≤ ----------- use linear interpolationF 2t ye f F yePanel Zones 1.5 8 11 NA NAFully Restrained Moment Connections 7For full penetration flange welds and bolted or welded web connection: beamdeformation limitsa. No panel zone yield 1 — 5 — 6 3 4b. Panel zone yield 0.8 2 2.5 2 2.51. m = 9 (1 – 1.7 P/P ye )2. m = 12 (1 – 1.7 P/P ye )3. m = 15 (1 – 1.7 P/P ye )4. m = 18 (1 – 1.7 P/P ye )5. m = 6 – 0.125 d b6. m = 7 – 0.125 d b7. If construction documents verify that notch-tough rated weldment was used, these values may be multiplied by two.8. For built-up numbers where strength is governed by the facing plates, use one-half these m values.9. If P/P ye > 0.5, assume column to be force-controlled.5-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)(5-12)In addition, if the strength of the panel zone is less than0.9 times the maximum shear force that can bedelivered by the beams, then the m for the beam shall beFigure 5-3m = 6.0 – 0.125 d bDoubler plateas requiredm = 2Stiffeners orcontinuity platesas required(5-13)Full-Pen Connection in FR Connectionwith Variable BehaviorFlange Plate and End Plate Connections. The strengthof these connections should be in accordance withstandard practice as given in AISC (1994a and 1994b).Additional information for these connections is givenbelow in Section 5.4.3.3.Column Base Plates to Concrete Pile Caps orFootings. The strength of connections between columnbase plates and concrete pile caps or footings usuallyexceeds the strength of the columns. The strength of thebase plate and its connection may be governed by thewelds or bolts, the dimensions of the plate, or theexpected yield strength, F ye , of the base plate. Theconnection between the base plate and the concrete maybe governed by shear or tension yield of the anchorbolts, loss of bond between the anchor bolts and theconcrete, or failure of the concrete. Expected strengthsfor each failure type shall be calculated by rationalanalysis or the provisions in AISC (1994b). The valuesfor m may be chosen from similar partially restrainedend plate actions given in Table 5-5.B. Nonlinear Static ProcedureThe NSP requires modeling of the complete loaddeformationrelationship to failure for each component.This may be based on experiment, or on a rationalanalysis, preferably verified by experiment. In lieu ofthese, the conservative approximate behavior depictedby Figure 5-1 may be used. The values for Q CE and θ yshown in Figure 5-1 are the same as those used in theLSP and given in Section 5.4.2.2. Deformation controlpoints and acceptance criteria for the Nonlinear Staticand Dynamic Procedures are given in Table 5-4.C. Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be modeled for this procedure. Guidelines for thisare given in the Commentary. Deformation limits aregiven in Table 5-4.5.4.2.4 Rehabilitation Measures for FRMoment FramesSeveral options are available for rehabilitation of FRmoment frames. In all cases, the compatibility of newand existing components and/or elements must bechecked at displacements consistent with thePerformance Level chosen. The rehabilitation measuresare as follows:• Add steel braces to one or more bays of each story toform concentric or eccentric braced frames.(Attributes and design criteria for braced frames aregiven in Section 5.5.) Braces significantly increasethe stiffness of steel frames. Care should be takenwhen designing the connections between the newbraces and the existing frame. The connectionshould be designed to carry the maximum probablebrace force, which may be approximated as 1.2times the expected strength of the brace.• Add ductile concrete or masonry shear walls or infillwalls to one or more bays of each story. Attributesand design requirements of concrete and masonryinfills are given in Sections 6.7 and 7.5, respectively.This greatly increases the stiffness and strength ofthe structure. Do not introduce torsional stress intothe system.FEMA 273 Seismic Rehabilitation Guidelines 5-15

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-4Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—Fully Restrained(FR) Moment Frames-----∆∆ yResidualStrengthRatioPlastic Rotation, Deformation LimitsPrimarySecondaryComponent/ActionBeams 1 :a.b-----2t f52< -----------F yed e c IO LS CP LS CP10 12 0.6 2 7 9 10 12b.b-----2t f95> -----------F ye5 7 0.2 1 3 4 4 552 b 95c. For ----------- ≤ ----- ≤ -----------F 2t ye f F yeuse linear interpolationColumns 2 :For P/P ye < 0.20a.b-----2t f52< -----------F ye10 12 0.6 2 7 9 10 12b.b-----2t f95> -----------F ye0.2 1 3 4 4 5c. For -----------52-----b 95≤ ≤ -----------F 2t ye f F yeuse linear interpolation1. Add θ y from Equations 5-1 or 5-2 to plastic end rotation to estimate chord rotation.2. Columns in moment or braced frames need only be designed for the maximum force that can be delivered.3. Deformation = 0.072 (1 – 1.7 P/P ye )4. Deformation = 0.100 (1 – 1.7 P/P ye )5. Deformation = 0.042 (1 – 1.7 P/P ye )6. Deformation = 0.060 (1 – 1.7 P/P ye )7. 0.043 – 0.0009 d b8. 0.035 – 0.0008 d b9. If P/P ye > 0.5, assume column to be force-controlled.5-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-4Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—Fully Restrained(FR) Moment Frames (continued)∆-----∆ yResidualStrengthRatioPlastic Rotation, Deformation LimitsPrimarySecondaryComponent/ActionFor 0.2 ≤ P/P ye ≤ 0.50 9b 52a. ----- < -----------2t f F yed e c IO LS CP LS CP— 3 — 4 0.2 0.04 — 5 — 6 0.019 0.031b.b-----2t f95> -----------F ye2 2.5 0.2 1 1.5 1.8 1.8 252 b 95c. For ----------- ≤ ----- ≤ -----------F 2t ye f F yeuse linear interpolationPlasticRotationa bPanel Zones 0.052 0.081 0.800 0.004 0.025 0.043 0.055 0.067ConnectionsFor full penetration flange weld, bolted orwelded web: beam deformation limitsa. No panel zone yield — 7 — 7 0.200 0.008 — 8 — 8 0.017 0.025b. Panel zone yield 0.009 0.017 0.400 0.003 0.005 0.007 0.010 0.0131. Add θ y from Equations 5-1 or 5-2 to plastic end rotation to estimate chord rotation.2. Columns in moment or braced frames need only be designed for the maximum force that can be delivered.3. Deformation = 0.072 (1 – 1.7 P/P ye )4. Deformation = 0.100 (1 – 1.7 P/P ye )5. Deformation = 0.042 (1 – 1.7 P/P ye )6. Deformation = 0.060 (1 – 1.7 P/P ye )7. 0.043 – 0.0009 d b8. 0.035 – 0.0008 d b9. If P/P ye > 0.5, assume column to be force-controlled.• Attach new steel frames to the exterior of thebuilding. This scheme has been used in the past andhas been shown to be very effective under certainconditions. Since this will change the distribution ofstiffness in the building, the seismic load path mustbe carefully checked. The connections between thenew and existing frames are particularly vulnerable.This approach may be structurally efficient, but itchanges the architectural appearance of the building.The advantage is that the rehabilitation may takeplace without disrupting the use of the building.• Reinforce the moment-resisting connections to forceplastic hinge locations in the beam material awayfrom the joint region. The idea behind this concept isthat the stresses in the welded connection will besignificantly reduced, thereby reducing thepossibility of brittle fractures. This may not beFEMA 273 Seismic Rehabilitation Guidelines 5-17

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)effective if weld material with very low toughnesswas used in the full-pen connection. Strainhardening at the new hinge location may producelarger stresses at the weld than expected. Also, manyfractures during past earthquakes are believed tohave occurred at stresses lower than yield. Variousmethods, such as horizontal cover plates, verticalstiffeners, or haunches, can be employed. Otherschemes that result in the removal of beam materialmay achieve the same purpose. Modification of allmoment-resisting connections could significantlyincrease (or decrease, in the case of materialremoval) the structure’s stiffness; therefore,recalculation of the seismic demands may berequired. Modification of selected joints should bedone in a rational manner that is justified byanalysis. Guidance on the design of thesemodifications is discussed in SAC (1995).• Adding damping devices may be a viablerehabilitation measure for FR frames. See Chapter 9of these Guidelines.5.4.3 Partially Restrained Moment Frames5.4.3.1 GeneralPartially restrained (PR) moment frames are thoseframes for which deformation of the beam-to-columnconnections contributes greater than 5% of the storydrift. A moment frame shall also be considered to be PRif the strength of the connections is less than thestrength of the weaker of the two members beingjoined. A PR connection usually has two or more failuremodes. The weakest failure mechanism shall beconsidered to govern the behavior of the joint. Thebeam and/or column need only resist the maximumforce (or moment) that can be delivered by theconnection. General design provisions for PR framesgiven in AISC (1994a) or BSSC (1995) shall applyunless superseded by these Guidelines. Equations forcalculating nominal design strength shall be used fordetermining the expected strength, except φ = 1, and F yeshall be used in place of F y .5.4.3.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresBeams, columns, and panel zones. Axial area, sheararea, moment of inertia, and panel zone stiffness shallbe determined as given in Section 5.4.2.2 for FRframes.Connections. The rotational stiffness K θ of each PRconnection shall be determined by experiment or byrational analysis based on experimental results. Thedeformation of the connection shall be included whencalculating frame displacements. Further discussion ofthis is given in the Commentary. In the absence of morerational analysis, the stiffness may be estimated by thefollowing approximate procedures:The rotational spring stiffness, K θ , may be estimated bywhereM CEfor:K θ =M------------ CE0.005= Expected moment strength, kip-in.(5-14)• PR connections that are encased in concrete for fireprotection, and where the nominal resistance, M CE ,determined for the connection includes thecomposite action provided by the concreteencasement• PR connections that are encased in masonry, wherecomposite action cannot be developed in theconnection resistance• Bare steel PR connectionsFor all other PR connections, the rotational springstiffness may be estimated byMK CEθ = ------------0.003The connection strength, M CE , is discussed inSection 5.4.3.3.(5-15)As a simplified alternative analysis method to an exactPR frame analysis, where connection stiffness ismodeled explicitly, the beam stiffness, EI b , may beadjusted by5-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)whereK θ = Equivalent rotational spring stiffness,kip-in./radM CE = Expected moment strength, kip-in.I b = Moment of inertia of the beam, in. 4h = Average story height of the columns, in.= Centerline span of the beam, in.l bEI b adjusted = --------------------------16h 1---------- + -------2l bKθEI b(5-16)This adjusted beam stiffness may be used in standardrigid-connection frame finite element analysis. Thejoint rotation of the column shall be used as the jointrotation of the beam at the joint with this simplifiedanalysis procedure.B. Nonlinear Static Procedure• Use elastic component properties as given inSection 5.4.3.2A.• Use appropriate nonlinear moment-curvature orload-deformation behavior for beams, beamcolumns,and panel zones as given in Section 5.4.2for FR frames.Use appropriate nonlinear moment-rotation behaviorfor PR connections as determined by experiment. Inlieu of experiment, or more rational analyticalprocedure based on experiment, the moment-rotationrelationship given in Figure 5-1 and Table 5-6 may beused. The parameters θ and θ y are rotation and yieldrotation. The value for θ y may be assumed to be 0.003or 0.005 in accordance with the provisions inSection 5.4.3.2A.Q and Q CE are the component moment and expectedyield moment, respectively. Approximate values ofM CE for common types of PR connections are given inSection 5.4.3.3B.C. Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be properly modeled based on experiment.5.4.3.3 Strength and DeformationAcceptance CriteriaA. Linear Static and Dynamic ProceduresThe strength and deformation acceptance criteria forthese methods require that the load and resistancerelationships given in Equations 3-18 and 3-19 inChapter 3 be satisfied. The expected strength and otherrestrictions for a beam or column shall be determined inaccordance with the provisions given above inSection 5.4.2.3 for FR frames.Evaluation of component acceptability requiresknowledge of the lower-bound component capacity,Q CL for Equation 3-19 and Q CE for Equation 3-18, andthe ductility factor, m, as given in Table 5-5 for use inEquation 3-18. Values for Q CE and Q CL for beams andcolumns in PR frames are the same as those given inSection 5.4.2.3 and Table 5-3 for FR frames. Values forQ CE for PR connections are given in this section.Control points and acceptance criteria for Figure 5-1 forPR frames are given in Table 5-6. Values for m aregiven in Table 5-5 for the Immediate Occupancy, LifeSafety, and Collapse Prevention Performance Levels.B. Nonlinear Static ProcedureThe NSP requires modeling of the complete loaddeformationrelationship to failure for each component.This may be based on experiment, or a rational analysis,preferably verified by experiment. In lieu of these, theconservative and approximate behavior depicted byFigure 5-1 may be used. The values for Q CE and θ y arethe same as those used in the LSP in Sections 5.4.2.2and 5.4.3.2. The deformation limits and nonlinearcontrol points, c, d, and e, shown in Figure 5-1 aregiven in Table 5-6.The expected strength, Q CE , for PR connections shallbe based on experiment or accepted methods of analysisas given in AISC (1994a and b) or in the Commentary.In lieu of these, approximate conservative expressionsfor Q CE for common types of PR connections are givenbelow.Riveted or Bolted Clip Angle Connection. This is abeam-to-column connection as defined in Figure 5-4.The expected moment strength of the connection, M CE ,may be conservatively determined by using the smallestvalue of M CE computed using Equations 5-17 through5-22.FEMA 273 Seismic Rehabilitation Guidelines 5-19

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-5Acceptance Criteria for Linear Procedures—Partially Restrained (PR) Moment Framesm Values for Linear MethodsPrimarySecondaryComponent/ActionPartially restrained moment connectionIO LS CP LS CPFor top and bottom clip angles 1a. Rivet or bolt shear failure 2 1.5 4 6 6 8b. Angle flexure failure 2 5 7 7 14c. Bolt tension failure 2 1 1.5 2.5 4 4For top and bottom T-stub 1a. Bolt shear failure 2 1.5 4 6 6 8b. T-stub flexure failure 2 5 7 7 14c. Bolt tension failure 2 1 1.5 2.5 4 4For composite top and clip angle bottom 1a. Yield and fracture of deck reinforcement 1 2 3 4 6b. Local yield and web crippling of column flange 1.5 4 6 5 7c. Yield of bottom flange angle 1.5 4 6 6 7d. Tensile yield of column connectors or OSL of angle 1 1.5 2.5 2.5 3.5e. Shear yield of beam flange connections 1 2.5 3.5 3.5 4.5For flange plates welded to column bolted or welded to beam 1a. Failure in net section of flange plate or shear failure of bolts or rivets 2 1.5 4 5 4 5b. Weld failure or tension failure on gross section of plate 0.5 1.5 2 1.5 2For end plate welded to beam bolted to columna. Yielding of end plate 2 5.5 7 7 7b. Yield of bolts 1.5 2 3 4 4c. Failure of weld 0.5 1.5 2 3 31. Assumed to have web plate or stiffened seat to carry shear. Without shear connection, this may not be downgraded to a secondary member. If d b >18 inches, multiply m values by 18/d b .2. For high-strength bolts, divide these values by two.If the shear connectors between the beam flange and theflange angle control the resistance of the connection:whereA b = Gross area of rivet or bolt, in. 2= Overall beam depth, in.d bQ CE = M CE = d b ( F ve A b N b )(5-17)F ve = Unfactored nominal shear strength of the boltsor rivets given in AISC (1994a), ksiN b = Least number of bolts or rivets connecting thetop or bottom flange to the angleIf the tensile capacity of the horizontal outstanding leg(OSL) of the connection controls the capacity, then P CEis the smaller ofP CE ≤ F ye A g(5-18)5-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-6Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—PartiallyRestrained (PR) Moment FramesPlasticRotation 1ResidualForceRatioPrimaryJoint RotationSecondarya b c IO LS CP LS CPTop and Bottom Clip Angles 1a. Rivet or bolt shear 2 0.036 0.048 0.200 0.008 0.020 0.030 0.030 0.040b. Angle flexure 0.042 0.084 0.200 0.010 0.025 0.035 0.035 0.070c. Bolt tension 0.016 0.025 1.000 0.005 0.008 0.013 0.020 0.020Top and Bottom T-Stub 1a. Rivet or bolt shear 2 0.036 0.048 0.200 0.008 0.020 0.030 0.030 0.040b. T-stub flexure 0.042 0.084 0.200 0.010 0.025 0.035 0.035 0.070c. Rivet or bolt tension 0.016 0.024 0.800 0.005 0.008 0.013 0.020 0.020Composite Top Angle Bottom 1a. Deck reinforcement 0.018 0.035 0.800 0.005 0.010 0.015 0.020 0.030b. Local yield column flange 0.036 0.042 0.400 0.008 0.020 0.030 0.025 0.035c. Bottom angle yield 0.036 0.042 0.200 0.008 0.020 0.030 0.025 0.035d. Connectors in tension 0.015 0.022 0.800 0.005 0.008 0.013 0.013 0.018e. Connections in shear 2 0.022 0.027 0.200 0.005 0.013 0.018 0.018 0.023Flange Plates Welded to Column Bolted or Welded to Beam 2a. Flange plate net section or 0.030 0.030 0.800 0.008 0.020 0.025 0.020 0.025shear in connectorsb. Weld or connector tension 0.012 0.018 0.800 0.003 0.008 0.010 0.010 0.015End Plate Bolted to Column Welded to Beama. End plate yield 0.042 0.042 0.800 0.010 0.028 0.035 0.035 0.035b. Yield of bolts 0.018 0.024 0.800 0.008 0.010 0.015 0.020 0.020c. Fracture of weld 0.012 0.018 0.800 0.003 0.008 0.010 0.015 0.0151. If d b > 18, multiply deformations by 18/d b . Assumed to have web plate to carry shear. Without shear connection, this may not be downgraded to asecondary member.2. For high-strength bolts, divide rotations by 2.P CE ≤ F te A e(5-19) andQ CE = M CE ≤ P CE ( d b + t a )(5-20)FEMA 273 Seismic Rehabilitation Guidelines 5-21

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)whereb ab awF ye= Dimension shown in Figure 5-4, in.= Length of the flange angle, in.= Expected yield strengthRiveted or Bolted T-Stub Connection. A riveted orbolted T-stub connection is a beam-to-columnconnection as depicted in Figure 5-5. The expectedmoment strength, M CE , may be determined by using thesmallest value of M CE computed using Equations 5-23through 5-25.Figure 5-4Clip Angle Connectionwhereb tA e = Effective net area of the OSL, in. 2A g = Gross area of the OSL, in. 2P = Force in the OSL, kips= Thickness of angle, in.t aIf the tensile capacity of the rivets or bolts attaching theOSL to the column flange control the capacity of theconnection:Q CE = M CE = ( d b + b a )( F te A c N b )where(5-21)Figure 5-5T-Stub Connection may be FR or PRConnectionA c = Rivet or bolt area, in. 2b aF teN b= Dimension in Figure 5-4, in.= Expected tensile strength of the bolts or rivets,ksi= Least number of bolts or rivets connecting topor bottom angle to column flangeFlexural yielding of the flange angles controls theexpected strength if:2wt aFyeQ CE = M CE = ------------------------ ( dt b + b a )4 b --- aa –2(5-22)If the shear connectors between the beam flange and theT-stub web control the resistance of the connection, useEquation 5-17.If the tension capacity of the bolts or rivets connectingthe T-stub flange to the column flange control theresistance of the connection:whereN bt sQ CE = M CE = ( d b + 2b t + t s )( F te A b N b )(5-23)= Number of fasteners in tension connecting theflanges of one T-stub to the column flange= Thickness of T-stub stem5-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)If tension in the stem of the T-stub controls theresistance, use Equations 5-18 and 5-19 with A g and A ebeing the gross and net areas of the T-stub stem.If flexural yielding of the flanges of the T-stub controlsthe resistance of the connection:2( dQ CE M b + t s )wt f Fye= CE = -------------------------------------2( b t – k 1 )(5-24)Stiffener asRequiredwherek 1b twt f= Distance from the center of the T-stub stem tothe edge of the T-stub flange fillet, in.= Distance between one row of fasteners in theT-stub flange and the centerline of the stem(Figure 5-5; different from b a in Figure 5-4)= Length of T-stub, in.= Thickness of T-stub flange, in.Flange Plate Connections. Flange plate connectionsare sometimes used as shown in Figure 5-6. Thisconnection may be considered to be fully restrained ifthe strength is sufficient to develop the strength of thebeam. The expected strength of the connection may becalculated aswhereP CEt pQ CE = M CE = P CE ( d b + t p )(5-25)= Expected strength of the flange plateconnection as governed by the net section ofthe flange plate or the shear capacity of thebolts or welds, kips= Thickness of flange plate, in.The strength of the welds must also be checked. Theflange plates may also be bolted to the beam; in thiscase, the strength of the bolts and the net section of theflange plates must also be checked.End Plate Connections. As shown in Figure 5-7, thesemay sometimes be considered to be FR if the strength isgreat enough to develop the expected strength of thebeam. The strength may be governed by the bolts thatare under combined shear and tension or bending in theFigure 5-6end plate. The design strength Q CE = M CE shall becomputed in accordance with AISC (1994b) or by anyother rational procedure supported by experimentalresults.Composite Partially Restrained Connections. Thesemay be used as shown in Figure 5-8. The equivalentrotational spring constant, K θ , shall be that given byEquation 5-14. The behavior of these connections iscomplex, with several possible failure mechanisms.Strength calculations are discussed in the Commentary.C. Nonlinear Dynamic ProcedureSee Section 5.4.2.3.Flange Plate Connection may be FR orPR Connection5.4.3.4 Rehabilitation Measures for PRMoment FramesThe rehabilitation measures for FR moment frames willoften work for PR moment frames as well (seeSection 5.4.2.4). PR moment frames are often tooflexible to provide adequate seismic performance.Adding concentric or eccentric bracing, or reinforcedconcrete or masonry infills, may be a cost-effectiverehabilitation measure.FEMA 273 Seismic Rehabilitation Guidelines 5-23

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)be rehabilitated by replacing rivets with high-strengthbolts, adding weldment to supplement rivets or bolts,welding stiffeners to connection pieces or combinationsof these measures.5.5 Steel Braced Frames5.5.1 GeneralThe seismic resistance of steel braced frames isprimarily derived from the axial force capacity of theircomponents. Steel braced frames act as vertical trusseswhere the columns are the chords and the beams andbraces are the web members. Braced frames may actalone or in conjunction with concrete or masonry walls,or steel moment frames, to form a dual system.Figure 5-7End Plate Connection may be FR or PRConnectionReinforcementor wire meshReinforcementor wire meshSteel braced frames may be divided into two types:concentric braced frames (CBF) and eccentric bracedframes (EBF). Columns, beams, braces, andconnections are the components of CBF and EBF. Alink beam is also a component of an EBF. Thecomponents are usually hot-rolled shapes. Thecomponents may be bare steel, steel with anonstructural coating for fire protection, steel withconcrete encasement for fire protection, or steel withmasonry encasement for fire protection.5.5.2 Concentric Braced Frames (CBF)5.5.2.1 GeneralConcentric braced frames are braced systems whoseworklines essentially intersect at points. Minoreccentricities, where the worklines intersect within thewidth of the bracing member are acceptable ifaccounted for in the design.5.5.2.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresBeams and Columns. Axial area, shear area, andmoment of inertia shall be calculated as given inSection 5.4.2.2.Figure 5-8Two Configurations of PR CompositeConnectionsConnections in PR moment frames are usually theweak, flexible, or both, components. Connections mayConnections. FR connections shall be modeled asgiven in Section 5.4.2.2. PR connections shall bemodeled as given in Section 5.4.3.2.Braces. Braces shall be modeled the same as columnsfor linear procedures.5-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)B. Nonlinear Static Procedure• Use elastic component properties as given inSection 5.4.2.2A.• Use appropriate nonlinear moment curvature orload-deformation behavior for beams, columns,braces, and connections to represent yielding andbuckling. Guidelines are given in Section 5.4.2.2 forbeams and columns and Section 5.4.3.2 for PRconnections.Braces. Use nonlinear load-deformation behavior forbraces as determined by experiment or analysissupported by experiment. In lieu of these, the loadversus axial deformation relationship given inFigure 5-1 and Table 5-8 may be used. The parameters∆ and ∆ y are axial deformation and axial deformation atbrace buckling. The reduction in strength of a braceafter buckling must be included in the model. Elastoplasticbrace behavior may be assumed for thecompression brace if the yield force is taken as theresidual strength after buckling, as indicated by theparameter c in Figure 5-1 and Table 5-8. Implications offorces higher than this lower-bound force must beconsidered.C. Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be properly based on experiment or generallyaccepted engineering practice. Guidelines for this aregiven in the Commentary.5.5.2.3 Strength and DeformationAcceptance CriteriaA. Linear Static and Dynamic ProceduresThe strength and deformation acceptance criteria forthese methods require that the load and resistancerelationships given in Equations 3-18 and 3-19 inChapter 3 be satisfied. The design strength and otherrestrictions for a beam and column shall be determinedin accordance with the provisions given inSection 5.4.2.3.Evaluation of component acceptability requiresknowledge of the component lower-bound capacity,Q CL , for Equation 3-19 and Q CE for Equation 3-18, andthe ductility factor, m, as given in Table 5-7 for use inEquation 3-10. Columns shall be considered to beforce-controlled members. Values for Q CE and Q CL forbeams and columns are the same as those given inSection 5.4.2.3 for FR frames. Q CE and Q CL for PRconnections are given in Section 5.4.3.3B. Braces aredeformation-controlled components where the expectedstrength for the brace in compression is computed in thesame manner as for columns given in Section 5.4.2.3.Table 5-7Acceptance Criteria for Linear Procedures—Braced Frames andSteel Shear Wallsm Values for Linear ProceduresPrimarySecondaryComponent/ActionIO LS CP LS CPConcentric Braced FramesColumns: 1a. Columns in compression 1 Force-controlled member, use Equation 3-15 or 3-16.b. Columns in tension 1 1 3 5 6 7Braces in Compression 2a. Double angles buckling in plane 0.8 6 8 7 9b. Double angles buckling out of plane 0.8 5 7 6 8c. W or I shape 0.8 6 8 6 8d. Double channel buckling in plane 0.8 6 8 7 9e. Double channel buckling out of plane 0.8 5 7 6 8f. Rectangular concrete-filled cold-formed tubes 0.8 5 7 5 7FEMA 273 Seismic Rehabilitation Guidelines 5-25

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-7Acceptance Criteria for Linear Procedures—Braced Frames andSteel Shear Walls (continued)m Values for Linear ProceduresPrimarySecondaryComponent/Actiong. Rectangular cold-formed tubesd1. --90≤ ---------t F y2.3.d--t190≥ ---------F y3. ---------90 d≤ --190≤ ---------F t y F yh. Circular hollow tubesd 15001. -- ≤ -----------t F yd2. --6000≥ -----------t F y1500-----------F yd≤ --t6000≤ -----------F yIO LS CP LS CP0.8 5 7 5 70.8 2 3 2 3Use linear interpolation0.8 5 7 5 70.8 2 3 2 3Use linear interpolationBraces in Tension 3 1 6 8 8 10Eccentric Braced Framesa. Beams Governed by linkb. Braces Force-controlled, use Equation 3-19c. Columns in compression Force-controlled, use Equation 3-19d. Columns in tension 1 3 5 6 7Link beam 4 2Ma. 5 -------------- CE< 1.6 d: 16, e: 18, c: 1.00eV CE2Mb. -------------- CE< 2.6eV CE2M CEc. 1.6 ≤ -------------- < 2.6eV CE1.5 9 13 13 15Same as for beam in FR moment frame; see Table 5-3Use linear interpolation5-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-7Acceptance Criteria for Linear Procedures—Braced Frames andSteel Shear Walls (continued)m Values for Linear ProceduresPrimarySecondaryComponent/ActionIO LS CP LS CPSteel Shear Walls 6 1.5 8 12 12 141. Columns in moment or braced frames need only be designed for the maximum force that can be delivered.2. Connections in braced frames should be able to carry 1.25 times the brace strength in compression, or the expected strength of the member in tension.Otherwise maximum value of m = 2.3. For tension-only bracing systems, divide these m values by 2.4. Assumes ductile detailing for flexural links.5. Link beams with three or more web stiffeners. If no stiffeners, use half of these values. For one or two stiffeners, interpolate.6. Applicable if stiffeners are provided to prevent shear buckling.For common cross bracing configurations where bothbraces are attached to a common gusset plate wherethey cross at their midpoints, the effective length ofeach brace may be taken as 0.5 times the total length ofthe brace including gusset plates for both axes ofbuckling. For other bracing configurations (chevron, V,single brace), if the braces are back-to-back shapesattached to common gusset plates, the length shall betaken as the total length of the brace including gussetplates, and K, the effective length factor, (AISC, 1994a)may be assumed to be 0.8 for in-plane buckling and 1.0for out-of-plane buckling.Restrictions on bracing members, gusset plates, braceconfiguration, and lateral bracing of link beams aregiven in the seismic provisions of AISC (1994a). If thespecial requirements of Section 22.11.9.2 of AISI(1986) are met, then 1.0 may be added to the brace mvalues given in Table 5-7.The strength of brace connections shall be the larger ofthe maximum force deliverable by the tension brace or1.25 times the maximum force deliverable by thecompression brace. If not, the connection shall bestrengthened, or the m values and deformation limitsshall be reduced to comparable values given forconnectors with similar limit states (see Table 5-5).Stitch plates for built-up members shall be spaced suchthat the largest slenderness ratio of the components ofthe brace is at most 0.4 times the governing slendernessratio of the brace as a whole. The stitches forcompressed members must be able to resist 0.5 timesthe maximum brace force where buckling of the bracewill cause shear forces in the stitches. If not, stitchplates shall be added, or the m values in Table 5-7 anddeformation limits in Table 5-8 shall be reduced by50%. Values of m need not be less than 1.0.B. Nonlinear Static ProcedureThe NSP requires modeling of the complete nonlinearforce-deformation relationship to failure for eachcomponent. This may be based on experiment, oranalysis verified by experiment. Guidelines are given inthe Commentary. In lieu of these, the conservativeapproximate behavior depicted in Figure 5-1 may beused. The values for Q CE and θ y are the same as thoseused for the LSP. Deformation parameters c, d, and e forFigure 5-1 and deformation limits are given inTable 5-8. The force-deformation relationship for thecompression brace should be modeled as accurately aspossible (see the Commentary). In lieu of this, the bracemay be assumed to be elasto-plastic, with the yieldforce equal to the residual force that corresponds to theparameter c in Figure 5-1 and Table 5-8. Thisassumption is an estimate of the lower-bound braceforce. Implications of forces higher than this must beconsidered.C. Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be modeled for this procedure. Guidelines for thisare given in the Commentary.FEMA 273 Seismic Rehabilitation Guidelines 5-27

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-8Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—Braced Framesand Steel Shear Walls∆-----∆ yResidualForceRatioPrimaryDeformationSecondaryComponent/Actiond e c IO LS CP LS CPConcentric Braced Framesa. Columns in compression 1 Force-controlled, use Equation 3-19b. Columns in tension 1 6 8 1.000 1 4 6 7 8Braces in Compression 2,3a. Two angles buckle in plane 1 10 0.2 0.8 6 8 8 9b. Two angles buckle out of plane 1 9 0.2 0.8 5 7 7 8c. W or I shape 1 9 0.2 0.8 6 8 8 9d. Two channels buckle in plane 1 10 0.2 0.8 6 8 8 9e. Two channels buckle out of plane 1 9 0.2 0.8 5 7 7 8f. Concrete-filled tubes 1 8 0.2 0.8 5 7 7 8g. Rectangular cold-formed tubesd 901 8 0.4 0.8 5 7 7 81. -- ≤ ---------t F y2.3.d--t190≥ ---------F y90 d 1903. --------- ≤ -- ≤ ---------F t y F yh. Circular hollow tubesd 15001. -- ≤ -----------t F yd 60002. -- ≥ -----------t F y1500-----------F yd≤ --t6000≤ -----------F y1 4 0.2 0.8 2 3 3 4Use linear interpolation1 10 0.4 0.8 5 7 6 91 4 0.2 0.8 2 3 3 4Use linear interpolationBraces in Tension 12 15 0.800 1 8 10 12 14Eccentric Braced Framesa. Beams Governed by linkb. Braces Force-controlled, use Equation 3-19c. Columns in compression Force-controlled, use Equation 3-19d. Columns in tension 6 8 1.000 1 4 6 7 85-28 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Table 5-8Modeling Parameters and Acceptance Criteria for Nonlinear Procedures—Braced Framesand Steel Shear Walls (continued)∆-----∆ yResidualForceRatioPrimaryDeformationSecondaryComponent/Actiond e c IO LS CP LS CPLink Beam 32M 16 18 0.80 1.5 12 15 15 17CEa. 4 -------------- ≤ 1.6eV CEb.c.2M-------------- CE≥ 2.6eV CE2M CE1.6 ≤ -------------- ≤ 2.6eV CESame as for beam in FR moment frame (see Table 5-4)Use linear interpolationSteel Shear Walls 5 15 17 .07 1.5 11 14 14 161. Columns in moment or braced frames need only be designed for the maximum force that can be delivered.2. ∆ c is the axial deformation at expected buckling load.3. Deformation is rotation angle between link and beam outside link or column. Assume ∆ y is 0.01 radians for short links.4. Link beams with three or more web stiffeners. If no stiffeners, use half of these values. For one or two stiffeners, interpolate.5. Applicable if stiffeners are provided to prevent shear buckling.5.5.2.4 Rehabilitation Measures forConcentric Braced FramesProvisions for moment frames may be applicable tobraced frames. Braces that are insufficient in strengthand/or ductility may be replaced or modified.Insufficient connections may also be modified.Columns may be encased in concrete to improve theirperformance. For further guidance, see Section 5.4.2.4and the Commentary.5.5.3 Eccentric Braced Frames (EBF)5.5.3.1 GeneralFor an EBF, the action lines of the braces do notintersect at the action line of the beam. The distancebetween the brace action lines where they intersect thebeam action line is the eccentricity, e. The beamsegment between these points is the link beam. Thestrength of the frame is governed by the strength of thelink beam.5.5.3.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresThe elastic stiffness of beams, columns, braces, andconnections are the same as those used for FR and PRmoment frames and CBF. The load-deformation modelfor a link beam must include shear deformation andflexural deformation.The elastic stiffness of the link beam, K e , iswhereK eK s K= ------------------ bK s + K bGAK ws = -----------e(5-26)(5-27)FEMA 273 Seismic Rehabilitation Guidelines 5-29

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)andwhereA w = (d b – 2t f ) t w , in. 2e = Length of link beam, in.G = Shear modulus, k/in. 2K eK bK sThe strength of the link beam may be governed byshear, flexure, or the combination of these.1.6M CEIf e ≤ ------------------V CEwhere= Stiffness of the link beam, k/in.= Flexural stiffness, kip/in.= Shear stiffness, kip/in.M CE = Expected moment, kip/in.2.6M CEIf e > ------------------V CEK b=12EI-------------- be 3Q CE = V CE = 0.6F ye t w A wQ CE = V CE = 2----------M CEe1.6M CE 2.6MIf ------------------ CE≤ e ≤ ------------------ , use linear interpolationV CE V CEbetween Equations 5-29 and 5-30.(5-28)(5-29)(5-30)The yield deformation is the link rotation as given byQθ CEy = ---------K e e(5-31)B. Nonlinear Static ProcedureThe NSP requires modeling of the complete nonlinearload-deformation relation to failure for eachcomponent. This may be based on experiment, orrational analysis verified by experiment. In lieu ofthese, the load versus deformation relationship given inFigure 5-1 and Table 5-8 may be used. Q CE and θ y arecalculated in accordance with provisions given in AISC(1994a) or by rational analysis.The nonlinear models used for beams, columns, andconnections for FR and PR moment frames, and for thebraces for a CBF, may be used.C. Nonlinear Dynamic ProcedureThe strength and deformation criteria require that theload and resistance relationships given inEquations 3-18 and 3-19 in Chapter 3 be satisfied. Thecomplete hysteretic behavior of each component mustbe modeled for this procedure. Guidelines for this aregiven in the Commentary.5.5.3.3 Strength and DeformationAcceptance CriteriaA. Linear Static and Dynamic ProceduresThe modeling assumptions given for a CBF are the sameas those for an EBF. Values for Q CE and θ y are given inSection 5.5.3.2 and m values are given in Table 5-7. Thestrength and deformation capacities of the link beam maybe governed by shear strength, flexural strength, or theirinteraction. The values of Q CE and θ y are the same asthose used in the LSP as given in Section 5.5.3.2A. Linksand beams are deformation-controlled components andmust satisfy Equation 3-18. Columns and braces are to beconsidered force-controlled members and must satisfyEquation 3-19.The requirements for link stiffeners, link-to-columnconnections, lateral supports of the link, the diagonalbrace and beam outside the link, and beam-to-columnconnections given in AISC (1994a) must be met. Thebrace should be able to carry 1.25 times the link strengthto ensure link yielding without brace or column buckling.If this is not satisfied for existing buildings, the designprofessional shall make extra efforts to verify that theexpected link strength will be reached before brace orcolumn buckling. This may require additional inspectionand material testing. Where the link beam is attached tothe column flange with full-pen welds, the provisions forthese connections is the same as for FR frame full-pen5-30 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)connections. The columns of an EBF are force-controlledmembers. The maximum force deliverable to a columnshould be calculated from the maximum brace forcesequal to 1.25 times the calculated strength of the brace.B. Nonlinear Static ProcedureThe NSP requirements for an EBF are the same as thosefor a CBF. Modeling of the nonlinear load deformationof the link beam should be based on experiment, orrational analysis verified by experiment. In lieu ofthese, the conservative approximate behavior depictedin Figure 5-1 may be used. Values for Q CE and θ y arethe same as those used for the LSP. Deformation limitsare given in Table 5-8.C. Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be properly modeled. This behavior must beverified by experiment. This procedure is notrecommended in most cases.5.5.3.4 Rehabilitation Measures for EccentricBraced FramesMany of the beams, columns, and braces may berehabilitated using procedures given for moment framesand CBFs. Cover plates and/or stiffeners may be usedfor these components. The strength of the link beammay be increased by adding cover plates to the beamflange(s), adding doubler plates or stiffeners to the web,or changing the brace configuration.5.6 Steel Plate Walls5.6.1 GeneralA steel plate wall develops its seismic resistancethrough shear stress in the plate wall. In essence, it is asteel shear wall. A solid steel plate, with or preferablywithout perforations, fills an entire bay betweencolumns and beams. The steel plate is welded to thecolumns on each side and to the beams above andbelow. Although these are not common, they have beenused to rehabilitate a few essential structures whereimmediate occupancy and operation of a facility ismandatory after a large earthquake. These walls work inconjunction with other existing elements to resistseismic load. However, due to their stiffness, theyattract much of the seismic shear. It is essential that thenew load paths be carefully established.5.6.2 Stiffness for Analysis5.6.2.1 Linear Static and DynamicProceduresThe most appropriate way to analyze a steel plate wallis to use a plane stress finite element model with thebeams and columns as boundary elements. The globalstiffness of the wall can be calculated. The modelingcan be similar to that used for a reinforced concreteshear wall. A simple approximate stiffness K w for thewall iswhereGa tK ww = --------------hG = Shear modulus of steel, ksia = Clear width of wall between columns, in.h = Clear height of wall between beams, in.t w = Thickness of plate wall, in.Other approximations of the wall stiffness based onprinciples of mechanics are acceptable.5.6.2.2 Nonlinear Static Procedure(5-32)The elastic part of the load-deformation relationship forthe wall is given in Section 5.6.2.1. The yield load,Q CE , is given in the next section. The completenonlinear load-deformation relationship should bebased on experiment or rational analysis. In lieu of this,the approximate simplified behavior may be modeledusing Figure 5-1 and Table 5-8.5.6.2.3 Nonlinear Dynamic ProcedureThe complete hysteretic behavior of each componentmust be properly modeled. This behavior must beverified by experiment. This procedure is notrecommended in most cases.5.6.3 Strength and Deformation AcceptanceCriteria5.6.3.1 Linear Static and DynamicProceduresThe strength and deformation acceptance criteria forthese methods require that the load and resistancerelationships given in Equations 3-18 and 3-19 inChapter 3 be satisfied. The design strength of the steelFEMA 273 Seismic Rehabilitation Guidelines 5-31

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)wall shall be determined using the appropriateequations in Part 6 of AISC (1994a). The wall can beassumed to be like the web of a plate girder. Designrestrictions for plate girder webs given in AISC(1994a), particularly those related to stiffener spacing,must be followed. Stiffeners should be spaced such thatbuckling of the wall does not occur. In this caseQ CE = V CE = 0.6F ye at wIn lieu of stiffeners, the steel wall may be encased inconcrete. If buckling is not prevented, equations forV CE given in AISC (1994a) for plate girders may beused. The m values for steel walls are given inTable 5-7. A steel shear wall is adeformation-controlled component.5.6.3.2 Nonlinear Static Procedure(5-33)The NSP requires modeling of the complete loaddeformationbehavior to failure. This may be based onexperiment or rational analysis. In lieu of these, theconservative approximate behavior depicted inFigure 5-1 may be used, along with parameters given inTable 5-8. The equation for Q CE is Equation 5-33. Theyield deformation isQθ CEy = ---------K w5.6.3.3 Nonlinear Dynamic Procedure(5-34)The complete hysteretic behavior of each componentmust be properly modeled. This behavior must beverified by experiment. This procedure is notrecommended in most cases.5.6.4 Rehabilitation MeasuresThis is not an issue because steel walls in existingconstruction are rare.5.7 Steel Frames with InfillsIt is common for older existing steel frame buildings tohave complete or partial infill walls of reinforcedconcrete or masonry. Due to the high wall stiffnessrelative to the frame stiffness, the infill walls will attractmost of the seismic shear. In many cases, because thesewalls are unreinforced or lightly reinforced, theirstrength and ductility may be inadequate.The engineering properties and acceptance criteria forthe infill walls are presented in Chapter 6 for concreteand Chapter 7 for masonry. The walls may beconsidered to carry all of the seismic shear in theseelements until complete failure of the walls hasoccurred. After that, the steel frames will resist theseismic forces. Before the loss of the wall, the steelframe adds confining pressure to the wall and enhancesits resistance. However, the actual effective forces onthe steel frame components are probably minimal. Asthe frame components begin to develop force they willdeform; however, the concrete or masonry on the otherside is stiffer so it picks up the load.The analysis of the component should be done in stagesand carried through each performance goal. At the pointwhere the infill has been deemed to fail—as given inChapter 6 or Chapter 7—the wall should be removedfrom the analytical model and the analysis resumedwith only the bare steel frame in place. At this point, theengineering properties and acceptance criteria for themoment frame given above in Section 5.4 areapplicable.5.8 Diaphragms5.8.1 Bare Metal Deck Diaphragms5.8.1.1 GeneralBare metal deck diaphragms are usually used for roofsof buildings where there are very light gravity loadsother than support of roofing materials. The metal deckunits are often composed of gage thickness steel sheets,from 22 gage down to 14 gage, two to three feet wide,and formed in a repeating pattern with ridges andvalleys. Rib depths vary from 1-1/2 to 3 inches in mostcases. Decking units are attached to each other and tothe structural steel supports by welds or, in some morerecent applications, by mechanical fasteners. In largeroof structures, these roofs may have supplementarydiagonal bracing. (See the description of horizontalsteel bracing in Section 5.8.4.)Chord and collector elements in these diaphragms areconsidered to be composed of the steel frame elementsattached to the diaphragm. Load transfer to frameelements that act as chords or collectors in modernframes is through shear connectors, puddle welds,screws, or shot pins.5-32 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)5.8.1.2 Stiffness for AnalysisA. Linear Static ProcedureThe distribution of forces for existing diaphragms isbased on flexible diaphragm assumption, withdiaphragms acting as simply supported between the stiffvertical lateral-force-resisting elements. Flexibilityfactors for various types of metal decks are availablefrom manufacturers’ catalogs. In systems for whichvalues are not available, values can be established byinterpolating between the most representative systemsfor which values are available. Flexibility can also becalculated using the Steel Deck Institute DiaphragmDesign Manual (Section 3). The analysis should verifythat the diaphragm strength is not exceeded for theelastic assumption to hold.All criteria for existing diaphragms mentioned aboveapply to stiffened or strengthened diaphragms.Interaction of new and existing elements ofstrengthened diaphragms must be considered to ensurestiffness compatibility. Load transfer mechanismsbetween new and existing diaphragm elements must beconsidered.Analyses should verify that diaphragm strength is notexceeded, so that elastic assumptions are still valid.B. Nonlinear Static ProcedureInelastic properties of diaphragms are usually notincluded in inelastic seismic analyses.More flexible diaphragms, such as bare metal deck ordeck-formed slabs with long spans between lateralforce-resistingelements, could be subject to inelasticaction. Procedures for developing models for inelasticresponse of wood diaphragms in unreinforced masonry(URM) buildings could be used as the basis for aninelastic model of a bare metal deck diaphragmcondition. A strain-hardening modulus of 3% could beused in the post-elastic region. If the weak link of thediaphragm is connection failure, then the elementnonlinearity cannot be incorporated into the model.5.8.1.3 Strength and DeformationAcceptance CriteriaMember capacities of steel deck diaphragms are givenin International Conference of Building Officials(ICBO) reports, in manufacturers’ literature, or in thepublications of the Steel Deck Institute (SDI). (See thereferences in Section 5.12 and CommentarySection C5.12.) Where allowable stresses are given,these may be multiplied by 2.0 in lieu of informationprovided by the manufacturer or other knowledgeablesources. If bare deck capacity is controlled byconnections to frame members or panel buckling, theninelastic action and ductility are limited. Therefore, thedeck should be considered to be a force-controlledmember.In many cases, diaphragm failure would not be a lifesafety consideration unless it led to a loss of bearingsupport or anchorage. Goals for higher performancewould limit the amount of damage to the connections toinsure that the load transfer mechanism was still intact.Deformations should be limited to below the thresholdof deflections that cause damage to other elements(either structural or nonstructural) at specifiedPerformance Levels.The m value for shear yielding, or panel or platebuckling is 1, 2, or 3 for the IO, LS, or CP PerformanceLevels, respectively. Weld and connector failure isforce-controlled.The SDI calculations procedure should be used forstrengths, or ICBO values with a multiplier may be usedto bring allowable values to expected strength levels.Specific references are given in Section 5.12 and in theCommentary, Section C5.12.Connections between metal decks and steel framingcommonly use puddle welds. Connection capacity mustbe checked for the ability to transfer the total diaphragmreaction into the steel framing. Connection capacitiesare provided in ICBO reports, manufacturers’ data, theSDI Manual, or the Welding Code for Sheet Steel, AWSD1.3. Other attachment systems, such as clips, aresometimes used.5.8.1.4 Rehabilitation MeasuresSee the Commentary.5.8.2 Metal Deck Diaphragms withStructural Concrete Topping5.8.2.1 GeneralMetal deck diaphragms with structural concrete toppingare frequently used on floors and roofs of buildingswhere there are typical floor gravity loads. The metaldeck may be either a composite deck, which hasindentations, or a noncomposite form deck. In bothtypes of deck, the slab and deck act together to resistFEMA 273 Seismic Rehabilitation Guidelines 5-33

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)diaphragm loads. The concrete fill may be either normalor lightweight concrete, with reinforcing composed ofwire mesh or small-diameter reinforcing steel.Additional slab reinforcing may be added at areas ofhigh stress. The metal deck units are composed of gagethickness steel sheets, two to three feet wide, and areformed in a repeating pattern with ridges and valleys.Decking units are attached to each other and tostructural steel supports by welds or, in some morerecent applications, by mechanical fasteners. Concretediaphragms in which the slab was formed and thebeams are encased in concrete for fire protection maybe considered to be similar to topped metal deckdiaphragms.Concrete has structural properties that significantly addto diaphragm stiffness and strength. Concretereinforcing ranges from light mesh reinforcement to aregular grid of small reinforcing bars (#3 or #4). Metaldecking is typically composed of corrugated sheet steelfrom 22 ga. down to 14 ga. Rib depths vary from 1-1/2to 3 inches in most cases. Attachment of the metal deckto the steel frame is usually accomplished using puddlewelds at one to two feet on center. For compositebehavior, shear studs are welded to the frame before theconcrete is cast.Chord and collector elements in these diaphragms areconsidered to be composed of the steel frame elementsattached to the diaphragm. Load transfer to frameelements that act as chords or collectors in modernframes is usually through puddle welds or headed studs.In older construction where the frame is encased for fireprotection, load transfer is made through bond.5.8.2.2 Stiffness for AnalysisA. Linear Static ProcedureFor existing diaphragms, the distribution of forces maybe based on a rigid diaphragm assumption if thediaphragm span-to-depth ratio is not greater than five toone. For greater ratios, justify with analysis. Diaphragmflexibility should be included in cases with larger spansand/or plan irregularities by three-dimensional analysisprocedures and shell finite elements for the diaphragms.Diaphragm stiffness can be calculated using the SDIDesign Manual, manufacturers’ catalogs, or with arepresentative concrete thickness.All procedures for existing diaphragms noted aboveapply to strengthened diaphragms as well. Interaction ofnew and existing elements of strengthened diaphragms(stiffness compatibility) must be considered. Loadtransfer mechanisms between new and existingdiaphragm components may need to be considered indetermining the flexibility of the diaphragm.All procedures for existing diaphragms noted aboveapply to new diaphragms. Interaction of newdiaphragms with the existing frames must beconsidered. Load transfer mechanisms between newdiaphragm components and existing frames may needto be considered in determining the flexibility of thediaphragm.For all diaphragms, the analyses must verify that thediaphragm strength is not exceeded, so that elasticassumptions are still valid.B. Nonlinear Static ProcedureInelastic properties of diaphragms are usually notincluded in inelastic seismic analyses, but could be ifthe connections are adequate. More flexiblediaphragms—such as bare metal deck or deck-formedslabs with long spans between lateral-force-resistingelements—could be subject to inelastic action.Procedures for developing models for inelastic responseof wood diaphragms in URM buildings could be used asthe basis for an inelastic model of a bare metal deck orlong span composite diaphragm condition. If the weaklink of the diaphragm is connection failure, the elementnonlinearity cannot be incorporated into the model.5.8.2.3 Strength and DeformationAcceptance CriteriaMember capacities of steel deck diaphragms withstructural concrete are given in manufacturers’ catalogs,ICBO reports, or the SDI Manual. If composite deckcapacity is controlled by shear connectors, inelasticaction and ductility are limited. It would be expectedthat there would be little or no inelastic action in steeldeck/concrete diaphragms, except in long spanconditions; however, perimeter transfer mechanismsand collector forces must be considered to be sure thatthis is the case.In many cases, diaphragm failure would not be a lifesafety consideration unless it led to a loss of bearingsupport or anchorage. Goals for higher performancewould limit the amount of damage to the connections orcracking in concrete-filled slabs in order to ensure thatthe load transfer mechanism was still intact.Deformations should be limited below the threshold of5-34 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)deflections that cause damage to other elements (eitherstructural or nonstructural) at specified PerformanceLevels.Connection failure is force-limited, so Equation 3-19must be used. Shear failure of the deck requirescracking of the concrete and/or tearing of the metaldeck, so the m values for IO, LS, and CP PerformanceLevels are 1, 2, and 3, respectively. See Section 5.8.6.3for acceptance criteria for collectors.SDI calculation procedures should be used forstrengths, or ICBO values with a multiplier of 2.0should be used to bring allowable values to a strengthlevel. The deck will be considered elastic in mostanalyses.Connector capacity must be checked for the ability totransfer the total diaphragm reaction into the supportingsteel framing. This load transfer can be achieved bypuddle welds and/or headed studs. For the connectionof the metal deck to steel framing, puddle welds tobeams are most common. Connector capacities areprovided in ICBO reports, manufacturers’ data, the SDIManual, or the Welding Code for Sheet Steel, AWSD1.3. Shear studs replace puddle welds to beams wherethey are required for composite action with supportingsteel beams.Headed studs are most commonly used for connectionof the concrete slab to steel framing. Connectorcapacities can be found using the AISC Manual of SteelConstruction, UBC, or manufacturers’ catalogs. Whensteel beams are designed to act compositely with theslab, shear connectors must have the capacity to transferboth diaphragm shears and composite beam shears. Inolder structures where the beams are encased inconcrete, load transfer may be provided through bondbetween the steel and concrete.5.8.2.4 Rehabilitation MeasuresSee the Commentary.5.8.3 Metal Deck Diaphragms withNonstructural Concrete Topping5.8.3.1 GeneralMetal deck diaphragms with nonstructural concrete fillare typically used on roofs of buildings where there arevery small gravity loads. The concrete fill, such as verylightweight insulating concrete (e.g., vermiculite), doesnot have usable structural properties. If the concrete isreinforced, reinforcing consists of wire mesh or smalldiameterreinforcing steel. Typically, the metal deck is aform deck or roof decks, so the only attachmentbetween the concrete and metal deck is through bondand friction. The concrete fill is not designed to actcompositely with the metal deck and has no positivestructural attachment. The metal deck units are typicallycomposed of gage thickness steel sheets, two to threefeet wide, and formed in a repeating pattern with ridgesand valleys. Decking units are attached to each otherand structural steel supports by welds or, in some morerecent applications, by mechanical fasteners.Consideration of any composite action must be donewith caution, after extensive investigation of fieldconditions. Material properties, force transfermechanisms, and other similar factors must be verifiedin order to include such composite action. Typically, thedecks are composed of corrugated sheet steel from 22gage down to 14 gage, and the rib depths vary from 9/16 to 3 inches in most cases. Attachment to the steelframe is usually through puddle welds, typically spacedat one to two feet on center. Chord and collectorelements in these diaphragms are composed of the steelframe elements attached to the diaphragm.5.8.3.2 Stiffness for AnalysisA. Linear Static ProcedureThe potential for composite action and modification ofload distribution must be considered. Flexibility of thediaphragm will depend on the strength and thickness ofthe topping. It may be necessary to bound the solutionin some cases, using both rigid and flexible diaphragmassumptions. Interaction of new and existing elementsof strengthened diaphragms (stiffness compatibility)must be considered, and the load transfer mechanismsbetween the new and existing diaphragm elements mayneed to be considered in determining the flexibility ofthe diaphragm. Similarly, the interaction of newdiaphragms with existing frames must be carefullyconsidered, as well as the load transfer mechanismsbetween them. Finally, the analyses must verify thatdiaphragm strength is not exceeded, so elasticassumptions are still valid.B. Nonlinear Static ProcedureInelastic properties of diaphragms are usually notincluded in inelastic seismic analyses. Whennonstructural topping is present its capacity must beverified. More flexible diaphragms, such as bare metalFEMA 273 Seismic Rehabilitation Guidelines 5-35

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)deck or decks with inadequate nonstructural topping,could be subject to inelastic action. Procedures fordeveloping models for inelastic response of wooddiaphragms in URM buildings could be used as thebasis for an inelastic model of a bare metal deckdiaphragm condition. If a weak link of the diaphragm isconnection failure, then the element nonlinearity cannotbe incorporated into the model.5.8.3.3 Strength and DeformationAcceptance CriteriaA. Linear Static ProcedureCapacities of steel deck diaphragms with nonstructuraltopping are provided by ICBO reports, bymanufacturers, or in general by the SDI Manual. Whenthe connection failure governs, or topping lacksadequate strength, inelastic action and ductility arelimited. As a limiting case, the diaphragm shear may becomputed using only the bare deck (see Section 5.8.1for bare decks). Generally, there should be little or noinelastic action in the diaphragms, provided theconnections to the framing members are adequate.In many cases, diaphragm failure would not be a lifesafety consideration unless it led to a loss of bearingsupport or anchorage. Goals for higher PerformanceLevels would limit the amount of damage to theconnections or cracking in concrete filled slabs, toensure that the load transfer mechanism was still intact.Deformations should be limited below the threshold ofdeflections that cause damage to other elements (eitherstructural or nonstructural) at specified performancelevels.Connection failure is force-limited, so Equation 3-19must be used. Shear failure of the deck requiresconcrete cracking and/or tearing of the metal deck, so mvalues for IO, LS, and CP are 1, 2, and 3, respectively.Panel buckling or plate buckling have m values of 1, 2,and 3 for IO, LS, and CP. SDI calculation proceduresshould be used for strengths, or ICBO values with amultiplier to bring allowable values to strength levels.5.8.3.4 Rehabilitation MeasuresSee the Commentary.5.8.4 Horizontal Steel Bracing (Steel TrussDiaphragms)5.8.4.1 GeneralHorizontal steel bracing (steel truss diaphragms) maybe used in conjunction with bare metal deck roofs andin conditions where diaphragm stiffness and/or strengthis inadequate to transfer shear forces. Steel trussdiaphragm elements are typically found in conjunctionwith vertical framing systems that are of structural steelframing. Steel trusses are more common in long spansituations, such as special roof structures for arenas,exposition halls, auditoriums, and industrial buildings.Diaphragms with a large span-to-depth ratio may oftenbe stiffened by the addition of steel trusses. Theaddition of steel trusses for diaphragms identified to bedeficient may provide a proper method of enhancement.Horizontal steel bracing (steel truss diaphragms) maybe made up of any of the various structural shapes.Often, the truss chord elements consist of wide flangeshapes that also function as floor beams to support thegravity loads of the floor. For lightly loaded conditions,such as industrial metal deck roofs without concrete fill,the diagonal members may consist of threaded rodelements, which are assumed to act only in tension. Forsteel truss diaphragms with large loads, diagonalelements may consist of wide flange members, tubes, orother structural elements that will act in both tensionand compression. Truss element connections aregenerally concentric, to provide the maximum lateralstiffness and ensure that the truss members act underpure axial load. These connections are generally similarto those of gravity-load-resisting trusses. Whereconcrete fill is provided over the metal decking,consideration of relative rigidities between the truss andconcrete systems may be necessary.5.8.4.2 Stiffness for AnalysisA. Linear Static ProcedureExisting truss diaphragm systems are modeled ashorizontal truss elements (similar to braced steelframes) where axial stiffness controls the deflections.Joints are often taken as pinned. Where joints providethe ability for moment resistance or where eccentricitiesare introduced at the connections, joint rigidities shouldbe considered. A combination of stiffness with that ofconcrete fill over metal decking may be necessary insome instances. Flexibility of truss diaphragms shouldbe considered in distribution of lateral loads to verticalelements.5-36 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)The procedures for existing diaphragms provided aboveapply to strengthened truss diaphragms. Interaction ofnew and existing elements of strengthened diaphragmsystems (stiffness compatibility) must be considered incases where steel trusses are added as part of a seismicupgrade. Load transfer mechanisms between new andexisting diaphragm elements must be considered indetermining the flexibility of the strengtheneddiaphragm.The procedures for existing truss diaphragmsmentioned above also apply to new diaphragms.Interaction of new truss diaphragms with existingframes must be considered. Load transfer mechanismsbetween new diaphragm elements and existing framesmay need to be considered in determining the flexibilityof the diaphragm/frame system.For modeling assumptions and limitations, see thepreceding comments related to truss joint modeling,force transfer, and interaction between diaphragmelements. Analyses are also needed to verify that elasticdiaphragm response assumptions are still valid.Acceptance criteria for the components of a trussdiaphragm are the same as for a CBF.B. Nonlinear Static ProcedureInelastic properties of truss diaphragms are usually notincluded in inelastic seismic analyses. In the case oftruss diaphragms, inelastic models similar to those ofbraced steel frames may be appropriate. Inelasticdeformation limits of truss diaphragms may be differentfrom those prescribed for braced steel frames (e.g.,more consistent with that of a concrete-toppeddiaphragm).5.8.4.3 Strength and DeformationAcceptance CriteriaMember capacities of truss diaphragm members may becalculated in a manner similar to those for braced steelframe members. It may be necessary to include gravityforce effects in the calculations for some members ofthese trusses. Lateral support conditions provided bymetal deck, with or without concrete fill, must beproperly considered. Force transfer mechanismsbetween various members of the truss at theconnections, and between trusses and frame elements,must be considered to verify the completion of the loadpath.In many cases, diaphragm distress would not be a lifesafety consideration unless it led to a loss of bearingsupport or anchorage. Goals for higher PerformanceLevels would limit the amount of damage to theconnections or bracing elements, to insure that the loadtransfer mechanism was still complete. Deformationsshould be limited below the threshold of deflections thatcause damage to other elements (either structural ornonstructural) at specified Performance Levels. Thesevalues must be established in conjunction with those ofbraced steel frames.The m values to be used are half of those forcomponents of a CBF as given in Table 5-7.A. Nonlinear Static ProcedureProcedures similar to those used for a CBF should beused, but deformation limits shall be half of those givenfor CBFs in Table 5-8.5.8.4.4 Rehabilitation MeasuresSee the Commentary.5.8.5 Archaic Diaphragms5.8.5.1 GeneralArchaic diaphragms in steel buildings generally consistof shallow brick arches that span between steel floorbeams, with the arches packed tightly between thebeams to provide the necessary resistance to thrustforces. Archaic steel diaphragm elements are almostalways found in older steel buildings in conjunctionwith vertical systems that are of structural steel framing.The brick arches were typically covered with a verylow-strength concrete fill, usually unreinforced. Inmany instances, various archaic diaphragm systemswere patented by contractors.5.8.5.2 Stiffness for AnalysisA. Linear Static ProcedureExisting archaic diaphragm systems are modeled as ahorizontal diaphragm with equivalent thickness ofarches and concrete fill. Development of truss elementsbetween steel beams and compression elements ofarches could also be considered. Flexibility of archaicdiaphragms should be considered in the distribution oflateral loads to vertical elements, especially if spans arelarge.FEMA 273 Seismic Rehabilitation Guidelines 5-37

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)All preceding comments for existing diaphragms applyfor archaic diaphragms. Interaction of new and existingelements of strengthened elements (stiffnesscompatibility) must be considered in cases where steeltrusses are added as part of a seismic upgrade. Loadtransfer mechanisms between new and existingdiaphragm elements must be considered in determiningthe flexibility of the strengthened diaphragm.For modeling assumptions and limitations, see thepreceding comments related to force transfer, andinteraction between diaphragm elements. Analyses arerequired to verify that elastic diaphragm responseassumptions are valid.B. Nonlinear Static ProcedureInelastic properties of archaic diaphragms should bechosen with caution for seismic analyses. For the caseof archaic diaphragms, inelastic models similar to thoseof archaic timber diaphragms in unreinforced masonrybuildings may be appropriate. Inelastic deformationlimits of archaic diaphragms should be lower than thoseprescribed for a concrete-filled diaphragm.5.8.5.3 Strength and DeformationAcceptance CriteriaMember capacities of archaic diaphragm componentscan be calculated assuming little or no tension capacityexcept for the steel beam members. Gravity forceeffects must be included in the calculations for allcomponents of these diaphragms. Force transfermechanisms between various members and betweenframe elements must be considered to verify thecompletion of the load path.In many cases, diaphragm distress could result in lifesafety considerations, due to possible loss of bearingsupport for the elements of the arches. Goals for higherperformance would limit the amount of diagonaltension stresses, to insure that the load transfermechanism was still complete. Deformations should belimited below the threshold of deflections that causedamage to other elements (either structural ornonstructural) at specified Performance Levels. Thesevalues must be established in conjunction with those forsteel frames. Archaic diaphragm components should beconsidered as force-limited, so Equation 3-19 must beused.5.8.5.4 Rehabilitation MeasuresSee the Commentary.5.8.6 Chord and Collector Elements5.8.6.1 GeneralChords and collectors for all the previously describeddiaphragms typically consist of the steel framing thatsupports the diaphragm. When structural concrete ispresent, additional slab reinforcing may act as the chordor collector for tensile loads, while the slab carrieschord or collector compression. When the steel framingacts as a chord or collector, it is typically attached to thedeck with spot welds or by mechanical fasteners. Whenreinforcing acts as the chord or collector, load transferoccurs through bond between the reinforcing bars andthe concrete.5.8.6.2 Stiffness for AnalysisModeling assumptions similar to those for equivalentframe members should be used.5.8.6.3 Strength and DeformationAcceptance CriteriaCapacities of chords and collectors are provided by theAISC LRFD Specifications (1994a) and ACI-318 (ACI,1995; see Chapter 6 for the citation) design guides.Inelastic action may occur, depending on theconfiguration of the diaphragm. It is desirable to designchord and collector components for a force that willdevelop yielding or ductile failure in either thediaphragm or vertical lateral-force-resisting system, sothat the chords and collectors are not the weak link inthe load path. In some cases, failure of chord andcollector components may result in a life safetyconsideration when beams act as the chords orcollectors and vertical support is compromised. Goalsfor higher performance would limit stresses and damagein chords and collectors, keeping the load path intact.In buildings where the steel framing members thatsupport the diaphragm act as collectors, the steelcomponents may be alternately in tension andcompression. If all connections to the diaphragm aresufficient, the diaphragm will prevent buckling of thechord member so values of m equal to 1, 6, and 8 maybe used for IO, LS, and CP, respectively. If thediaphragm provides only limited support againstbuckling of the chord or collector, values of m equal to1, 2, and 3 should be used. Where chords or collectorscarry gravity loads along with seismic loads, theyshould be checked as members with combined loadingusing Equations 5-10 and 5-11. Welds and connectors5-38 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)joining the diaphragms to the collectors should beconsidered to be force-controlled.5.8.6.4 Rehabilitation MeasuresSee the Commentary.5.9 Steel Pile Foundations5.9.1 GeneralSteel piles are one of the most common components forbuilding foundations. Wide flange shapes (H piles) orstructural tubes, with and without concrete infills, arethe most commonly used shapes. Piles are usuallydriven in groups. A reinforced concrete pile cap is thencast over each group, and a steel column with a baseplate is attached to the pile cap with anchor bolts.The piles provide strength and stiffness to thefoundation in one of two ways. Where very strong soilor rock lies at not too great a distance below thebuilding site, the pile forces are transferred directly tothe soil or rock at the bearing surface. Where thiscondition is not met, the piles are designed to transfertheir load to the soil through friction. The design of theentire foundation is covered in Chapter 4 of theseGuidelines. The design of the steel piles is covered inthe following subsections.5.9.2 Stiffness for AnalysisIf the pile cap is below grade, the foundation attainsmuch of its stiffness from the pile cap bearing againstthe soil. Equivalent soil springs may be derived asdiscussed in Chapter 4. The piles may also providesignificant stiffness through bending and bearingagainst the soil. The effective pile contribution tostiffness is decreased if the piles are closely spaced; thisgroup effect must be taken into account whencalculating foundation and strength. For a more detaileddescription, see the Commentary, Section C5.9.2, andChapter 4 of these Guidelines.5.9.3 Strength and Deformation AcceptanceCriteriaBuckling of steel piles is not a concern, since the soilprovides lateral support. The moments in the piles maybe calculated in one of two ways. The first is an elasticmethod that requires finding the effective point offixity; the pile is then designed as a cantilever column.The second, a nonlinear method, requires a computerprogram that is available at no cost. Details are given inthe Commentary, Section C5.9.2.Once the axial force and maximum bending momentsare known, the pile strength acceptance criteria are thesame as for a steel column, as given in Equation 5-10.The expected axial and flexural strengths inEquation 5-10 are computed for an unbraced lengthequal to zero. Note that Equation 5-11 does not apply tosteel piles. Exceptions to these criteria, whereliquefaction is a concern, are discussed in theCommentary, Section C5.9.2.5.9.4 Rehabilitation Measures for Steel PileFoundationsRehabilitation of the pile cap is covered in Chapter 6.Chapter 4 covers general criteria for the rehabilitationof the foundation element. In most cases, it is notpossible to rehabilitate the existing piles. Increasedstiffness and strength may be gained by drivingadditional piles near existing groups and then adding anew pile cap. Monolithic behavior can be gained byconnecting the new and old pile caps with epoxieddowels, or other means.5.10 DefinitionsBeam: A structural member whose primary functionis to carry loads transverse to its longitudinal axis;usually a horizontal member in a seismic frame system.Braced frame: An essentially vertical truss system ofconcentric or eccentric type that resists lateral forces.Concentric braced frame (CBF): A braced frame inwhich the members are subjected primarily to axialforces.Connection: A link between components or elementsthat transmits actions from one component or elementto another component or element. Categorized by typeof action (moment, shear, or axial), connection links arefrequently nonductile.Continuity plates: Column stiffeners at the top andbottom of the panel zone.Diagonal bracing: Inclined structural memberscarrying primarily axial load, employed to enable astructural frame to act as a truss to resist horizontalloads.FEMA 273 Seismic Rehabilitation Guidelines 5-39

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)Dual system: A structural system included inbuildings with the following features:• An essentially complete space frame providessupport for gravity loads.• Resistance to lateral load is provided by concrete orsteel shear walls, steel eccentrically braced frames(EBF), or concentrically braced frames (CBF) alongwith moment-resisting frames (Special MomentFrames, or Ordinary Moment Frames) that arecapable of resisting at least 25% of the lateral loads.• Each system is also designed to resist the total lateralload in proportion to its relative rigidity.Eccentric braced frame (EBF): A diagonal bracedframe in which at least one end of each diagonal bracingmember connects to a beam a short distance from eithera beam-to-column connection or another brace end.Joint: An area where two or more ends, surfaces, oredges are attached. Categorized by the type of fasteneror weld used and the method of force transfer.Lateral support member: A member designed toinhibit lateral buckling or lateral-torsional buckling of acomponent.Link: In an EBF, the segment of a beam that extendsfrom column to brace, located between the end of adiagonal brace and a column, or between the ends oftwo diagonal braces of the EBF. The length of the linkis defined as the clear distance between the diagonalbrace and the column face, or between the ends of twodiagonal braces.Moment frame: A building frame system in whichseismic shear forces are resisted by shear and flexure inmembers and joints of the frame.Nominal strength: The capacity of a structure orcomponent to resist the effects of loads, as determinedby (1) computations using specified material strengthsand dimensions, and formulas derived from acceptedprinciples of structural mechanics, or (2) field tests orlaboratory tests of scaled models, allowing formodeling effects, and differences between laboratoryand field conditions.Ordinary Moment Frame (OMF): A moment framesystem that meets the requirements for OrdinaryMoment Frames as defined in seismic provisions fornew construction in AISC (1994a), Chapter 5.P-∆ effect: The secondary effect of column axialloads and lateral deflection on the shears and momentsin various components of a structure.Panel zone: The area of a column at the beam-tocolumnconnection delineated by beam and columnflanges.Required strength: The load effect (force, moment,stress, as appropriate) acting on a component orconnection, determined by structural analysis from thefactored loads (using the most appropriate critical loadcombinations).Resistance factor: A reduction factor applied tomember resistance that accounts for unavoidabledeviations of the actual strength from the nominalvalue, and the manner and consequences of failure.Link intermediate web stiffeners:stiffeners placed within the link.Vertical webSlip-critical joint: A bolted joint in which slipresistance of the connection is required.Link rotation angle: The angle of plastic rotationbetween the link and the beam outside of the linkderived using the specified base shear, V.LRFD (Load and Resistance Factor Design):A method of proportioning structural components(members, connectors, connecting elements, andassemblages) using load and resistance factors such thatno applicable limit state is exceeded when the structureis subjected to all design load combinations.Special Moment Frame (SMF): A moment framesystem that meets the special requirements for frames asdefined in seismic provisions for new construction.Structural system: An assemblage of load-carryingcomponents that are joined together to provide regularinteraction or interdependence.V-braced frame: A concentric braced frame (CBF)in which a pair of diagonal braces located either aboveor below a beam is connected to a single point withinthe clear beam span. Where the diagonal braces are5-40 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)below the beam, the system also is referred to as an“inverted V-brace frame,” or “chevron bracing.”X-braced frame: A concentric braced frame (CBF)in which a pair of diagonal braces crosses near the midlengthof the braces.Y-braced frame: An eccentric braced frame (EBF)in which the stem of the Y is the link of the EBFsystem.5.11 SymbolsThis list may not contain symbols defined at their firstuse if not used thereafter.A b Gross area of bolt or rivet, in.2A c Rivet area, in.2A e Effective net area, in.2A f Flange area of member, in.2A g Gross area, in.2A st Area of link stiffener, in.2A w Effective area of weld, in.2C b Coefficient to account for effect of nonuniformmoment; given in AISC (1994a)E Young’s modulus of elasticity, 29,000 ksiF EXX Classification strength of weld metal, ksiF te Expected tensile strength, ksiF v Design shear strength of bolts or rivets, ksiF y Specified minimum yield stress for the type ofsteel being used, ksiF yb F y of a beam, ksiF yc F y of a column, ksiF ye Expected yield strength, ksiF yf F y of a flange, ksiG Shear modulus of steel, 11,200 ksiI b Moment of inertia of a beam, in.4I c Moment of inertia of a columnK Length factor for brace (see AISC, 1994a)K e Stiffness of a link beam, kip/in.K s Rotational stiffness of a connection,kip-in./radK wK θLL pL rM CEM CExM CEyM pM xM yN bStiffness of wall, kip/in.Rotational stiffness of a partially restrainedconnection, kip-in./radLength of bracing member, in.The limiting unbraced length between points oflateral restraint for the full plastic momentcapacity to be effective (see AISC, 1994a)The limiting unbraced length between points oflateral support beyond which elastic lateraltorsional buckling of the beam is the failuremode (see AISC, 1994a)Expected flexural strength of a member orjoint, kip-in.Expected bending strength of a member aboutthe x-axis, kip-in.Expected bending strength of a member abouty-axis, kip-in.Plastic bending moment, kip-in.Bending moment in a member for the x-axis,kip-in.Bending moment in a member for the y-axis,kip-in.Number of bolts or rivetsP Axial force in a member, kipsPR Partially restrainedP cr Critical compression strength of bracing, kipsP CL Lower-bound axial strength of column, kipsP u Required axial strength of a column or a link,kipsP ye Expected yield axial strength of a member =F ye A g , kipsQ CE Expected strength of a component or element atthe deformation level under consideration in adeformation-controlled actionQ CL Lower-bound estimate of the strength of acomponent or element at the deformation levelunder consideration for a force-controlledactionV CE Expected shear strength of a member, kipsV CE Shear strength of a link beam, kipsV ya Nominal shear strength of a member modifiedby the axial load magnitude, kipsZ Plastic section modulus, in. 3aClear width of wall between columnsFEMA 273 Seismic Rehabilitation Guidelines 5-41

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)bb ab cfb fb tdd bd cd vd zehhhh ch vk vl bl cmm em xm yrr ytt at bft cft ft pt pWidth of compression element, in.Connection dimensionColumn flange width, in.Flange width, in.Connection dimensionOverall depth of member, in.Overall beam depth, in.Overall column depth, in.Bolt or rivet diameter, in.Overall panel zone depth between continuityplates, in.EBF link length, in.Average story height above and below a beamcolumnjointClear height of wall between beamsDistance from inside of compression flange toinside of tension flange, in.Assumed web depth for stability, in.Height of story vShear buckling coefficientLength of beamLength of columnA modification factor used in the acceptancecriteria of deformation-controlled componentsor elements, indicating the available ductility ofa component actionEffective mValue of m for bending about x-axis of amemberValue of m for bending about y-axis of amemberGoverning radius of gyration, in.Radius of gyration about y axis, in.Thickness of link stiffener, in.Thickness of angle, in.Thickness of beam flange, in.Thickness of column flange, in.Thickness of flange, in.Thickness of panel zone including doublerplates, in.Thickness of flange plate, in.t wt wt zww z∆∆ i∆ yθθ iθ yκλλ pλ rρρ lpThickness of web, in.Thickness of plate wallThickness of panel zone (doubler plates notnecessarily included), in.Length of flange angleWidth of panel zone between column flanges,in.Generalized deformation, unitlessInter-story displacement (drift) of story idivided by the story heightGeneralized yield deformation, unitlessGeneralized deformation, radiansInter-story drift ratio, radiansGeneralized yield deformation, radiansA reliability coefficient used to reducecomponent strength values for existingcomponents based on the quality of knowledgeabout the components’ properties (seeSection 2.7.2)Slenderness parameterLimiting slenderness parameter for compactelementLimiting slenderness parameter fornoncompact elementRatio of required axial force (P u ) to nominalshear strength (V y ) of a linkYield deformation of a link beamφ Resistance factor = 1.05.12 ReferencesACI, 1995, Building Code Requirements for ReinforcedConcrete: ACI 318-95, American Concrete Institute,Detroit, Michigan.AISC, 1994a, Manual of Steel Construction, Load andResistance Factor Design Specification for StructuralSteel Buildings (LRFD), Volume I, Structural Members,Specifications and Codes, American Institute of SteelConstruction, Chicago, Illinois.AISC, 1994b, Manual of Steel Construction, Load andResistance Factor Design, Volume II, Connections,5-42 Seismic Rehabilitation Guidelines FEMA 273

Chapter 5: Steel and Cast Iron(Systematic Rehabilitation)American Institute of Steel Construction, Chicago,Illinois.AISI, 1973, The Criteria for Structural Applications forSteel Cables for Building, 1973 Edition, American Ironand Steel Institute, Washington, D.C.AISI, 1986, Specification for the Design of Cold-Formed Steel Structural Members, August 10, 1986edition with December 11, 1989 Addendum, AmericanIron and Steel Institute, Chicago, Illinois.ASCE, 1990, Specification for the Design of Cold-Formed Steel Stainless Steel Structural Members,Report No. ASCE-8, American Society of CivilEngineers, New York, New York.BSSC, 1992, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No. FEMA178), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Reports No.FEMA 222A and 223A), Washington, D.C.SAC, 1995, Interim Guidelines: Evaluation, Repair,Modification and Design of Welded Steel MomentFrame Structures, Report No. FEMA 267, developedby the SEAOC, ATC, and CUREE Joint Venture(Report No. SAC-95-02) for the Federal EmergencyManagement Agency, Washington, D.C.SDI, latest edition, SDI Design Manual for CompositeDecks, Form Decks and Roof Decks, Steel DiaphragmInstitute.SJI, 1990, Standard Specification, Load Tables andWeight Tables for Steel Joists and Joist Girders, SteelJoist Institute, 1990 Edition.FEMA 273 Seismic Rehabilitation Guidelines 5-43

6.Concrete(Systematic Rehabilitation)6.1 ScopeEngineering procedures for estimating the seismicperformance of lateral-force-resisting concretecomponents and elements are described in this chapter.Methods are applicable for concrete components thatare either (1) existing components of a building system,(2) rehabilitated components of a building system, or(3) new components that are added to an existingbuilding system.Information needed for Systematic Rehabilitation ofconcrete buildings, as described in Chapter 2 andChapter 3, is presented herein. Symbols usedexclusively in this chapter are defined in Section 6.15.Section 6.2 provides a brief historical perspective of theuse of concrete in building construction; acomprehensive historical perspective is contained in theCommentary. In Section 6.3, material and componentproperties are discussed in detail. Important propertiesof in-place materials and components are described interms of physical attributes as well as how to determineand measure them. Guidance is provided on how to usethe values in Tables 6-1 to 6-3 that might be used asdefault assumptions for material properties in apreliminary analysis.General analysis and design assumptions andrequirements are covered in Section 6.4. Critical modesof failure for beams, columns, walls, diaphragms, andfoundations are discussed in terms of shear, bending,and axial forces. Components that are usuallycontrolled by deformation are described in generalterms. Other components that have limiting behaviorcontrolled by force levels are presented along withAnalysis Procedures.Sections 6.5 through 6.13 cover the majority of thevarious structural concrete elements, including frames,braced frames, shear walls, diaphragms, andfoundations. Modeling procedures, acceptance criteria,and rehabilitation measures for each component arediscussed.6.2 Historical PerspectiveThe components of concrete seismic resisting elementsare columns, beams, slabs, braces, collectors,diaphragms, shear walls, and foundations. There hasbeen a constant evolution in form, function, concretestrength, concrete quality, reinforcing steel strength,quality and detailing, forming techniques, and concreteplacement techniques. All of these factors have asignificant impact on the seismic resistance of aconcrete building. Innovations such as prestressed andprecast concrete, post tensioning, and lift slabconstruction have created a multivariant inventory ofexisting concrete structures.The practice of seismic resistant design is relativelynew to most areas of the United States, even thoughsuch practice has been evolving in California for thepast 70 years. It is therefore important to investigate thelocal practices relative to seismic design when trying toanalyze a specific building. Specific benchmark yearscan generally be determined for the implementation ofseismic resistant design in most locations, but cautionshould be exercised in assuming optimisticcharacteristics for any specific building.Particularly with concrete materials, the date of originalbuilding construction has significant influence onseismic performance. In the absence of deleteriousconditions or materials, concrete gains compressivestrength from the time it is originally cast and in-place.Strengths typically exceed specified design values (28-day or similar). Early uses of concrete did not specifyany design strength, and low-strength concrete was notuncommon. Also, early use of concrete in buildingsoften employed reinforcing steel with relatively lowstrength and ductility, limited continuity, and reducedbond development. Continuity between specificexisting components and elements (e.g., beams andcolumns, diaphragms and shear walls) is alsoparticularly difficult to assess, given the presence ofconcrete cover and other barriers to inspection. Also,early use of concrete was expanded by use ofproprietary structural system designs and constructiontechniques. Some of these systems are described in theCommentary Section C6.2. The design professional iscautioned to fully examine available constructiondocuments and in-place conditions in order to properlyanalyze and characterize historical concrete elementsand components of buildings.As indicated in Chapter 1, great care should beexercised in selecting the appropriate rehabilitationapproaches and techniques for application to historicFEMA 273 Seismic Rehabilitation Guidelines 6-1

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-1Tensile and Yield Properties of Concrete Reinforcing Bars for Various Periods60 70 75Grade33 40 50Structural 1 Intermediate 1 Hard 1Year 3 Minimum Yield 2 (psi) 33,000 40,000 50,000 50,000 60,000 75,000Minimum Tensile 2 (psi) 55,000 70,000 80,000 90,000 80,000 100,0001911-1959 x x x1959-1966 x x x x x1966-1972 x x x1972-1974 x x x1974-1987 x x x x1987-present x x x x xGeneral Note: An entry “x” indicates the grade was available in those years.Specific Notes: 1. The terms structural, intermediate, and hard became obsolete in 1968.2. Actual yield and tensile strengths may exceed minimum values.3. Until about 1920, a variety of proprietary reinforcing steels were used. Yield strengths are likely to be in the range from 33,000 psi to 55,000psi, but higher values are possible Plain and twisted square bars were sometimes used between 1900 and 1949.buildings in order to preserve their uniquecharacteristics.Tables 6-1 and 6-2 contain a summary of reinforcingsteel properties that might be expected to be encountered.Table 6-1 provides a range of properties for use whereonly the year of construction is known. Where bothASTM designations and year of construction are known,use Table 6-2. Properties of Welded Wire Fabric forvarious periods of construction can be obtained from theWire Reinforcement Institute. Possible concrete strengthsas a function of time are given in Table 6-3. A moredetailed historical treatment is provided in Section C6.2of the Commentary, and the reader is encouraged toreview the referenced documents..6.3 Material Properties andCondition Assessment6.3.1 GeneralQuantification of in-place material properties andverification of existing system configuration andcondition are necessary to analyze a building properly.This section identifies properties requiringconsideration and provides guidelines for determiningthe properties of buildings. Also described is the needfor a thorough condition assessment and utilization ofknowledge gained in analyzing component and systembehavior. Personnel involved in material propertyquantification and condition assessment shall beexperienced in the proper implementation of testingpractices, and interpretation of results.The extent of in-place materials testing and conditionassessment needed is related to the availability andaccuracy of construction (as-built) records, quality ofmaterials and construction, and physical condition.Documentation of properties and grades of materialused in component/connection construction isinvaluable and may be effectively used to reduce theamount of in-place testing required. The designprofessional is encouraged to research and acquire allavailable records from original construction.6.3.2 Properties of In-Place Materials andComponents6.3.2.1 Material PropertiesMechanical properties of component and connectionmaterial strongly influence the structural behaviorunder load. Mechanical properties of greatest interestfor concrete elements and components include thefollowing:• Concrete compressive and tensile strengths andmodulus of elasticity6-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-2Tensile and Yield Properties of Concrete Reinforcing Bars for Various ASTM Specificationsand PeriodsStructural 1 Intermediate 1 Hard 1ASTMSteelTypeYearRange 3A15 Billet 1911-1966A16 Rail 4 1913-1966A61 Rail 4 1963-1966A160 Axle 1936-1964A160 Axle 1965-1966A408 Billet 1957-1966A431 Billet 1959-1966A432 Billet 1959-1966A615 Billet 1968-1972A615 Billet 1974-1986A615 Billet 1987-1997A616 Rail 4 1968-1997A617 Axle 1968-1997A706 Low- 1974-Alloy 5 1997A955 Stainless 1996-1997Grade 33 40 50 60 70 75MinimumYield 2(psi)33,000 40,000 50,000 50,000 60,000 75,000Minimum 55,000 70,000 80,000 90,000 80,000 100,000Tensile 2(psi)x x xx x xx x x xx x xxx x xxx x xxxx x xxxxxxxxGeneral Note: An entry “x” indicates the grade was available in those years.Specific Notes: 1. The terms structural, intermediate, and hard became obsolete in 1968.2. Actual yield and tensile strengths may exceed minimum values.3. Until about 1920, a variety of proprietary reinforcing steels were used. Yield strengths are likely to be in therange from 33,000 psi to 55,000 psi, but higher values are possible Plain and twisted square bars weresometimes used between 1900 and 1949.4. Rail bars should be marked with the letter “R.” Bars marked “s!” (ASTM 616) have supplementaryrequirements for bend tests.5. ASTM steel is marked with the letter “W.”FEMA 273 Seismic Rehabilitation Guidelines 6-3

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-3 Compressive Strength of Structural Concrete (psi) 1Time Frame Footings Beams Slabs Columns Walls1900–1919 1000–2500 2000–3000 1500–3000 1500–3000 1000–25001920–1949 1500–3000 2000–3000 2000–3000 2000–4000 2000–30001950–1969 2500–3000 3000–4000 3000–4000 3000–6000 2500–40001970–Present 3000–4000 3000–5000 3000–5000 3000–10000 2 3000–50001. Concrete strengths are likely to be highly variable within any given older structure.2. Exceptional cases of very high strength concrete may be found.• Yield and ultimate strength of conventional andprestressing reinforcing steel and metal connectionhardware• Ductility, toughness, and fatigue properties• Metallurgical condition of the reinforcing steel,including carbon equivalent, presence of anydegradation such as corrosion, bond with concrete,and chemical composition.The effort required to determine these propertiesdepends on the availability of accurate updatedconstruction documents and drawings, quality and typeof construction (absence of degradation), accessibility,and condition of materials. The method of analysis(e.g., Linear Static Procedure, Nonlinear StaticProcedure) to be used in the rehabilitation may alsoinfluence the scope of the testingIn general, the determination of material properties(other than connection behavior) is best accomplishedthrough removal of samples and laboratory analysis.Sampling shall take place in primary gravity- andlateral-force-resisting components. Where possible,sampling shall occur in regions of reduced stress tolimit the effects of reduced sectional area. The size ofthe samples and removal practices to be followed arereferenced in the Commentary. The frequency ofsampling, including the minimum number of tests forproperty determination, is addressed in Section 6.3.2.4.Generally, mechanical properties for both concrete andreinforcing steel can be established from combined coreand specimen sampling at similar locations, followedby laboratory testing. For concrete, the samplingprogram shall consist of the removal of standardvertical or horizontal cores. Core drilling shall bepreceded by nondestructive location of the reinforcingsteel, and shall avoid damaging the existing reinforcingsteel as much as practicable. Core holes shall be filledwith comparable-strength concrete or grout. Forconventional reinforcing and bonded prestressing steel,sampling shall consist of the removal of local barsegments (extreme care shall be taken with removal ofany prestressing steels). Depending on the location andamount of bar removed, replacement spliced materialshall be installed to maintain continuity.6.3.2.2 Component PropertiesStructural elements often utilize both primary andsecondary components to perform their load- anddeformation-resisting function. Behavior of thecomponents, including beams, columns, and walls, isdictated by such properties as cross-sectionaldimensions and area, reinforcing steel location, widthto-thicknessand slenderness ratios, lateral bucklingresistance, and connection details. This behavior mayalso be altered by the presence of degradation orphysical damage. The following component propertiesshall be established during the condition assessmentphase of the seismic rehabilitation process to aid inevaluating component behavior (see Section 6.3.3 forassessment guidelines):• Original and current cross-sectional dimensions• As-built configuration and physical condition ofprimary component end connections, andintermediate connections such as those betweendiaphragms and supporting beams/girders• Size, anchorage, and thickness of other connectormaterials, including metallic anchor bolts, embeds,bracing components, and stiffening materials,commonly used in precast and tilt-up construction6-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)• Characteristics that may influence the continuity,moment-rotation, or energy dissipation and loadtransfer behavior of connections• Confirmation of load transfer capability atcomponent-to-element connections, and overallelement/structure behaviorThese properties may be needed to characterizebuilding performance properly in the seismic analysis.The starting point for assessing component propertiesand condition should be retrieval of availableconstruction documents. Preliminary review of thesedocuments shall be performed to identify primaryvertical- (gravity) and lateral load-carrying elementsand systems, and their critical components andconnections. In the absence of a complete set ofbuilding drawings, the design professional mustperform a thorough inspection of the building toidentify these elements, systems, and components asindicated in Section 6.3.3.In the absence of degradation, component dimensionsand properties from original drawings may be used instructural analyses without introducing significant error.Variance from nominal dimensions, such as reinforcingsteel size and effective area, is usually small.6.3.2.3 Test Methods to Quantify PropertiesTo obtain the desired in-place mechanical properties ofmaterials and components, it is necessary to use provendestructive and nondestructive testing methods. Certainfield tests—such as estimation of concrete compressivestrength from hardness and impact resistance tests—may be performed, but laboratory testing shall be usedwhere strength is critical. Critical properties of concretecommonly include the compressive and tensile strength,modulus of elasticity, and unit weight. Samples ofconcrete and reinforcing and connector steel shall alsobe examined for physical condition (seeSection 6.3.3.2).Accurate determination of existing concrete strengthproperties is typically achieved through removal of coresamples and performance of laboratory destructivetesting. Removal of core samples should employ theprocedures contained in ASTM C 42. Testing shouldfollow the procedures contained in ASTM C 42, C 39,and C 496. The measured strength from testing must becorrelated to in-place concrete compressive strength;the Commentary provides further guidance oncorrelating core strength to in-place strength and otherrecommendations. The Commentary providesreferences for various test methods that may be used toestimate material properties.Accurate determination of existing reinforcing steelstrength properties is typically achieved throughremoval of bar or tendon length samples andperformance of laboratory destructive testing. Theprimary strength measures for reinforcing andprestressing steels are the tensile yield strength andultimate strength, as used in the structural analysis.Strength values may be obtained by using theprocedures contained in ASTM A 370. Prestressingmaterials must also meet the supplemental requirementsin ASTM A 416, A 421, or A 722, depending on materialtype. The chemical composition may also bedetermined from the retrieved samples.Particular test methods that may be used for connectorsteels include wet and dry chemical composition tests,and direct tensile and compressive strength tests. Foreach test, industry standards published by ASTM,including Standard A 370, exist and shall be followed.For embedded connectors, the strength of the materialmay also be assessed in situ using the provisions ofASTM E 488. The Commentary provides references forthese tests.Usually, the reinforcing steel system used inconstruction of a specific building is of a common gradeand strength. Occasionally one grade of reinforcementis used for small-diameter bars (e.g., those used forstirrups and hoops) and another grade for largediameterbars (e.g., those used for longitudinalreinforcement). Furthermore, it is possible that anumber of different concrete design strengths (or“classes”) have been employed. In developing a testingprogram, the design professional shall consider thepossibility of varying concrete classes. Historicalresearch and industry documents also contain insight onmaterial mechanical properties used in differentconstruction eras. Section 6.3.2.5 provides strength datafor most primary concrete and reinforcing steels used.This information, with laboratory and field test data,may be used to gain confidence in in situ strengthproperties.6.3.2.4 Minimum Number of TestsIn order to quantify in-place properties accurately, it isimportant that a minimum number of tests be conductedon primary components in the lateral-force-resistingsystem. As stated previously, the minimum number ofFEMA 273 Seismic Rehabilitation Guidelines 6-5

Chapter 6: Concrete(Systematic Rehabilitation)tests is dictated by available data from originalconstruction, the type of structural system employed,desired accuracy, and quality/condition of in-placematerials. The accessibility of the structural system mayalso influence the testing program scope. The focus ofthis testing shall be on primary lateral-force-resistingcomponents and on specific properties needed foranalysis. The test quantities provided in this section areminimum numbers; the design professional shoulddetermine whether further testing is needed to evaluateas-built conditions.Testing is not required on components other than thoseof the lateral-force-resisting system. If the existinglateral-force-resisting system is being replaced in therehabilitation process, minimum material testing isneeded to qualify properties of existing materials at newconnection points.A. Concrete MaterialsFor each concrete element type (such as a shear wall), aminimum of three core samples shall be taken andsubjected to compression tests. A minimum of six testsshall be done for the complete concrete buildingstructure, subject to the limitations noted below. Ifvarying concrete classes/grades were employed inbuilding construction, a minimum of three samples andtests shall be performed for each class. Test results shallbe compared with strength values specified in theconstruction documents. The core strength shall beconverted to in situ concrete compressive strength (f c ' )as in Section C6.3.2.3 of the Commentary. The unitweight and modulus of elasticity shall be derived orestimated during strength testing. Samples should betaken at random locations in components critical tostructural behavior of the building. Tests shall also beperformed on samples from components that aredamaged or degraded, to quantify their condition. If testvalues less than the specified strength in theconstruction documents are found, further strengthtesting shall be performed to determine the cause oridentify whether the condition is localized.The minimum number of tests to determinecompressive and tensile strength shall also conform tothe following criteria.• For concrete elements for which the specified designstrength is known and test results are not available, aminimum of three cores/tests shall be conducted foreach floor level, 400 cubic yards of concrete, or10,000 square feet of surface area (estimatedsmallest of the three).• For concrete elements for which the design strengthis unknown and test results are not available, aminimum of six cores/tests shall be conducted foreach floor level, 400 cubic yards of concrete, or10,000 square feet of surface area (use smallestnumber). Where the results indicate that differentclasses of concrete were employed, the degree oftesting shall be increased to confirm class use.• A minimum of three samples shall be removed forsplitting tensile strength determination, if alightweight aggregate concrete were used forprimary components. Additional tests may bewarranted, should the coefficient of variation in testresults exceed 14%.If a sample population greater than the minimumspecified is used in the testing program and thecoefficient of variation in test results is less than 14%,the mean strength derived may be used as the expectedstrength in the analysis. If the coefficient of variationfrom testing is greater than 14%, additional samplingand testing should be performed to improve theaccuracy of testing or understanding of in situ materialstrength. The design professional (and subcontractedtesting agency) shall carefully examine test results toverify that suitable sampling and testing procedureswere followed, and that appropriate values for theanalysis were selected from the data. In general, theexpected concrete strength shall not exceed the meanless one standard deviation in situations wherevariability is greater than 14%.In addition to destructive sampling and testing, furtherquantification of concrete strength may be estimated viaultrasonics, or another nondestructive test method (seethe Commentary). Because these methods do not yieldaccurate strength values directly, they should be usedfor confirmation and comparison only and shall not besubstituted for core sampling and laboratory testing.B. Conventional Reinforcing and Connector SteelsIn terms of defining reinforcing and connector steelstrength properties, the following guidelines shall befollowed. Connector steel is defined as additionalstructural or bolting steel material used to secure precastand other concrete shapes to the building structure.Both yield and ultimate strengths shall be determined.A minimum of three tensile tests shall be conducted on6-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)conventional reinforcing steel samples from a buildingfor strength determination, subject to the followingsupplemental conditions.• If original construction documents definingproperties exist, and if an Enhanced RehabilitationObjective (greater than the BSO) is desired, at leastthree strength coupons shall be randomly removedfrom each element or component type (e.g., slabs,walls, beams) and tested.• If original construction documents definingproperties do not exist, but the approximate date ofconstruction is known and a common material gradeis confirmed (e.g., all bars are Grade 60 steel), atleast three strength coupons shall be randomlyremoved from each element or component type (e.g.,beam, wall) for every three floors of the building. Ifthe date of construction is unknown, at least six suchsamples/tests, for every three floors, shall beperformed. This is required to satisfy the BSO.All sampled steel shall be replaced with new fullyspliced and connected material, unless an analysisconfirms that replacement of function is not required.C. Prestressing SteelsThe sampling of prestressing steel tendons forlaboratory testing shall be accomplished with extremecare; only those prestressed components that are a partof the lateral-force-resisting system shall be considered.Components in diaphragms should generally beexcluded from testing. If limited information existsregarding original materials and the prestressing forceapplied, the design professional must attempt toquantify properties for analysis. Tendon removals shallbe avoided if possible in prestressed members. Only aminimum number of tendon samples for laboratorytesting shall be taken.Determination of material properties may be possible,without tendon removal or prestress removal, by carefulsampling of either the tendon grip or extension beyondthe anchorage.All sampled steel shall be replaced with new fullyconnected and stressed material and anchoragehardware unless an analysis confirms that replacementof function is not required.D. GeneralFor other material properties, such as hardness andductility, no minimum number of tests is prescribed.Similarly, standard test procedures may not exist. Thedesign professional shall examine the particular needfor this type of testing and establish an adequateprotocol. In general, it is recommended that a minimumof three tests be conducted to determine any property. Ifoutliers (results with coefficients of variation greaterthan 15%) are detected, additional tests shall beperformed until an accurate representation of theproperty is gained.6.3.2.5 Default PropertiesMechanical properties for materials and componentsshall be based on available historical data for theparticular structure and tests on in-place conditions.Should extenuating circumstances prevent minimummaterial sampling and testing from being performed,default strength properties may be used. Defaultmaterial and component properties have beenestablished for concrete compressive strength andreinforcing steel tensile and yield strengths frompublished literature; these are presented in Tables 6-1 to6-3. These default values are generally conservative,representing values reduced from mean strength inorder to address variability. However, the selection of adefault strength for concrete shall be made with carebecause of the multitude of mix designs and materialsused in the construction industry.For concrete default compressive strength, lower-boundvalues from Table 6-3 may be used. The defaultcompressive strength shall be used to establish otherstrength and performance characteristics for theconcrete as needed in the structural analysis.Reinforcing steel tensile properties are presented inTables 6-1 and 6-2. Because of lower variability, thelower-bound tabulated values may be used withoutfurther reduction. For Rehabilitation Objectives inwhich default values are assumed for existingreinforcing steel, no welding or mechanical coupling ofnew reinforcing to the existing reinforcing steel ispermitted. For buildings constructed prior to 1950, thebond strength developed between reinforcing steel andconcrete may be less than present-day strength. Thetensile lap splices and development length of older plainreinforcing should be considered as 50% of the capacityof present-day tabulated values (such as in ACI 318-95FEMA 273 Seismic Rehabilitation Guidelines 6-7

Chapter 6: Concrete(Systematic Rehabilitation)[ACI, 1995]) unless further justified through testing andassessment (CRSI, 1981).For connector materials, the nominal strength fromdesign and construction documents may be used. In theabsence of this information, the default yield strengthfor steel connector material may be taken as 27,000 psi.Default values for prestressing steel in prestressedconcrete construction shall not be used, unlesscircumstances prevent material sampling/testing frombeing performed. In this case, it may be prudent to add anew lateral-force-resisting system to the building.6.3.3 Condition Assessment6.3.3.1 GeneralA condition assessment of the existing building and siteconditions shall be performed as part of the seismicrehabilitation process. The goal of this assessment isthreefold:• To examine the physical condition of primary andsecondary components and the presence of anydegradation• To verify the presence and configuration ofcomponents and their connections, and thecontinuity of load paths between components,elements, and systems• To review other conditions that may influenceexisting building performance, such as neighboringparty walls and buildings, nonstructural componentsthat may contribute to resistance, and any limitationsfor rehabilitationThe physical condition of existing components andelements, and their connections, must be examined forpresence of degradation. Degradation may includeenvironmental effects (e.g., corrosion, fire damage,chemical attack), or past or current loading effects (e.g.,overload, damage from past earthquakes, fatigue,fracture). The condition assessment shall also examinefor configurational problems observed in recentearthquakes, including effects of discontinuouscomponents, construction deficiencies, poor fit-up, andductility problems.Component orientation, plumbness, and physicaldimensions should be confirmed during an assessment.Connections in concrete components, elements, andsystems require special consideration and evaluation.The load path for the system must be determined, andeach connection in the load path(s) must be evaluated.This includes diaphragm-to-component andcomponent-to-component connections. Where theconnection is attached to one or more components thatare expected to experience significant inelasticresponse, the strength and deformation capacity ofconnections must be evaluated. The condition anddetailing of at least one of each connection type shouldbe investigated.The condition assessment also affords an opportunity toreview other conditions that may influence concreteelements and systems, and overall buildingperformance. Of particular importance is theidentification of other elements and components thatmay contribute to or impair the performance of theconcrete system in question, including infills,neighboring buildings, and equipment attachments.Limitations posed by existing coverings, wall andceiling space, infills, and other conditions shall also bedefined.6.3.3.2 Scope and ProceduresThe scope of the condition assessment should includeall primary structural elements and componentsinvolved in gravity and lateral load resistance, aslimited by accessibility. The knowledge and insightgained from the condition assessment is invaluable tothe understanding of load paths and the ability ofcomponents to resist and transfer these loads. Thedegree of assessment performed also affects the κ factorthat is used in the analysis, and the type of analysis (seeSection 6.3.4).A. Visual InspectionDirect visual inspection provides the most valuableinformation, as it can be used to quickly identify anyconfigurational issues, and it allows the measurement ofcomponent dimensions, and the determination whetherdegradation is present. The continuity of load paths maybe established through viewing of components andconnection condition. From visual inspection, the needfor other test methods to quantify the presence anddegree of degradation may be established. Thedimensions of accessible primary components shall bemeasured and compared with available designinformation. Similarly, the configuration and conditionof all connections (exposed surfaces) shall be verifiedwith permanent deformations or other noted anomalies.6-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Industry-accepted procedures are cited in theCommentary.Visual inspection of the specific building should includeall elements and components constructed of concrete,including foundations, vertical and horizontal framemembers, diaphragms (slabs), and connections. As aminimum, a representative sampling of at least 20% ofthe elements, components, and connections shall bevisually inspected at each floor level. If significantdamage or degradation is found, the assessment sampleshall be increased to all critical components of similartype in the building. The damage should be quantifiedusing supplemental methods cited in this chapter andthe Commentary.If coverings or other obstructions exist, indirect visualinspection through the obstruction may be conducted byusing drilled holes and a fiberscope. If this method isnot appropriate, then exposure will be necessary.Exposure is defined as local minimized removal ofcover concrete and other materials to allow inspectionof reinforcing system details; all damaged concretecover shall be replaced after inspection. The followingguidelines shall be used for assessing primaryconnections in the building.• If detailed design drawings exist, exposure of at leastthree different primary connections shall occur, withthe connection sample including different types(e.g., beam-column, column-foundation, beamdiaphragm).If no deviations from the drawingsexist, the sample may be considered representativeof installed conditions. If deviations are noted, thenexposure of at least 25% of the specific connectiontype is necessary to identify the extent of deviation.• In the absence of accurate drawings, exposure of atleast three connections of each primary connectiontype shall occur for inspection. If common detailingis observed, this sample may be consideredrepresentative. If many different details ofdeviations are observed, increased connectioninspection is warranted until an accurateunderstanding of building construction and behavioris gained.B. Additional TestingThe physical condition of components and connectorsmay also dictate the need for certain destructive andnondestructive test methods. Such methods may beused to determine the degree of damage or presence ofcontamination, and to improve understanding of theinternal condition and quality of the concrete. Furtherguidelines and procedures for destructive andnondestructive tests that may be used in the conditionassessment are provided in the Commentary. Thefollowing paragraphs identify those nondestructiveexamination (NDE) methods having the greatest useand applicability to condition assessment.• Surface NDE methods include infraredthermography, delamination sounding, surfacehardness measurement, and crack mapping. Thesemethods may be used to find surface degradation incomponents such as service-induced cracks,corrosion, and construction defects.• Volumetric NDE methods, including radiographyand ultrasonics, may be used to identify the presenceof internal discontinuities, as well as to identify lossof section. Impact-echo ultrasonics is particularlyuseful because of ease of implementation andproven capability in concrete.• Structural condition and performance may beassessed through on-line monitoring using acousticemissions and strain gauges, and in-place static ordynamic load tests. Monitoring is used to determineif active degradation or deformations are occurring,while nondestructive load testing provides directinsight on load-carrying capacity.• Locating, sizing, and initial assessment of thereinforcing steel may be completed usingelectromagnetic methods (such as pachometer).Further assessment of suspected corrosion activityshould utilize electrical half-cell potential andresistivity measurements.• Where it is absolutely essential, the level of prestressremaining in an unbonded prestressed system maybe measured using lift-off testing (assuming originaldesign and installation data are available), or anothernondestructive method such as “coring stress relief”(ASCE, 1990).The Commentary provides general background andreferences for these methods.6.3.3.3 Quantifying ResultsThe results of the condition assessment shall be used inthe preparation of building system models in theevaluation of seismic performance. To aid in this effort,FEMA 273 Seismic Rehabilitation Guidelines 6-9

Chapter 6: Concrete(Systematic Rehabilitation)the results shall be quantified, with the followingspecific topics addressed:• Component section properties and dimensions• Component configuration and presence of anyeccentricities or permanent deformation• Connection configuration and presence of anyeccentricities• Presence and effect of alterations to the structuralsystem since original construction (e.g., doorwayscut into shear walls)• Interaction of nonstructural components and theirinvolvement in lateral load resistanceAs previously noted, the acceptance criteria for existingcomponents depend on the design professional’sknowledge of the condition of the structural system andmaterial properties. All deviations noted betweenavailable construction records and as-built conditionsshall be accounted for and considered in the structuralanalysis. Again, some removal of cover concrete isrequired during this stage to confirm reinforcing steelconfiguration.Gross component section properties in the absence ofdegradation have been found to be statistically close tonominal. Unless concrete cracking, reinforcingcorrosion, or other mechanisms are observed in thecondition assessment to be causing damage or reducedcapacity, the cross-sectional area and other sectionalproperties shall be taken as those from the designdrawings. If some sectional material loss has occurred,the loss shall be quantified via direct measurement. Thesectional properties shall then be reduced accordingly,using the principles of structural mechanics. If thedegradation is significant, further analysis orrehabilitative measures shall be undertaken.6.3.4 Knowledge (κ ) FactorAs described in Section 2.7, computation of componentcapacities and allowable deformations involves the useof a knowledge (κ) factor. For cases where a LinearStatic Procedure (LSP) will be used in the analysis, twopossible values for κ exist (0.75 and 1.0). For nonlinearprocedures, the design professional must obtain an indepthunderstanding of the building structural systemand condition to support the use of a κ factor of 1.0.This section further describes the requirements andselection criteria for a κ factor specific to concretestructural components.If the concrete structural system is exposed and goodaccess exists, significant knowledge regardingconfiguration and behavior may be gained throughcondition assessment. In general, a κ factor of 1.0 canbe used when a thorough assessment is performed onthe primary/secondary components and load paths, andthe requirements of Sections 2.7 and 6.3.3 are met. Thisassessment should include exposure of at least onesample of each primary component connection type andcomparison with construction documents. However, iforiginal reinforcing steel shop drawings, materialspecifications, and field inspection or quality controlrecords are available, this effort is not required.If incomplete knowledge of as-built component orconnection configuration exists because a smallersampling is performed than that required for κ = 1.0, κshall be reduced to 0.75. Rehabilitation requires that aminimum sampling be performed from whichknowledge of as-built conditions can be surmised.Where a κ of 0.75 cannot be justified, no seismicresistance capacity may be used for existingcomponents.If all required testing for κ = 1.0 is done and thefollowing situations prevail, κ shall be reduced to 0.75.• Construction documents for the concrete structureare not available or are incomplete.• Components are found degraded during assessment,for which further testing is required to qualifybehavior and to use κ = 1.0.• Components have high variability in mechanicalproperties (up to a coefficient of variation of 25%).• Components shown in construction documents lacksufficient structural detail to allow proper analysis.• Components contain archaic or proprietary materialand their materials condition is uncertain.6.3.5 Rehabilitation IssuesUpon determining that concrete elements in an existingbuilding are deficient for the desired RehabilitationObjective, the next step is to define rehabilitation orreplacement alternatives. If replacement of the elementis selected, design of the new element shall be in6-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)accordance with local building codes and the NEHRPRecommended Provisions for new buildings (BSSC,1995).6.3.6 ConnectionsConnections between existing concrete components andany components added to rehabilitate the originalstructure are critical to overall seismic performance.The design professional is strongly encouraged toexamine as-built connections and perform any physicaltesting/inspection to assess their performance. All newconnections shall be subject to the quality controlprovisions contained in these Guidelines. In addition,for connectors that are not cast-in-place, such as anchorbolts, a minimum of five samples from each connectortype shall be tested after installation. Connectors thatrely on ductility shall be tested according toSection 2.13. (See also Section 6.4.6.)6.4 General Assumptions andRequirements6.4.1 Modeling and Design6.4.1.1 General ApproachDesign approaches for an existing or rehabilitatedbuilding generally shall follow procedures ofACI 318-95 (ACI, 1995), except as otherwise indicatedin these Guidelines, and shall emphasize the following.• Brittle or low-ductility failure modes shall beidentified as part of the analysis. These typicallyinclude behavior in direct or nearly-directcompression, shear in slender components and incomponent connections, torsion in slendercomponents, and reinforcement development,splicing, and anchorage. It is preferred that thestresses, forces, and moments acting to cause thesefailure modes be determined from consideration ofthe probable resistances at the locations fornonlinear action.• Analysis of reinforced concrete components shallinclude an evaluation of demands and capacities atall sections along the length of the component.Particular attention shall be paid to locations wherelateral and gravity loads produce maximum effects;where changes in cross section or reinforcementresult in reduced strength; and where abrupt changesin cross section or reinforcement, including splices,may produce stress concentrations resulting inpremature failure.6.4.1.2 StiffnessComponent stiffnesses shall be calculated according toaccepted principles of mechanics. Sources of flexibilityshall include flexure, shear, axial load, andreinforcement slip from adjacent connections andcomponents. Stiffnesses should be selected to representthe stress and deformation levels to which thecomponents will be subjected, considering volumechange effects (temperature and shrinkage) combinedwith design earthquake and gravity load effects.A. Linear ProceduresWhere design actions are determined using the linearprocedures of Chapter 3, component effectivestiffnesses shall correspond to the secant value to theyield point for the component, except that higherstiffnesses may be used where it is demonstrated byanalysis to be appropriate for the design loading. Theeffective stiffness values in Table 6-4 should be used,except where little nonlinear behavior is expected ordetailed evaluation justifies different values. Thesesame stiffnesses may be appropriate for the initialstiffness for use in the nonlinear procedures ofChapter 3.B. Nonlinear ProceduresWhere design actions are determined using thenonlinear procedures of Chapter 3, component loaddeformationresponse shall be represented by nonlinearload-deformation relations, except that linear relationsare acceptable where nonlinear response will not occurin the component. The nonlinear load-deformationrelation shall be based on experimental evidence or maybe taken from quantities specified in Sections 6.5through 6.13. The nonlinear load-deformation relationfor the Nonlinear Static Procedure (NSP) may becomposed of line segments or curves defining behaviorunder monotonically increasing lateral deformation.The nonlinear load-deformation relation for theNonlinear Dynamic Procedure (NDP) may becomposed of line segments or curves, and shall definebehavior under monotonically increasing lateraldeformation and under multiple reversed deformationcycles.Figure 6-1 illustrates a generalized load-deformationrelation that may be applicable for most concretecomponents evaluated using the NSP. The relation isFEMA 273 Seismic Rehabilitation Guidelines 6-11

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-4Effective Stiffness ValuesComponent Flexural Rigidity Shear Rigidity Axial RigidityBeams—nonprestressed 0.5E c I g 0.4E c A w —Beams—prestressed E c I g 0.4E c A w —Columns in compression 0.7E c I g 0.4E c A w E c A gColumns in tension 0.5E c I g 0.4E c A w E s A sWalls—uncracked (on inspection) 0.8E c I g 0.4E c A w E c A gWalls—cracked 0.5E c I g 0.4E c A w E c A gFlat Slabs—nonprestressed See Section 6.5.4.2 0.4E c A g —Flat Slabs—prestressed See Section 6.5.4.2 0.4E c A g —Note: I gfor T-beams may be taken as twice the value of I gof the web alone, or may be based on the effective width as defined in Section 6.4.1.3.For shear stiffness, the quantity 0.4E chas been used to represent the shear modulus G.described by linear response from A (unloadedcomponent) to an effective yield B. Subsequently, thereis linear response, at reduced stiffness, from B to C,with sudden reduction in lateral load resistance to D,response at reduced resistance to E, and final loss ofresistance thereafter. The slope from A to B shall beaccording to Section 6.4.1.2A. The slope from B to C,ignoring effects of gravity loads acting through lateraldisplacements, typically may be taken as equal tobetween zero and 10% of the initial slope. C has anordinate equal to the strength of the component and anabscissa equal to the deformation at which significantstrength degradation begins. It is permissible torepresent the load-deformation relation by linesconnecting points A, B, and C, provided that thecalculated response is not beyond C. It is alsoacceptable to use more refined relations where they arejustified by experimental evidence. Sections 6.5through 6.13 recommend numerical values for thepoints identified in Figure 6-1.Typically, the responses shown in Figure 6-1 areassociated with flexural response or tension response.In this case, the resistance at Q/Q CE = 1.0 is the yieldvalue, and subsequent strain hardening accommodatesstrain hardening in the load-deformation relation as themember is deformed toward the expected strength.When the response shown in Figure 6-1 is associatedwith compression, the resistance at Q/Q CE = 1.0typically is the value at which concrete begins to spall,and strain hardening in well-confined sections may beassociated with strain hardening of the longitudinalQQ CE1.0Figure 6-1AB(a) DeformationQQ CE1.0ABθda(b) Deformation ratioorbeGeneralized Load-Deformation Relationreinforcement and the confined concrete. When the∆∆hCDCDEEcc6-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)response shown in Figure 6-1 is associated with shear,the resistance at Q/Q CE = 1.0 typically is the value atwhich the design shear strength is reached, and no strainhardening follows.Figure 6-1 shows two different ways to define thedeformations, as follows:(a) Deformation, or Type I. In this curve, deformationsare expressed directly using terms such as strain,curvature, rotation, or elongation. The parameters a andb refer to those portions of the deformation that occurafter yield; that is, the plastic deformation. Theparameter c is the reduced resistance after the suddenreduction from C to D. Parameters a, b, and c aredefined numerically in various tables in this chapter.(b) Deformation Ratio, or Type II. In this curve,deformations are expressed in terms such as shear angleand tangential drift ratio. The parameters d and e referto total deformations measured from the origin.Parameters c, d, and e are defined numerically invarious tables in this chapter.6.4.1.3 Flanged ConstructionIn components and elements consisting of a web andflange that act integrally, the combined stiffness andstrength for flexural and axial loading shall becalculated considering a width of effective flange oneach side of the web equal to the smaller of (1) theprovided flange width, (2) eight times the flangethickness, (3) half the distance to the next web, and (4)one-fifth of the span for beams or one-half the totalheight for walls. When the flange is in compression,both the concrete and reinforcement within the effectivewidth shall be considered effective in resisting flexureand axial load. When the flange is in tension,longitudinal reinforcement within the effective widthshall be considered fully effective for resisting flexureand axial loads, provided that proper splice lengths inthe reinforcement can be verified. The portion of theflange extending beyond the width of the web shall beassumed ineffective in resisting shear.6.4.2 Design Strengths and Deformabilities6.4.2.1 GeneralActions in a structure shall be classified as being eitherdeformation-controlled or force-controlled, as definedin Chapter 3. General procedures for calculating designstrengths for deformation-controlled and forcecontrolledactions shall be according to Sections 6.4.2.2and 6.4.2.3.Components shall be classified as having low,moderate, or high ductility demands according toSection 6.4.2.4.Where strength and deformation capacities are derivedfrom test data, the tests shall be representative ofproportions, details, and stress levels for thecomponent. General requirements for testing arespecified in Section 2.13.1.Strengths and deformation capacities given in thischapter are for earthquake loadings involving threefully reversed deformation cycles to the designdeformation levels, in addition to similar cycles tolesser deformation levels. In some cases—includingsome short-period buildings, and buildings subjected toa long-duration design earthquake—a building may beexpected to be subjected to more numerous cycles to thedesign deformation levels. The increased number ofcycles may lead to reductions in resistance anddeformation capacity. The effects on strength anddeformation capacity of more numerous deformationcycles should be considered in design. Largeearthquakes will cause more numerous cycles.6.4.2.2 Deformation-Controlled ActionsDeformation-controlled actions are defined inSection 3.2.2.4. Strengths used in design fordeformation-controlled actions generally are denotedQ CE and shall be taken as equal to expected strengthsobtained experimentally or calculated using acceptedmechanics principles. Expected strength is defined asthe mean maximum resistance expected over the rangeof deformations to which the component is likely to besubjected. When calculations are used to define meanexpected strength, expected material strength—including strain hardening—is to be taken into account.The tensile stress in yielding longitudinal reinforcementshall be assumed to be at least 1.25 times the nominalyield stress. Procedures specified in ACI 318 may beused to calculate strengths used in design, except thatthe strength reduction factor, φ, shall be taken as equalto unity, and other procedures specified in theseGuidelines shall govern where applicable.6.4.2.3 Force-Controlled ActionsForce-controlled actions are defined in Chapter 3.Strengths used in design for force-controlled actionsFEMA 273 Seismic Rehabilitation Guidelines 6-13

Chapter 6: Concrete(Systematic Rehabilitation)generally are denoted Q CL and shall be taken as equal tolower bound strengths obtained experimentally orcalculated using established mechanics principles.Lower bound strength is defined generally as the lowerfive percentile of strengths expected. Where thestrength degrades with continued cycling or increasedlateral deformations, the lower bound strength isdefined as the expected minimum value within therange of deformations and loading cycles to which thecomponent is likely to be subjected. When calculationsare used to define lower bound strengths, lower boundestimates of material properties are to be assumed.Procedures specified in ACI 318 may be used tocalculate strengths used in design, except otherprocedures specified in the Guidelines shall governwhere applicable (see Section 6.3.2.5).6.4.2.4 Component Ductility DemandClassificationSome strength calculation procedures in this chapterrequire definition of component ductility demandclassification. For this purpose, components shall alsobe classified as having low, moderate, or high ductilitydemands, based on the maximum value of the demandcapacity ratio (DCR; see Section 2.9.1) from the linearprocedures of Chapter 3, or the calculated displacementductility from the nonlinear procedures of Chapter 3.Table 6-5 defines the relation.Table 6-5Component Ductility DemandClassificationMaximum value of DCR ordisplacement ductility Descriptor< 2 Low Ductility Demand2 to 4 Moderate Ductility Demand> 4 High Ductility Demand6.4.3 Flexure and Axial LoadsFlexural strength and deformability of members withand without axial loads shall be calculated according toaccepted procedures. Strengths and deformabilities ofcomponents with monolithic flanges shall be calculatedconsidering concrete and developed longitudinalreinforcement within the effective flange width definedin Section 6.4.1.3. Strengths and deformabilities shallbe determined considering available development oflongitudinal reinforcement.Without confining transverse reinforcement, maximumusable strain at extreme concrete compression fibershall not exceed 0.002 for components in nearly purecompression and 0.005 for other components. Largerstrains are permitted where transverse reinforcementprovides confinement. Maximum allowablecompression strains for confined concrete shall bebased on experimental evidence and shall considerlimitations posed by fracture of transversereinforcement, buckling of longitudinal reinforcement,and degradation of component resistance at largedeformation levels. Maximum compression strain shallnot exceed 0.02, and maximum longitudinalreinforcement tension strain shall not exceed 0.05.Where longitudinal reinforcement has embedment ordevelopment length into adjacent components that isinsufficient for development of reinforcementstrength—as in beams with bottom bars embedded ashort distance into beam-column joints—flexuralstrength shall be calculated based on limiting stresscapacity of the embedded bar as defined inSection 6.4.5.Where flexural deformation capacities are calculatedfrom basic mechanics principles, reduction indeformation capacity due to applied shear shall be takeninto consideration.6.4.4 Shear and TorsionStrengths in shear and torsion shall be calculatedaccording to ACI 318 (ACI, 1995), except as notedbelow and in Sections 6.5 and 6.9.Within yielding regions of components with moderateor high ductility demands, shear and torsion strengthshall be calculated according to accepted procedures forductile components (for example, the provisions ofChapter 21 of ACI 318-95). Within yielding regions ofcomponents with low ductility demands, and outsideyielding regions, shear strength may be calculated usingaccepted procedures normally used for elastic response(for example, the provisions of Chapter 11 of ACI 318).Within yielding regions of components with moderateor high ductility demands, transverse reinforcementshall be assumed ineffective in resisting shear or torsionwhere: (1) longitudinal spacing of transversereinforcement exceeds half the component effectivedepth measured in the direction of shear, or (2)perimeter hoops are either lap spliced or have hooksthat are not adequately anchored in the concrete core.6-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Within yielding regions of components with lowductility demands, and outside yielding regions,transverse reinforcement shall be assumed ineffective inresisting shear or torsion where the longitudinal spacingof transverse reinforcement exceeds the componenteffective depth measured in the direction of shear.Shear friction strength shall be calculated according toACI 318-95, taking into consideration the expectedaxial load due to gravity and earthquake effects. Whererehabilitation involves addition of concrete requiringoverhead work with dry-pack, the shear frictioncoefficient µ shall be taken as equal to 70% of the valuespecified by ACI 318-95.6.4.5 Development and Splices ofReinforcementDevelopment strength of straight bars, hooked bars, andlap splices shall be calculated according to the generalprovisions of ACI 318-95, with the followingmodifications:Within yielding regions of components with moderateor high ductility demands, details and strengthprovisions for new straight developed bars, hookedbars, and lap spliced bars shall be according toChapter 21 of ACI 318-95. Within yielding regions ofcomponents with low ductility demands, and outsideyielding regions, details and strength provisions for newconstruction shall be according to Chapter 12 ofACI 318-95, except requirements and strengthprovisions for lap splices may be taken as equal to thosefor straight development of bars in tension withoutconsideration of lap splice classifications.Where existing development, hook, and lap splicelength and detailing requirements are not according tothe requirements of the preceding paragraph, maximumstress capacity of reinforcement shall be calculatedaccording to Equation 6-1.f sl b= ---fl yd(6-1)where f s = bar stress capacity for the development,hook, or lap splice length l b provided; l d = lengthrequired by Chapter 12 or Chapter 21 (as appropriate)of ACI 318-95 for development, hook, or lap splicelength, except splices may be assumed to be equivalentto straight bar development in tension; and f y = yieldstrength of reinforcement. Where transversereinforcement is distributed along the developmentlength with spacing not exceeding one-third of theeffective depth, the developed reinforcement may beassumed to retain the calculated stress capacity to largeductility levels. For larger spacings of transversereinforcement, the developed stress shall be assumed todegrade from f s to 0.2f s at ductility demand or DCRequal to 2.0.Strength of straight, discontinuous bars embedded inconcrete sections (including beam-column joints) withclear cover over the embedded bar not less than 3d b maybe calculated according to Equation 6-2.2500f s = -----------ld e ≤ f y b(6-2)where f s = maximum stress (in psi) that can bedeveloped in an embedded bar having embedmentlength l e (in inches), d b = diameter of embedded bar (ininches), and f y = bar yield stress (in psi). When theexpected stress equals or exceeds f s as calculated above,and f s is less than f y , the developed stress shall beassumed to degrade from f s to 0.2f s at ductility demandor DCR equal to 2.0. In beams with short bottom barembedments into beam-column joints, flexural strengthshall be calculated considering the stress limitation ofEquation 6-2, and modeling parameters and acceptancecriteria shall be according to Section 6.5.2.Doweled bars added in seismic rehabilitation may beassumed to develop yield stress when all the followingare satisfied: (1) drilled holes for dowel bars are cleanedwith a stiff brush that extends the length of the hole;(2) embedment length l e is not less than 10d b ; and(3) minimum spacing of dowel bars is not less than 4l e ,and minimum edge distance is not less than 2l e . Otherdesign values for dowel bars shall be verified by testdata. Field samples shall be obtained to ensure designstrengths are developed per Section 6.3.3.6.4.6 Connections to Existing ConcreteConnections used to connect two or more componentsmay be classified according to their anchoring systemsas cast-in-place systems or as post-installed systems.FEMA 273 Seismic Rehabilitation Guidelines 6-15

Chapter 6: Concrete(Systematic Rehabilitation)6.4.6.1 Cast-In-Place SystemsThe capacity of the connection should be not less than1.25 times the smaller of (1) the force corresponding todevelopment of the minimum probable strength of thetwo interconnected components, and (2) the componentactions at the connection. Shear forces, tension forces,bending moments, and prying actions shall beconsidered. Design values for connection anchoragesshall be ultimate values, and shall be taken as suggestedin ACI Report 355.1R-91, or as specified in the latestversion of the locally adopted strength design buildingcode.The capacity of anchors placed in areas where crackingis expected shall be reduced by a factor of 0.5.6.4.6.2 Post-Installed SystemsThe capacity should be calculated according toSection 6.4.6.1. See the Commentary for exceptions.6.4.6.3 Quality ControlSee Commentary for this section.6.5 Concrete Moment Frames6.5.1 Types of Concrete Moment FramesConcrete moment frames are those elements composedprimarily of horizontal framing components (beamsand/or slabs) and vertical framing components(columns) that develop lateral load resistance throughbending of horizontal and vertical framing components.These elements may act alone to resist lateral loads, orthey may act in conjunction with shear walls, bracedframes, or other elements to form a dual system.The provisions in Section 6.5 are applicable to framesthat are cast monolithically, including monolithicconcrete frames rehabilitated or created by the additionof new material. Frames covered under this sectioninclude reinforced concrete beam-column momentframes, prestressed concrete beam-column momentframes, and slab-column moment frames. Sections 6.6,6.7, and 6.10 apply to precast concrete frames, infilledconcrete frames, and concrete braced frames,respectively.6.5.1.1 Reinforced Concrete Beam-ColumnMoment FramesReinforced concrete beam-column moment frames arethose frames that satisfy the following conditions:1. Framing components are beams (with or withoutslabs) and columns.2. Beams and columns are of monolithic constructionthat provides for moment transfer between beamsand columns.3. Primary reinforcement in components contributingto lateral load resistance is nonprestressed.The frames include Special Moment Frames,Intermediate Moment Frames, and Ordinary MomentFrames as defined in the 1994 NEHRP RecommendedProvisions (BSSC, 1995), as well as frames notsatisfying the requirements of these Provisions. Thisclassification includes existing construction, newconstruction, and existing construction that has beenrehabilitated.6.5.1.2 Post-Tensioned Concrete Beam-Column Moment FramesPost-tensioned concrete beam-column moment framesare those frames that satisfy the following conditions:1. Framing components are beams (with or withoutslabs) and columns.2. Beams and columns are of monolithic constructionthat provides for moment transfer between beamsand columns.3. Primary reinforcement in beams contributing tolateral load resistance includes post-tensionedreinforcement with or without nonprestressedreinforcement.This classification includes existing construction, newconstruction, and existing construction that has beenrehabilitated.6.5.1.3 Slab-Column Moment FramesSlab-column moment frames are those frames thatsatisfy the following conditions:1. Framing components are slabs (with or withoutbeams in the transverse direction) and columns.6-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)2. Slabs and columns are of monolithic constructionthat provides for moment transfer between slabs andcolumns.3. Primary reinforcement in slabs contributing tolateral load resistance includes nonprestressedreinforcement, prestressed reinforcement, or both.The slab-column frame may or may not have beenintended in the original design to be part of the lateralload-resistingsystem. This classification includesexisting construction, new construction, and existingconstruction that has been rehabilitated.6.5.2 Reinforced Concrete Beam-ColumnMoment Frames6.5.2.1 General ConsiderationsThe analysis model for a beam-column frame elementshall represent strength, stiffness, and deformationcapacity of beams, columns, beam-column joints, andother components that may be part of the frame,including connections with other elements. Potentialfailure in flexure, shear, and reinforcement developmentat any section along the component length shall beconsidered. Interaction with other elements, includingnonstructural elements and components, shall beincluded.The analytical model generally can represent a beamcolumnframe using line elements with propertiesconcentrated at component centerlines. Where beamand column centerlines do not coincide, the effects onframing shall be considered. Where minor eccentricitiesoccur (i.e., the centerline of the narrower componentfalls within the middle third of the adjacent framingcomponent measured transverse to the framingdirection), the effect of the eccentricity can be ignored.Where larger eccentricities occur, the effect shall berepresented either by reductions in effective stiffnesses,strengths, and deformation capacities, or by directmodeling of the eccentricity.The beam-column joint in monolithic constructiongenerally shall be represented as a stiff or rigid zonehaving horizontal dimensions equal to the columncross-sectional dimensions and vertical dimensionequal to the beam depth, except that a wider joint maybe assumed where the beam is wider than the columnand where justified by experimental evidence. Themodel of the connection between the columns andfoundation shall be selected based on the details of thecolumn-foundation connection and rigidity of thefoundation-soil system.Action of the slab as a diaphragm interconnectingvertical elements shall be represented. Action of theslab as a composite beam flange is to be considered indeveloping stiffness, strength, and deformationcapacities of the beam component model, according toSection 6.4.1.3.Inelastic deformations in primary components shall berestricted to flexure in beams (plus slabs, if present) andcolumns. Other inelastic deformations are permitted insecondary components. Acceptance criteria areprovided in Section 6.5.2.4.6.5.2.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresBeams shall be modeled considering flexural and shearstiffnesses, including in monolithic construction theeffect of the slab acting as a flange. Columns shall bemodeled considering flexural, shear, and axialstiffnesses. Joints shall be modeled as stiff components,and may in most cases be considered rigid. Effectivestiffnesses shall be according to Section 6.4.1.2.B. Nonlinear Static ProcedureNonlinear load-deformation relations shall follow thegeneral guidelines of Section 6.4.1.2.Beams and columns may be modeled usingconcentrated plastic hinge models, distributed plastichinge models, or other models whose behavior has beendemonstrated to adequately represent importantcharacteristics of reinforced concrete beam and columncomponents subjected to lateral loading. The modelshall be capable of representing inelastic response alongthe component length, except where it is shown byequilibrium that yielding is restricted to the componentends. Where nonlinear response is expected in a modeother than flexure, the model shall be established torepresent these effects.Monotonic load-deformation relations shall beaccording to the generalized relation shown inFigure 6-1, except that different relations are permittedwhere verified by experiments. In that figure, point Bcorresponds to significant yielding, C corresponds tothe point where significant lateral load resistance can beassumed to be lost, and E corresponds to the pointwhere gravity load resistance can be assumed to be lost.FEMA 273 Seismic Rehabilitation Guidelines 6-17

Chapter 6: Concrete(Systematic Rehabilitation)The overall load-deformation relation shall beestablished so that the maximum resistance is consistentwith the design strength specifications of Sections 6.4.2and 6.5.2.3.For beams and columns, the generalized deformation inFigure 6-1 may be either the chord rotation or theplastic hinge rotation. For beam-column joints, anacceptable measure of the generalized deformation isshear strain. Values of the generalized deformation atpoints B, C, and D may be derived from experiments orrational analyses, and shall take into account theinteractions between flexure, axial load, and shear.Alternately, where the generalized deformation is takenas rotation in the flexural plastic hinge zone in beamsand columns, the plastic hinge rotation capacities shallbe as defined by Tables 6-6 and 6-7. Where thegeneralized deformation is shear distortion of the beamcolumnjoint, shear angle capacities shall be as definedby Table 6-8.C. Nonlinear Dynamic ProcedureFor the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby experimental evidence. The relation of Figure 6-1may be taken to represent the envelope relation for theanalysis. Unloading and reloading properties shallrepresent significant stiffness and strength degradationcharacteristics.6.5.2.3 Design StrengthsComponent strengths shall be computed according tothe general requirements of Section 6.4.2, as modifiedin this section.The maximum component strength shall be determinedconsidering potential failure in flexure, axial load,shear, torsion, development, and other actions at allpoints along the length of the component under theactions of design gravity and lateral load combinations.For columns, the contribution of concrete to shearstrength, V c, may be calculated according toEquation 6-3.V c⎛ N3.5λ k u ⎞= ⎜ + ----------------- ⎟⎝ 2000A g ⎠f c ′ b w d(6-3)in which k = 1.0 in regions of low ductility demand and0 in regions of moderate and high ductility demand,λ = 0.75 for lightweight aggregate concrete and 1.0 fornormal weight aggregate concrete, and N u = axialcompression force in pounds (= 0 for tension force). Allunits are expressed in pounds and inches. Where axialforce is calculated from the linear procedures ofChapter 3, compressive axial load for use inEquation 6-3 should be taken as equal to the valuecalculated considering design gravity load only, andtensile axial load should be taken as equal to the valuecalculated from the analysis considering design loadcombinations, including gravity and earthquake loadingaccording to Section 3.2.8.For columns satisfying the detailing and proportioningrequirements of Chapter 21 of ACI 318, the shearstrength equations of ACI 318 may be used.For beam-column joints, the nominal cross-sectionalarea, A j , shall be defined by a joint depth equal to thecolumn dimension in the direction of framing and ajoint width equal to the smallest of (1) the columnwidth, (2) the beam width plus the joint depth, and(3) twice the smaller perpendicular distance from thelongitudinal axis of the beam to the column side.Design forces shall be calculated based on developmentof flexural plastic hinges in adjacent framing members,including effective slab width, but need not exceedvalues calculated from design gravity and earthquakeload combinations. Nominal joint shear strength V nshall be calculated according to the general proceduresof ACI 318, modified as described below.Q CL = V n = λγ f c ′ A j , psi(6-4)in which λ = 0.75 for lightweight aggregate concreteand 1.0 for normal weight aggregate concrete, A j is theeffective horizontal joint area with dimension asdefined above, and γ is as defined in Table 6-9.6.5.2.4 Acceptance CriteriaA. Linear Static and Dynamic ProceduresAll actions shall be classified as being eitherdeformation-controlled or force-controlled, as definedin Chapter 3. In primary components, deformationcontrolledactions shall be restricted to flexure in beams(with or without slab) and columns. In secondarycomponents, deformation-controlled actions shall berestricted to flexure in beams (with or without slab),plus restricted actions in shear and reinforcementdevelopment, as identified in Tables 6-10 through 6-12.6-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-6Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete BeamsModeling Parameters 3 Acceptance Criteria 3Plastic Rotation Angle, radiansComponent TypePlastic RotationAngle, radiansResidualStrengthRatioPrimaryPerformance LevelSecondaryConditionsa b c IO LS CP LS CPi. Beams controlled by flexure 1ρ – ρ′ Trans. V------------- Reinf.ρ 2 -------------------balb w d f c ′≤ 0.0 C ≤ 3 0.025 0.05 0.2 0.005 0.02 0.025 0.02 0.05≤ 0.0 C ≥ 6 0.02 0.04 0.2 0.005 0.01 0.02 0.02 0.04≥ 0.5 C ≤ 3 0.02 0.03 0.2 0.005 0.01 0.02 0.02 0.03≥ 0.5 C ≥ 6 0.015 0.02 0.2 0.005 0.005 0.015 0.015 0.02≤ 0.0 NC ≤ 3 0.02 0.03 0.2 0.005 0.01 0.02 0.02 0.03≤ 0.0 NC ≥ 6 0.01 0.015 0.2 0.0 0.005 0.01 0.01 0.015≥ 0.5 NC ≤ 3 0.01 0.015 0.2 0.005 0.01 0.01 0.01 0.015≥ 0.5 NC ≥ 6 0.005 0.01 0.2 0.0 0.005 0.005 0.005 0.01ii. Beams controlled by shear 1Stirrup spacing ≤ d/2 0.0 0.02 0.2 0.0 0.0 0.0 0.01 0.02Stirrup spacing > d/2 0.0 0.01 0.2 0.0 0.0 0.0 0.005 0.01iii. Beams controlled by inadequate development or splicing along the span 1Stirrup spacing ≤ d/2 0.0 0.02 0.0 0.0 0.0 0.0 0.01 0.02Stirrup spacing > d/2 0.0 0.01 0.0 0.0 0.0 0.0 0.005 0.01iv. Beams controlled by inadequate embedment into beam-column joint 10.015 0.03 0.2 0.01 0.01 0.015 0.02 0.031. When more than one of the conditions i, ii, iii, and iv occurs for a given component, use the minimum appropriate numerical value from the table.2. Under the heading “Transverse Reinforcement,” “C” and “NC” are abbreviations for conforming and nonconforming details, respectively. A componentis conforming if, within the flexural plastic region, closed stirrups are spaced at ≤ d/3, and if, for components of moderate and high ductility demand, thestrength provided by the stirrups (V s ) is at least three-fourths of the design shear. Otherwise, the component is considered nonconforming.3. Linear interpolation between values listed in the table is permitted.All other actions shall be defined as being forcecontrolledactions.Design actions on components shall be determined asprescribed in Chapter 3. Where the calculated DCRvalues exceed unity, the following actions preferablyshall be determined using limit analysis principles asprescribed in Chapter 3: (1) moments, shears, torsions,and development and splice actions corresponding todevelopment of component strength in beams andFEMA 273 Seismic Rehabilitation Guidelines 6-19

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-7Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete ColumnsModeling Parameters 4 Acceptance Criteria 4Plastic Rotation Angle, radiansComponent TypePlastic RotationAngle, radiansResidualStrengthRatioPrimaryPerformance LevelSecondaryConditionsa b c IO LS CP LS CPi. Columns controlled by flexure 1-----------PA g f c ′Trans.Reinf. 2-------------------Vb w d f c ′≤ 0.1 C ≤ 3 0.02 0.03 0.2 0.005 0.01 0.02 0.015 0.03≤ 0.1 C ≥ 6 0.015 0.025 0.2 0.005 0.01 0.015 0.01 0.025≥ 0.4 C ≤ 3 0.015 0.025 0.2 0.0 0.005 0.015 0.010 0.025≥ 0.4 C ≥ 6 0.01 0.015 0.2 0.0 0.005 0.01 0.01 0.015≤ 0.1 NC ≤ 3 0.01 0.015 0.2 0.005 0.005 0.01 0.005 0.015≤ 0.1 NC ≥ 6 0.005 0.005 – 0.005 0.005 0.005 0.005 0.005≥ 0.4 NC ≤ 3 0.005 0.005 – 0.0 0.0 0.005 0.0 0.005≥ 0.4 NC ≥ 6 0.0 0.0 – 0.0 0.0 0.0 0.0 0.0ii. Columns controlled by shear 1,3Hoop spacing ≤ d/2,or -----------P≤ 0.1A g f c ′0.0 0.015 0.2 0.0 0.0 0.0 0.01 0.015Other cases 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0iii. Columns controlled by inadequate development or splicing along the clear height 1,3Hoop spacing ≤ d/2 0.01 0.02 0.4 1 1 1 0.01 0.02Hoop spacing > d/2 0.0 0.01 0.2 1 1 1 0.005 0.011,3iv. Columns with axial loads exceeding 0.70P oConforming reinforcement over the 0.015 0.025 0.02 0.0 0.005 0.001 0.01 0.02entire lengthAll other cases 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.01. When more than one of the conditions i, ii, iii, and iv occurs for a given component, use the minimum appropriate numerical value from the table.2. Under the heading “Transverse Reinforcement,” “C” and “NC” are abbreviations for conforming and nonconforming details, respectively. A componentis conforming if, within the flexural plastic hinge region, closed hoops are spaced at ≤ d/3, and if, for components of moderate and high ductility demand,the strength provided by the stirrups (V s ) is at least three-fourths of the design shear. Otherwise, the component is considered nonconforming.3. To qualify, hoops must not be lap spliced in the cover concrete, and hoops must have hooks embedded in the core or other details to ensure that hoopswill be adequately anchored following spalling of cover concrete.4. Linear interpolation between values listed in the table is permitted.6-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-8Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete Beam-Column JointsModeling Parameters 4 Acceptance Criteria 4Plastic Rotation Angle, radiansComponent TypeShear Angle,radiansResidualStrengthRatioPrimaryPerformance LevelSecondaryConditionsd e c IO LS CP LS CPi. Interior jointsTrans.-----------P 2Reinf.A 1 -----V 3g f c ′V n≤ 0.1 C ≤ 1.2 0.015 0.03 0.2 0.0 0.0 0.0 0.02 0.03≤ 0.1 C ≥ 1.5 0.015 0.03 0.2 0.0 0.0 0.0 0.015 0.02≥ 0.4 C ≤ 1.2 0.015 0.025 0.2 0.0 0.0 0.0 0.015 0.025≥ 0.4 C ≥ 1.5 0.015 0.02 0.2 0.0 0.0 0.0 0.015 0.02≤ 0.1 NC ≤ 1.2 0.005 0.02 0.2 0.0 0.0 0.0 0.015 0.02≤ 0.1 NC ≥ 1.5 0.005 0.015 0.2 0.0 0.0 0.0 0.01 0.015≥ 0.4 NC ≤ 1.2 0.005 0.015 0.2 0.0 0.0 0.0 0.01 0.015≥ 0.4 NC ≥ 1.5 0.005 0.015 0.2 0.0 0.0 0.0 0.01 0.015ii. Other jointsTrans.-----------P 2Reinf. 1 -----V 3A g f c ′V n≤ 0.1 C ≤ 1.2 0.01 0.02 0.2 0.0 0.0 0.0 0.015 0.02≤ 0.1 C ≥ 1.5 0.01 0.015 0.2 0.0 0.0 0.0 0.01 0.015≥ 0.4 C ≤ 1.2 0.01 0.02 0.2 0.0 0.0 0.0 0.015 0.02≥ 0.4 C ≥ 1.5 0.01 0.015 0.2 0.0 0.0 0.0 0.01 0.015≤ 0.1 NC ≤ 1.2 0.005 0.01 0.2 0.0 0.0 0.0 0.005 0.01≤ 0.1 NC ≥ 1.5 0.005 0.01 0.2 0.0 0.0 0.0 0.005 0.01≥ 0.4 NC ≤ 1.2 0.0 0.0 – 0.0 0.0 0.0 0.0 0.0≥ 0.4 NC ≥ 1.5 0.0 0.0 – 0.0 0.0 0.0 0.0 0.01. Under the heading “Transverse Reinforcement,” “C” and “NC” are abbreviations for conforming and nonconforming details, respectively. A joint isconforming if closed hoops are spaced at ≤ h c /3 within the joint. Otherwise, the component is considered nonconforming. Also, to qualify as conformingdetails under ii, hoops must not be lap spliced in the cover concrete, and must have hooks embedded in the core or other details to ensure that hoops willbe adequately anchored following spalling of cover concrete.2. This is the ratio of the design axial force on the column above the joint to the product of the gross cross-sectional area of the joint and the concretecompressive strength. The design axial force is to be calculated using limit analysis procedures, as described in Chapter 3.3. This is the ratio of the design shear force to the shear strength for the joint. The design shear force is to be calculated according to Section 6.5.2.3.4. Linear interpolation between values listed in the table is permitted.FEMA 273 Seismic Rehabilitation Guidelines 6-21

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-9Values of γ for Joint Strength CalculationInterior jointwithouttransversebeamsValue of γExterior jointwithouttransversebeamsInterior joint withExterior jointtransversewith transverseρ"beamsbeamsKnee joint

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-10Numerical Acceptance Criteria for Linear Procedures—Reinforced Concrete Beamsm factors 3Component TypePrimarySecondaryPerformance LevelConditionsIO LS CP LS CPi. Beams controlled by flexure 1ρ – ρ′ Trans. V------------- Reinf.ρ 2 -------------------balb w d f c ′≤ 0.0 C ≤ 3 2 6 7 6 10≤ 0.0 C ≥ 6 2 3 4 3 5≥ 0.5 C ≤ 3 2 3 4 3 5≥ 0.5 C ≥ 6 2 2 3 2 4≤ 0.0 NC ≤ 3 2 3 4 3 5≤ 0.0 NC ≥ 6 1 2 3 2 4≥ 0.5 NC ≤ 3 2 3 3 3 4≥ 0.5 NC ≥ 6 1 2 2 2 3ii. Beams controlled by shear 1Stirrup spacing ≤ d/2 – – – 3 4Stirrup spacing > d/2 – – – 2 3iii. Beams controlled by inadequate development or splicing along the span 1Stirrup spacing ≤ d/2 – – – 3 4Stirrup spacing > d/2 – – – 2 3iv. Beams controlled by inadequate embedment into beam-column joint 12 2 3 3 41. When more than one of the conditions i, ii, iii, and iv occurs for a given component, use the minimum appropriate numerical value from the table.2. Under the heading “Transverse Reinforcement,” “C” and “NC” are abbreviations for conforming and nonconforming details, respectively. A componentis conforming if, within the flexural plastic region, closed stirrups are spaced at ≤ d/3, and if, for components of moderate and high ductility demand, thestrength provided by the stirrups (V s ) is at least three-fourths of the design shear. Otherwise, the component is considered nonconforming.3. Linear interpolation between values listed in the table is permitted.• Post-tensioning existing beams, columns, orjoints using external post-tensionedreinforcement. Post-tensioned reinforcement shallbe unbonded within a distance equal to twice theeffective depth from sections where inelastic actionis expected. Anchorages shall be located away fromregions where inelastic action is anticipated, andshall be designed considering possible forcevariations due to earthquake loading.• Modification of the element by selective materialremoval from the existing element. Examplesinclude: (1) where nonstructural elements orcomponents interfere with the frame, removing orseparating the nonstructural elements or componentsto eliminate the interference; (2) weakening, usuallyby removal of concrete or severing of longitudinalreinforcement, to change response mode from anonductile mode to a more ductile mode (e.g.,FEMA 273 Seismic Rehabilitation Guidelines 6-23

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-11Numerical Acceptance Criteria for Linear Procedures—Reinforced Concrete Columnsm factors 4Component TypePrimarySecondaryPerformance LevelConditionsIO LS CP LS CPi. Columns controlled by flexure 1P-----------A g f c ′Trans.Reinf. 2V-------------------b w d f c ′≤ 0.1 C ≤ 3 2 3 4 3 4≤ 0.1 C ≥ 6 2 3 3 3 3≥ 0.4 C ≤ 3 1 2 2 2 2≥ 0.4 C ≥ 6 1 1 2 1 2≤ 0.1 NC ≤ 3 2 2 3 2 3≤ 0.1 NC ≥ 6 2 2 2 2 2≥ 0.4 NC ≤ 3 1 1 2 1 2≥ 0.4 NC ≥ 6 1 1 1 1 1ii. Columns controlled by shear 1,3Hoop spacing ≤ d/2,Por ----------- ≤ 0.1A g f c ′– – – 2 3Other cases – – – 1 1iii. Columns controlled by inadequate development or splicing along the clear height 1,3Hoop spacing ≤ d/2 – – – 3 4Hoop spacing > d/2 – – – 2 31,3iv. Columns with axial loads exceeding 0.70P oConforming reinforcement over the entire1 1 2 2 2lengthAll other cases – – – 1 11. When more than one of the conditions i, ii, iii, and iv occurs for a given component, use the minimum appropriate numerical value from the table.2. Under the heading “Transverse Reinforcement,” “C” and “NC” are abbreviations for conforming and nonconforming details, respectively. A componentis conforming if, within the flexural plastic hinge region, closed hoops are spaced at ≤ d/3, and if, for components of moderate and high ductility demand,the strength provided by the stirrups (V s ) is at least three-fourths of the design shear. Otherwise, the component is considered nonconforming.3. To qualify, hoops must not be lap spliced in the cover concrete, and must have hooks embedded in the core or other details to ensure that hoops will beadequately anchored following spalling of cover concrete.4. Linear interpolation between values listed in the table is permitted.weakening of beams to promote formation of astrong-column, weak-beam system); and (3)segmenting walls to change stiffness and strength.6-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-12Numerical Acceptance Criteria for Linear Procedures—Reinforced Concrete Beam-ColumnJointsm factors 4Component TypePrimary 5SecondaryPerformance LevelConditionsIO LS CP LS CPi. Interior joints-----------PA g f′c2Trans.Reinf. 1 -----VV 3n≤ 0.1 C ≤ 1.2 – – – 3 4≤ 0.1 C ≥ 1.5 – – – 2 3≥ 0.4 C ≤ 1.2 – – – 3 4≥ 0.4 C ≥ 1.5 – – – 2 3≤ 0.1 NC ≤ 1.2 – – – 2 3≤ 0.1 NC ≥ 1.5 – – – 2 3≥ 0.4 NC ≤ 1.2 – – – 2 3≥ 0.4 NC ≥ 1.5 – – – 2 3ii. Other joints-----------PA g f′c2Trans.Reinf. 1 -----VV 3n≤ 0.1 C ≤ 1.2 – – – 3 4≤ 0.1 C ≥ 1.5 – – – 2 3≥ 0.4 C ≤ 1.2 – – – 3 4≥ 0.4 C ≥ 1.5 – – – 2 3≤ 0.1 NC ≤ 1.2 – – – 2 3≤ 0.1 NC ≥ 1.5 – – – 2 3≥ 0.4 NC ≤ 1.2 – – – 1 1≥ 0.4 NC ≥ 1.5 – – – 1 11. Under the heading “Transverse Reinforcement,” “C” and “NC” are abbreviations for conforming and nonconforming details, respectively. A joint isconforming if closed hoops are spaced at ≤ h c /3 within the joint. Otherwise, the component is considered nonconforming. Also, to qualify as conformingdetails under ii, hoops must not be lap spliced in the cover concrete, and must have hooks embedded in the core or other details to ensure that hoops willbe adequately anchored following spalling of cover concrete.2. This is the ratio of the design axial force on the column above the joint to the product of the gross cross-sectional area of the joint and the concretecompressive strength. The design axial force is to be calculated using limit analysis procedures as described in Chapter 3.3. This is the ratio of the design shear force to the shear strength for the joint. The design shear force is to be calculated according to Section 6.5.2.3.4. Linear interpolation between values listed in the table is permitted.5. All interior joints are force-controlled, and no m factors apply.FEMA 273 Seismic Rehabilitation Guidelines 6-25

Chapter 6: Concrete(Systematic Rehabilitation)• Improvement of deficient existing reinforcementdetails. This approach involves removal of coverconcrete, modification of existing reinforcementdetails, and casting of new cover concrete. Concreteremoval shall avoid unintended damage to coreconcrete and the bond between existingreinforcement and core concrete. New coverconcrete shall be designed and constructed toachieve fully composite action with the existingmaterials.• Changing the building system to reduce thedemands on the existing element. Examplesinclude addition of supplementary lateral-forceresistingelements such as walls or buttresses,seismic isolation, and mass reduction.• Changing the frame element to a shear wall,infilled frame, or braced frame element byaddition of new material. Connections betweennew and existing materials shall be designed totransfer the forces anticipated for the design loadcombinations. Where the existing concrete framecolumns and beams act as boundary elements andcollectors for the new shear wall or braced frame,these shall be checked for adequacy, consideringstrength, reinforcement development, anddeformability. Diaphragms, including drag strutsand collectors, shall be evaluated and, if necessary,rehabilitated to ensure a complete load path to thenew shear wall or braced frame element.Rehabilitated frames shall be evaluated according to thegeneral principles and requirements of this chapter. Theeffects of rehabilitation on stiffness, strength, anddeformability shall be taken into account in ananalytical model of the rehabilitated structure.Connections required between existing and newelements shall satisfy the requirements of Section 6.4.6and other requirements of the Guidelines. An existingframe rehabilitated according to procedures listed aboveshall satisfy the relevant specific requirements ofChapter 6.6.5.3 Post-Tensioned Concrete Beam-Column Moment Frames6.5.3.1 General ConsiderationsThe analysis model for a post-tensioned concrete beamcolumnframe element shall be established followingthe guidelines established in Section 6.5.2.1 forreinforced concrete beam-column moment frames. Inaddition to potential failure modes described inSection 6.5.2.1, the analysis model shall considerpotential failure of tendon anchorages.The linear procedures and the NSP described inChapter 3 apply directly to frames with post-tensionedbeams in which the following conditions are satisfied:1. The average prestress, f pc , calculated for an areaequal to the product of the shortest cross-sectionaldimension and the perpendicular cross-sectionaldimension of the beam, does not exceed the greaterof 350 psi or f c ′ /12 at locations of nonlinear action.2. Prestressing tendons do not provide more than onequarterof the strength for both positive momentsand negative moments at the joint face.3. Anchorages for tendons have been demonstrated toperform satisfactorily for seismic loadings. Theseanchorages must occur outside hinging areas orjoints.Alternative procedures are required where theseconditions are not satisfied.6.5.3.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresBeams shall be modeled considering flexural and shearstiffnesses, including in monolithic and compositeconstruction the effect of the slab acting as a flange.Columns shall be modeled considering flexural, shear,and axial stiffnesses. Joints shall be modeled as stiffcomponents, and may in most cases be considered rigid.Effective stiffnesses shall be according toSection 6.4.1.2.B. Nonlinear Static ProcedureNonlinear load-deformation relations shall follow thegeneral guidelines of Section 6.4.1.2 and the reinforcedconcrete frame guidelines of Section 6.5.2.2B.Values of the generalized deformation at points B, C,and D in Figure 6-1 may be derived from experimentsor rational analyses, and shall take into account theinteractions between flexure, axial load, and shear.Alternately, where the generalized deformation is takenas rotation in the flexural plastic hinge zone, and wherethe three conditions of Section 6.5.3.1 are satisfied,beam plastic hinge rotation capacities may be as defined6-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)by Table 6-6. Columns and joints may be modeled asdescribed in Section 6.5.2.2.C. Nonlinear Dynamic ProcedureFor the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby experimental evidence. The relation of Figure 6-1may be taken to represent the envelope relation for theanalysis. Unloading and reloading properties shallrepresent significant stiffness and strength degradationcharacteristics as influenced by prestressing.6.5.3.3 Design StrengthsComponent strengths shall be computed according tothe general requirements of Section 6.4.2 and theadditional requirements of Section 6.5.2.3. Effects ofprestressing on strength shall be considered. Fordeformation-controlled actions, prestress shall beassumed to be effective for the purpose of determiningthe maximum actions that may be developed associatedwith nonlinear response of the frame. For forcecontrolledactions, the effects on strength of prestressloss shall also be considered as a design condition,where these losses are possible under design loadcombinations including inelastic deformation reversals.6.5.3.4 Acceptance CriteriaAcceptance criteria shall follow the criteria forreinforced concrete beam-column frames, as specifiedin Section 6.5.2.4.Tables 6-6, 6-7, 6-8, 6-10, 6-11, and 6-12 presentacceptability values for use in the four procedures ofChapter 3. The values in these tables for beams applyonly if the beams satisfy the three conditions ofSection 6.5.3.1.6.5.3.5 Rehabilitation MeasuresRehabilitation measures include the general approacheslisted in Section 6.5.2.5, as well as other approachesbased on rational procedures.Rehabilitated frames shall be evaluated according to thegeneral principles and requirements of this chapter. Theeffects of rehabilitation on stiffness, strength, anddeformability shall be taken into account in ananalytical model of the rehabilitated building.Connections required between existing and newelements shall satisfy the requirements of Section 6.4.6and other requirements of the Guidelines.6.5.4 Slab-Column Moment Frames6.5.4.1 General ConsiderationsThe analysis model for a slab-column frame elementshall represent strength, stiffness, and deformationcapacity of slabs, columns, slab-column connections,and other components that may be part of the frame.Potential failure in flexure, shear, shear-momenttransfer, and reinforcement development at any sectionalong the component length shall be considered.Interaction with other elements, including nonstructuralelements and components, shall be included.The analytical model can represent the slab-columnframe, using line elements with properties concentratedat component centerlines, or a combination of lineelements (to represent columns) and plate-bendingelements (to represent the slab). Three approaches arespecifically recognized.• Effective beam width model. Columns and slabsare represented by frame elements that are rigidlyinterconnected at the slab-column joint.• Equivalent frame model. Columns and slabs arerepresented by frame elements that areinterconnected by connection springs.• Finite element model. The columns are representedby frame elements and the slab is represented byplate-bending elements.In any model, the effects of changes in cross section,including slab openings, shall be considered.The model of the connection between the columns andfoundation shall be selected based on the details of thecolumn-foundation connection and rigidity of thefoundation-soil system.Action of the slab as a diaphragm interconnectingvertical elements shall be represented.Inelastic deformations in primary components shall berestricted to flexure in slabs and columns, plus limitednonlinear response in slab-column connections. Otherinelastic deformations are permitted in secondarycomponents. Acceptance criteria are in Section 6.5.4.4.FEMA 273 Seismic Rehabilitation Guidelines 6-27

Chapter 6: Concrete(Systematic Rehabilitation)6.5.4.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresSlabs shall be modeled considering flexural, shear, andtension (in the slab adjacent to the column) stiffnesses.Columns shall be modeled considering flexural, shear,and axial stiffnesses. Joints shall be modeled as stiffcomponents, and may in most cases be considered rigid.The effective stiffnesses of components shall beadjusted on the basis of experimental evidence torepresent effective stiffnesses according to the generalprinciples of Section 6.4.1.2.B. Nonlinear Static ProcedureNonlinear load-deformation relations shall follow thegeneral guidelines of Section 6.4.1.2.Slabs and columns may be modeled using concentratedplastic hinge models, distributed plastic hinge models,or other models whose behavior has been demonstratedto adequately represent important characteristics ofreinforced concrete slab and column componentssubjected to lateral loading. The model shall be capableof representing inelastic response along the componentlength, except where it is shown by equilibrium thatyielding is restricted to the component ends. Slabcolumnconnections preferably will be modeledseparately from the slab and column components, sothat potential failure in shear and moment transfer canbe identified. Where nonlinear response is expected in amode other than flexure, the model shall be establishedto represent these effects.Monotonic load-deformation relations shall beaccording to the generalized relation shown inFigure 6-1, with definitions according toSection 6.5.2.2B. The overall load-deformation relationshall be established so that the maximum resistance isconsistent with the design strength specifications ofSections 6.4.2 and 6.5.4.3. Where the generalizeddeformation shown in Figure 6-1 is taken as the flexuralplastic hinge rotation for the column, the plastic hingerotation capacities shall be as defined by Table 6-7.Where the generalized deformation shown in Figure 6-1is taken as the rotation of the slab-column connection,the plastic rotation capacities shall be as defined byTable 6-13.C. Nonlinear Dynamic ProcedureThe general approach shall be according to thespecification of Section 6.5.2.2C.6.5.4.3 Design StrengthsComponent strengths shall be according to the generalrequirements of Section 6.4.2, as modified in thissection.The maximum component strength shall be determinedconsidering potential failure in flexure, axial load,shear, torsion, development, and other actions at allpoints along the length of the component under theactions of design gravity and lateral load combinations.The strength of slab-column connections shall also bedetermined and incorporated in the analytical model.The flexural strength of a slab to resist moment due tolateral deformations shall be calculated as M nCS –M gCS , where M nCS is the design flexural strength of thecolumn strip and M gCS is the column strip moment dueto gravity loads. M gCS is to be calculated according tothe procedures of ACI 318-95 (ACI, 1995) for thedesign gravity load specified in Chapter 3.For columns, the shear strength may be evaluatedaccording to Section 6.5.2.3.Shear and moment transfer strength of the slab-columnconnection shall be calculated considering thecombined action of flexure, shear, and torsion acting inthe slab at the connection with the column. Anacceptable procedure is to calculate the shear andmoment transfer strength as described below.For interior connections without transverse beams, andfor exterior connections with moment about an axisperpendicular to the slab edge, the shear and momenttransfer strength may be taken as equal to the minimumof two strengths: (1) the strength calculated consideringeccentricity of shear on a slab critical section due tocombined shear and moment, as prescribed inACI 318-95; and (2) the moment transfer strength equalto ΣM n /γ f , where ΣM n = the sum of positive andnegative flexural strengths of a section of slab betweenlines that are two and one-half slab or drop panelthicknesses (2.5h) outside opposite faces of the columnor capital; γ f = the fraction of the moment resisted byflexure per ACI 318-95; and h = slab thickness.For moment about an axis parallel to the slab edge atexterior connections without transverse beams, wherethe shear on the slab critical section due to gravity loadsdoes not exceed 0.75V c , or the shear at a corner support6-28 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-13Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Two-Way Slabs and Slab-Column ConnectionsModeling Parameters 4 Acceptance Criteria 4Plastic Rotation Angle, radiansComponent TypePlastic RotationAngle, radiansResidualStrengthRatioPrimaryPerformance LevelSecondaryConditionsa b c IO LS CP LS CPi. Slabs controlled by flexure, and slab-column connections 1V Continuity----- g 2 Reinforcement 3V o≤ 0.2 Yes 0.02 0.05 0.2 0.01 0.015 0.02 0.03 0.05≥ 0.4 Yes 0.0 0.04 0.2 0.0 0.0 0.0 0.03 0.04≤ 0.2 No 0.02 0.02 – 0.01 0.015 0.02 0.015 0.02≥ 0.4 No 0.0 0.0 – 0.0 0.0 0.0 0.0 0.0ii. Slabs controlled by inadequate development or splicing along the span 10.0 0.02 0.0 0.0 0.0 0.0 0.01 0.02iii. Slabs controlled by inadequate embedment into slab-column joint 10.015 0.03 0.2 0.01 0.01 0.015 0.02 0.031. When more than one of the conditions i, ii, and iii occurs for a given component, use the minimum appropriate numerical value from the table.2. V g = the gravity shear acting on the slab critical section as defined by ACI 318; V o = the direct punching shear strength as defined by ACI 318.3. Under the heading “Continuity Reinforcement,” assume “Yes” where at least one of the main bottom bars in each direction is effectively continuousthrough the column cage. Where the slab is post-tensioned, assume “Yes” where at least one of the post-tensioning tendons in each direction passesthrough the column cage. Otherwise, assume “No.”4. Interpolation between values shown in the table is permitted.does not exceed 0.5 V c , the moment transfer strengthmay be taken as equal to the flexural strength of asection of slab between lines that are a distance, c 1 ,outside opposite faces of the column or capital. V c is thedirect punching shear strength defined by ACI 318-95.6.5.4.4 Acceptance CriteriaA. Linear Static and Dynamic ProceduresAll component actions shall be classified as being eitherdeformation-controlled or force-controlled, as definedin Chapter 3. In primary components, deformationcontrolledactions shall be restricted to flexure in slabsand columns, and shear and moment transfer in slabcolumnconnections. In secondary components,deformation-controlled actions shall also be permittedin shear and reinforcement development, as identifiedin Table 6-14. All other actions shall be defined asbeing force-controlled actions.Design actions on components shall be determined asprescribed in Chapter 3. Where the calculated DCRvalues exceed unity, the following actions preferablyshall be determined using limit analysis principles asprescribed in Chapter 3: (1) moments, shears, torsions,and development and splice actions corresponding todevelopment of component strength in slabs andcolumns; and (2) axial load in columns, consideringFEMA 273 Seismic Rehabilitation Guidelines 6-29

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-14Numerical Acceptance Criteria for Linear Procedures—Two-Way Slabs and Slab-ColumnConnectionsm factorsComponent TypePrimarySecondaryPerformance LevelConditionsIO LS CP LS CPi. Slabs controlled by flexure, and slab-column connections 1V Continuity Reinforcement 3----- g 2V o≤ 0.2 Yes 2 2 3 3 4≥ 0.4 Yes 1 1 1 2 3≤ 0.2 No 2 2 3 2 3≥ 0.4 No 1 1 1 1 1ii. Slabs controlled by inadequate development or splicing along the span 1– – – 3 4iii. Slabs controlled by inadequate embedment into slab-column joint 12 2 3 3 41. When more than one of the conditions i, ii, and iii occurs for a given component, use the minimum appropriate numerical value from the table.2. V g = the gravity shear acting on the slab critical section as defined by ACI 318; V o = the direct punching shear strength as defined by ACI 318.3. Under the heading “Continuity Reinforcement,” assume “Yes” where at least one of the main bottom bars in each direction is effectively continuousthrough the column cage. Where the slab is post-tensioned, assume “Yes” where at least one of the post-tensioning tendons in each direction passesthrough the column cage. Otherwise, assume “No.”likely plastic action in components above the level inquestion.Design actions shall be compared with design strengthsto determine which components develop their designstrengths. Those components that do not reach theirdesign strengths may be assumed to satisfy theperformance criteria for those components.Components that reach their design strengths shall befurther evaluated according to this section to determineperformance acceptability.Where the average of the DCRs of columns at a levelexceeds the average value of slabs at the same level,and exceeds the greater of 1.0 and m/2, the element isdefined as a weak story element. In this case, follow theprocedure for weak story elements described inSection 6.5.2.4A.Calculated component actions shall satisfy therequirements of Chapter 3. Tables 6-11 and 6-14present m values.B. Nonlinear Static and Dynamic ProceduresInelastic response shall be restricted to thosecomponents and actions listed in Tables 6-7 and 6-13,except where it is demonstrated that other inelasticaction can be tolerated considering the selectedPerformance Levels.Calculated component actions shall satisfy therequirements of Chapter 3. Maximum permissibleinelastic deformations are listed in Tables 6-7 and 6-13.Where inelastic action is indicated for a component oraction not listed in these tables, the performance shallbe deemed unacceptable. Alternative approaches or6-30 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)values are permitted where justified by experimentalevidence and analysis.6.5.4.5 Rehabilitation MeasuresRehabilitation measures include the general approacheslisted in Section 6.5.2.5, plus other approaches based onrational procedures.Rehabilitated frames shall be evaluated according to thegeneral principles and requirements of this chapter. Theeffects of rehabilitation on stiffness, strength, anddeformability shall be taken into account in ananalytical model of the rehabilitated building.Connections required between existing and newelements shall satisfy requirements of Section 6.4.6 andother requirements of the Guidelines.6.6 Precast Concrete Frames6.6.1 Types of Precast Concrete FramesPrecast concrete frames are those elements that areconstructed from individually made beams andcolumns, that are assembled to create gravity-loadcarryingsystems. These systems are sometimesexpected to directly resist lateral loads, and are alwaysrequired to deform in a manner that is compatible withthe structure as a whole.The provisions of this section are applicable to precastconcrete frames that emulate cast-in-place momentframes, precast concrete beam-column moment framesother than emulated cast-in-place moment frames, andprecast concrete frames not expected to directly resistlateral loads.6.6.1.1 Precast Concrete Frames thatEmulate Cast-in-Place MomentFramesEmulated moment frames of precast concrete are thoseprecast beam-column systems that are interconnectedusing reinforcing and wet concrete in such a way as tocreate a system that will act to resist lateral loads in amanner similar to cast-in-place concrete systems. Thesesystems are recognized and accepted by the 1994NEHRP Recommended Provisions (BSSC, 1995), andare based on ACI 318, which requires safety andserviceability levels expected from monolithicconstruction. There are insufficient research and testingdata at this time to qualify systems assembled using dryjoints as emulated moment frames.6.6.1.2 Precast Concrete Beam-ColumnMoment Frames other than EmulatedCast-in-Place Moment FramesFrames of this classification are assembled using dryjoints; that is, connections are made by bolting,welding, post-tensioning, or other similar means.Frames of this nature may act alone to resist lateralloads, or they may act in conjunction with shear walls,braced frames, or other elements to form a dual system.The appendix to Chapter 6 of the 1994 NEHRPRecommended Provisions (BSSC, 1995) contains a trialversion of code provisions for new construction of thisnature, but it was felt to be premature in 1994 to baseactual provisions on the material in the appendix.6.6.1.3 Precast Concrete Frames NotExpected to Resist Lateral LoadsDirectlyFrames of this classification are assembled using dryjoints similar to those of Section 6.6.1.2, but are notexpected to participate in resisting the lateral loadsdirectly or significantly. Shear walls, braced frames, orsteel moment frames are expected to provide the entirelateral load resistance, but the precast concrete“gravity” frame system must be able to deform in amanner that is compatible with the structure as a whole.Conservative assumptions shall be made concerning therelative fixity of joints.6.6.2 Precast Concrete Frames that EmulateCast-in-Place Moment Frames6.6.2.1 General ConsiderationsThe analysis model for an emulated beam-columnframe element shall represent strength, stiffness, anddeformation capacity of beams, columns, beam-columnjoints, and other components that may be part of theframe. Potential failure in flexure, shear, andreinforcement development at any section along thecomponent length shall be considered. Interaction withother elements, including nonstructural elements andcomponents, shall be included. All other considerationsof Section 6.5.2.1 shall be taken into account. Inaddition, special care shall be taken to consider theeffects of shortening due to creep, and prestressing andpost-tensioning on member behavior.6.6.2.2 Stiffness for AnalysisStiffness for analysis shall be as defined inSection 6.5.2.2. The effects of prestressing shall beFEMA 273 Seismic Rehabilitation Guidelines 6-31

Chapter 6: Concrete(Systematic Rehabilitation)considered when computing the effective stiffnessvalues using Table 6-4.6.6.2.3 Design StrengthsComponent strength shall be computed according to therequirements of Section 6.5.2.3, with the additionalrequirement that the following factors be included in thecalculation of strength:1. Effects of prestressing that are present, including,but not limited to, reduction in rotation capacity,secondary stresses induced, and amount of effectiveprestress force remaining2. Effects of construction sequence, including thepossibility that the moment connections may havebeen constructed after dead load had been applied toportions of the structure3. Effects of restraint that may be present due tointeraction with interconnected wall or bracecomponents6.6.2.4 Acceptance CriteriaAcceptance criteria for precast concrete frames thatemulate cast-in-place moment frames are as describedin Section 6.5.2.4, except that the factors defined inSection 6.6.2.3 shall also be considered.6.6.2.5 Rehabilitation MeasuresRehabilitation measures for emulated cast-in-placemoment frames are given in Section 6.5.2.5. Specialconsideration shall be given to the presence ofprestressing strand when installing new elements andwhen adding new rigid elements to the existing system.6.6.3 Precast Concrete Beam-ColumnMoment Frames other than EmulatedCast-in-Place Moment Frames6.6.3.1 General ConsiderationsThe analysis model for precast concrete beam-columnmoment frames other than emulated moment framesshall be established following Section 6.5.2.1 forreinforced concrete beam-column moment frames, withadditional consideration of the special nature of the dryjoints used in assembling the precast system. Therequirements given in the appendix to Chapter 6 of the1994 NEHRP Recommended Provisions for this type ofstructural system should be adhered to where possible,and the philosophy and approach should be employedwhen designing new connections for existingcomponents. See also Section 6.4.6.6.6.3.2 Stiffness for AnalysisStiffness for analysis shall be as defined inSections 6.5.2.2 and 6.6.2.2. Flexibilities associatedwith connections should be included in the analyticalmodel. See also Section 6.4.6.6.6.3.3 Design StrengthsComponent strength shall be computed according to therequirements of Sections 6.5.2.3 and 6.6.2.3, with theadditional requirements that the connections complywith the appendix to Chapter 6 of the 1994 NEHRPRecommended Provisions, and connection strengthshall be represented. See also Section 6.4.6.6.6.3.4 Acceptance CriteriaAcceptance criteria for precast concrete beam-columnmoment frames other than emulated cast-in-placemoment frames are given in Sections 6.5.2.4 and6.6.2.4, with the additional requirement that theconnections meet the requirements of Section 6.A.4 ofthe appendix to Chapter 6 of the 1994 NEHRPRecommended Provisions. See also Section 6.4.6.6.6.3.5 Rehabilitation MeasuresRehabilitation measures for the frames of this sectionshall meet the requirements of Section 6.6.2.5. Specialconsideration shall be given to connections that arestressed beyond their elastic limit.6.6.4 Precast Concrete Frames NotExpected to Resist Lateral LoadsDirectly6.6.4.1 General ConsiderationsThe analysis model for precast concrete frames that arenot expected to resist significant lateral loads directlyshall include the effects of deformations that the lateralload-resistingsystem will experience. The generalconsiderations of Sections 6.5.2.1 and 6.6.3.1 shall beincluded.6.6.4.2 Stiffness for AnalysisThe stiffness for analysis considers possible resistancethat may develop under lateral deformation. In somecases it may be appropriate to assume zero lateral6-32 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)stiffness. However, the Northridge earthquakegraphically demonstrated that there are practically nosituations where the precast column can be consideredto be completely pinned top and bottom, and as aconsequence, not resisting any shear from buildingdrift. Several parking structures collapsed as a result ofthis defect. Conservative assumptions should be made.6.6.4.3 Design StrengthsComponent strength shall be computed according to therequirements of Section 6.6.3.3. All components shallhave sufficient strength and ductility to transmitinduced forces from one member to another and to thedesignated lateral-force-resisting system.6.6.4.4 Acceptance CriteriaAcceptance criteria for components in precast concreteframes not expected to directly resist lateral loads aregiven in Section 6.6.3.4. All moments, shear forces, andaxial loads induced through the deformation of theintended lateral-force-resisting system shall be checkedfor acceptability by appropriate criteria in thereferenced section.6.6.4.5 Rehabilitation MeasuresRehabilitation measures for the frames discussed in thissection shall meet the requirements of Section 6.6.3.5.6.7 Concrete Frames with Infills6.7.1 Types of Concrete Frames with InfillsConcrete frames with infills are those framesconstructed with complete gravity-load-carrying framesinfilled with masonry or concrete, constructed in such away that the infill and the concrete frame interact whensubjected to design load combinations.Infills may be considered to be isolated infills if they areisolated from the surrounding frame according to theminimum gap requirements described in Section 7.5.1.If all infills in a frame are isolated infills, the frameshould be analyzed as an isolated frame according toprovisions given elsewhere in this chapter, and theisolated infill panels shall be analyzed according to therequirements of Chapter 7.The provisions are applicable to frames with existinginfills, frames that are rehabilitated by addition orremoval of material, and concrete frames that arerehabilitated by the addition of new infills.6.7.1.1 Types of FramesThe provisions are applicable to frames that are castmonolithically and frames that are precast. Types ofconcrete frames are described in Sections 6.5, 6.6, and6.10.6.7.1.2 Masonry InfillsTypes of masonry infills are described in Chapter 7.6.7.1.3 Concrete InfillsThe construction of concrete-infilled frames is verysimilar to that for masonry-infilled frames, except thatthe infill is of concrete instead of masonry units. Inolder existing buildings, the concrete infill commonlycontains nominal reinforcement, which is unlikely toextend into the surrounding frame. The concrete islikely to be of lower quality than that used in the frame,and should be investigated separately frominvestigations of the frame concrete.6.7.2 Concrete Frames with Masonry Infills6.7.2.1 General ConsiderationsThe analysis model for a concrete frame with masonryinfills shall be sufficiently detailed to represent strength,stiffness, and deformation capacity of beams, slabs,columns, beam-column joints, masonry infills, and allconnections and components that may be part of theelement. Potential failure in flexure, shear, anchorage,reinforcement development, or crushing at any sectionshall be considered. Interaction with other nonstructuralelements and components shall be included.Behavior of a concrete frame with masonry infillresisting lateral forces within its plane may becalculated based on linear elastic behavior if it can bedemonstrated that the wall will not crack whensubjected to design lateral forces. In this case, theassemblage of frame and infill should be considered tobe a homogeneous medium for stiffness computations.Behavior of cracked concrete frames with masonryinfills may be represented by a diagonally braced framemodel in which the columns act as vertical chords, thebeams act as horizontal ties, and the infill is modeledusing the equivalent compression strut analogy.Requirements for the equivalent compression strutanalogy are described in Chapter 7.FEMA 273 Seismic Rehabilitation Guidelines 6-33

Chapter 6: Concrete(Systematic Rehabilitation)Frame components shall be evaluated for forcesimparted to them through interaction of the frame withthe infill, as specified in Chapter 7. In frames with fullheightmasonry infills, the evaluation shall include theeffect of strut compression forces applied to the columnand beam, eccentric from the beam-column joint. Inframes with partial-height masonry infills, theevaluation shall include the reduced effective length ofthe columns in the noninfilled portion of the bay.In frames having infills in some bays and no infill inother bays, the restraint of the infill shall be representedas described above, and the noninfilled bays shall bemodeled as frames according to the specifications ofthis chapter. Where infills create a discontinuous wall,the effects on overall building performance shall beconsidered.6.7.2.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresGeneral aspects of modeling are described inSection 6.7.2.1. Beams and columns in infilled portionsmay be modeled considering axial tension andcompression flexibilities only. Noninfilled portionsshall be modeled according to procedures described fornoninfilled frames. Effective stiffnesses shall beaccording to Section 6.4.1.2.B. Nonlinear Static ProcedureNonlinear load-deformation relations shall follow thegeneral guidelines of Section 6.4.1.2.Beams and columns in infilled portions may bemodeled using nonlinear truss elements. Beams andcolumns in noninfilled portions may be modeled usingprocedures described in this chapter. The model shall becapable of representing inelastic response along thecomponent lengths.Monotonic load-deformation relations shall beaccording to the generalized relation shown inFigure 6-1, except different relations are permittedwhere verified by tests. Numerical quantities inFigure 6-1 may be derived from tests or rationalanalyses following the general guidelines of Chapter 2,and shall take into account the interactions betweenframe and infill components. Alternatively, thefollowing may be used for monolithic reinforcedconcrete frames.1. For beams and columns in noninfilled portions offrames, where the generalized deformation is takenas rotation in the flexural plastic hinge zone, theplastic hinge rotation capacities shall be as definedby Table 6-17.2. For masonry infills, the generalized deformationsand control points shall be as defined in Chapter 7.3. For beams and columns in infilled portions offrames, where the generalized deformation is takenas elongation or compression displacement of thebeams or columns, the tension and compressionstrain capacities shall be as specified in Table 6-15.C. Nonlinear Dynamic ProcedureFor the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby tests. Unloading and reloading properties shallrepresent significant stiffness and strength degradationcharacteristics.6.7.2.3 Design StrengthsStrengths of reinforced concrete components shall beaccording to the general requirements of Section 6.4.2,as modified by other specifications of this chapter.Strengths of masonry infills shall be according to therequirements of Chapter 7. Strengths shall considerlimitations imposed by beams, columns, and joints inunfilled portions of frames; tensile and compressivecapacity of columns acting as boundary elements ofinfilled frames; local forces applied from the infill to theframe; strength of the infill; and connections withadjacent elements. .6-34 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-15Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Reinforced Concrete Infilled FramesModeling Parameters 4Acceptance CriteriaTotal StrainResidualStrengthRatioTotal StrainComponent TypePrimarySecondaryPerformance LevelConditionsd e c IO LS CP LS CPi. Columns modeled as compression chords 3Columns confined along entire 0.02 0.04 0.4 0.003 0.015 0.020 0.03 0.04length 2All other cases 0.003 0.01 0.2 0.002 0.002 0.003 0.01 0.01ii. Columns modeled as tension chords 3Columns with well-confined 0.05 0.05 0.0 0.01 0.03 0.04 0.04 0.05splices, or no splicesAll other casesSeenote 10.03 0.2 See note 1 0.02 0.031. Splice failure in a primary component can result in loss of lateral load resistance. For these cases, refer to the generalized procedure of Section 6.4.2. Forprimary actions, Collapse Prevention Performance Level shall be defined as the deformation at which strength degradation begins. Life SafetyPerformance Level shall be taken as three-quarters of that value.2 A column may be considered to be confined along its entire length when the quantity of transverse reinforcement along the entire story height includingthe joint is equal to three-quarters of that required by ACI 318 for boundary elements of concrete shear walls. The maximum longitudinal spacing of setsof hoops shall not exceed h/3 nor 8d b .3. In most infilled walls, load reversals will result in both conditions i and ii applying to a single column, but for different loading directions.4. Interpolation is not permitted.FEMA 273 Seismic Rehabilitation Guidelines 6-35

Chapter 6: Concrete(Systematic Rehabilitation)6.7.2.4 Acceptance CriteriaA. Linear Static and Dynamic ProceduresAll component actions shall be classified as eitherdeformation-controlled or force-controlled, as definedin Chapter 3. In primary components, deformationcontrolledactions shall be restricted to flexure and axialactions in beams, slabs, and columns, and lateraldeformations in masonry infill panels. In secondarycomponents, deformation-controlled actions shall berestricted to those actions identified for the isolatedframe in this chapter and for the masonry infill inChapter 7.Design actions shall be determined as prescribed inChapter 3. Where calculated DCR values exceed unity,the following actions preferably shall be determinedusing limit analysis principles as prescribed inChapter 3: (1) moments, shears, torsions, anddevelopment and splice actions corresponding todevelopment of component strength in beams, columns,or masonry infills; and (2) column axial loadcorresponding to development of the flexural capacityof the infilled frame acting as a cantilever wall.Design actions shall be compared with design strengthsto determine which components develop their designstrengths. Those components that have design actionsless than design strengths may be assumed to satisfy theperformance criteria for those components.Components that reach their design strengths shall befurther evaluated according to this section to determineperformance acceptability.Calculated component actions shall satisfy therequirements of Chapter 3. Refer to Section 7.5.2.2 form values for masonry infills. Refer to other sections ofthis chapter for m values for concrete frames; m valuesfor columns modeled as tension and compressionchords are in Table 6-16.Table 6-16Numerical Acceptance Criteria for Linear Procedures—Reinforced Concrete Infilled Framesm factors 3Component TypePrimarySecondaryPerformance LevelConditionsIO LS CP LS CPi. Columns modeled as compression chords 2Columns confined along entire length 1 1 3 4 4 5All other cases 1 1 1 1 1ii. Columns modeled as tension chords 2Columns with well-confined splices, or no3 4 5 5 6splicesAll other cases 1 2 2 3 41. A column may be considered to be confined along its entire length when the quantity of transverse reinforcement along the entire story height includingthe joint is equal to three-quarters of that required by ACI 318 for boundary elements of concrete shear walls. The maximum longitudinal spacing of setsof hoops shall not exceed h/3 nor 8d b .2. In most infilled walls, load reversals will result in both conditions i and ii applying to a single column, but for different loading directions.3. Interpolation is not permitted.6-36 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)B. Nonlinear Static and Dynamic ProceduresInelastic response shall be restricted to thosecomponents and actions that are permitted for isolatedframes in this chapter and for masonry infills inChapter 7.Design actions shall be compared with design strengthsto determine which components develop their designstrengths. Those components that have design actionsless than design strengths may be assumed to satisfy theperformance criteria for those components.Components that reach their design strengths shall befurther evaluated, according to Section 6.5.2.4B, todetermine performance acceptability.Calculated component actions shall not exceed thenumerical values listed in Table 6-15, the relevanttables for isolated frames given in this chapter, and therelevant tables for masonry infills given in Chapter 7.Where inelastic action is indicated for a component oraction not listed in Tables 6-10 through 6-12, theperformance shall be deemed unacceptable. Alternativeapproaches or values are permitted where justified byexperimental evidence and analysis.6.7.2.5 Rehabilitation MeasuresRehabilitation measures include the general approacheslisted for isolated frames in this chapter, the measureslisted for masonry infills in Section 7.5, and otherapproaches based on rational procedures. Both in-planeand out-of-plane loading shall be considered.The following methods should be considered.• Jacketing existing beams, columns, or joints withnew reinforced concrete, steel, or fiber wrapoverlays. The new materials shall be designed andconstructed to act compositely with the existingconcrete. Where reinforced concrete jackets areused, the design shall provide detailing to enhanceductility. Component strength shall be taken to notexceed any limiting strength of connections withadjacent components. Jackets designed to provideincreased connection strength and improvedcontinuity between adjacent components arepermitted.• Post-tensioning existing beams, columns, orjoints using external post-tensionedreinforcement. Vertical post-tensioning may beuseful for increasing tensile capacity of columnsacting as boundary zones. Anchorages shall belocated away from regions where inelastic action isanticipated, and shall be designed consideringpossible force variations due to earthquake loading.• Modification of the element by selective materialremoval from the existing element. Either the infillcan be completely removed from the frame, or gapscan be provided between the frame and the infill. Inthe latter case, the gap requirements of Chapter 7shall be satisfied.• Improvement of deficient existing reinforcementdetails. This approach involves removal of coverconcrete, modification of existing reinforcementdetails, and casting of new cover concrete. Concreteremoval shall avoid unintended damage to coreconcrete and the bond between existingreinforcement and core concrete. New coverconcrete shall be designed and constructed toachieve fully composite action with the existingmaterials.• Changing the building system to reduce thedemands on the existing element. Examplesinclude the addition of supplementary lateral-forceresistingelements such as walls, steel braces, orbuttresses; seismic isolation; and mass reduction.Rehabilitated frames shall be evaluated according to thegeneral principles and requirements of this chapter. Theeffects of rehabilitation on stiffness, strength, anddeformability shall be taken into account in theanalytical model. Connections required betweenexisting and new elements shall satisfy the requirementsof Section 6.4.6 and other requirements of theGuidelines.6.7.3 Concrete Frames with Concrete Infills6.7.3.1 General ConsiderationsThe analysis model for a concrete frame with concreteinfills shall be sufficiently detailed to represent thestrength, stiffness, and deformation capacity of beams,slabs, columns, beam-column joints, concrete infills,and all connections and components that may be part ofthe elements. Potential failure in flexure, shear,anchorage, reinforcement development, or crushing atany section shall be considered. Interaction with othernonstructural elements and components shall beincluded.FEMA 273 Seismic Rehabilitation Guidelines 6-37

Chapter 6: Concrete(Systematic Rehabilitation)The numerical model should be established consideringthe relative stiffness and strength of the frame and theinfill, as well as the level of deformations andassociated damage. For low deformation levels, and forcases where the frame is relatively flexible, it may besuitable to model the infilled frame as a solid shearwall, although openings should be considered wherethey occur. In other cases, it may be more suitable tomodel the frame-infill system using a braced-frameanalogy such as that described for concrete frames withmasonry infills in Section 6.7.2. Some judgment isnecessary to determine the appropriate type andcomplexity of the analytical model.Frame components shall be evaluated for forcesimparted to them through interaction of the frame withthe infill, as specified in Chapter 7. In frames with fullheightinfills, the evaluation shall include the effect ofstrut compression forces applied to the column andbeam eccentric from the beam-column joint. In frameswith partial-height infills, the evaluation shall includethe reduced effective length of the columns in thenoninfilled portion of the bay.In frames having infills in some bays and no infills inother bays, the restraint of the infill shall be representedas described above, and the noninfilled bays shall bemodeled as frames according to the specifications ofthis chapter. Where infills create a discontinuous wall,the effects on overall building performance shall beconsidered.6.7.3.2 Stiffness for AnalysisA. Linear Static and Dynamic ProceduresGeneral aspects of modeling are described inSection 6.7.3.1. Effective stiffnesses shall be accordingto the general principles of Section 6.4.1.2.B. Nonlinear Static ProcedureGeneral aspects of modeling are described inSection 6.7.3.1. Nonlinear load-deformation relationsshall follow the general guidelines of Section 6.4.1.2.Monotonic load-deformation relations shall beaccording to the generalized relation shown inFigure 6-1, except different relations are permittedwhere verified by tests. Numerical quantities inFigure 6-1 may be derived from tests or rationalanalyses following the general guidelines ofSection 2.13, and shall take into account theinteractions between frame and infill components. Theguidelines of Section 6.7.2.2 may be used to guidedevelopment of modeling parameters for concreteframes with concrete infills.C. Nonlinear Dynamic ProcedureFor the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby tests. Unloading and reloading properties shallrepresent significant stiffness and strength degradationcharacteristics.6.7.3.3 Design StrengthsStrengths of reinforced concrete components shall beaccording to the general requirements of Section 6.4.2,as modified by other specifications of this chapter.Strengths shall consider limitations imposed by beams,columns, and joints in unfilled portions of frames;tensile and compressive capacity of columns acting asboundary elements of infilled frames; local forcesapplied from the infill to the frame; strength of theinfill; and connections with adjacent elements.Strengths of existing concrete infills shall bedetermined considering shear strength of the infillpanel. For this calculation, procedures specified inSection 6.8.2.3 shall be used for calculation of the shearstrength of a wall segment.Where the frame and concrete infill are assumed to actas a monolithic wall, flexural strength shall be based oncontinuity of vertical reinforcement in both (1) thecolumns acting as boundary elements, and (2) the infillwall, including anchorage of the infill reinforcement inthe boundary frame.6.7.3.4 Acceptance CriteriaThe acceptance criteria for concrete frames withconcrete infills should be guided by relevant acceptancecriteria of Sections 6.7.2.4, 6.8, and 6.9.6.7.3.5 Rehabilitation MeasuresRehabilitation measures include the general approacheslisted for masonry infilled frames in Section 6.7.2.5.Strengthening of the existing infill may be consideredas an option for rehabilitation. Shotcrete can be appliedto the face of an existing wall to increase the thicknessand shear strength. For this purpose, the face of theexisting wall should be roughened, a mat of reinforcing6-38 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)steel should be doweled into the existing structure, andshotcrete should be applied to the desired thickness.Rehabilitated frames shall be evaluated according to thegeneral principles and requirements of this chapter. Theeffects of rehabilitation on stiffness, strength, anddeformability shall be taken into account in theanalytical model. Connections required betweenexisting and new elements shall satisfy the requirementsof Section 6.4.6 and other requirements of theGuidelines.6.8 Concrete Shear Walls6.8.1 Types of Concrete Shear Walls andAssociated ComponentsConcrete shear walls consist of planar vertical elementsthat normally serve as the primary lateral-load-resistingelements when they are used in concrete structures. Ingeneral, shear walls (or wall segments) are consideredto be slender if their aspect ratio (height/length) is Š 3.0,and they are considered to be short if their aspect ratio is≤ 1.5. Slender shear walls are normally controlled byflexural behavior; short walls are normally controlledby shear behavior. The response of walls withintermediate aspect ratios is influenced by both flexureand shear.The provisions given here are applicable to all shearwalls in all types of structural systems that incorporateshear walls. This includes isolated shear walls, shearwalls used in dual (wall-frame) systems, coupled shearwalls, and discontinuous shear walls. Shear walls areconsidered to be solid walls if they have small openingsthat do not significantly influence the strength orinelastic behavior of the wall. Perforated shear walls arecharacterized by a regular pattern of large openings inboth horizontal and vertical directions that create aseries of pier and deep beam elements. In thediscussions and tables that appear in the followingsections, these vertical piers and horizontal beams willboth be referred to as wall segments.Provisions are also included for coupling beams andcolumns that support discontinuous shear walls. Theseare special frame components that are associated morewith shear walls than with the normal frame elementscovered in Section 6.5.6.8.1.1 Monolithic Reinforced ConcreteShear Walls and Wall SegmentsMonolithic reinforced concrete (RC) shear walls consistof vertical cast-in-place elements, usually with aconstant cross section, that typically form open orclosed shapes around vertical building shafts. Shearwalls are also used frequently along portions of theperimeter of the building. The wall reinforcement isnormally continuous in both the horizontal and verticaldirections, and bars are typically lap spliced for tensioncontinuity. The reinforcement mesh may also containhorizontal ties around vertical bars that are concentratedeither near the vertical edges of a wall with constantthickness, or in boundary members formed at the walledges. The amount and spacing of these ties isimportant for determining how well the concrete at thewall edge is confined, and thus for determining thelateral deformation capacity of the wall.In general, slender reinforced concrete shear walls willbe governed by flexure and will tend to form a plasticflexural hinge near the base of the wall under severelateral loading. The ductility of the wall will be afunction of the percentage of longitudinal reinforcementconcentrated near the boundaries of the wall, the levelof axial load, the amount of lateral shear required tocause flexural yielding, and the thickness andreinforcement used in the web portion of the shear wall.In general, higher axial load stresses and higher shearstresses will reduce the flexural ductility and energyabsorbing capability of the shear wall. Squat shear wallswill normally be governed by shear. These walls willnormally have a limited ability to deform beyond theelastic range and continue to carry lateral loads. Thus,these walls are typically designed either asdisplacement-controlled components with low ductilitycapacities or as force-controlled components.Shear walls or wall segments with axial loads greaterthan 0.35 P o shall not be considered effective inresisting seismic forces. The maximum spacing ofhorizontal and vertical reinforcement shall not exceed18 inches. Walls with horizontal and verticalreinforcement ratios less than 0.0025, but withreinforcement spacings less than 18 inches, shall bepermitted where the shear force demand does notexceed the reduced nominal shear strength of the wallcalculated in accordance with Section 6.8.2.3.FEMA 273 Seismic Rehabilitation Guidelines 6-39

Chapter 6: Concrete(Systematic Rehabilitation)6.8.1.2 Reinforced Concrete ColumnsSupporting Discontinuous ShearWallsIn shear wall buildings it is not uncommon to find thatsome walls are terminated either to create commercialspace in the first story or to create parking spaces in thebasement. In such cases, the walls are commonlysupported by columns. Such designs are notrecommended in seismic zones because very largedemands may be placed on these columns duringearthquake loading. In older buildings such columnswill often have “standard” longitudinal and transversereinforcement; the behavior of such columns duringpast earthquakes indicates that tightly spaced closed tieswith well-anchored 135-degree hooks will be requiredfor the building to survive severe earthquake loading.6.8.1.3 Reinforced Concrete Coupling BeamsReinforced concrete coupling beams are used to linktwo shear walls together. The coupled walls aregenerally much stiffer and stronger than they would beif they acted independently. Coupling beams typicallyhave a small span-to-depth ratio, and their inelasticbehavior is normally affected by the high shear forcesacting in these components. Coupling beams in mostolder reinforced concrete buildings will commonly have“conventional” reinforcement that consists oflongitudinal flexural steel and transverse steel for shear.In some, more modern buildings, or in buildings wherecoupled shear walls are used for seismic rehabilitation,the coupling beams may use diagonal reinforcement asthe primary reinforcement for both flexure and shear.The inelastic behavior of coupling beams that usediagonal reinforcement has been shown experimentallyto be much better with respect to retention of strength,stiffness, and energy dissipation capacity than theobserved behavior of coupling beams with conventionalreinforcement.6.8.2 Reinforced Concrete Shear Walls,Wall Segments, Coupling Beams, andRC Columns SupportingDiscontinuous Shear Walls6.8.2.1 General Modeling ConsiderationsThe analysis model for an RC shear wall element shallbe sufficiently detailed to represent the stiffness,strength, and deformation capacity of the overall shearwall. Potential failure in flexure, shear, andreinforcement development at any point in the shearwall shall be considered. Interaction with otherstructural and nonstructural elements shall be included.In most cases, shear walls and wall elements may bemodeled analytically as equivalent beam-columnelements that include both flexural and sheardeformations. The flexural strength of beam-columnelements shall include the interaction of axial load andbending. The rigid connection zone at beamconnections to this equivalent beam-column elementwill need to be long enough to properly represent thedistance from the wall centroid—where the beamcolumnelement is placed in the computer model—tothe edge of the wall. Unsymmetrical wall sections shallmodel the different bending capacities for the twoloading directions.For rectangular shear walls and wall segments withh w ⁄ l w ≤ 2.5 , and flanged wall sections withh w ⁄ l w ≤ 3.5 , shear deformations become moresignificant. For such cases, either a modified beamcolumnanalogy or a multiple-node, multiple-springapproach should be used (references are given in theCommentary). Because shear walls usually respond insingle curvature over a story height, the use of onemultiple-spring element per story is recommended formodeling shear walls. For wall segments, whichtypically deform into a double curvature pattern, thebeam-column element is usually preferred. If amultiple-spring model is used for a wall segment, thenit is recommended that two elements be used over thelength of the wall segment.A beam element that incorporates both bending andshear deformations shall be used to model couplingbeams. It is recommended that the element inelasticresponse should account for the loss of shear strengthand stiffness during reversed cyclic loading to largedeformations. Coupling beams that have diagonalreinforcement satisfying the 1994 NEHRPRecommended Provisions (BSSC, 1995) willcommonly have a stable hysteretic response under largeload reversals. Therefore, these members couldadequately be modeled with beam elements used fortypical frame analyses.Columns supporting discontinuous shear walls may bemodeled with beam-column elements typically used inframe analysis. This element should also account forshear deformations, and care must be taken to ensurethat the model properly reflects the potentially rapid6-40 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)reduction in shear stiffness and strength these columnsmay experience after the onset of flexural yielding.The diaphragm action of concrete slabs thatinterconnect shear walls and frame columns shall beproperly represented.6.8.2.2 Stiffness for AnalysisThe stiffness of all the elements discussed in thissection depends on the material properties, componentdimensions, reinforcement quantities, boundaryconditions, and current state of the member with respectto cracking and stress levels. All of these aspects shouldbe considered when defining the effective stiffness ofan element. General values for effective stiffness aregiven in Table 6-4. To obtain a proper distribution oflateral forces in bearing wall buildings, all of the wallsshall be assumed to be either cracked or uncracked. Inbuildings where lateral load resistance is provided byeither structural walls only, or a combination of wallsand frame members, all shear walls and wall segmentsdiscussed in this section should be considered to becracked.For coupling beams, the values given in Table 6-4 fornonprestressed beams should be used. Columnssupporting discontinuous shear walls will experiencesignificant changes in axial load during lateral loadingof the shear wall they support. Thus, the stiffness valuesfor these column elements will need to change betweenthe values given for columns in tension andcompression, depending on the direction of the lateralload being resisted by the shear wall.A. Linear Static and Dynamic ProceduresShear walls and associated components shall bemodeled considering axial, flexural, and shear stiffness.For closed and open wall shapes, such as box, T, L, I,and C sections, the effective tension or compressionflange widths on each side of the web shall be taken asthe smaller of: (1) one-fifth of the wall height, (2) halfthe distance to the next web, or (3) the provided widthof the flange. The calculated stiffnesses to be used inanalysis shall be in accordance with the requirements ofSection 6.4.1.2.Joints between shear walls and frame elements shall bemodeled as stiff components and shall be consideredrigid in most cases.B. Nonlinear Static and Dynamic ProceduresNonlinear load-deformation relations shall follow thegeneral procedures described in Section 6.4.1.2.Monotonic load-deformation relationships foranalytical models that represent shear walls, wallelements, coupling beams, and RC columns that supportdiscontinuous shear walls shall be of the general shapesdefined in Figure 6-1. For both of the load-deformationrelationships in Figure 6-1, point B corresponds tosignificant yielding, point C corresponds to the pointwhere significant lateral resistance is assumed to belost, and point E corresponds to the point where gravityload resistance is assumed to be lost.The load-deformation relationship in Figure 6-1(a)should be referred to for shear walls and wall segmentshaving inelastic behavior under lateral loading that isgoverned by flexure, as well as columns supportingdiscontinuous shear walls. For all of these members, thex-axis of Figure 6-1(a) should be taken as the rotationover the plastic hinging region at the end of the member(Figure 6-2). The hinge rotation at point B correspondsto the yield point, θ y , and is given by the followingexpression:where:M yE cIl p⎛M y ⎞θ y = ⎜-------⎟ lp⎝E c I⎠= Yield moment capacity of the shear wall orwall segment= Concrete modulus= Member moment of inertia, as discussedabove= Assumed plastic hinge length(6-5)For analytical models of shear walls and wall segments,the value of l p shall be set equal to 0.5 times the flexuraldepth of the element, but less than one story height forshear walls and less than 50% of the element length forwall segments. For RC columns supportingdiscontinuous shear walls, l p shall be set equal to 0.5times the flexural depth of the component.FEMA 273 Seismic Rehabilitation Guidelines 6-41

Chapter 6: Concrete(Systematic Rehabilitation)θPlastic hinge rotation = θFor shear walls and wall segments whose inelasticresponse is controlled by shear, it is more appropriate touse drift as the deformation value in Figure 6-1(b). Forshear walls, this drift is actually the story drift as shownin Figure 6-3. For wall segments, Figure 6-3 essentiallyrepresents the member drift.Figure 6-2Figure 6-3Values for the variables a, b, and c, which are requiredto define the location of points C, D, and E inFigure 6-1(a), are given in Table 6-17.Figure 6-4Plastic Hinge Rotation in Shear Wallwhere Flexure Dominates InelasticResponseStory Drift in Shear Wall where ShearDominates Inelastic ResponseChord Rotation:θ = ∆LθLChord Rotation for Shear Wall CouplingBeams∆∆LFor coupling beams, the deformation measure to beused in Figure 6-1(b) is the chord rotation for themember, as defined in Figure 6-4. Chord rotation is themost representative measure of the deformed state of acoupling beam, whether its inelastic response isgoverned by flexure or by shear.Values for the variables d, e, and c, which are requiredto find the points C, D, and E in Figure 6-1(b), are givenin Tables 6-17 and 6-18 for the appropriate members.Linear interpolation between tabulated values shall beused if the member under analysis has conditions thatare between the limits given in the tables.For the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby experimental evidence. The relationships inFigure 6-1 may be taken to represent the envelope forthe analysis. The unloading and reloading stiffnessesand strengths, and any pinching of the load-versusrotationhysteresis loops, shall reflect the behaviorexperimentally observed for wall elements similar tothe one under investigation.6.8.2.3 Design StrengthsThe discussions in the following paragraphs shall applyto shear walls, wall segments, coupling beams, andcolumns supporting discontinuous shear walls. Ingeneral, component strengths shall be computedaccording to the general requirements of Section 6.4.2,except as modified here. The yield and maximumcomponent strength shall be determined considering thepotential for failure in flexure, shear, or developmentunder combined gravity and lateral load.Nominal flexural strength of shear walls or wallsegments shall be determined using the fundamentalprinciples given in Chapter 10 of Building CodeRequirements for Structural Concrete, ACI 318-95(ACI, 1995). For calculation of nominal flexuralstrength, the effective compression and tension flangewidths defined in Section 6.8.2.2A shall be used, exceptthat the first limit shall be changed to one-tenth of thewall height. When determining the flexural yieldstrength of a shear wall, as represented by point B in6-42 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-17Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Members Controlled by FlexureConditionsi. Shear walls and wall segments( A s – A s ′ )f y + P Shear----------------------------------- --------------------t w l w f c ′ t w l w f c ′ConfinedBoundary 1Plastic HingeRotation(radians)ResidualStrengthRatioAcceptable Plastic Hinge Rotation(radians)Component TypePrimarySecondaryPerformance Levela b c IO LS CP LS CP≤ 0.1 ≤3 Yes 0.015 0.020 0.75 0.005 0.010 0.015 0.015 0.020≤ 0.1 ≥ 6 Yes 0.010 0.015 0.40 0.004 0.008 0.010 0.010 0.015≥ 0.25 ≤ 3 Yes 0.009 0.012 0.60 0.003 0.006 0.009 0.009 0.012≥ 0.25 ≥ 6 Yes 0.005 0.010 0.30 0.001 0.003 0.005 0.005 0.010≤ 0.1 ≤ 3 No 0.008 0.015 0.60 0.002 0.004 0.008 0.008 0.015≤ 0.1 ≥ 6 No 0.006 0.010 0.30 0.002 0.004 0.006 0.006 0.010≥ 0.25 ≤ 3 No 0.003 0.005 0.25 0.001 0.002 0.003 0.003 0.005≥ 0.25 ≥ 6 No 0.002 0.004 0.20 0.001 0.001 0.002 0.002 0.004ii. Columns supporting discontinuous shear wallsTransverse reinforcement 2Conforming 0.010 0.015 0.20 0.003 0.007 0.010 n.a. n.a.Nonconforming 0.0 0.0 0.0 0.0 0.0 0.0 n.a. n.a.ChordRotation(radians)d eiii. Shear wall coupling beamsLongitudinal reinforcement and Sheartransverse reinforcement 3--------------------t w l w f c ′Conventional longitudinal≤ 3 0.025 0.040 0.75 0.006 0.015 0.025 0.025 0.040reinforcement with conformingtransverse reinforcement≥ 6 0.015 0.030 0.50 0.005 0.010 0.015 0.015 0.030Conventional longitudinal≤ 3 0.020 0.035 0.50 0.006 0.012 0.020 0.020 0.035reinforcement with nonconformingtransverse reinforcement≥ 6 0.010 0.025 0.25 0.005 0.008 0.010 0.010 0.025Diagonal reinforcement n.a. 0.030 0.050 0.80 0.006 0.018 0.030 0.030 0.0501. Requirements for a confined boundary are the same as those given in ACI 318-95.2. Requirements for conforming transverse reinforcement are: (a) closed stirrups over the entire length of the column at a spacing ≤ d/2, and (b) strength ofclosed stirrups V s ≥ required shear strength of column.3. Conventional longitudinal reinforcement consists of top and bottom steel parallel to the longitudinal axis of the beam. Conforming transversereinforcement consists of: (a) closed stirrups over the entire length of the beam at a spacing ≤ d/3, and (b) strength of closed stirrups V s ≥ 3/4 of requiredshear strength of beam.Figure 6-1(a), only the longitudinal steel in theboundary of the wall should be included. If the walldoes not have a boundary member, then only thelongitudinal steel in the outer 25% of the wall sectionFEMA 273 Seismic Rehabilitation Guidelines 6-43

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-18Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures—Members Controlled by ShearDrift Ratio (%),or ChordRotation(radians) 1ResidualStrengthRatioAcceptable Drift (%) or ChordRotation (radians) 1Component TypePrimaryPerformance LevelSecondaryConditionsd e c IO LS CP LS CPi. Shear walls and wall segmentsAll shear walls and wall segments 2 0.75 2.0 0.40 0.40 0.60 0.75 0.75 1.5ii. Shear wall coupling beamsLongitudinal reinforcement andtransverse reinforcement 3Conventional longitudinalreinforcement with conformingtransverse reinforcementConventional longitudinalreinforcement withnonconforming transversereinforcementShear--------------------t w l w f c ′≤ 3 0.018 0.030 0.60 0.006 0.012 0.015 0.015 0.024≥ 6 0.012 0.020 0.30 0.004 0.008 0.010 0.010 0.016≤ 3 0.012 0.025 0.40 0.006 0.008 0.010 0.010 0.020≥ 6 0.008 0.014 0.20 0.004 0.006 0.007 0.007 0.0121. For shear walls and wall segments, use drift; for coupling beams, use chord rotation; refer to Figures 6-3 and 6-4.2. For shear walls and wall segments where inelastic behavior is governed by shear, the axial load on the member must be ≤ 0.15 A g f c ' ; otherwise, themember must be treated as a force-controlled component.3. Conventional longitudinal reinforcement consists of top and bottom steel parallel to the longitudinal axis of the beam. Conforming transversereinforcement consists of: (a) closed stirrups over the entire length of the beam at a spacing ≤ d/3, and (b) strength of closed stirrups V s ≥ 3/4 of requiredshear strength of beam.shall be included in the calculation of the yield strength.When calculating the nominal flexural strength of thewall, as represented by point C in Figure 6-1(a), alllongitudinal steel (including web reinforcement) shallbe included in the calculation. For both of the momentcalculations described here, the yield strength of thelongitudinal reinforcement should be taken as 125% ofthe specified yield strength to account for materialoverstrength and strain hardening. For all momentstrength calculations, the axial load acting on the wallshall include gravity loads as defined in Chapter 3.The nominal flexural strength of a shear wall or wallsegment shall be used to determine the maximum shearforce likely to act in shear walls, wall segments, andcolumns supporting discontinuous shear walls. Forcantilever shear walls and columns supportingdiscontinuous shear walls, the design shear force isequal to the magnitude of the lateral force required todevelop the nominal flexural strength at the base of thewall, assuming the lateral force is distributed uniformlyover the height of the wall. For wall segments, thedesign shear force is equal to the shear corresponding tothe development of the positive and negative nominalmoment strengths at opposite ends of the wall segment.The nominal shear strength of a shear wall or wallsegment shall be determined based on the principles andequations given in Section 21.6 of ACI 318-95. Thenominal shear strength of RC columns supportingdiscontinuous shear walls shall be determined based onthe principles and equations given in Section 21.3 ofACI 318-95. For all shear strength calculations, 1.0times the specified reinforcement yield strength should6-44 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)be used. There should be no difference between theyield and nominal shear strengths, as represented bypoints B and C in Figure 6-1.When a shear wall or wall segment has a transversereinforcement percentage, ρ n , less than the minimumvalue of 0.0025 but greater than 0.0015, the shearstrength of the wall shall be analyzed using theACI 318-95 equations noted above. For transversereinforcement percentages less than 0.0015, thecontribution from the wall reinforcement to the shearstrength of the wall shall be held constant at the valueobtained using ρ n = 0.0015 (Wood, 1990).Splice lengths for primary longitudinal reinforcementshall be evaluated using the procedures given inSection 6.4.5. Reduced flexural strengths shall beevaluated at locations where splices govern the usablestress in the reinforcement. The need for confinementreinforcement in shear wall boundary members shall beevaluated by the procedure in the Uniform BuildingCode (ICBO, 1994), or the method recommended byWallace (1994 and 1995) for determining maximumlateral deformations in the wall and the resultingmaximum compression strains in the wall boundary.The nominal flexural and shear strengths of couplingbeams reinforced with conventional reinforcement shallbe evaluated using the principles and equationscontained in Chapter 21 of ACI 318-95. The nominalflexural and shear strengths of coupling beamsreinforced with diagonal reinforcement shall beevaluated using the procedure defined in the 1994NEHRP Recommended Provisions. In both cases, 125%of the specified yield strength for the longitudinal anddiagonal reinforcement should be used.The nominal shear and flexural strengths of columnssupporting discontinuous shear walls shall be evaluatedas defined in Section 6.5.2.3.6.8.2.4 Acceptance CriteriaA. Linear Static and Dynamic ProceduresAll shear walls, wall segments, coupling beams, andcolumns supporting discontinuous shear walls shall beclassified as either deformation- or force-controlled, asdefined in Chapter 3. For columns supportingdiscontinuous shear walls, deformation-controlledactions shall be restricted to flexure. In the othercomponents or elements noted here, deformationcontrolledactions shall be restricted to flexure or shear.All other actions shall be defined as being forcecontrolledactions.Design actions (flexure, shear, or force transfer at rebaranchorages and splices) on components shall bedetermined as prescribed in Chapter 3. Whendetermining the appropriate value for the designactions, proper consideration should be given to gravityloads and to the maximum forces that can betransmitted considering nonlinear action in adjacentcomponents. For example, the maximum shear at thebase of a shear wall cannot exceed the shear required todevelop the nominal flexural strength of the wall.Tables 6-19 and 6-20 present m values for use inEquation 3-18. Alternate m values are permitted wherejustified by experimental evidence and analysis.B. Nonlinear Static and Dynamic ProceduresInelastic response shall be restricted to those elementsand actions listed in Tables 6-17 and 6-18, except whereit is demonstrated that other inelastic actions can betolerated considering the selected Performance Levels.For members experiencing inelastic behavior, themagnitude of other actions (forces, moments, or torque)in the member shall correspond to the magnitude of theaction causing inelastic behavior. The magnitude ofthese other actions shall be shown to be below theirnominal capacities.For members experiencing inelastic response, themaximum plastic hinge rotations, drifts, or chordrotation angles shall not exceed the values given inTables 6-17 and 6-18, for the particular PerformanceLevel being evaluated. Linear interpolation betweentabulated values shall be used if the member underanalysis has conditions that are between the limits givenin the tables. If the maximum plastic hinge rotation,drift, or chord rotation angle exceeds the correspondingvalue obtained either directly from the tables or byinterpolation, the member shall be considered to bedeficient, and either the member or the structure willneed to be rehabilitated.6.8.2.5 Rehabilitation MeasuresAll of the rehabilitation measures listed here for shearwalls assume that a proper evaluation will be made ofthe wall foundation, diaphragms, and connectionsbetween existing structural elements and any elementsadded for rehabilitation purposes. Connectionrequirements are given in Section 6.4.6.FEMA 273 Seismic Rehabilitation Guidelines 6-45

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-19Numerical Acceptance Criteria for Linear Procedures—Members Controlled by Flexurem factorsComponent TypePrimarySecondaryPerformance LevelConditionsIO LS CP LS CPi. Shear walls and wall segments( A s– A s ′ )f y+ P-----------------------------------t w l w f c ′Shear--------------------t w l w f c ′ConfinedBoundary 1≤ 0.1 ≤ 3 Yes 2 4 6 6 8≤ 0.1 ≥ 6 Yes 2 3 4 4 6≥ 0.25 ≤ 3 Yes 1.5 3 4 4 6≥ 0.25 ≥ 6 Yes 1 2 2.5 2.5 4≤ 0.1 ≤ 3 No 2 2.5 4 4 6≤ 0.1 ≥ 6 No 1.5 2 2.5 2.5 4≥ 0.25 ≤ 3 No 1 1.5 2 2 3≥ 0.25 ≥ 6 No 1 1 1.5 1.5 2ii. Columns supporting discontinuous shear wallsTransverse reinforcement 2Conforming 1 1.5 2 n.a. n.a.Nonconforming 1 1 1 n.a. n.a.iii. Shear wall coupling beamsLongitudinal reinforcement and transversereinforcement 3Shear--------------------t w l w f c ′Conventional longitudinal reinforcement withconforming transverse reinforcement≤ 3 2 4 6 6 9≥ 6 1.5 3 4 4 7Conventional longitudinal reinforcement with≤ 3 1.5 3.5 5 5 8nonconforming transverse reinforcement≥ 6 1.2 1.8 2.5 2.5 4Diagonal reinforcement n.a. 2 5 7 7 101. Requirements for a confined boundary are the same as those given in ACI 318-95.2. Requirements for conforming transverse reinforcement are: (a) closed stirrups over the entire length of the column at a spacing ≤ d/2, and (b) strength ofclosed stirrups V s ≥ required shear strength of column.3. Conventional longitudinal reinforcement consists of top and bottom steel parallel to the longitudinal axis of the beam. Conforming transversereinforcement consists of: (a) closed stirrups over the entire length of the beam at a spacing ≤ d/3, and (b) strength of closed stirrups V s ≥ 3/4 of requiredshear strength of beam.6-46 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)Table 6-20Numerical Acceptance Criteria for Linear Procedures—Members Controlled by Shearm factorsComponent TypePrimarySecondaryPerformance LevelConditionsIO LS CP LS CPi. Shear walls and wall segmentsAll shear walls and wall segments 1 2 2 3 2 3ii. Shear wall coupling beamsLongitudinal reinforcement and transverse reinforcement 2Conventional longitudinal reinforcement with conformingtransverse reinforcementConventional longitudinal reinforcement withnonconforming transverse reinforcementShear--------------------t w l w f c ′≤ 3 1.5 3 4 4 6≥ 6 1.2 2 2.5 2.5 3.5≤ 3 1.5 2.5 3 3 4≥ 6 1 1.2 1.5 1.5 2.51. For shear walls and wall segments where inelastic behavior is governed by shear, the axial load on the member must be ≤ 0.15 A g f c ' , the longitudinalreinforcement must be symmetrical, and the maximum shear stress must be ≤ 6 , otherwise the shear shall be considered to be a force-controlledaction.2. Conventional longitudinal reinforcement consists of top and bottom steel parallel to the longitudinal axis of the beam. Conforming transversereinforcement consists of: (a) closed stirrups over the entire length of the beam at a spacing ≤ d/3, and (b) strength of closed stirrups V s ≥ 3/4 of requiredshear strength of beam.f c ′• Addition of wall boundary members. Shear wallsor wall segments that have insufficient flexuralstrength may be strengthened by the addition ofboundary members. These members could be castin-placereinforced concrete elements or steelsections. In both cases, proper connections must bemade between the existing wall and the addedmembers. Also, the shear capacity of therehabilitated wall will need to be reevaluated.• Addition of confinement jackets at wallboundaries. The flexural deformation capacity of ashear wall can be improved by increasing theconfinement at the wall boundaries. This is mosteasily achieved by the addition of a steel orreinforced concrete jacket. For both types of jackets,the longitudinal steel should not be continuous fromstory to story unless the jacket is also being used toincrease the flexural capacity. The minimumthickness for a concrete jacket shall be three inches.Carbon fiber wrap may also be an effective methodfor improving the confinement of concrete incompression.• Reduction of flexural strength. In some cases itmay be desirable to reduce the flexural capacity of ashear wall to change the governing failure modefrom shear to flexure. This is most easilyaccomplished by saw-cutting a specified number oflongitudinal bars near the edges of the shear wall.• Increased shear strength of wall. The shearstrength provided by the web of a shear wall can beincreased by casting additional reinforced concreteadjacent to the wall web. The new concrete shouldbe at least four inches thick and should containhorizontal and vertical reinforcement. The newconcrete will need to be properly bonded to theexisting web of the shear wall. The use of carbonFEMA 273 Seismic Rehabilitation Guidelines 6-47

Chapter 6: Concrete(Systematic Rehabilitation)fiber sheets, epoxied to the concrete surface, canalso increase the shear capacity of a shear wall.• Confinement jackets to improve deformationcapacity of coupling beams and columnssupporting discontinuous shear walls. The use ofconfinement jackets has been discussed above forwall boundaries and in Section 6.5 for frameelements. The same procedures can be used toincrease both the shear capacity and the deformationcapacity of coupling beams and columns supportingdiscontinuous shear walls.• Infilling between columns supportingdiscontinuous shear walls. Where a discontinuousshear wall is supported on columns that lack eithersufficient strength or deformation capacity to satisfydesign criteria, the opening between these columnsmay be infilled to make the wall continuous. Theinfill and existing columns should be designed tosatisfy all the requirements for new wallconstruction. This may require strengthening of theexisting columns by adding a concrete or steel jacketfor strength and increased confinement. The openingbelow a discontinuous shear wall could also be“infilled” with steel bracing. The bracing membersshould be sized to satisfy all design requirementsand the columns should be strengthened with a steelor a reinforced concrete jacket.6.9 Precast Concrete Shear Walls6.9.1 Types of Precast Shear WallsPrecast concrete shear walls typically consist of storyhighor half-story-high precast wall segments that aremade continuous through the use of either mechanicalconnectors or reinforcement splicing techniques, and,usually a cast-in-place connection strip. Connectionsbetween precast segments are typically made along boththe horizontal and vertical edges of a wall segment. Tiltupconstruction should be considered to be a specialtechnique for precast wall construction. There arevertical joints between adjacent panels and horizontaljoints at the foundation level and where the roof or floordiaphragm connects with the tilt-up panel.If the reinforcement connections are made to bestronger than the adjacent precast panels, the lateralload response behavior of the precast wall system willbe comparable to that for monolithic shear walls. Thisdesign approach is known as cast-in-place emulation.An alternate design approach is to allow the inelasticaction to occur at the connections between precastpanels, an approach known as jointed construction. Theprovisions given here are intended for use with all typesof precast wall systems.6.9.1.1 Cast-In-Place EmulationFor this design approach, the connections betweenprecast wall elements are designed and detailed to bestronger than the panels they connect. Thus, when theprecast shear wall is subjected to lateral loading, anyyielding and inelastic behavior should take place in thepanel elements away from the connections. If thereinforcement detailing in the panel is similar to that forcast-in-place shear walls, then the inelastic response ofa precast shear wall should be very similar to that for acast-in-place wall.Modern building codes permit the use of precast shearwall construction in high seismic zones if it satisfies thecriteria for cast-in-place emulation. For such structures,the shear walls and wall segments can be evaluated bythe criteria defined in Section 6.8.6.9.1.2 Jointed ConstructionFor most older structures that contain precast shearwalls, and for some modern construction, inelasticactivity can be expected in the connections betweenprecast wall panels during severe lateral loading.Because joints between precast shear walls in olderbuildings have often exhibited brittle behavior duringinelastic load reversals, jointed construction had notbeen permitted in high seismic zones. Therefore, whenevaluating older buildings that contain precast shearwalls that are likely to respond as jointed construction,the permissible ductilities and rotation capacities givenin Section 6.8 will have to be reduced.For some modern structures, precast shear walls havebeen constructed with special connectors that aredetailed to exhibit ductile response and energyabsorption characteristics. Many of these connectors areproprietary and only limited experimental evidenceconcerning their inelastic behavior is available.Although this type of construction is clearly safer thanjointed construction in older buildings, the experimentalevidence is not sufficient to permit the use of the sameductility and rotation capacities given for cast-in-placeconstruction. Thus, the permissible values given inSection 6.8 will need to be reduced.6-48 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)6.9.1.3 Tilt-up ConstructionTilt-up construction should be considered to be aspecial case of jointed construction. The walls for mostbuildings constructed by the tilt-up method are longerthan their height. Shear would usually govern their inplanedesign, and their shear strength should beanalyzed as force-controlled action. The major concernfor most tilt-up construction is the connection betweenthe tilt-up wall and the roof diaphragm. That connectionshould be carefully analyzed to be sure the diaphragmforces can be safely transmitted to the precast wallsystem.6.9.2 Precast Concrete Shear Walls andWall Segments6.9.2.1 General Modeling ConsiderationsThe analysis model for a precast concrete shear wall orwall segment shall represent the stiffness, strength, anddeformation capacity of the overall member, as well asthe connections and joints between any precast panelcomponents that compose the wall. Potential failure inflexure, shear, and reinforcement development at anypoint in the shear wall panels or connections shall beconsidered. Interaction with other structural andnonstructural elements shall be included.In most cases, precast concrete shear walls and wallsegments within the precast panels may be modeledanalytically as equivalent beam-columns that includeboth flexural and shear deformations. The rigidconnection zone at beam connections to theseequivalent beam-columns must properly represent thedistance from the wall centroid—where the beamcolumnis placed—to the edge of the wall or wallsegment. Unsymmetrical precast wall sections shallmodel the different bending capacities for the twoloading directions.For precast shear walls and wall segments where sheardeformations will have a more significant effect onbehavior, a multiple spring model should be used.The diaphragm action of concrete slabs interconnectingprecast shear walls and frame columns shall be properlyrepresented.6.9.2.2 Stiffness for AnalysisThe modeling assumptions defined in Section 6.8.2.2for monolithic concrete shear walls and wall segmentsshall also be used for precast concrete walls. Inaddition, the analytical model shall adequately modelthe stiffness of the connections between the precastcomponents that compose the wall. This may beaccomplished by softening the model used to representthe precast panels to account for flexibility in theconnections. An alternative procedure would be to addspring elements to simulate axial, shear, and rotationaldeformations within the connections between panels.A. Linear Static and Dynamic ProceduresThe modeling procedures given in Section 6.8.2.2A,combined with a procedure for including connectiondeformations as noted above, shall be used.B. Nonlinear Static and Dynamic ProceduresNonlinear load-deformation relations shall follow thegeneral guidelines of Section 6.4.1.2. The monotonicload-deformation relationships for analytical modelsthat represent precast shear walls and wall elementswithin precast panels shall be represented by one of thegeneral shapes defined in Figure 6-1. Values for plastichinge rotations or drifts at points B, C, and E for the twogeneral shapes are defined below. The strength levels atpoints B and C should correspond to the yield strengthand nominal strength, as defined in Section 6.8.2.3. Theresidual strength for the line segment D–E is definedbelow.For precast shear walls and wall segments whoseinelastic behavior under lateral loading is governed byflexure, the general load-deformation relationship inFigure 6-1(a) will be referred to. For these members,the x-axis of Figure 6-1(a) should be taken as therotation over the plastic hinging region at the end of themember (Figure 6-2). If the requirements for cast-inplaceemulation are satisfied, the value of the hingerotation at point B corresponds to the yield rotation, θ y ,and is given by Equation 6-5. The same expressionshould also be used for wall segments within a precastpanel if flexure controls the inelastic response of thesegment.If the precast wall is of jointed construction and flexuregoverns the inelastic response of the member, then thevalue of θ y will need to be increased to account forrotation in the joints between panels or between thepanel and the foundation.For precast shear walls and wall segments whoseinelastic behavior under lateral loading is governed byshear, the general load-deformation relationship inFEMA 273 Seismic Rehabilitation Guidelines 6-49

Chapter 6: Concrete(Systematic Rehabilitation)Figure 6-1(b) will be referred to. For these members,the x-axis of Figure 6-1(b) should be taken as the storydrift for shear walls, and as the element drift for wallsegments (Figure 6-3).For construction classified as cast-in-place emulation,the values for the variables a, b, and c, which arerequired to define the location of points C, D, and E inFigure 6-1(a), are given in Table 6-17. For constructionclassified as jointed construction, the values of a, b, andc given in Table 6-17 shall be reduced to 50% of thegiven values, unless there is experimental evidenceavailable to justify higher values. In no case, however,shall values larger than those given in Table 6-17 beused.For construction classified as cast-in-place emulation,values for the variables d, e, and c, which are requiredto find the points C, D, and E in Figure 6-1(b), are givenin Tables 6-17 and 6-18 for the appropriate memberconditions. For construction classified as jointedconstruction, the values of d, e, and c given inTables 6-17 and 6-18 shall be reduced to 50% of thegiven values unless there is experimental evidenceavailable to justify higher values. In no case, however,shall values larger than those given in Tables 6-17 and6-18 be used.For Tables 6-17 and 6-18, linear interpolation betweentabulated values shall be used if the member underanalysis has conditions that are between the limits givenin the tables.For the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby experimental evidence. The relationships inFigure 6-1 may be taken to represent the envelope forthe analysis. The unloading and reloading stiffnessesand strengths, and any pinching of the load versusrotation hysteresis loops, shall reflect the behaviorexperimentally observed for wall elements similar tothe one under investigation.6.9.2.3 Design StrengthsThe strength of precast concrete shear walls and wallsegments within the panels shall be computed accordingto the general requirement of Section 6.4.2, except asmodified here. For cast-in-place emulation types ofconstruction, the strength calculation procedures givenin Section 6.8.2.3 shall be followed.For jointed construction, calculations of axial, shear,and flexural strength of the connections between panelsshall be based on known or assumed material propertiesand the fundamental principles of structural mechanics.Yield strength for steel reinforcement of connectionhardware used in the connections shall be increased to125% of its specified yield value when calculating theaxial and flexural strength of the connection region. Theunmodified specified yield strength of thereinforcement and connection hardware shall be usedwhen calculating the shear strength of the connectionregion.In older construction, particular attention must be givento the technique used for splicing reinforcementextending from adjacent panels into the connection.These connections may be insufficient and can oftengovern the strength of the precast shear wall system. Ifsufficient detail is not given on the design drawings,concrete should be removed in some connections toexpose the splicing details for the reinforcement.For all precast concrete shear walls of jointedconstruction, no difference shall be taken between thecomputed yield and nominal strengths in flexure andshear. Thus, the values for strength represented by thepoints B and C in Figure 6-1 shall be computedfollowing the procedures given in Section 6.8.2.3.6.9.2.4 Acceptance CriteriaA. Linear Static and Dynamic ProceduresFor precast shear wall construction that emulates castin-placeconstruction and for wall segments within aprecast panel, the acceptance criteria defined inSection 6.8.2.4A shall be followed. For precast shearwall construction defined as jointed construction, theacceptance criteria procedure given in Section 6.8.2.4Ashall be followed. However, the m values given inTables 6-19 and 6-20 shall be reduced by 50%, unlessexperimental evidence justifies the use of a larger value.In no case shall an m value be taken as less than 1.0.B. Nonlinear Static and Dynamic ProceduresInelastic response shall be restricted to those shear walls(and wall segments) and actions listed in Tables 6-17and 6-18, except where it is demonstrated that otherinelastic action can be tolerated considering the selectedPerformance Levels. For members experiencinginelastic behavior, the magnitude of the other actions(forces, moments, or torques) in the member shallcorrespond to the magnitude of the action causing the6-50 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)inelastic behavior. The magnitude of these other actionsshall be shown to be below their nominal capacities.For precast shear walls of the cast-in-place emulationtype of construction, and for wall segments within aprecast panel, the maximum plastic hinge rotationangles or drifts during inelastic response shall notexceed the values given in Tables 6-17 and 6-18. Forprecast shear walls of jointed construction, themaximum plastic hinge rotation angles or drifts duringinelastic response shall not exceed one-half of thevalues given in Tables 6-17 and 6-18, unlessexperimental evidence is available to justify a highervalue. However, in no case shall deformation valueslarger than those given in these tables be used forjointed type construction.If the maximum deformation value exceeds thecorresponding tabular value, the element shall beconsidered to be deficient and either the element orstructure will need to be rehabilitated.6.9.2.5 Rehabilitation MeasuresPrecast concrete shear wall systems may suffer fromsome of the same deficiencies as cast-in-place walls.These may include inadequate flexural capacity,inadequate shear capacity with respect to flexuralcapacity, lack of confinement at wall boundaries, andinadequate splice lengths for longitudinal reinforcementin wall boundaries. All of these deficiencies can berehabilitated by the use of one of the measuresdescribed in Section 6.8.2.5. A few deficiencies uniqueto precast wall construction are inadequate connectionsbetween panels, to the foundation, and to floor or roofdiaphragms.• Enhancement of connections between adjacent orintersecting precast wall panels. A combination ofmechanical and cast-in-place details may be used tostrengthen connections between precast panels.Mechanical connectors may include steel shapes andvarious types of drilled-in anchors. Cast-in-placestrengthening methods generally involve exposingthe reinforcing steel at the edges of adjacent panels,adding vertical and transverse (tie) reinforcement,and placing new concrete.• Enhancement of connections between precastwall panels and foundations. The shear capacity ofthe wall panel-to-foundation connection can bestrengthened by the use of supplemental mechanicalconnectors or by using a cast-in-place overlay withnew dowels into the foundation. The overturningmoment capacity of the panel-to-foundationconnection can be strengthened by using drilled-indowels within a new cast-in-place connection at theedges of the panel. Adding connections to adjacentpanels may also eliminate some of the forcestransmitted through the panel-to-foundationconnection.• Enhancement of connections between precastwall panels and floor or roof diaphragms. Theseconnections can be strengthened by using eithersupplemental mechanical devices or cast-in-placeconnectors. Both in-plane shear and out-of-planeforces will need to be considered whenstrengthening these connections.6.10 Concrete Braced Frames6.10.1 Types of Concrete Braced FramesReinforced concrete braced frames are those frameswith monolithic reinforced concrete beams, columns,and diagonal braces that are coincident at beam-columnjoints. Components are nonprestressed. Under lateralloading, the braced frame resists loads primarilythrough truss action.Masonry infills may be present in braced frames. Wheremasonry infills are present, requirements for masonryinfilled frames as specified in Section 6.7 also apply.The provisions are applicable to existing reinforcedconcrete braced frames, and existing reinforcedconcrete braced frames rehabilitated by addition orremoval of material.6.10.2 General Considerations in Analysisand ModelingThe analysis model for a reinforced concrete bracedframe shall represent the strength, stiffness, anddeformation capacity of beams, columns, braces, and allconnections and components that may be part of theelement. Potential failure in tension, compression(including instability), flexure, shear, anchorage, andreinforcement development at any section along thecomponent length shall be considered. Interaction withother structural and nonstructural elements andcomponents shall be included.FEMA 273 Seismic Rehabilitation Guidelines 6-51

Chapter 6: Concrete(Systematic Rehabilitation)The analytical model generally can represent theframing, using line elements with propertiesconcentrated at component centerlines. Generalconsiderations relative to the analytical model aresummarized in Section 6.5.2.1.In frames having braces in some bays and no braces inother bays, the restraint of the brace shall be representedas described above, and the nonbraced bays shall bemodeled as frames according to the specifications ofthis chapter. Where braces create a verticallydiscontinuous frame, the effects on overall buildingperformance shall be considered.Inelastic deformations in primary components shall berestricted to flexure and axial load in beams, columns,and braces. Other inelastic deformations are permittedin secondary components. Acceptance criteria arepresented in Section 6.10.5.6.10.3 Stiffness for Analysis6.10.3.1 Linear Static and DynamicProceduresBeams, columns, and braces in braced portions of theframe may be modeled considering axial tension andcompression flexibilities only. Nonbraced portions offrames shall be modeled according to proceduresdescribed elsewhere for frames. Effective stiffnessesshall be according to Section 6.4.1.2.6.10.3.2 Nonlinear Static ProcedureNonlinear load-deformation relations shall follow thegeneral guidelines of Section 6.4.1.2.Beams, columns, and braces in braced portions may bemodeled using nonlinear truss components. Beams andcolumns in nonbraced portions may be modeled usingprocedures described elsewhere in this chapter. Themodel shall be capable of representing inelasticresponse along the component lengths, as well as withinconnections.Numerical quantities in Figure 6-1 may be derived fromtests or rational analyses. Alternately, the guidelines ofSection 6.7.2.2B may be used, with braces modeled ascolumns per Table 6-15.6.10.3.3 Nonlinear Dynamic ProcedureFor the NDP, the complete hysteresis behavior of eachcomponent shall be modeled using properties verifiedby tests. Unloading and reloading properties shallrepresent significant stiffness and strength degradationcharacteristics.6.10.4 Design StrengthsComponent strengths shall be computed according tothe general requirements of Section 6.4.2 and theadditional requirements of Section 6.5.2.3. Thepossibility of instability of braces in compression shallbe considered.6.10.5 Acceptance Criteria6.10.5.1 Linear Static and DynamicProceduresAll component actions shall be classified as being eitherdeformation-controlled or force-controlled, as definedin Chapter 3. In primary components, deformationcontrolledactions shall be restricted to flexure and axialactions in beams and columns, and axial actions inbraces. In secondary components, deformationcontrolledactions shall be restricted to those actionsidentified for the braced or isolated frame in thischapter.Calculated component actions shall satisfy therequirements of Chapter 3. Refer to other sections ofthis chapter for m values for concrete frames, exceptthat m values for beams, columns, and braces modeledas tension and compression components may be takenas equal to values specified for columns in Table 6-16.Values of m shall be reduced from values in that tablewhere component buckling is a consideration. Alternateapproaches or values are permitted where justified byexperimental evidence and analysis.6.10.5.2 Nonlinear Static and DynamicProceduresCalculated component actions shall not exceed thenumerical values listed in Table 6-15 or the relevanttables for isolated frames given elsewhere in thischapter. Where inelastic action is indicated for acomponent or action not listed in these tables, theperformance shall be deemed unacceptable. Alternateapproaches or values are permitted where justified byexperimental evidence and analysis.6-52 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)6.10.6 Rehabilitation MeasuresRehabilitation measures include the general approacheslisted for other elements in this chapter, plus otherapproaches based on rational procedures.Rehabilitated frames shall be evaluated according to thegeneral principles and requirements of this chapter. Theeffects of rehabilitation on stiffness, strength, anddeformability shall be taken into account in theanalytical model. Connections required betweenexisting and new elements shall satisfy requirements ofSection 6.4.6 and other requirements of the Guidelines.6.11 Concrete Diaphragms6.11.1 Components of Concrete DiaphragmsCast-in-place concrete diaphragms transmit inertialforces from one location in a structure to a verticallateral-force-resisting element. A concrete diaphragm isgenerally a floor or roof slab, but can be a structuraltruss in the horizontal plane.Diaphragms are made up of slabs that transmit shearforces, struts that provide continuity around openings,collectors that gather force and distribute it, and chordsthat are located at the edges of diaphragms and thatresist tension and compression forces.6.11.1.1 SlabsThe primary function of any slab that is part of a flooror roof system is to support gravity loads. A slab mustalso function as part of the diaphragm to transmit theshear forces associated with the load transfer. Theseinternal shear forces are generated when the slab is theload path for forces that are being transmitted from onevertical lateral-force-resisting system to another, orwhen the slab is functioning to provide bracing to otherportions of the building that are being loaded out ofplane. Included in this section are all versions of castin-placeconcrete floor systems, and concrete-on-metaldeck systems.6.11.1.2 Struts and CollectorsStruts and collectors are built into diaphragms inlocations where there are defined stress demands thatexceed the typical stress capacity of the diaphragm.These locations occur around openings in thediaphragms, along defined load paths between lateralload-resistingelements, and at intersections of portionsof floors that have plan irregularities. Struts andcollectors may occur within the slab thickness or mayhave the form of cast-in-place beams that aremonolithic with the slabs. The forces that they resist areprimarily axial in nature, but may also include shear andbending forces.6.11.1.3 Diaphragm ChordsDiaphragm chords generally occur at the edges of ahorizontal diaphragm and function to resist bendingstresses in the diaphragm. Tensile forces typically aremost critical, but compressive forces in thin slabs couldbe a problem. Exterior walls can serve this function ifthere is adequate horizontal shear capacity between theslab and wall. When evaluating an existing building,special care should be taken to evaluate the condition ofthe lap splices. Where the splices are not confined byclosely-spaced transverse reinforcement, splice failureis possible if stress levels reach critical values. Inrehabilitation construction, new laps should be confinedby closely-spaced transverse reinforcement.6.11.2 Analysis, Modeling, and AcceptanceCriteria6.11.2.1 General ConsiderationsThe analysis model for a diaphragm shall represent thestrength, stiffness, and deformation capacity of eachcomponent and the diaphragm as a whole. Potentialfailure in flexure, shear, buckling, and reinforcementdevelopment at any point in the diaphragm shall beconsidered.The analytical model of the diaphragm can typically betaken as a continuous or simple span horizontal beamthat is supported by elements of varying stiffness. Thebeam may be rigid or semi-rigid. Most computermodels assume a rigid diaphragm. Few cast-in-placediaphragms would be considered flexible, whereas athin concrete slab on a metal deck might be semi-rigiddepending on the length-to-width ratio of thediaphragm.6.11.2.2 Stiffness for AnalysisDiaphragm stiffness shall be modeled according toSection 6.11.2.1 and shall be determined using a linearelastic model and gross section properties. The modulusof elasticity used shall be that of the concrete asspecified in Section 8.5.1 of ACI 318-95. When thelength-to-width ratio of the diaphragm exceeds 2.0(where the length is the distance between verticalFEMA 273 Seismic Rehabilitation Guidelines 6-53

Chapter 6: Concrete(Systematic Rehabilitation)elements), the effects of diaphragm deflection shall beconsidered when assigning lateral forces to the resistingvertical elements. The concern is for relatively flexiblevertical members that may be displaced by thediaphragm, and for relatively stiff vertical members thatmay be overloaded due to the same diaphragmdisplacement.6.11.2.3 Design StrengthsComponent strengths shall be according to the generalrequirements of Section 6.4.2, as modified in thissection.The maximum component strength shall be determinedconsidering potential failure in flexure, axial load,shear, torsion, development, and other actions at allpoints in the component under the actions of designgravity and lateral load combinations. The shearstrength shall be as specified in Section 21.6.4 ofACI 318-95. Strut, collector, and chord strengths shallbe determined according to Section 6.5.2.3 of theseGuidelines.6.11.2.4 Acceptance CriteriaAll component actions shall be classified as eitherdeformation-controlled or force-controlled, as definedin Chapter 3. Diaphragm shear shall be considered asbeing a force-controlled component and shall have aDCR not greater than 1.25. Acceptance criteria for allother component actions shall be as defined inSection 6.5.2.4A, with m values taken according tosimilar components in Tables 6-10 and 6-11 for use inEquation 3-18. Analysis shall be restricted to linearprocedures.6.11.3 Rehabilitation MeasuresCast-in-place concrete diaphragms can have a widevariety of deficiencies; see Chapter 10 and FEMA 178(BSSC, 1992a). Two general alternatives may be usedto correct deficiencies: either improving the strengthand ductility, or reducing the demand in accordancewith FEMA 172 (BSSC, 1992b). Individualcomponents can be strengthened or improved by addingadditional reinforcement and encasement. Diaphragmthickness may be increased, but the added weight mayoverload the footings and increase the seismic load.Demand can be lowered by adding additional lateralforce-resistingelements, introducing additionaldamping, or base isolating the structure. All correctivemeasures taken shall be based upon engineeringmechanics, taking into account load paths anddeformation compatibility requirements of thestructure.6.12 Precast Concrete Diaphragms6.12.1 Components of Precast ConcreteDiaphragmsSection 6.11 provided a general overview of concretediaphragms. Components of precast concretediaphragms are similar in nature and function to thoseof cast-in-place diaphragms, with a few criticaldifferences. One is that precast diaphragms do notpossess the inherent unity of cast-in-place monolithicconstruction. Additionally, precast components may behighly stressed due to prestressed forces. These forcescause long-term shrinkage and creep, which shorten thecomponent over time. This shortening tends to fractureconnections that restrain the component.Precast concrete diaphragms can be classified as toppedor untopped. A topped diaphragm is one that has had aconcrete topping slab poured over the completedhorizontal system. Most floor systems have a toppingsystem, but some hollow core floor systems do not.The topping slab generally bonds to the top of theprecast elements, but may have an inadequate thicknessat the center of the span, or may be inadequatelyreinforced. Also, extensive cracking of joints may bepresent along the panel joints. Shear transfer at theedges of precast concrete diaphragms is especiallycritical.Some precast roof systems are constructed as untoppedsystems. Untopped precast concrete diaphragms havebeen limited to lower seismic zones by recent versionsof the Uniform Building Code. This limitation has beenimposed because of the brittleness of connections andlack of test data concerning the various precast systems.Special consideration shall be given to diaphragmchords in precast construction.6.12.2 Analysis, Modeling, and AcceptanceCriteriaAnalysis and modeling of precast concrete diaphragmsshall conform to Section 6.11.2.2, with the addedrequirement that special attention be paid to consideringthe segmental nature of the individual components.Component strengths shall be determined according toSection 6.11.2.3, with the following exception. Welded6-54 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)connection strength shall be determined using the latestversion of the Precast Concrete Institute (PCI)Handbook, assuming that the connections have littleductility unless test data are available to document theassumed ductility.Acceptance criteria shall be as defined inSection 6.11.2.4; the criteria of Section 6.4.6.2, whereapplicable, shall also be included.6.12.3 Rehabilitation MeasuresSection 6.11.3 provides guidance for rehabilitationmeasures for concrete diaphragms in general. Specialcare shall be taken to overcome the segmental nature ofprecast concrete diaphragms, and to avoid fracturingprestressing strands when adding connections.6.13 Concrete Foundation Elements6.13.1 Types of Concrete FoundationsFoundations serve to transmit loads from the verticalstructural subsystems (columns and walls) of a buildingto the supporting soil or rock. Concrete foundations forbuildings are classified as either shallow or deepfoundations. Shallow foundations include spread orisolated footings; strip or line footings; combinationfootings; and concrete mat footings. Deep foundationsinclude pile foundations and cast-in-place piers.Concrete grade beams may be present in both shallowand deep foundation systems.These provisions are applicable to existing foundationelements and to new materials or elements that arerequired to rehabilitate an existing building.6.13.1.1 Shallow FoundationsExisting spread footings, strip footings, andcombination footings may be reinforced orunreinforced. Vertical loads are transmitted to the soilby direct bearing; lateral loads are transmitted by acombination of friction between the bottom of thefooting and the soil, and passive pressure of the soil onthe vertical face of the footing.Concrete mat footings must be reinforced to resist theflexural and shear stresses resulting from thesuperimposed concentrated and line structural loads andthe distributed resisting soil pressure under the footing.Lateral loads are resisted primarily by friction betweenthe soil and the bottom of the footing, and by passivepressure developed against foundation walls that arepart of the system.6.13.1.2 Deep FoundationsA. Driven Pile FoundationsConcrete pile foundations are composed of a reinforcedconcrete pile cap supported on driven piles. The pilesmay be concrete (with or without prestressing), steelshapes, steel pipes, or composite (concrete in a drivensteel shell). Vertical loads are transmitted to the pilingby the pile cap, and are resisted by direct bearing of thepile tip in the soil or by skin friction or cohesion of thesoil on the surface area of the pile. Lateral loads areresisted by passive pressure of the soil on the verticalface of the pile cap, in combination with interaction ofthe piles in bending and passive soil pressure on the pilesurface. In poor soils, or soils subject to liquefaction,bending of the piles may be the only dependableresistance to lateral loads.B. Cast-in-Place Pile FoundationsCast-in-place concrete pile foundations consist ofreinforced concrete placed in a drilled or excavatedshaft. The shaft may be formed or bare. Segmented steelcylindrical liners are available to form the shaft in weaksoils and allow the liner to be removed as the concrete isplaced. Various slurry mixes are often used to protectthe drilled shaft from caving soils; the slurry is thendisplaced as the concrete is placed by the tremiemethod. Cast-in-place pile or pier foundations resistvertical and lateral loads in a manner similar to that ofdriven pile foundations.6.13.2 Analysis of Existing FoundationsThe analytical model for concrete buildings, withcolumns or walls cast monolithically with thefoundation, is sometimes assumed to have the verticalstructural elements fixed at the top of the foundation.When this is assumed, the foundations and supportingsoil must be capable of resisting the induced moments.When columns are not monolithic with theirfoundations, or are designed so as to not resist flexuralmoments, they may be modeled with pinned ends. Insuch cases, the column base must be evaluated for theresulting axial and shear forces as well as the ability toaccommodate the necessary end rotation of thecolumns. The effects of base fixity of columns must betaken into account at the point of maximumdisplacement of the superstructure.FEMA 273 Seismic Rehabilitation Guidelines 6-55

Chapter 6: Concrete(Systematic Rehabilitation)Overturning moments and economics may dictate theuse of more rigorous Analysis Procedures. When this isthe case, appropriate vertical, lateral, and rotational soilsprings shall be incorporated in the analytical model asdescribed in Section 4.4.2. The spring characteristicsshall be based on the material in Chapter 4, and on therecommendations of the geotechnical consultant.Rigorous analysis of structures with deep foundations insoft soils will require special soil/pile interaction studiesto determine the probable location of the point of fixityin the foundation and the resulting distribution of forcesand displacements in the superstructure. In theseanalyses, the appropriate representation of theconnection of the pile to the pile cap is required.Buildings designed for gravity loads only may have anominal (about six inches) embedment of the pileswithout any dowels into the pile cap. These piles mustbe modeled as being “pinned” to the cap. Unless theconnection can be identified from the availableconstruction documents, the “pinned” connectionshould be assumed in any analytical model.When the foundations are included in the analyticalmodel, the responses of the foundation componentsmay be derived by any of the analytical methodsprescribed in Chapter 3, as modified by therequirements of Section 6.4. When the structuralelements of the analytical model are assumed to bepinned or fixed at the foundation level, the reactions(axial loads, shears, and moments) of those elementsshall be used to evaluate the individual components ofthe foundation system.6.13.3 Evaluation of Existing ConditionAllowable soil capacities (subgrade modulus, bearingpressure, passive pressure) are a function of the chosenPerformance Level, and will be as prescribed inChapter 4 or as established with project-specific data bya geotechnical consultant. All components of existingfoundation elements, and all new material, components,or elements required for rehabilitation, will beconsidered to be force-controlled (m = 1.0) based on themechanical and analytical properties in Section 6.3.3.However, the capacity of the foundation componentsneed not exceed 1.25 times the capacity of thesupported vertical structural component or element(column or wall). The amount of foundationdisplacement that is acceptable for the given structureshould be determined by the design engineer, and is afunction of the desired Performance Level.6.13.4 Rehabilitation MeasuresThe following general rehabilitation measures areapplicable to existing foundation elements. Otherapproaches, based on rational procedures, may also beutilized.6.13.4.1 Rehabilitation Measures for ShallowFoundations• Enlarging the existing footing by lateraladditions. The existing footing will continue toresist the loads and moment acting at the time ofrehabilitation (unless temporarily removed). Theenlarged footing is to resist subsequent loads andmoments produced by earthquakes if the lateraladditions are properly tied into the existing footing.Shear transfer and moment development must beaccomplished in the additions.• Underpinning the footing. Underpinning involvesthe removal of unsuitable soil under an existingfooting, coupled with replacement using concrete,soil cement, suitable soil, or other material, and mustbe properly staged in small increments so as not toendanger the stability of the structure. Thistechnique also serves to enlarge an existing footingor to extend it to a more competent soil stratum.• Providing tension hold-downs. Tension ties (soiland rock anchors—prestressed and unstressed) aredrilled and grouted into competent soils andanchored in the existing footing to resist uplift.Increased soil bearing pressures produced by the tiesmust be checked against values associated with thedesired Performance Level. Piles or drilled piersmay also be utilized.• Increasing effective depth of footing. This methodinvolves pouring new concrete to increase shear andmoment capacity of existing footing. Newhorizontal reinforcement can be provided, ifrequired, to resist increased moments.• Increasing the effective depth of a concrete matfoundation with a reinforced concrete overlay.This method involves pouring an integral toppingslab over the existing mat to increase shear andmoment capacity. The practicality must be checkedagainst possible severe architectural restrictions.• Providing pile supports for concrete footings ormat foundations. Addition of piles requires careful6-56 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)design of pile length and spacing to avoidoverstressing the existing foundations. Thetechnique may only be feasible in a limited numberof cases for driven piles, but special augered systemshave been developed and are used regularly.• Changing the building structure to reduce thedemand on the existing elements. This methodinvolves removing mass or height of the building oradding other materials or components (such asenergy dissipation devices) to reduce the loadtransfer at the base level. The addition of new shearwalls or braces will generally reduce the demand onexisting foundations.• Adding new grade beams. Grade beams may beused to tie existing footings together when poor soilexists, to provide fixity to column bases, and todistribute lateral loads between individual footings,pile caps, or foundation walls.• Improving existing soil. Grouting techniques maybe used to improve existing soil.6.13.4.2 Rehabilitation Measures for DeepFoundations• Providing additional piles or piers. The addition ofpiles or piers may require extension and additionalreinforcement of existing pile caps. See thecomments in previous sections for extending anexisting footing.• Increasing the effective depth of the pile cap.Addition of new concrete and reinforcement to thetop of the cap is done to increase shear and momentcapacity.• Improving soil adjacent to existing pile cap. SeeSection 4.6.1.• Increasing passive pressure bearing area of pilecap. Addition of new reinforced concreteextensions to the existing pile cap provides morevertical foundation faces and greater loadtransferability.• Changing the building system to reduce thedemands on the existing elements. Introduction ofnew lateral-load-resisting elements may reducedemand.• Adding batter piles or piers. Batter piles or piersmay be used to resist lateral loads. It should be notedthat batter piles have performed poorly in recentearthquakes when liquefiable soils were present.This is especially important to consider aroundwharf structures and in areas that have a high watertable. See Sections 4.2.2.2, 4.3.2, and 4.4.2.2B.• Increasing tension tie capacity from pile or pierto superstructure.6.14 DefinitionsThe definitions used in this chapter generally followthose of BSSC (1995) as well as those published in ACI318. Many of the definitions that are independent ofmaterial type are provided in Chapter 2.6.15 SymbolsA g Gross area of column, in. 2A jA sA′ sA wE cE sII gLM gCSEffective cross-sectional area within a joint,in. 2 , in a plane parallel to plane ofreinforcement generating shear in the joint.The joint depth shall be the overall depth ofthe column. Where a beam frames into asupport of larger width, the effective width ofthe joint shall not exceed the smaller of:(1) beam width plus the joint depth, and(2) twice the smaller perpendicular distancefrom the longitudinal axis of the beam to thecolumn side.Area of nonprestressed tensionreinforcement, in. 2Area of compression reinforcement, in. 2Area of the web cross section, = b w dModulus of elasticity of concrete, psiModulus of elasticity of reinforcement, psiMoment of inertiaMoment of inertia of gross concrete sectionabout centroidal axis, neglectingreinforcementLength of member along which deformationsare assumed to occurMoment acting on the slab column stripaccording to ACI 318FEMA 273 Seismic Rehabilitation Guidelines 6-57

Chapter 6: Concrete(Systematic Rehabilitation)M nM nCSM yN uPP oQQ CEQ CLVV cV gV nV oV sV uabb wcc 1ddd bNominal moment strength at sectionNominal moment strength of the slab columnstripYield moment strength at sectionFactored axial load normal to cross sectionoccurring simultaneously with V u . To betaken as positive for compression, negativefor tension, and to include effects of tensiondue to creep and shrinkage.Axial force in a member, lbsNominal axial load strength at zeroeccentricityGeneralized loadExpected strength of a component or elementat the deformation level under considerationfor deformation-controlled actionsLower-bound estimate of the strength of acomponent or element at the deformationlevel under consideration for force-controlledactionsDesign shear force at sectionNominal shear strength provided by concreteShear acting on slab critical section due togravity loadsNominal shear strength at sectionShear strength of slab at critical sectionNominal shear strength provided by shearreinforcementFactored shear force at sectionParameter used to measure deformationcapacityParameter used to measure deformationcapacityWeb width, in.Parameter used to measure residual strengthSize of rectangular or equivalent rectangularcolumn, capital, or bracket measured in thedirection of the span for which moments arebeing determined, in.Parameter used to measure deformationcapacityDistance from extreme compression fiber tocentroid of tension reinforcement, in.Nominal diameter of bar, in.ef c ′f pcf sf yhhh ch wkl bl dl el pl wmt w∆γγ fθθ yκParameter used to measure deformationcapacityCompressive strength of concrete, psiAverage compressive stress in concrete due toeffective prestress force only (after allowancefor all prestress losses)Stress in reinforcement, psiYield strength of tension reinforcementHeight of member along which deformationsare measuredOverall thickness of member, in.Gross cross-sectional dimension of columncore measured in the direction of joint shear,in.Total height of wall from base to top, in.Coefficient used for calculation of columnshear strengthProvided length of straight development, lapsplice, or standard hook, in.Development length for a straight bar, in.Length of embedment of reinforcement, in.Length of plastic hinge used for calculation ofinelastic deformation capacity, in.Length of entire wall or a segment of wallconsidered in the direction of shear force, in.Modification factor used in the acceptancecriteria of deformation-controlledcomponents or elements, indicating theavailable ductility of a component actionThickness of wall web, in.Generalized deformation, consistent unitsCoefficient for calculation of joint shearstrengthFraction of unbalanced moment transferredby flexure at slab-column connectionsGeneralized deformation, radiansYield rotation, radiansA reliability coefficient used to reducecomponent strength values for existingcomponents, based on the quality ofknowledge about the components’ properties(see Section 2.7.2)6-58 Seismic Rehabilitation Guidelines FEMA 273

Chapter 6: Concrete(Systematic Rehabilitation)λ Correction factor related to unit weight ofconcreteµ Coefficient of frictionρ Ratio of nonprestressed tension reinforcementρ′ Ratio of nonprestressed compressionreinforcementρ″ Reinforcement ratio for transverse jointreinforcementρ bal Reinforcement ratio producing balancedstrain conditionsρ n Ratio of distributed shear reinforcement in aplane perpendicular to the direction of theapplied shear6.16 ReferencesACI, 1989, Building Code Requirements for ReinforcedConcrete, Report No. ACI 318-89, American ConcreteInstitute, Detroit, Michigan.ACI, 1991, State-of-the-Art Report on Anchorage toConcrete, Report No. 355.1R-91, ACI Committee 355,ACI Manual of Concrete Practice, American ConcreteInstitute, Detroit, Michigan.ACI, 1994, Guide for Evaluation of Concrete StructuresPrior to Rehabilitation, Report No. ACI 364.1R-94,American Concrete Institute, Detroit, Michigan.ACI, 1995, Building Code Requirements for ReinforcedConcrete: ACI 318-95, American Concrete Institute,Detroit, Michigan.ASCE, 1917, “Final Report of Special Committee onConcrete and Reinforced Concrete,” Transactions,American Society of Civil Engineers, New York, NewYork, Vol. 81, pp. 1101–1205.ASCE, 1924, “Report of the Joint Committee onStandard Specifications for Concrete and ReinforcedConcrete,” Proceedings, American Society of CivilEngineers, New York, New York, pp. 1153–1285.ASCE, 1940, “Concrete Specifications Number,”Proceedings, American Society of Civil Engineers, NewYork, New York, Vol. 66, No. 6, Part 2.ASCE, 1990, Standard Guideline for StructuralCondition Assessment of Existing Buildings,Standard 11-90, American Society of Civil Engineers,New York, New York.ASTM, latest edition, standards with the followingnumbers, A370, A416, A421, A722, C39, C42, C496,E488, American Society of Testing Materials,Philadelphia, Pennsylvania.BSSC, 1992a, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 178), Washington, D.C.BSSC, 1992b, NEHRP Handbook of Techniques for theSeismic Rehabilitation of Existing Buildings, developedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 172), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos. FEMA222A and 223A), Washington, D.C.Corley, G., 1996, “Northridge Earthquake of January17, 1994 Reconnaissance Report—Concrete ParkingStructures,” Earthquake Spectra, EERI Publication95-03/2, Earthquake Engineering Research Institute,Oakland, California.CRSI, 1981, Evaluation of Reinforcing Steel Systems inOld Reinforced Concrete Structures, ConcreteReinforcing Steel Institute, Chicago, Illinois.Fleischman, R. B., et al., 1996, “Seismic Behavior ofPrecast Parking Structure Diaphragms,” Proceedings ofStructures Congress XIV, American Society of CivilEngineers, New York, New York, Vol. 2,pp. 1139–1146.ICBO, 1994, Uniform Building Code, Vol. 2: StructuralEngineering Design Provisions, InternationalConference of Building Officials, Whittier, California.Lord, A. R., 1928, “A Handbook of ReinforcedConcrete Building Design, in Accordance with the 1928Joint Standard Building Code,” authorized reprint fromthe copyrighted Proceedings of the American ConcreteInstitute, Detroit, Michigan, Vol. 24.FEMA 273 Seismic Rehabilitation Guidelines 6-59

Chapter 6: Concrete(Systematic Rehabilitation)Newlon, Jr., H., ed., 1976, A Selection of HistoricAmerican Papers on Concrete 1876–1926, PublicationNo. SP-52, American Concrete Institute, Detroit,Michigan.Taylor, F. W., Thompson, S. E., and Smulski, E., 1925,Concrete, Plain and Reinforced, Vol. 1, Theory andDesign of Concrete and Reinforced Structures, fourthedition, John Wiley & Sons, Inc., New York, New York.Wallace, J. W., 1994, “New Methodology for SeismicDesign of RC Shear Walls,” Journal of the StructuralEngineering Division, American Society of CivilEngineers, New York, New York, Vol. 120, No. 3,pp. 863–884.Wallace, J. W., 1995, “Seismic Design of RC StructuralWalls, Part I: New Code Format,” Journal of theStructural Engineering Division, American Society ofCivil Engineers, New York, New York, Vol. 121, No. 1,pp. 75–87.Wood, S. L., 1990, “Shear Strength of Low-RiseReinforced Concrete Walls,” ACI Structural Journal,American Concrete Institute, Detroit, Michigan,Vol. 87, No. 1, pp. 99–107.6-60 Seismic Rehabilitation Guidelines FEMA 273

7.Masonry(Systematic Rehabilitation)7.1 ScopeThis chapter describes engineering procedures forestimating seismic performance of vertical lateralforce-resistingmasonry elements. The methods areapplicable for masonry wall and infill panels that areeither existing or rehabilitated elements of a buildingsystem, or new elements that are added to an existingbuilding system.This chapter presents the information needed forsystematic rehabilitation of masonry buildings asdepicted in Step 3 of the Process Flow chart shown inFigure 1-1. A brief historical perspective is given inSection 7.2, with an expanded version in CommentarySection C7.2. Masonry material properties for new andexisting construction are discussed in Section 7.3.Attributes of masonry walls and masonry infills aregiven in Sections 7.4 and 7.5, respectively. Masonrycomponents are classified by their behavior;unreinforced components precede reinforcedcomponents, and in-plane action is separated from outof-planeaction. For each component type, theinformation needed to model stiffness is presented first,followed by recommended strength and deformationacceptance criteria for various performance levels.These attributes are presented in a format for direct usewith the Linear and Nonlinear Static Proceduresprescribed in Chapter 3. Guidelines for anchorage tomasonry walls and masonry foundation elements aregiven in Sections 7.6 and 7.7, respectively. Section 7.8provides definitions for terms used in this chapter, andSection 7.9 lists the symbols used in Chapter 7equations. Applicable reference standards are listed inSection 7.10.Portions of a masonry building that are not subject tosystematic rehabilitation provisions of this chapter—such as parapets, cladding, or partition walls—shall beconsidered with the Simplified Rehabilitation options ofChapter 10 or with the provisions for nonstructuralcomponents addressed in Chapter 11.The provisions of this chapter are intended for solid orhollow clay-unit masonry, solid or hollow concrete-unitmasonry, and hollow clay tile. Stone or glass blockmasonry is not covered in this chapter.The properties and behavior of steel, concrete, andtimber floor or roof diaphragms are addressed inChapters 5, 6, and 8, respectively. Connections tomasonry walls are addressed in Section 7.6 for caseswhere behavior of the connection is dependent onproperties of the masonry. Attributes for masonryfoundation elements are briefly described inSection 7.7.Unreinforced masonry buildings with flexible floordiaphragms may be evaluated by using the proceduresgiven in Appendix C of FEMA 178 (BSSC, 1992) if thesimplified rehabilitation approach of Chapter 10 isfollowed.7.2 Historical PerspectiveConstruction of existing masonry buildings in theUnited States dates back to the 1500s in thesoutheastern and southwestern parts of the country, tothe 1770s in the central and eastern parts, and to the1850s in the western half of the nation. The stock ofexisting masonry buildings in the United States largelycomprises structures constructed in the last 150 years.Since the types of units, mortars, and constructionmethods have changed over this course of time,knowing the vintage of a masonry building may beuseful in identifying the characteristics of theconstruction. Although structural properties cannot beinferred solely from age, some background on typicalmaterials and methods for a given period can help toimprove engineering judgment, and provide somedirection in the assessment of an existing building.As indicated in Chapter 1, great care should beexercised in selecting the appropriate rehabilitationapproaches and techniques for application to historicbuildings in order to preserve their uniquecharacteristics.Section C7.2 of the Commentary provides an extensivehistorical perspective on various masonry materials andconstruction practices.FEMA 273 Seismic Rehabilitation Guidelines 7-1

Chapter 7: Masonry(Systematic Rehabilitation)7.3 Material Properties andCondition Assessment7.3.1 GeneralThe methods specified in Section 7.3.2 fordetermination of mechanical properties of existingmasonry construction shall be used as the basis forstiffness and strength attributes of masonry walls andinfill panels, along with the methods described inSections 7.4 and 7.5. Properties of new masonrycomponents that are added to an existing structuralsystem shall be based on values given in BSSC (1995).Minimum requirements for determining in situ masonrycompressive, tensile, and shear strength, as well aselastic and shear moduli, are provided in Section 7.3.2.Recommended procedures for measurement of eachmaterial property are described in correspondingsections of the Commentary. Data from testing of inplacematerials shall be expressed in terms of meanvalues for determination of expected componentstrengths, Q CE , and lower bound strengths, Q CL , withthe linear or nonlinear procedures described inChapter 3.In lieu of in situ testing, default values of materialstrength and modulus, as given in Section 7.3.2, shall beassigned to masonry components in good, fair, and poorcondition. Default values represent typical lower boundestimates of strength or stiffness for all masonrynationwide, and thus should not be construed asexpected values for a specific structure. As specified inSection 7.3.4, certain in situ tests are needed to attainthe comprehensive level of knowledge (a κ value of1.00) needed in order to use the nonlinear procedures ofChapter 3. Thus, unless noted otherwise, general use ofthe default values without in situ testing is limited to thelinear procedures of Chapter 3.Procedures for defining masonry structural systems,and assessing masonry condition, shall be conducted inaccordance with provisions stated in Section 7.3.3.Requirements for either a minimum or a comprehensivelevel of evaluation, as generally stated in Section 2.7,are further refined for masonry components inSection 7.3.4.7.3.2 Properties of In-Place Materials7.3.2.1 Masonry Compressive StrengthExpected masonry compressive strength, f me , shall bemeasured using one of the following three methods.1. Test prisms shall be extracted from an existing walland tested per Section 1.4.B.3 of the MasonryStandards Joint Committee’s Building CodeRequirements for Masonry Structures (MSJC,1995a).2. Prisms shall be fabricated from actual extractedmasonry units, and a surrogate mortar designed onthe basis of a chemical analysis of actual mortarsamples. The test prisms shall be tested perSection 1.4.B.3 of the Specification for MasonryStructures (MSJC, 1995b).3. Two flat jacks shall be inserted into slots cut intomortar bed joints and pressurized until peak stress isreached.For each of the three methods, the expectedcompressive strength shall be based on the net mortaredarea.If the masonry unit strength and the mortar type areknown, f me values may be taken from Tables 1 and 2 ofMSJC (1995a) for clay or concrete masonry constructedafter 1960. The f me value shall be obtained bymultiplying the table values by a factor that representsboth the ratio of expected to lower bound strength andthe height-to-thickness ratio of the prism (seeCommentary Section C7.3.2.1).In lieu of material tests, default values for masonryprism compressive strength shall be taken to not exceed900 psi for masonry in good condition, 600 psi formasonry in fair condition, and 300 psi for masonry inpoor condition.7.3.2.2 Masonry Elastic Modulus inCompressionExpected values of elastic modulus for masonry incompression, E me , shall be measured using one of thefollowing two methods:1. Test prisms shall be extracted from an existing wall,transported to a laboratory, and tested in7-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)compression. Stresses and deformations shall bemeasured to infer modulus values.2. Two flat jacks shall be inserted into slots cut intomortar bed joints, and pressurized up to nominallyone half of the expected masonry compressivestrength. Deformations between the two flat jacksshall be measured to infer compressive strain, andthus elastic modulus.In lieu of prism tests, values for the modulus ofelasticity of masonry in compression shall be taken as550 times the expected masonry compressive strength,f me .7.3.2.3 Masonry Flexural Tensile StrengthExpected flexural tensile strength, f te , for out-of-planebending shall be measured using one of the followingthree methods:1. Test samples shall be extracted from an existingwall, and subjected to minor-axis bending using thebond-wrench method.2. Test samples shall be tested in situ using the bondwrenchmethod.3. Sample wall panels shall be extracted and subjectedto minor-axis bending in accordance with ASTME 518.In lieu of material tests, default values of masonryflexural tensile strength for walls or infill panels loadednormal to their plane shall be taken to not exceed 20 psifor masonry in good condition, 10 psi for masonry infair condition, and zero psi for masonry in poorcondition. For masonry constructed after 1960 withcement-based mortars, default values of flexural tensilestrength can be based on values from Table 8.3.10.5.1of BSSC (1995).Flexural tensile strength for unreinforced masonry(URM) walls subjected to in-plane lateral forces shallbe assumed to be equal to that for out-of-plane bending,unless testing is done to define expected tensilestrength.7.3.2.4 Masonry Shear StrengthFor URM components, expected masonry shearstrength, v me , shall be measured using the in-place sheartest. Expected shear strength shall be determined inaccordance with Equation 7-1.whereP CE(7-1)= Expected gravity compressive force appliedto a wall or pier component stressconsidering load combinations given inEquations 3-14, 3-15, and 3-16A n = Area of net mortared/grouted section, in. 2= Average bed-joint shear strength, psiv te⎛ P0.75 0.75v CE ⎞⎜ te + --------- ⎟⎝ A n ⎠v me = --------------------------------------------------1.5The 0.75 factor on the v te term may be waived for singlewythe masonry, or if the collar joint is known to beabsent or in very poor condition.Values for the mortar shear strength, v te , shall notexceed 100 psi for the determination of v me inEquation 7-1.Average bed-joint shear strength, v te , shall bedetermined from individual shear strength test values,v to , in accordance with Equation 7-2.Vv testto = ---------- – pA D + Lb(7-2)where V test is the load at first movement of a masonryunit, A b is the net mortared area of the bed joints aboveand below the test brick, and p D+L is the estimatedgravity stress at the test location.In lieu of material tests, default values of shear strengthof URM components shall be taken to not exceed 27 psifor running bond masonry in good condition, 20 psi forrunning bond masonry in fair condition, and 13 psi forrunning bond masonry in poor condition. These valuesshall also be used for masonry in other than runningbond if fully grouted. For masonry in other than runningbond and partially grouted or ungrouted, shear strengthshall be reduced by 60% of these values. For masonryconstructed after 1960 with cement-based mortars,FEMA 273 Seismic Rehabilitation Guidelines 7-3

Chapter 7: Masonry(Systematic Rehabilitation)default values of shear strength can be based on valuesin BSSC (1995) for unreinforced masonry.The in-place shear test shall not be used to estimateshear strength of reinforced masonry (RM)components. The expected shear strength of RMcomponents shall be in accordance withSection 7.4.4.2A.7.3.2.5 Masonry Shear ModulusThe expected shear modulus of uncracked,unreinforced, or reinforced masonry, G me , shall beestimated as 0.4 times the elastic modulus incompression. After cracking, the shear modulus shall betaken as a fraction of this value based on the amount ofbed joint sliding or the opening of diagonal tensioncracks.7.3.2.6 Strength and Modulus of ReinforcingSteelThe expected yield strength of reinforcing bars, f ye ,shall be based on mill test data, or tension tests of actualreinforcing bars taken from the subject building.Tension tests shall be done in accordance with ASTMA 615.In lieu of tension tests of reinforcing bars, defaultvalues of yield stress shall be determined perSection 6.3.2.5. These values shall also be consideredas lower bound values, f y , to be used to estimate lowerbound strengths, Q CL .The expected modulus of elasticity of steelreinforcement, E se , shall be assumed to be 29,000,000psi.7.3.2.7 Location and Minimum Number ofTestsThe number and location of material tests shall beselected to provide sufficient information to adequatelydefine the existing condition of materials in thebuilding. Test locations shall be identified in thosemasonry components that are determined to be criticalto the primary path of lateral-force resistance.A visual inspection of masonry condition shall be donein conjunction with any in situ material tests to assessuniformity of construction quality. For masonry withconsistent quality, the minimum number of tests foreach masonry type, and for each three floors ofconstruction or 3000 square feet of wall surface, shallbe three, if original construction records are availablethat specify material properties, or six, if originalconstruction records are not available. At least two testsshould be done per wall, or line of wall elementsproviding a common resistance to lateral forces. Aminimum of eight tests should be done per building.Tests should be taken at locations representative of thematerial conditions throughout the entire building,taking into account variations in workmanship atdifferent story levels, variations in weathering of theexterior surfaces, and variations in the condition of theinterior surfaces due to deterioration caused by leaksand condensation of water and/or the deleterious effectsof other substances contained within the building.For masonry with perceived inconsistent quality,additional tests shall be done as needed to estimatematerial strengths in regions where properties aresuspected to differ. Nondestructive conditionassessment tests per Section 7.3.3.2 may be used toquantify variations in material strengths.An increased sample size may be adopted to improvethe confidence level. The relation between sample sizeand confidence shall be as defined in ASTM E 22.If the coefficient of variation in test measurementsexceeds 25%, additional tests shall be done. If thevariation does not reduce below this limit, use of thetest data shall be limited to the Linear Static Proceduresof Chapter 3.If mean values from in situ material tests are less thanthe default values prescribed in Section 7.3.2, additionaltests shall be done. If the mean continues to be less thanthe default values, the measured values shall be used,and shall be used only with the Linear Static Proceduresof Chapter 3.7.3.3 Condition Assessment7.3.3.1 Visual ExaminationThe size and location of all masonry shear and bearingwalls shall be determined. The orientation andplacement of the walls shall be noted. Overalldimensions of masonry components shall be measured,or determined from plans, including wall heights,lengths, and thicknesses. Locations and sizes of windowand door openings shall be measured, or determined7-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)from plans. The distribution of gravity loads to bearingwalls should be estimated.The wall type shall be identified as reinforced orunreinforced, composite or noncomposite, and/orgrouted, partially grouted, or ungrouted. For RMconstruction, the size and spacing of horizontal andvertical reinforcement should be estimated. Formultiwythe construction, the number of wythes shouldbe noted, as well as the distance between wythes (thethickness of the collar joint or cavity), and theplacement of interwythe ties. The condition andattachment of veneer wythes should be noted. Forgrouted construction, the quality of grout placementshould be assessed. For partially grouted walls, thelocations of grout placement should be identified.The type and condition of the mortar and mortar jointsshall be determined. Mortar shall be examined forweathering, erosion, and hardness, and to identify thecondition of any repointing, including cracks, internalvoids, weak components, and/or deteriorated or erodedmortar. Horizontal cracks in bed joints, vertical cracksin head joints and masonry units, and diagonal cracksnear openings shall be noted.The examination shall identify vertical components thatare not straight. Bulging or undulations in walls shall beobserved, as well as separation of exterior wythes, outof-plumbwalls, and leaning parapets or chimneys.Connections between masonry walls, and betweenmasonry walls and floors or roofs, shall be examined toidentify details and condition. If construction drawingsare available, a minimum of three connections shall beinspected for each general connection type (e.g., floorto-wall,wall-to-wall). If no deviations from thedrawings are found, the sample may be consideredrepresentative. If drawings are unavailable, orsignificant deviations are noted between the drawingsand constructed work, then a random sample ofconnections shall be inspected until a representativepattern of connections can be identified.7.3.3.2 Nondestructive TestsNondestructive tests may be used to supplement thevisual observations required in Section 7.3.3.1. One, ora combination, of the following nondestructive tests,shall be done to meet the requirements of acomprehensive evaluation as stated in Section 7.3.4:• mechanical pulse velocity• impact echo• radiographyThe location and number of nondestructive tests shallbe in accordance with the requirements ofSection 7.3.2.7. Descriptive information concerningthese test procedures is provided in the Commentary,Section C7.3.3.2.7.3.3.3 Supplemental TestsAncillary tests are recommended, but not required, toenhance the level of confidence in masonry materialproperties, or to assess condition. These are described inthe Commentary to this section.7.3.4 Knowledge (κ) FactorIn addition to those characteristics specified inSection 2.7.2, a knowledge factor, κ, equal to 0.75,representing a minimum level of knowledge of thestructural system, shall be used if a visual examinationof masonry structural components is done per therequirements of Section 7.3.3.1. A knowledge factor, κ,equal to 1.00, shall be used only with a comprehensivelevel of knowledge of the structural system (as definedin Section 2.7.2).7.4 Engineering Properties ofMasonry WallsThis section provides basic engineering information forassessing attributes of structural walls, and includesstiffness assumptions, strength acceptance criteria, anddeformation acceptance criteria for the ImmediateOccupancy, Life Safety, and Collapse PreventionPerformance Levels. Engineering properties given formasonry walls shall be used with the analytical methodsprescribed in Chapter 3, unless otherwise noted.Masonry walls shall be categorized as primary orsecondary elements. Walls that are considered to be partof the lateral-force system, and may or may not supportgravity loads, shall be primary elements. Walls that arenot considered as part of the lateral-force-resistingsystem, but must remain stable while supporting gravityloads during seismic excitation, shall be secondaryelements.• ultrasonic pulse velocityFEMA 273 Seismic Rehabilitation Guidelines 7-5

Chapter 7: Masonry(Systematic Rehabilitation)7.4.1 Types of Masonry WallsThe procedures set forth in this section are applicable tobuilding systems comprising any combination ofexisting masonry walls, masonry walls enhanced forseismic rehabilitation, and new walls added to anexisting building for seismic rehabilitation. In addition,any of these three categories of masonry elements canbe used in combination with existing, rehabilitated, ornew lateral-force-resisting elements of other materialssuch as steel, concrete, or timber.When analyzing a system comprising existing masonrywalls, rehabilitated masonry walls, and/or new masonrywalls, expected values of strength and stiffness shall beused.7.4.1.1 Existing Masonry WallsExisting masonry walls considered in Section 7.4 shallinclude all structural walls of a building system that arein place prior to seismic rehabilitation.Wall types shall include unreinforced or reinforced;ungrouted, partially grouted, or fully grouted; andcomposite or noncomposite. Existing walls subjected tolateral forces applied parallel with their plane shall beconsidered separately from walls subjected to forcesapplied normal to their plane, as described inSections 7.4.2 through 7.4.5.Material properties for existing walls shall beestablished per Section 7.3.2. Prior to rehabilitation,masonry structural walls shall be assessed for conditionper the procedures set forth in Sections 7.3.3.1, 7.3.3.2,or 7.3.3.3. Existing masonry walls shall be assumed tobehave in the same manner as new masonry walls,provided that the condition assessment demonstratesequivalent quality of construction.7.4.1.2 New Masonry WallsNew masonry walls shall include all new elementsadded to an existing lateral-force-resisting system. Walltypes shall include unreinforced or reinforced;ungrouted, partially grouted, or fully grouted; andcomposite or noncomposite. Design of newlyconstructed walls shall follow the requirements set forthin BSSC (1995).When analyzing a system of new and existing walls,expected values of strength and stiffness shall be usedfor the newly constructed walls. Any capacity reductionfactors given in BSSC (1995) shall not be used, andmean values of material strengths shall be used in lieuof lower bound estimates.New walls subjected to lateral forces applied parallelwith their plane shall be considered separately fromwalls subjected to forces applied normal to their plane,as described in Sections 7.4.2 through 7.4.5.7.4.1.3 Enhanced Masonry WallsEnhanced masonry walls shall include existing wallsthat are rehabilitated with the methods given in thissection. Unless stated otherwise, methods are applicableto both unreinforced and reinforced walls, and areintended to improve performance of masonry wallssubjected to both in-plane and out-of-plane lateralforces.Enhanced walls subjected to lateral forces appliedparallel with their plane shall be considered separatelyfrom walls subjected to forces applied normal to theirplane, as described in Sections 7.4.2 through 7.4.5.A. Infilled OpeningsAn infilled opening shall be considered to actcompositely with the surrounding masonry if thefollowing provisions are met.1. The sum of the lengths of all openings in thedirection of in-plane shear force in a singlecontinuous wall is less than 40% of the overalllength of the wall.2. New and old masonry units shall be interlaced at theboundary of the infilled opening with full toothing,or adequate anchorage shall be provided to give anequivalent shear strength at the interface of new andold units.Stiffness assumptions, strength criteria, and acceptabledeformations for masonry walls with infilled openingsshall be the same as given for nonrehabilitated solidmasonry walls, provided that differences in elasticmoduli and strengths for the new and old masonries areconsidered for the composite section.B. Enlarged OpeningsOpenings in a masonry shear wall may be enlarged byremoving portions of masonry above or below windowsor doors. This is done to increase the height-to-lengthaspect ratio of piers so that the limit state may be altered7-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)from shear to flexure. This method is only applicable toURM walls.Stiffness assumptions, strength criteria, and acceptabledeformations for URM walls with enlarged openingsshall be the same as given for existing perforatedmasonry walls, provided that the cutting operation doesnot cause any distress.C. ShotcreteAn existing masonry wall with an application ofshotcrete shall be considered to behave as a compositesection, as long as adequate anchorage is provided atthe shotcrete-masonry interface for shear transfer.Stresses in the masonry and shotcrete shall bedetermined considering the difference in elastic modulifor each material. Alternatively, the masonry may beneglected if the new shotcrete layer is designed to resistall of the force, and minor cracking of the masonry isacceptable.Stiffness assumptions, strength criteria, and acceptabledeformations for masonry components with shotcreteshall be the same as for new reinforced concretecomponents, with due consideration to possiblevariations in boundary conditions.D. Coatings for URM WallsA coated masonry wall shall be considered to behave asa composite section, as long as adequate anchorage isprovided at the interface between the coating and themasonry wall. Stresses in the masonry and coating shallbe determined considering the difference in elasticmoduli for each material. If stresses exceed expectedstrengths of the coating material, then the coating shallbe considered ineffective.Stiffness assumptions, strength criteria, and acceptabledeformations for coated masonry walls shall be thesame as for existing URM walls.E. Reinforced Cores for URM WallsA reinforced-cored masonry wall shall be considered tobehave as a reinforced masonry wall, provided thatsufficient bonding exists between the new reinforcementand the grout, and between the grout and the coredsurface. Vertical reinforcement shall be anchored at thebase of the wall to resist its full tensile strength.Grout in new reinforced cores should consist ofcementitious materials whose hardened properties arecompatible with those of the surrounding masonry.Adequate shear strength must exist, or be provided, sothat the strength of the new vertical reinforcement canbe developed.Stiffness assumptions, strength criteria, and acceptabledeformations for URM walls with reinforced cores shallbe the same as for existing reinforced walls.F. Prestressed Cores for URM WallsA prestressed-cored masonry wall with unbondedtendons shall be considered to behave as a URM wallwith increased vertical compressive stress.Losses in prestressing force due to creep and shrinkageof the masonry shall be accounted for.Stiffness assumptions, strength criteria, and acceptabledeformations for URM walls with unbondedprestressing tendons shall be the same as for existingunreinforced masonry walls subjected to verticalcompressive stress.G. Grout InjectionsAny grout used for filling voids and cracks shall havestrength, modulus, and thermal properties compatiblewith the existing masonry.Inspection shall be made during the grouting to ensurethat voids are completely filled with grout.Stiffness assumptions, strength criteria, and acceptabledeformations for masonry walls with grout injectionsshall be the same as for existing unreinforced orreinforced walls.H. RepointingBond strength of new mortar shall be equal to or greaterthan that of the original mortar. Compressive strength ofnew mortar shall be equal to or less than that of theoriginal mortar.Stiffness assumptions, strength criteria, and acceptabledeformations for repointed masonry walls shall be thesame as for existing masonry walls.FEMA 273 Seismic Rehabilitation Guidelines 7-7

Chapter 7: Masonry(Systematic Rehabilitation)I. Braced Masonry WallsMasonry walls may be braced with external structuralelements to reduce span lengths for out-of-planebending. Adequate strength shall be provided in thebracing element and connections to resist the transfer offorces from the masonry wall to the bracing element.Out-of-plane deflections of braced walls resulting fromthe transfer of vertical floor or roof loadings shall beconsidered.Stiffness assumptions, strength criteria, and acceptabledeformations for braced masonry walls shall be thesame as for existing masonry walls. Due considerationshall be given to the reduced span of the masonry wall.J. Stiffening ElementsMasonry walls may be stiffened with external structuralmembers to increase the out-of-plane stiffness andstrength. The stiffening member shall be proportionedto resist a tributary portion of lateral load appliednormal to the plane of a masonry wall. Adequateconnections at the ends of the stiffening element shallbe provided to transfer the reaction of force. Flexibilityof the stiffening element shall be considered whenestimating lateral drift of a masonry wall panel forPerformance Levels.Stiffness assumptions, strength criteria, and acceptabledeformations for stiffened masonry walls shall be thesame as for existing masonry walls. Due considerationshall be given to the stiffening action that the newelement provides.7.4.2 URM In-Plane Walls and PiersInformation is given in this section for depicting theengineering properties of URM walls subjected tolateral forces applied parallel with their plane.Requirements of this section shall apply to cantileveredshear walls that are fixed against rotation at their base,and piers between window or door openings that arefixed against rotation at their top and base.Stiffness and strength criteria are presented that areapplicable for use with both the Linear Static andNonlinear Static Procedures prescribed in Chapter 3.accordance with the guidelines of this subsection. Thelateral stiffness of masonry walls subjected to lateral inplaneforces shall be determined considering bothflexural and shear deformations.The masonry assemblage of units, mortar, and groutshall be considered to be a homogeneous medium forstiffness computations with an expected elastic modulusin compression, E me , as specified in Section 7.3.2.2.For linear procedures, the stiffness of a URM wall orpier resisting lateral forces parallel with its plane shallbe considered to be linear and proportional with thegeometrical properties of the uncracked section.For nonlinear procedures, the in-plane stiffness of URMwalls or piers shall be based on the extent of cracking.Story shears in perforated shear walls shall bedistributed to piers in proportion to the relative lateraluncracked stiffness of each pier.Stiffnesses for existing, enhanced, and new walls shallbe determined using the same principles of mechanics.7.4.2.2 Strength Acceptance CriteriaUnreinforced masonry walls and piers shall beconsidered as deformation-controlled components iftheir expected lateral strength limited by bed-jointsliding shear stress or rocking (the lesser of valuesgiven by Equations 7-3 and 7-4) is less than the lowerbound lateral strength limited by diagonal tension or toecompressive stress (the lesser of values given byEquations 7-5 or 7-6). Otherwise, these componentsshall be considered as force-controlled components.A. Expected Lateral Strength of Walls and PiersExpected lateral strength of existing URM walls or piercomponents shall be based on expected bed-jointsliding shear strength, or expected rocking strength, inaccordance with Equations 7-3 and 7-4, respectively.The strength of such URM walls or piers shall be thelesser of:Q CE = V bjs = v me A n(7-3)7.4.2.1 StiffnessThe lateral stiffness of masonry wall and piercomponents shall be determined based on the minimumnet sections of mortared and grouted masonry inLQ CE = V r = 0.9αP ⎛ CE-------⎞⎝h ⎠ eff(7-4)7-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)whereA n = Area of net mortared/grouted section, in. 2h effLP CEv meV bjsV rα= Height to resultant of lateral force= Length of wall or pier= Expected vertical axial compressive force perload combinations in Equations 3-2 and 3-3= Expected bed-joint sliding shear strength perSection 7.3.2.4, psi= Lateral strength of wall or pier based on bedjointshear strength, pounds= Lateral rocking strength of wall or piercomponent, pounds= Factor equal to 0.5 for fixed-free cantileverwall, or equal to 1.0 for fixed-fixed pierwhere A n , h eff , L, and α are the same as given forEquations 7-3 and 7-4, and:f af ′ dtP CLV dtV tc= Upper bound of vertical axial compressivestress from Equation 3-2, psi= Lower bound of masonry diagonal tensionstrength, psi= Lower bound of masonry compressivestrength, psi= Lower bound of vertical compressive forcefrom load combination of Equation 3-3,pounds= Lateral strength limited by diagonal tensionstress, pounds= Lateral strength limited by toe compressivestress, poundsf ′ mf ′ dtf ′ mExpected lateral strength of newly constructed wall orpier components shall be based on the NEHRPRecommended Provisions (BSSC, 1995), with theexception that capacity reduction factors shall be takenas equal to 1.0.B. Lower Bound Lateral Strength of Walls and PiersLower bound lateral strength of existing URM walls orpier components shall be limited by diagonal tensionstress or toe compressive stress, in accordance withEquations 7-5 and 7-6, respectively. The lateral strengthof URM walls or piers shall be the lesser of Q CL valuesgiven by these two equations.If L /h eff is larger than 0.67 and less than 1.00, then:Q CL V dt f dt ′ A ⎛ Ln------- ⎞f= =1 + ------ a⎝h eff⎠ f dt ′(7-5)For determination of V dt , the bed-joint shear strength,v me , may be substituted for the diagonal tensionstrength, , in Equation 7-5.The lower bound masonry compressive strength, ,shall be taken as the expected strength, f me , determinedper Section 7.3.2.1, divided by 1.6.If the Linear Static Procedures of Section 3.3 are used,lateral forces from gravity and seismic effects shall beless than the lower bound lateral strength, Q CL , asrequired by Equation 3-19.C. Lower Bound Vertical Compressive Strength ofWalls and PiersLower bound vertical compressive strength of existingURM walls or pier components shall be limited bymasonry compressive stress per Equation 7-7.Q CL V tc αP ⎛ LCL------- ⎞⎛ f1a ⎞= =– --------------⎝h eff⎠⎜ ⎟⎝ 0.7f ′ m ⎠(7-6)Q CL = P c = 0.80( 0.85f m ′ A n )where f ′ m is equal to the expected strength, f me ,determined per Section 7.3.2.1, divided by 1.6.(7-7)If the Linear Static Procedures of Section 3.3 are used,vertical forces from gravity and seismic effects shall beless than the lower bound lateral strength, Q CL , as statedin Equation 3-19.FEMA 273 Seismic Rehabilitation Guidelines 7-9

Chapter 7: Masonry(Systematic Rehabilitation)7.4.2.3 Deformation Acceptance CriteriaA. Linear ProceduresTable 7-1Linear Static Procedure—m Factors for URM In-Plane Walls and Piersm FactorsLimitingBehavioral Mode Primary SecondaryIO LS CP LS CPBed-Joint Sliding 1 3 4 6 8Rocking (1.5h eff /L)>1 (3h eff /L)>1.5 (4h eff /L)>2 (6h eff /L)>3 (8h eff /L)>4Note: Interpolation is permitted between table values.If the linear procedures of Section 3.3 are used, theproduct of expected strength, Q CE , of those componentsclassified as deformation-controlled, multiplied by mfactors given in Table 7-1 for particular PerformanceLevels and κ factors given in Section 2.7.2, shall exceedthe sum of unreduced seismic forces, Q E , and gravityforces, Q G , per Equation 3-18. The limiting behaviormode in Table 7-1 shall be identified from the lower ofthe two expected strengths as determined fromEquations 7-3 and 7-4.For determination of m factors from Table 7-1, thevertical compressive stress, f ae , shall be based on anexpected value of gravity compressive force given bythe load combinations given in Equations 3-2 and 3-3.QQ exp1.0Figure 7-1de∆ effh effIdealized Force-Deflection Relation forWalls, Pier Components, and InfillPanelscB. Nonlinear ProceduresIf the Nonlinear Static Procedure given in Section 3.3.3is used, deformation-controlled wall and piercomponents shall be assumed to deflect to nonlinearlateral drifts as given in Table 7-2. Variables d and e,representing nonlinear deformation capacities forprimary and secondary components, are expressed interms of story drift ratio percentages, as defined inFigure 7-1.The limiting behavior mode in Table 7-2shall be identified from the lower of the two expectedstrengths as determined from Equations 7-3 and 7-4.For components of primary lateral-force-resistingelements, collapse shall be considered at lateral driftpercentages exceeding values of d in the table, and theLife Safety Performance Level shall be considered atapproximately 75% of the d value. For components ofsecondary elements, collapse shall be considered atlateral drift percentages exceeding the values of e in thetable, and the Life Safety Performance Level shall beconsidered at approximately 75% of the e value in thetable. Drift percentages based on these criteria are givenin Table 7-2.If the Nonlinear Dynamic Procedure given inSection 3.3.4 is used, nonlinear force-deflectionrelations for wall and pier components shall beestablished based on the information given in Table 7-2,or on a more comprehensive evaluation of the hystereticcharacteristics of those components.7.4.3 URM Out-of-Plane WallsAs required by Section 2.11.7, URM walls shall beconsidered to resist out-of-plane excitation as isolated7-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)Table 7-2 Nonlinear Static Procedure—Simplified Force-Deflection Relations for URM In-Plane Wallsand PiersAcceptance CriteriaLimiting BehavioralMode Primary Secondaryc%d%e%Bed-Joint Sliding 0.6 0.4 0.8 0.1 0.3 0.4 0.6 0.8Rocking 0.6 0.4h eff /L 0.8h eff /L 0.1 0.3h eff /L 0.4h eff /L 0.6h eff /L 0.8h eff /LIO%LS%CP%LS%CP%Note: Interpolation is permitted between table values.components spanning between floor levels, and/orspanning horizontally between columns or pilasters.Out-of-plane walls shall not be analyzed with the Linearor Nonlinear Static Procedures prescribed in Chapter 3.7.4.3.1 StiffnessThe stiffness of out-of-plane walls shall be neglectedwith analytical models of the global structural system ifin-plane walls or infill panels exist, or are placed, in theorthogonal direction.7.4.3.2 Strength Acceptance CriteriaThe Immediate Occupancy Performance Level shall belimited by flexural cracking of out-of-plane walls.Unless arching action is considered, flexural crackingshall be limited by the expected tensile stress valuesgiven in Section 7.3.2.3 for existing walls and in BSSC(1995) for new construction.Arching action shall be considered if, and only if,surrounding floor, roof, column, or pilaster elementshave sufficient stiffness and strength to resist thrustsfrom arching of a wall panel, and a conditionassessment has been done to ensure that there are nogaps between a wall panel and the adjacent structure.Due consideration shall be given to the condition of thecollar joint when estimating the effective thickness of awall.7.4.3.3 Deformation Acceptance CriteriaThe Life Safety and Collapse Prevention Levels permitflexural cracking in URM walls subjected to out-ofplaneloading, provided that cracked wall segments willremain stable during dynamic excitation. Stability shallbe checked using analytical time-step integrationmodels with realistic depiction of acceleration timehistories at the top and base of a wall panel. Wallsspanning vertically, with a height-to-thickness (h/t)ratio less than that given in Table 7-3, need not bechecked for dynamic stability.Table 7-37.4.4 Reinforced Masonry In-Plane Wallsand Piers7.4.4.1 StiffnessPermissible h/t Ratios for URM Outof-PlaneWalls0.24g

Chapter 7: Masonry(Systematic Rehabilitation)Stiffnesses for existing and new walls shall be assumedto be the same.7.4.4.2 Strength Acceptance Criteria forReinforced Masonry (RM)Strength of RM wall or pier components in flexure,shear, and axial compression shall be determined perthe requirements of this section. The assumptions,procedures, and requirements of this section shall applyto both existing and newly constructed RM wall or piercomponents.Reinforced masonry walls and piers shall be consideredas deformation-controlled components if their expectedlateral strength for flexure per Section 7.4.4.2A is lessthan the lower bound lateral strength limited by shearper Section 7.4.4.2B. Vertical compressive behavior ofreinforced masonry wall or pier components shall beassumed to be a force-controlled action. Methods fordetermining lower bound axial compressive strength aregiven in Section 7.4.4.2D.A. Expected Flexural Strength of Walls and PiersExpected flexural strength of an RM wall or pier shallbe determined on the basis of the followingassumptions.• Stress in reinforcement below the expected yieldstrength, f ye , shall be taken as the modulus ofelasticity, E se , times the steel strain. Forreinforcement strains larger than thosecorresponding to the expected yield strength, thestress in the reinforcement shall be consideredindependent of strain and equal to the expected yieldstrength, f ye .• Tensile strength of masonry shall be neglected incalculating the flexural strength of a reinforcedmasonry cross section.• Flexural compression stress in masonry shall beassumed to be distributed across an equivalentrectangular stress block. Masonry stress of 0.85times the expected compressive strength, f me , shallbe distributed uniformly over an equivalentcompression zone bounded by edges of the crosssection and with a depth equal to 85% of the depthfrom the neutral axis to the fiber of maximumcompressive strain.• Strains in the reinforcement and masonry shall beconsidered linear across the wall or pier crosssection. For purposes of determining forces inreinforcing bars distributed across the section, themaximum compressive strain in the masonry shallbe assumed to be equal to 0.003.B. Lower-Bound Shear Strength of Walls and PiersLower-bound shear strength of RM wall or piercomponents, V CL , shall be determined usingEquation 7-8.where:V mLV sL(7-8)The lower bound shear strength of an RM wall or piershall not exceed shear forces given by Equations 7-9and 7-10.For M/Vd v less than 0.25:For M/Vd v greater than or equal to 1.00:where:= Lower bound shear strength provided bymasonry, lb= Lower bound shear strength provided byreinforcement, lbA n = Area of net mortared/grouted section, in. 2f ′ mMVd vQ CL = V CL = V mL + V sLV CL ≤ 6 f m ′ A nV CL ≤ 4 f m ′ A n= Compressive strength of masonry, psi= Moment on the masonry section, in.-lb= Shear on the masonry section, lb= Wall length in direction of shear force, in.Lower-bound shear strength, V mL , resisted by themasonry shall be determined using Equation 7-11.(7-9)(7-10)7-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)V mL = 4.0 – 1.75⎛--------M ⎞ A⎝Vd ⎠ n f m + 0.25P CLv(7-11)where M/Vd v need not be taken greater than 1.0, andP CL is the lower-bound vertical compressive force inpounds based on the load combinations given inEquations 3-2 and 3-3.Lower-bound shear strength, V sL , resisted by thereinforcement shall be determined using Equation 7-12.where:V sL0.5 A v= ⎛-----⎞ fy d⎝ s ⎠ vA v = Area of shear reinforcement, in. 2s = Spacing of shear reinforcement, in.f y = Lower-bound yield strength of shearreinforcement, psi(7-12)C. Strength Considerations for Flanged WallsWall intersections shall be considered effective intransferring shear when either condition (1) or (2), andcondition (3), as noted below, are met:1. The face shells of hollow masonry units are removedand the intersection is fully grouted.2. Solid units are laid in running bond, and 50% of themasonry units at the intersection are interlocked.3. Reinforcement from one intersecting wall continuespast the intersection a distance not less than 40 bardiameters or 24 inches.The width of flange considered effective incompression on each side of the web shall be taken asequal to six times the thickness of the web, or shall beequal to the actual flange on either side of the web wall,whichever is less.The width of flange considered effective in tension oneach side of the web shall be taken as equal to 3/4 of thewall height, or shall be equal to the actual flange oneither side of the web wall, whichever is less.D. Lower Bound Vertical Compressive Strength ofWalls and PiersLower bound vertical compressive strength of existingRM walls or pier components shall be determined usingEquation 7-13.where:f ′ mf yQ CL = P c = 0.8[ 0.85f m ′ ( A n – A s ) + A s f y ](7-13)= Lower bound masonry compressive strengthequal to expected strength, f me , determinedper Section 7.3.2.1, divided by 1.6= Lower bound reinforcement yield strengthper Section 7.3.2.6If the Linear Static Procedures of Section 3.3.1 areused, vertical forces from gravity and seismic effectsshall be less than the lower bound lateral strength, Q CL ,as required by Equation 3-19.7.4.4.3 Deformation Acceptance CriteriaA. Linear ProceduresIf the linear procedures of Section 3.3 are used, theproduct of expected strength, Q CE , of those componentsclassified as deformation-controlled, multiplied by mfactors given in Table 7-4 for particular PerformanceLevels and κ factors given in Section 2.7.2, shall exceedthe sum, Q UD ,of unreduced seismic forces, Q E , andgravity forces, Q G , as in Equations 3-14 and 3-18.For determination of m factors from Table 7-4, the ratioof vertical compressive stress to expected compressivestrength, f ae /f me , shall be based on an expected value ofgravity compressive force per the load combinationsgiven in Equations 3-2 and 3-3.B. Nonlinear ProceduresIf the Nonlinear Static Procedure given in Section 3.3.3is used, deformation-controlled wall and piercomponents shall be assumed to deflect to nonlinearlateral drifts as given in Table 7-5. Variables d and e,representing nonlinear deformation capacities forprimary and secondary components, are expressed interms of story drift ratio percentages as defined inFigure 7-1.For determination of the c, d, and e values and theacceptable drift levels using Table 7-5, the verticalFEMA 273 Seismic Rehabilitation Guidelines 7-13

Chapter 7: Masonry(Systematic Rehabilitation)compressive stress, f ae , shall be based on an expectedvalue of gravity compressive force per the loadcombinations given in Equations 3-2 and 3-3.For components of primary lateral-force-resistingelements, collapse shall be considered at lateral driftpercentages exceeding values of d in Table 7-5, and theLife Safety Performance Level shall be considered atapproximately 75% of the d value. For components ofsecondary elements, collapse shall be considered atlateral drift percentages exceeding the values of e in thetable, and the Life Safety Performance Level shall beconsidered at approximately 75% of the e value in thetable. Story drift ratio percentages based on thesecriteria are given in Table 7-5.If the Nonlinear Dynamic Procedure given inSection 3.3.4 is used, nonlinear force-deflectionrelations for wall and pier components shall beestablished based on the information given in Table 7-5,or on a more comprehensive evaluation of the hystereticcharacteristics of those components.Acceptable deformations for existing and new wallsshall be assumed to be the same.7.4.5 RM Out-of-Plane WallsAs required by Section 2.11.7, RM walls shall beconsidered to resist out-of-plane excitation as isolatedcomponents spanning between floor levels, and/orspanning horizontally between columns or pilasters.Out-of-plane walls shall not be analyzed with the Linearor Nonlinear Static Procedures prescribed in Chapter 3,but shall resist lateral inertial forces as given inSection 2.11.7, or respond to earthquake motions asdetermined with the Nonlinear Dynamic Procedure andsatisfy the deflection criteria given in Section 7.4.5.3.7.4.5.1 StiffnessOut-of-plane RM walls shall be considered as localelements spanning between individual story levels.The stiffness of out-of-plane walls shall be neglectedwith analytical models of the global structural system ifin-plane walls exist or are placed in the orthogonaldirection.Uncracked sections based on the net mortared/groutedarea shall be considered for determination ofgeometrical properties, provided that net flexural tensilestress does not exceed expected tensile strength, f te , perSection 7.3.2.3. Stiffness shall be based on a crackedsection for a wall whose net flexural tensile stressexceeds expected tensile strength.Stiffnesses for existing and new reinforced out-of-planewalls shall be assumed to be the same.7.4.5.2 Strength Acceptance CriteriaOut-of-plane RM walls shall be sufficiently strong inflexure to resist the transverse loadings prescribed inSection 2.11.7 for all Performance Levels. Expectedflexural strength shall be based on the assumptionsgiven in Section 7.4.4.2A. For walls with an h/t ratioexceeding 20, the effects of deflections on momentsshall be considered.Strength of new walls and existing walls shall beassumed to be the same.7.4.5.3 Deformation Acceptance CriteriaIf the Nonlinear Dynamic Procedure is used, thefollowing performance criteria shall be based on themaximum deflection normal to the plane of a transversewall.• The Immediate Occupancy Performance Level shallbe met when significant visual cracking of an RMwall occurs. This limit state shall be assumed tooccur at a lateral story drift ratio of approximately2%.• The Life Safety Performance Level shall be metwhen masonry units are dislodged and fall out of thewall. This limit state shall be assumed to occur at alateral drift of a story panel equal to approximately3%.• The Collapse Prevention Performance Level shall bemet when the post-earthquake damage state is on theverge of collapse. This limit state shall be assumedto occur at a lateral story drift ratio of approximately5%.Acceptable deformations for existing and new wallsshall be assumed to be the same.7.5 Engineering Properties ofMasonry InfillsThis section provides basic engineering information forassessing attributes of masonry infill panels, including7-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)Table 7-4Linear Static Procedure—m Factors for Reinforced Masonry In-Plane Wallsf ae /f meL/h eff ρ g f ye /f me IO LS CP LS CPm FactorsPrimarySecondary0.00 0.5 0.01 4.0 7.0 8.0 8.0 10.00.05 2.5 5.0 6.5 8.0 10.00.20 1.5 2.0 2.5 4.0 5.01.0 0.01 4.0 7.0 8.0 8.0 10.00.05 3.5 6.5 7.5 8.0 10.00.20 1.5 3.0 4.0 6.0 8.02.0 0.01 4.0 7.0 8.0 8.0 10.00.05 3.5 6.5 7.5 8.0 10.00.20 2.0 3.5 4.5 7.0 9.00.038 0.5 0.01 3.0 6.0 7.5 8.0 10.00.05 2.0 3.5 4.5 7.0 9.00.20 1.5 2.0 2.5 4.0 5.01.0 0.01 4.0 7.0 8.0 8.0 10.00.05 2.5 5.0 6.5 8.0 10.00.20 1.5 2.5 3.5 5.0 7.02.0 0.01 4.0 7.0 8.0 8.0 10.00.05 3.5 6.5 7.5 8.0 10.00.20 1.5 3.0 4.0 6.0 8.00.075 0.5 0.01 2.0 3.5 4.5 7.0 9.00.05 1.5 3.0 4.0 6.0 8.00.20 1.0 2.0 2.5 4.0 5.01.0 0.01 2.5 5.0 6.5 8.0 10.00.05 2.0 3.5 4.5 7.0 9.00.20 1.5 2.5 3.5 5.0 7.02.0 0.01 4.0 7.0 8.0 8.0 10.00.05 2.5 5.0 6.5 8.0 10.00.20 1.5 3.0 4.0 4.0 8.0Note: Interpolation is permitted between table values.stiffness assumptions, and the strength acceptance anddeformation acceptance criteria for the ImmediateOccupancy, Life Safety, and Collapse PreventionPerformance Levels. Engineering properties given formasonry infills shall be used with the analyticalmethods prescribed in Chapter 3, unless notedotherwise.Masonry infill panels shall be considered as primaryelements of a lateral-force-resisting system. If thesurrounding frame can remain stable following the lossof an infill panel, infill panels shall not be subject tolimits set by the Collapse Prevention PerformanceLevel.FEMA 273 Seismic Rehabilitation Guidelines 7-15

Chapter 7: Masonry(Systematic Rehabilitation)Table 7-5Nonlinear Static Procedure—Simplified Force-Deflection Relations for Reinforced MasonryShear WallsAcceptance CriteriaPrimarySecondaryf ae /f me L/h eff ρ g f ye /f me cd%e%IO%LS%CP%LS%CP%0.00 0.5 0.01 0.5 2.6 5.3 1.0 2.0 2.6 3.9 5.30.05 0.6 1.1 2.2 0.4 0.8 1.1 1.6 2.20.20 0.7 0.5 1.0 0.2 0.4 0.5 0.7 1.01.0 0.01 0.5 2.1 4.1 0.8 1.6 2.1 3.1 4.10.05 0.6 0.8 1.6 0.3 0.6 0.8 1.2 1.60.20 0.7 0.3 0.6 0.1 0.2 0.3 0.5 0.62.0 0.01 0.5 1.6 3.3 0.6 1.2 1.6 2.5 3.30.05 0.6 0.6 1.3 0.2 0.5 0.6 0.9 1.30.20 0.7 0.2 0.4 0.1 0.2 0.2 0.3 0.40.038 0.5 0.01 0.4 1.0 2.0 0.4 0.8 1.0 1.5 2.00.05 0.5 0.7 1.4 0.3 0.5 0.7 1.0 1.40.20 0.6 0.4 0.9 0.2 0.3 0.4 0.7 0.91.0 0.01 0.4 0.8 1.5 0.3 0.6 0.8 1.1 1.50.05 0.5 0.5 1.0 0.2 0.4 0.5 0.7 1.00.20 0.6 0.3 0.6 0.1 0.2 0.3 0.4 0.62.0 0.01 0.4 0.6 1.2 0.2 0.4 0.6 0.9 1.20.05 0.5 0.4 0.7 0.1 0.3 0.4 0.5 0.70.20 0.6 0.2 0.4 0.1 0.1 0.2 0.3 0.40.075 0.5 0.01 0.3 0.6 1.2 0.2 0.5 0.6 0.9 1.20.05 0.4 0.5 1.0 0.2 0.4 0.5 0.8 1.00.20 0.5 0.4 0.8 0.1 0.3 0.4 0.6 0.81.0 0.01 0.3 0.4 0.9 0.2 0.3 0.4 0.7 0.90.05 0.4 0.4 0.7 0.1 0.3 0.4 0.5 0.70.20 0.5 0.2 0.5 0.1 0.2 0.2 0.4 0.52.0 0.01 0.3 0.3 0.7 0.1 0.2 0.3 0.5 0.70.05 0.4 0.3 0.5 0.1 0.2 0.3 0.4 0.50.20 0.5 0.2 0.3 0.1 0.1 0.2 0.2 0.3Note: Interpolation is permitted between table values.7-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)7.5.1 Types of Masonry InfillsProcedures set forth in this section are applicable toexisting panels, panels enhanced for seismicrehabilitation, and new panels added to an existingframe. Infills shall include panels built partially or fullywithin the plane of steel or concrete frames, andbounded by beams and columns around theirperimeters.Infill panel types considered in these Guidelines includeunreinforced clay-unit masonry, concrete masonry, andhollow-clay tile masonry. Infills made of stone or glassblock are not addressed.Infill panels that are considered to be isolated from thesurrounding frame must have sufficient gaps at top andsides to accommodate maximum lateral framedeflections. Isolated panels shall be restrained in thetransverse direction to insure stability under normalforces. Panels that are in tight contact with the frameelements on all four sides are termed shear infill panels.For panels to be considered under this designation, anygaps between an infill and a surrounding frame shall befilled to provide tight contact.Frame members and connections surrounding infillpanels shall be evaluated for frame-infill interactioneffects. These effects shall include forces transferredfrom an infill panel to beams, columns, andconnections, and bracing of frame members across apartial length.7.5.1.1 Existing Masonry InfillsExisting masonry infills considered in this section shallinclude all structural infills of a building system that arein place prior to seismic rehabilitation.Infill types included in this section consist ofunreinforced and ungrouted panels, and composite ornoncomposite panels. Existing infill panels subjected tolateral forces applied parallel with their plane shall beconsidered separately from infills subjected to forcesapplied normal to their plane, as described inSections 7.5.2 and 7.5.3.Material properties for existing infills shall beestablished per Section 7.3.2. Prior to rehabilitation,masonry infills shall be assessed for condition perprocedures set forth in Sections 7.3.3.1, 7.3.3.2, or7.3.3.3. Existing masonry infills shall be assumed tobehave the same as new masonry infills, provided that acondition assessment demonstrates equivalent qualityof construction.7.5.1.2 New Masonry InfillsNew masonry infills shall include all new panels addedto an existing lateral-force-resisting system forstructural rehabilitation. Infill types shall includeunreinforced, ungrouted, reinforced, grouted andpartially grouted, and composite or noncomposite.When analyzing a system of new infills, expectedvalues of strength and stiffness shall be used. Nocapacity reduction factors shall be used, and expectedvalues of material strengths shall be used in lieu oflower bound estimates.7.5.1.3 Enhanced Masonry InfillsEnhanced masonry infill panels shall include existinginfills that are rehabilitated with the methods given inthis section. Unless stated otherwise, methods areapplicable to unreinforced infills, and are intended toimprove performance of masonry infills subjected toboth in-plane and out-of-plane lateral forces.Masonry infills that are enhanced in accordance withthe minimum standards of this section shall beconsidered using the same Analysis Procedures andperformance criteria as for new infills.Guidelines from the following sections, pertaining toenhancement methods for unreinforced masonry walls,shall also apply to unreinforced masonry infill panels:(1) “Infilled Openings,” Section 7.4.1.3A; (2)“Shotcrete,” Section 7.4.1.3C; (3) “Coatings for URMWalls,” Section 7.4.1.3D; (4) “Grout Injections,”Section 7.4.1.3G; (5) “Repointing,” Section 7.4.1.3H;and (6) “Stiffening Elements,” Section 7.4.1.3J. Inaddition, the following two enhancement methods shallalso apply to masonry infill panels.A. Boundary Restraints for Infill PanelsInfill panels not in tight contact with perimeter framemembers shall be restrained for out-of-plane forces.This may be accomplished by installing steel angles orplates on each side of the infills, and welding or boltingthe angles or plates to the perimeter frame members.B. Joints Around Infill PanelsGaps between an infill panel and the surrounding frameshall be filled if integral infill-frame action is assumedfor in-plane response.FEMA 273 Seismic Rehabilitation Guidelines 7-17

Chapter 7: Masonry(Systematic Rehabilitation)7.5.2 In-Plane Masonry Infills7.5.2.1 StiffnessThe elastic in-plane stiffness of a solid unreinforcedmasonry infill panel prior to cracking shall berepresented with an equivalent diagonal compressionstrut of width, a, given by Equation 7-14. Theequivalent strut shall have the same thickness andmodulus of elasticity as the infill panel it represents.analyses that consider the nonlinear behavior of theinfilled frame system after the masonry is cracked.The equivalent compression strut analogy shall be usedto represent the elastic stiffness of a perforatedunreinforced masonry infill panel, provided that theequivalent strut properties are determined fromappropriate stress analyses of infill walls withrepresentative opening patterns.a=0.175( λ 1 h col ) – 0.4 r inf(7-14)Stiffnesses for existing and new infills shall be assumedto be the same.whereandh col1--E me t inf sin2θ4λ 1 = -------------------------------4E fe I col h inf= Column height between centerlines ofbeams, in.h inf = Height of infill panel, in.E fe = Expected modulus of elasticity of framematerial, psiE me = Expected modulus of elasticity of infillmaterial, psiI col = Moment of inertia of column, in. 4For noncomposite infill panels, only the wythes in fullcontact with the frame elements shall be consideredwhen computing in-plane stiffness, unless positiveanchorage capable of transmitting in-plane forces fromframe members to all masonry wythes is provided on allsides of the walls.Stiffness of cracked unreinforced masonry infill panelsshall be represented with equivalent struts, providedthat the strut properties are determined from detailed7.5.2.2 Strength Acceptance CriteriaA. Infill Shear StrengthThe transfer of story shear across a masonry infill panelconfined within a concrete or steel frame shall beconsidered as a deformation-controlled action.Expected in-plane panel shear strength shall bedetermined per the requirements of this section.Expected infill shear strength, V ine , shall be calculatedas the product of the net mortared and grouted area ofthe infill panel, A ni , times the expected shear strength ofthe masonry, f vie , in accordance with Equation 7-15.where:L inf = Length of infill panel, in.A ni = Area of net mortared/grouted section acrossr inf = Diagonal length of infill panel, in.infill panel, in. 2ft inf = Thickness of infill panel and equivalent strut, vie = Expected shear strength of masonry infill, psiin.θ = Angle whose tangent is the infill height-tolengthaspect ratio, radianstaken to not exceed the expected masonry bed-jointExpected shear strength of existing infills, f vie , shall be= Coefficient used to determine equivalentshear strength, v me , as determined per Section 7.3.2.4.λ 1width of infill strutQ CE = V ine = A ni f vie(7-15)Shear strength of newly constructed infill panels, f vie ,shall not exceed values given in Section 8.7.4 of BSSC(1995) for zero vertical compressive stress.For noncomposite infill panels, only the wythes in fullcontact with the frame elements shall be consideredwhen computing in-plane strength, unless positiveanchorage capable of transmitting in-plane forces fromframe members to all masonry wythes is provided on allsides of the walls.7-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)B. Required Strength of Column Members Adjacentto Infill PanelsUnless a more rigorous analysis is done, the expectedflexural and shear strengths of column membersadjacent to an infill panel shall exceed forces resultingfrom one of the following conditions:where:htanθ infb = ----------------------------aL inf – ------------sin θ b(7-19)1. The application of the horizontal component of theexpected infill strut force applied at a distance, l ceff ,from the top or bottom of the infill panel equal to:2. The shear force resulting from development ofexpected beam flexural strengths at the ends of abeam member with a reduced length equal to l beffl ceff=a-------------cosθ c(7-16)The requirements of this section shall be waived if theexpected masonry shear strength, v me , as measured pertest procedures of Section 7.3.2.4, is less than 50 psi.where:tan θ cah inf – -------------cosθ= ---------------------------- cL inf(7-17)2. The shear force resulting from development ofexpected column flexural strengths at the top andbottom of a column with a reduced height equal tol ceffThe reduced column length, l ceff , in Equation 7-16 shallbe equal to the clear height of opening for a captivecolumn braced laterally with a partial height infill.The requirements of this section shall be waived if theexpected masonry shear strength, v me , as measured pertest procedures of Section 7.3.2.4, is less than 50 psi.C. Required Strength of Beam Members Adjacent toInfill PanelsThe expected flexural and shear strengths of beammembers adjacent to an infill panel shall exceed forcesresulting from one of the following conditions:1. The application of the vertical component of theexpected infill strut force applied at a distance, l beff ,from the top or bottom of the infill panel equal to:l ceffa= ------------sinθ b(7-18)7.5.2.3 Deformation Acceptance CriteriaA. Linear ProceduresIf the linear procedures of Section 3.3.1 are used, theproduct of expected infill strength, V ine , multiplied by mfactors given in Table 7-6 for particular PerformanceLevels and κ factors given in Section 2.7.2, shall exceedthe sum of unreduced seismic forces, Q E , and gravityforces, Q G , per Equation 3-18. For the case of an infillpanel, Q E shall be the horizontal component of theunreduced axial force in the equivalent strut member.For determination of m factors per Table 7-6, the ratioof frame to infill strengths shall be determinedconsidering the expected lateral strength of eachcomponent. If the expected frame strength is less than0.3 times the expected infill strength, the confiningeffects of the frame shall be neglected and the masonrycomponent shall be evaluated as an individual wallcomponent per Sections 7.4.2 or 7.4.4.B. Nonlinear ProceduresIf the Nonlinear Static Procedure given in Section 3.3.3is used, infill panels shall be assumed to deflect tononlinear lateral drifts as given in Table 7-7. Thevariable d, representing nonlinear deformationcapacities, is expressed in terms of story drift ratio inpercent as defined in Figure 7-1.For determination of the d values and the acceptabledrift levels using Table 7-7, the ratio of frame to infillstrengths shall be determined considering the expectedlateral strength of each component. If the expectedframe strength is less than 0.3 times the expected infillstrength, the confining effects of the frame shall beneglected and the masonry component shall beFEMA 273 Seismic Rehabilitation Guidelines 7-19

Chapter 7: Masonry(Systematic Rehabilitation)Table 7-6 Linear Static Procedure—m Factorsfor Masonry Infill Panelsβ =V--------- freV ine0.3 ≤ β < 0.70.7 ≤ β < 1.3β ≥ 1.3L m Factors-------- infh inf IO LS CP0.5 1.0 4.0 n.a.1.0 1.0 3.5 n.a.2.0 1.0 3.0 n.a.0.5 1.5 6.0 n.a.1.0 1.2 5.2 n.a.2.0 1.0 4.5 n.a.0.5 1.5 8.0 n.a.1.0 1.2 7.0 n.a.2.0 1.0 6.0 n.a.Note: Interpolation is permitted between table values.evaluated as an individual wall component perSections 7.4.2 or 7.4.4.The Immediate Occupancy Performance Level shall bemet when significant visual cracking of an unreinforcedmasonry infill occurs. The Life Safety PerformanceLevel shall be met when substantial cracking of themasonry infill occurs, and the potential for the panel, orsome portion of it, to drop out of the frame is high.Acceptable story drift ratio percentages correspondingto these general Performance Levels are given inTable 7-7.If the surrounding frame can remain stable followingthe loss of an infill panel, infill panels shall not besubject to limits set by the Collapse PreventionPerformance Level.If the Nonlinear Dynamic Procedure given inSection 3.3.4 is used, nonlinear force-deflectionrelations for infill panels shall be established based onthe information given in Table 7-7, or on a morecomprehensive evaluation of the hystereticcharacteristics of those components.Acceptable deformations for existing and new infillsshall be assumed to be the same.Table 7-7β =V--------- freV ine0.3 ≤ β < 0.70.7 ≤ β < 1.3β ≥ 1.3Nonlinear Static Procedure—Simplified Force-Deflection Relations for Masonry Infill PanelsAcceptance CriteriaL-------- infdeLSCPh infc%%%%0.5 n.a. 0.5 n.a. 0.4 n.a.1.0 n.a. 0.4 n.a. 0.3 n.a.2.0 n.a. 0.3 n.a. 0.2 n.a.0.5 n.a. 1.0 n.a. 0.8 n.a.1.0 n.a. 0.8 n.a. 0.6 n.a.2.0 n.a. 0.6 n.a. 0.4 n.a.0.5 n.a. 1.5 n.a. 1.1 n.a.1.0 n.a. 1.2 n.a. 0.9 n.a.2.0 n.a. 0.9 n.a. 0.7 n.a.Note: Interpolation is permitted between table values.7.5.3 Out-of-Plane Masonry InfillsUnreinforced infill panels with h inf /t inf ratios less thanthose given in Table 7-8, and meeting the requirementsfor arching action given in the following section, neednot be analyzed for transverse seismic forces.7.5.3.1 StiffnessOut-of-plane infill panels shall be considered as localelements spanning vertically between floor levels and/or horizontally across bays of frames.7-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)Table 7-8LowSeismicZoneMaximum h inf /t inf Ratios for whichNo Out-of-Plane Analysis isNecessaryModerateSeismicZoneIO 14 13 8LS 15 14 9CP 16 15 10HighSeismicZoneThe stiffness of infill panels subjected to out-of-planeforces shall be neglected with analytical models of theglobal structural system if in-plane walls or infill panelsexist, or are placed, in the orthogonal direction.Flexural stiffness for uncracked masonry infillssubjected to transverse forces shall be based on theminimum net sections of mortared and groutedmasonry. Flexural stiffness for unreinforced, crackedinfills subjected to transverse forces shall be assumed tobe equal to zero unless arching action is considered.Arching action shall be considered if, and only if, thefollowing conditions exist.• The panel is in tight contact with the surroundingframe components.For infill panels not meeting the requirements forarching action, deflections shall be determined with theprocedures given in Sections 7.4.3 or 7.4.5.Stiffnesses for existing and new infills shall be assumedto be the same.7.5.3.2 Strength Acceptability CriteriaMasonry infill panels shall resist out-of-plane inertialforces as given in Section 2.11.7. Transversely loadedinfills shall not be analyzed with the Linear orNonlinear Static Procedures prescribed in Chapter 3.The lower bound transverse strength of a URM infillpanel shall exceed normal pressures as prescribed inSection 2.11.7. Unless arching action is considered, thelower bound strength of a URM infill panel shall belimited by the lower bound masonry flexural tensionstrength, f t ′ , which may be taken as 0.7 times theexpected tensile strength, f te , as determined perSection 7.3.2.3.Arching action shall be considered only when therequirements stated in the previous section are met. Insuch case, the lower bound transverse strength of aninfill panel in pounds per square foot, q in , shall bedetermined using Equation 7-21.• The product of the elastic modulus, E fe , times themoment of inertia, I f , of the most flexible framecomponent exceeds a value of 3.6 x 10 9 lb-in. 2 .0.7f m ′ λ 2Q CL = q in = -------------------- × 144⎛h -------- inf ⎞⎜ ⎟⎝t inf ⎠(7-21)• The frame components have sufficient strength toresist thrusts from arching of an infill panel.• The h inf /t inf ratio is less than or equal to 25.For such cases, mid-height deflection normal to theplane of an infill panel, ∆ inf , divided by the infill height,h inf , shall be determined in accordance withEquation 7-20.wheref ′ m= Lower bound of masonry compressivestrength equal to f me /1.6, psiλ 2 = Slenderness parameter as defined in Table 7-9Table 7-9Values of λ 2 for Use inEquation 7-21∆-------- infh inf=⎛0.002 h inf⎞⎜--------⎟⎝t inf ⎠-----------------------------------------------------⎛1 1 0.002 h inf⎞ 2+ – ⎜--------⎟⎝t inf ⎠(7-20)h inf /t inf 5 10 15 25λ 2 0.129 0.060 0.034 0.013FEMA 273 Seismic Rehabilitation Guidelines 7-21

Chapter 7: Masonry(Systematic Rehabilitation)7.5.3.3 Deformation Acceptance CriteriaThe Immediate Occupancy Performance Level shall bemet when significant visual cracking of a URM infilloccurs. This limit state shall be assumed to occur at anout-of-plane drift of a story panel equal toapproximately 2%.The Life Safety Performance Level shall be met whensubstantial damage to a URM infill occurs, and thepotential for the panel, or some portion of it, to drop outof the frame is high. This limit state shall be assumed tooccur at an out-of-plane lateral story drift ratio equal toapproximately 3%.If the surrounding frame can remain stable followingthe loss of an infill panel, infill panels shall not besubject to limits set by the Collapse PreventionPerformance Level.Acceptable deformations of existing and new wallsshall be assumed to be the same.7.6 Anchorage to Masonry Walls7.6.1 Types of AnchorsAnchors considered in Section 7.6.2 shall include plateanchors, headed anchor bolts, and bent bar anchor boltsembedded into clay-unit and concrete masonry.Anchors in hollow-unit masonry must be embedded ingrout.Pullout and shear strength of expansion anchors shall beverified by tests.7.6.2 Analysis of AnchorsAnchors embedded into existing or new masonry wallsshall be considered as force-controlled components.Lower bound values for strengths with respect topullout, shear, and combinations of pullout and shear,shall be based on Section 8.3.12 of BSSC (1995) forembedded anchors.The minimum effective embedment length forconsiderations of pullout strength shall be as defined inSection 8.3.12 of BSSC (1995). When the embedmentlength is less than four bolt diameters or two inches(50.8 mm), the pullout strength shall be taken as zero.The minimum edge distance for considerations of fullshear strength shall be 12 bar diameters. Shear strengthof anchors with edge distances equal to or less than oneinch (25.4 mm) shall be taken as zero. Linearinterpolation of shear strength for edge distancesbetween these two bounds is permitted.7.7 Masonry Foundation Elements7.7.1 Types of Masonry FoundationsMasonry foundations are common in older buildingsand are still used for some modern construction. Suchfoundations may include footings and foundation wallsconstructed of stone, clay brick, or concrete block.Generally, masonry footings are unreinforced;foundation walls may or may not be reinforced.Spread footings transmit vertical column and wall loadsto the soil by direct bearing. Lateral forces aretransferred through friction between the soil and themasonry, as well as by passive pressure of the soilacting on the vertical face of the footing.7.7.2 Analysis of Existing FoundationsA lateral-force analysis of a building system shallinclude the deformability of the masonry footings, andthe flexibility of the soil under them. The strength andstiffness of the soil shall be checked per Section 4.4.Masonry footings shall be modeled as elasticcomponents with little or no inelastic deformationcapacity, unless verification tests are done to proveotherwise. For the Linear Static Procedure, masonryfootings shall be considered to be force-controlledcomponents (m equals 1.0).Masonry retaining walls shall resist active and passivesoil pressures per Section 4.5. Stiffness, and strengthand acceptability criteria for masonry retaining wallsshall be the same as for other masonry walls subjectedto transverse loadings, as addressed in Sections 7.4.3and 7.4.5.7.7.3 Rehabilitation MeasuresIn addition to those rehabilitation measures provided forconcrete foundation elements in Section 6.13.4,masonry foundation elements may also be rehabilitatedwith the following options:• Injection grouting of stone foundations• Reinforcing of URM foundations7-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)• Prestressing of masonry foundations• Enlargement of footings by placement of reinforcedshotcrete• Enlargement of footings with additional reinforcedconcrete sectionsProcedures for rehabilitation shall follow provisions forenhancement of masonry walls where applicable perSection 7.4.1.3.7.8 DefinitionsBearing wall: A wall that supports gravity loads of atleast 200 pounds per linear foot from floors and/orroofs.Bed joint: The horizontal layer of mortar on which amasonry unit is laid.Cavity wall: A masonry wall with an air spacebetween wythes. Wythes are usually joined by wirereinforcement, or steel ties. Also known as anoncomposite wall.Clay-unit masonry: Masonry constructed with solid,cored, or hollow units made of clay. Hollow clay unitsmay be ungrouted, or grouted.Clay tile masonry: Masonry constructed with hollowunits made of clay tile. Typically, units are laid withcells running horizontally, and are thus ungrouted. Insome cases, units are placed with cells runningvertically, and may or may not be grouted.Collar joint: Vertical longitudinal joint betweenwythes of masonry or between masonry wythe andback-up construction that may be filled with mortar orgrout.Composite masonry wall: Multiwythe masonrywall acting with composite action.Concrete masonry: Masonry constructed with solidor hollow units made of concrete. Hollow concrete unitsmay be ungrouted, or grouted.Head joint: Vertical mortar joint placed betweenmasonry units in the same wythe.Hollow masonry unit: A masonry unit whose netcross-sectional area in every plane parallel to thebearing surface is less than 75% of the gross crosssectionalarea in the same plane.Infill: A panel of masonry placed within a steel orconcrete frame. Panels separated from the surroundingframe by a gap are termed “isolated infills.” Panels thatare in tight contact with a frame around its fullperimeter are termed “shear infills.”In-plane wall: See shear wall.Masonry: The assemblage of masonry units, mortar,and possibly grout and/or reinforcement. Types ofmasonry are classified herein with respect to the type ofthe masonry units, such as clay-unit masonry, concretemasonry, or hollow-clay tile masonry.Nonbearing wall: A wall that supports gravity loadsless than as defined for a bearing wall.Noncomposite masonry wall: Multiwythe masonrywall acting without composite action.Out-of-plane wall: A wall that resists lateral forcesapplied normal to its plane.Parapet: Portions of a wall extending above the roofdiaphragm. Parapets can be considered as flanges toroof diaphragms if adequate connections exist or areprovided.Partially grouted masonry wall: A masonry wallcontaining grout in some of the cells.Perforated wall or infill panel: A wall or panel notmeeting the requirements for a solid wall or infill panel.Pier: A vertical portion of masonry wall between twohorizontally adjacent openings. Piers resist axialstresses from gravity forces, and bending momentsfrom combined gravity and lateral forces.Reinforced masonry (RM) wall: A masonry wallthat is reinforced in both the vertical and horizontaldirections. The sum of the areas of horizontal andvertical reinforcement must be at least 0.002 times thegross cross-sectional area of the wall, and the minimumarea of reinforcement in each direction must be not lessthan 0.0007 times the gross cross-sectional area of thewall. Reinforced walls are assumed to resist loadsFEMA 273 Seismic Rehabilitation Guidelines 7-23

Chapter 7: Masonry(Systematic Rehabilitation)through resistance of the masonry in compression andthe reinforcing steel in tension or compression.Reinforced masonry is partially grouted or fullygrouted.Running bond: A pattern of masonry where the headjoints are staggered between adjacent courses by morethan a third of the length of a masonry unit. Also refersto the placement of masonry units such that head jointsin successive courses are horizontally offset at leastone-quarter the unit length.Shear wall: A wall that resists lateral forces appliedparallel with its plane. Also known as an in-plane wall.Solid masonry unit: A masonry unit whose netcross-sectional area in every plane parallel to thebearing surface is 75% or more of the gross crosssectionalarea in the same plane.Solid wall or solid infill panel: A wall or infill panelwith openings not exceeding 5% of the wall surfacearea. The maximum length or height of an opening in asolid wall must not exceed 10% of the wall width orstory height. Openings in a solid wall or infill panelmust be located within the middle 50% of a wall lengthand story height, and must not be contiguous withadjacent openings.Stack bond: In contrast to running bond, usually aplacement of units such that the head joints insuccessive courses are aligned vertically.Transverse wall: A wall that is oriented transverse tothe in-plane shear walls, and resists lateral forcesapplied normal to its plane. Also known as an out-ofplanewall.Unreinforced masonry (URM) wall: A masonrywall containing less than the minimum amounts ofreinforcement as defined for masonry (RM) walls. Anunreinforced wall is assumed to resist gravity andlateral loads solely through resistance of the masonrymaterials.Wythe: A continuous vertical section of a wall, onemasonry unit in thickness.7.9 SymbolsA b Sum of net mortared area of bed joints aboveand below test unit, in. 2A es Area of equivalent strut for masonry infill, in. 2A n Area of net mortared/grouted section of wall orpier, in. 2A ni Area of net mortared/grouted section ofmasonry infill, in. 2A s Area of reinforcement, in. 2E fe Expected elastic modulus of frame material, psiE me Elastic modulus of masonry in compression asdetermined per Section 7.3.2.2, psiE se Expected elastic modulus of reinforcing steelper Section 7.3.2.6, psiG me Shear modulus of masonry as determined perSection 7.3.2.5, psiI Moment of inertia of section, in. 4I col Moment of inertia of column section, in. 4IfLL infMM/VP cP CEP CLQ CEQ CLQ EMoment of inertia of smallest frame memberconfining infill panel, in. 4Length of wall, in.Length of infill panel, in.Moment on masonry section, in.-lbRatio of expected moment to shear acting onwall or pierLower bound of vertical compressive strengthfor wall or pier, lbExpected vertical axial compressive force forload combinations in Equations 3-14 and 3-15,lbLower bound of vertical compressive force forload combination of Equation 3-3, lbExpected strength of a component or element atthe deformation level under consideration in adeformation-controlled actionLower bound estimate of the strength of acomponent or element at the deformation levelunder consideration for a force-controlledactionUnreduced earthquake demand forces used inEquation 3-147-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 7: Masonry(Systematic Rehabilitation)Q GVV bjsV CLV dtV freV ineV mLV rV sLV tcV testacdd vef af aef mef tef vief yf yef ′ dtf m ′Gravity force acting on component as defined inSection 3.2.8Shear on masonry section, lbExpected shear strength of wall or pier based onbed-joint shear stress, lbLower bound shear capacity, lbLower bound of shear strength based ondiagonal tension for wall or pier, lbExpected story shear strength of bare frame, lbExpected shear strength of infill panel, lbLower bound shear strength provided bymasonry, lbExpected shear strength of wall or pier based onrocking, lbLower bound shear strength provided by shearreinforcement, lbLower bound of shear strength based on toecompressive stress for wall or pier, lbMeasured force at first movement of a masonryunit with in-place shear test, lbEquivalent width of infill strut, in.Fraction of strength loss for secondary elementsas defined in Figure 7-1Wall, pier, or infill inelastic drift percentage asdefined in Figure 7-1Length of component in direction of shearforce, in.Wall, pier, or infill inelastic drift percentage asdefined in Figure 7-1Lower bound of vertical compressive stress, psiExpected vertical compressive stress, psiExpected compressive strength of masonry asdetermined in Section 7.3.2.1, psiExpected masonry flexural tensile strength asdetermined in Section 7.3.2.3, psiExpected shear strength of masonry infill, psiLower bound of yield strength of reinforcingsteel, psiExpected yield strength of reinforcing steel asdetermined in Section 7.3.2.6, psiLower bound of masonry diagonal tensionstrength, psiLower bound of masonry compressive strength,psif ′ thh colh effh infLower bound masonry tensile strength, psiHeight of a column, pilaster, or wall, in.Height of column between beam centerlines, in.Height to resultant of lateral force for wall orpier, in.Height of infill panel, in.k Lateral stiffness of wall or pier, lb/in.l beff Assumed distance to infill strut reaction pointfor beams as shown in Figure C7-5, in.l ceff Assumed distance to infill strut reaction pointfor columns as shown in Figure C7-4, in.p D+L Expected gravity stress at test location, psiq ine Expected transverse strength of an infill panel,psfr inf Diagonal length of infill panel, in.t Least thickness of wall or pier, in.t inf Thickness of infill panel, in.v me Expected masonry shear strength as determinedby Equation 7-1, psiv Average bed-joint shear strength, psiteBed-joint shear stress from single test, psiv to∆ infαβθθ bθ cκλ 1Deflection of infill panel at mid-length whensubjected to transverse loads, in.Factor equal to 0.5 for fixed-free cantileveredshear wall, or 1.0 for fixed-fixed pierRatio of expected frame strength to expectedinfill strengthAngle between infill diagonal and horizontalaxis, tan θ = L inf /h inf , radiansAngle between lower edge of compressive strutand beam as shown in Figure C7-5, radiansAngle between lower edge of compressive strutand beam as shown in Figure C7-4, radiansA reliability coefficient used to reducecomponent strength values for existingcomponents based on the quality of knowledgeabout the components’ properties (seeSection 2.7.2)Coefficient used to determine equivalent widthof infill strutFEMA 273 Seismic Rehabilitation Guidelines 7-25

Chapter 7: Masonry(Systematic Rehabilitation)λ 2ρ gInfill slenderness factorRatio of area of total wall or pier reinforcementto area of gross sectionPart 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos. FEMA222A and 223A), Washington, D.C.7.10 ReferencesASTM, latest editions, Standards having the numbersA615, E22, E518, American Society for TestingMaterials, Philadelphia, Pennsylvania.BSSC, 1992, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 178), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Masonry Standards Joint Committee (MSJC), 1995a,Building Code Requirements for Masonry Structures,ACI 530-95/ASCE 5-95/TMS 402-95, AmericanConcrete Institute, Detroit, Michigan; AmericanSociety of Civil Engineers, New York, New York; andThe Masonry Society, Boulder, Colorado.Masonry Standards Joint Committee (MSJC), 1995b,Specification for Masonry Structures, ACI 530.1-95/ASCE 6-95/TMS 602-95, American Concrete Institute,Detroit, Michigan; American Society of CivilEngineers, New York, New York; and The MasonrySociety, Boulder, Colorado.7-26 Seismic Rehabilitation Guidelines FEMA 273

8.Wood and Light Metal Framing(Systematic Rehabilitation)8.1 ScopeThis chapter presents the general methods forrehabilitating wood frame buildings and/or wood andlight metal framed elements of other types of buildingsthat are presented in other sections of this document.Refer to Chapter 2 for the general methodology andissues regarding performance goals, and the decisionsand steps necessary for the engineer to develop arehabilitation scheme. The Linear Static Procedure(LSP) presented in Chapter 3 is the methodrecommended for the systematic analysis of woodframe buildings. However, properties of the idealizedelastic and inelastic performance of the variouselements and connections are included so that nonlinearprocedures can be used if desired.A general history of the development of wood framingmethods is presented in Section 8.2, along with thefeatures likely to be found in buildings of different ages.A more complete historical perspective is included inthe Commentary. The evaluation and assessment ofvarious structural elements of wood frame buildings isfound in Section 8.3. For a description and discussionof connections between the various components andelements, see Section 8.3.2.2B. Properties of shearwalls and other lateral-force-resisting systems such asbraced frames are described and discussed inSection 8.4, along with various retrofit or strengtheningmethods. Horizontal floor and roof diaphragms andbraced systems are discussed in Section 8.5, which alsocovers engineering properties and methods ofupgrading or strengthening the elements. Woodfoundations and pole structures are described inSection 8.6. For additional information regardingfoundations, see Chapter 4. Definitions of terms are inSection 8.7; symbols are in Section 8.8. Referencematerials for both new and existing materials areprovided in Section 8.9.8.2 Historical Perspective8.2.1 GeneralWood frame construction has evolved over millennia;wood is the primary building material of mostresidential and small commercial structures in theUnited States. It has often been used for the framing ofroofs and/or floors, in combination with other materials,for other types of buildings.Establishing the age and recognizing the location of abuilding can be helpful in determining what types oflateral-force-resisting systems may be present.Information regarding the establishment of a building’sage and a discussion of the evolution of framingsystems can be found in Section C8.2 of theCommentary.As indicated in Chapter 1, great care should beexercised in selecting the appropriate rehabilitationapproaches and techniques for application to historicbuildings in order to preserve their uniquecharacteristics.8.2.2 Building AgeBased on the approximate age of a building, variousassumptions can be made about the design and featuresof construction. Older wood frame structures thatpredate building codes and standards usually do nothave the types of elements considered essential forpredictable seismic performance. These elements willgenerally have to be added, or the existing elementsupgraded by the addition of lateral-load-resistingcomponents to the existing structure, in order to obtaina predictable performance.If the age of a building is known, the code in effect atthe time of construction and the general quality of theconstruction usual for the time can be helpful inevaluating an existing building. The level ofmaintenance of a building may be a helpful guide indetermining the extent of loss of a structure’s capacityto resist loads.8.2.3 Evolution of Framing MethodsThe earliest wood frame buildings built by Europeanimmigrants to the United States were built with post andbeam or frame construction adopted from Europe andthe British Isles. This was followed by the developmentof balloon framing in about 1830 in the Midwest, whichspread to the East Coast by the 1860s. This, in turn, wasfollowed by the development of western or platformframing shortly after the turn of the century. Platformframing is the system currently in use for multistoryconstruction.FEMA 273 Seismic Rehabilitation Guidelines 8-1

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Drywall or wallboard was first introduced in about1920; however, its use was not widespread until afterWorld War II, when gypsum lath (button board) alsocame into extensive use as a replacement for wood lath.With the exception of public schools in high seismicareas, modern wood frame structures detailed to resistseismic loads were generally not built prior to 1934. Formost wood frame structures, either general seismicprovisions were not provided—or the codes thatincluded them were not enforced—until the mid-1950sor later, even in the most active seismic areas. (Thistime frame varies somewhat depending on localconditions and practice.)Buildings constructed after 1970 in high seismic areasusually included a well-defined lateral-force-resistingsystem as a part of the design. However, site inspectionsand code enforcement varied greatly, so that theinclusion of various features and details on the plansdoes not necessarily mean that they are in place or fullyeffective. Verification is needed to ensure that goodconstruction practices were followed.Until about 1950, wood residential buildings werefrequently constructed on raised foundations and insome cases included a short stud wall, called a “cripplewall,” between the foundation and the first floorframing. This occurs on both balloon framed andplatform framed buildings. There may be an extrademand on these cripple walls, because most interiorpartition walls do not continue to the foundation.Special attention is required for these situations.Adequate bracing must be provided for cripple walls aswell as the attachment of the sill plate to the foundation.In more recent times, light gage metal studs and joistshave been used in lieu of wood framing for somestructures. Lateral-load resistance is either provided bymetal straps attached to the studs and top and bottomtracks, or by structural panels attached with sheet metalscrews to the studs and the top and bottom track in amanner similar to wood construction. The metal studsand joists vary in size, gage, and configurationdepending on the manufacturer and the loadingconditions.For systems using structural panels for bracing, seeSection 8.4 for analysis and acceptance criteria. For theall-metal systems using steel strap braces, see Chapter 5for guidance.8.3 Material Properties andCondition Assessment8.3.1 GeneralEach structural element in an existing building iscomposed of a material capable of resisting andtransferring applied loads to the foundation systems.One material group historically used in buildingconstruction is wood. Various grades and species ofwood have been used in a cut dimension form,combined with other structural materials (e.g., steel/wood elements), or in multiple layers of construction(e.g., glued-laminated wood components). Woodmaterials have also been manufactured into hardboard,plywood, and particleboard products, which may havestructural or nonstructural functions in construction.The condition of the in-place wood materials willgreatly influence the future behavior of woodcomponents in the building system.Quantification of in-place material properties andverification of existing system configuration andcondition are necessary to properly analyze thebuilding. The focus of this effort shall be given to theprimary vertical- and lateral-load-resisting elements andcomponents thereof. These primary components may beidentified through initial analysis and application ofloads to the building model.The extent of in-place materials testing and conditionassessment that must be accomplished is related toavailability and accuracy of construction documentsand as-built records, the quality of materials used andconstruction performed, and physical condition. Aspecific problem with wood construction is thatstructural wood components are often covered withother components, materials, or finishes; in addition,their behavior is influenced by past loading history.Knowledge of the properties and grades of materialused in original component/connection fabrication isinvaluable, and may be effectively used to reduce theamount of in-place testing required. The designprofessional is encouraged to research and acquire allavailable records from original construction, includingdesign calculations.Connection configuration also has a very importantinfluence on response to applied loads and motions. Alarge number of connector types exist, the mostprevalent being nails and through-bolts. However, morerecent construction has included metal straps and8-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)hangers, clip angles, and truss plates. An understandingof connector configuration and mechanical propertiesmust be gained to analyze properly the anticipatedperformance of the building construction.8.3.2 Properties of In-Place Materials andComponents8.3.2.1 Material PropertiesWood has dramatically different properties in its threeorthotropic axes (parallel to grain, transverse to grain,and radial). These properties vary with wood species,grade, and density. The type, grade, and condition of thecomponent(s) must be established in order to computestrength and deformation characteristics. Mechanicalproperties and configuration of component andconnection material dictate the structural behavior ofthe component under load. The effort required todetermine these properties is related to the availabilityof original and updated construction documents,original quality of construction, accessibility, conditionof materials, and the analytical procedure to be used inthe rehabilitation (e.g., LSP for wood construction).The first step in quantifying properties is to establish thespecies and grade of wood through review ofconstruction documents or direct inspection. If thewood is not easily identified visually or by the presenceof a stamped grade mark on the wood surface, thensamples can be taken for laboratory testing andidentification (see below). The grade of the wood alsomay be established from grade marks, the size andpresence of knots, splits and checks, the slope of thegrain, and the spacing of growth rings through the useof appropriate grade rules. The grading shall beperformed using a specific grading handbook for theassumed wood species and application (e.g.,Department of Commerce American Softwood LumberStandard PS 20-70 (NIST, 1986), the National GradingRules for Dimension Lumber of the National GradingRules Committee), or through the use of the ASTM(1992) D245 grading methodology.In general, the determination of material species andproperties (other than inter-component connectionbehavior) is best accomplished through removal ofsamples coupled with laboratory analysis by experts inwood science. Sampling shall take place in regions ofreduced stress, such as mid-depth of members. Somelocal repair may be necessary after sampling.The properties of adhesives used in fabrication ofcertain component types (e.g., laminated products) mustalso be evaluated. Such adhesives may be adverselyaffected by exposure to moisture and other conditionsin-service. Material properties may also be affected bycertain chemical treatments (e.g., fire retardant)originally applied to protect the component fromenvironmental conditions.8.3.2.2 Component PropertiesA. ElementsStructural elements of the lateral-force-resisting systemcomprise primary and secondary components, whichcollectively define element strength and resistance todeformation. Behavior of the components—includingshear walls, beams, diaphragms, columns, and braces—is dictated by physical properties such as area; materialgrade; thickness, depth, and slenderness ratios; lateraltorsional buckling resistance; and connection details.The following component properties shall beestablished during a condition assessment in the initialstages of the seismic rehabilitation process, to aid inevaluating component strength and deformationcapacities (see Section 8.3.3 for assessment guidelines):• Original cross sectional shape and physicaldimensions (e.g., actual dimensions for 2" x 4" stud)for the primary members of the structure• Size and thickness of additional connected materials,including plywood, bracing, stiffeners; chord, sills,struts, and hold-down posts• Modifications to members (e.g., notching, holes,splits, cracks)• Location and dimension of braced frames and shearwalls; type, grade, nail size, and spacing of holddownsand drag/strut members• Current physical condition of members, includingpresence of decay or deformation• Confirmation of component(s) behavior with overallelement behaviorThese primary component properties are needed toproperly characterize building performance in theseismic analysis. The starting point for establishingcomponent properties should be the availableconstruction documents. Preliminary review of theseFEMA 273 Seismic Rehabilitation Guidelines 8-3

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)documents shall be performed to identify primaryvertical- (gravity-) and lateral-load-carrying elementsand systems, and their critical components andconnections. Site inspections should be conducted toverify conditions and to assure that remodeling has notchanged the original design concept. In the absence of acomplete set of building drawings, the designprofessional must perform a thorough inspection of thebuilding to identify these elements, systems, andcomponents as indicated in Section 8.3.3. Wherereliable record drawings do not exist, an as-built set ofplans for the building must be created.B. ConnectionsThe method of connecting the various elements of thestructural system is critical to its performance. The typeand character of the connections must be determined bya review of the plans and a field verification of theconditions.The following connections shall be established during acondition assessment to aid in the evaluation of thestructural behavior:• Connections between horizontal diaphragms andshear walls and braced frames• Size and character of all drag ties and struts,including splice connections used to collect loadsfrom the diaphragms to deliver to shear walls orbraced frames• Connections at splices in chord members ofhorizontal diaphragms• Connections of horizontal diaphragms to exterior orinterior concrete or masonry walls for both in-planeand out-of-plane loads• Connections of cross-tie members for concrete ormasonry buildings• Connections of shear walls to foundations• Method of through-floor transfer of wall shears inmultistory buildings8.3.2.3 Test Methods to Quantify PropertiesTo obtain the desired in-place mechanical properties ofmaterials and components, including expected strength,it is often necessary to use proven destructive andnondestructive testing methods.Of greatest interest to wood building systemperformance are the expected orthotropic strengths ofthe installed materials for anticipated actions (e.g.,flexure). Past research and accumulation of data byindustry groups have led to published mechanicalproperties for most wood types and sizes (e.g.,dimensional solid-sawn lumber, and glued-laminated or“glulam” beams). Section 8.3.2.5 addresses theseestablished default strengths and distortion properties.This information may be used, together with tests fromrecovered samples, to establish rapidly the expectedstrength properties for use in component strength anddeformation analyses. Where possible, the load historyfor the building shall be assessed for possible influenceon component strength and deformation properties.To quantify material properties and analyze theperformance of archaic wood construction, shear walls,and diaphragm action, more extensive sampling andtesting may be necessary. This testing should includefurther evaluation of load history and moisture effectson properties, and the examination of wall anddiaphragm continuity, and suitability of in-placeconnectors.Where it is desired to use an existing assembly and littleor no information as to its performance is available, acyclic load test of a mock-up of the existing structuralelements can be utilized to determine the performanceof various assemblies, connections, and load transferconditions. The establishment of the parameters givenin Tables 8-1 and 8-2 can be determined from theresults of the cyclic load tests. See Section 2.13 for anexplanation of the backbone curve and theestablishment of parameters.8.3.2.4 Minimum Number of TestsIn order to accurately quantify expected strength andother in-place properties, it is important that a minimumnumber of tests be conducted on representativecomponents. The minimum number of tests is dictatedby available data from original construction, the type ofstructural system employed, desired accuracy, andquality/condition of in-place materials. Visual access tothe structural system also influences testing programdefinition. As an alternative, the design professionalmay elect to utilize the default strength properties, perSection 8.3.2.5 provisions, as opposed to conductingthe specified testing. However, these default values8-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)should only be used for the LSP. It is stronglyencouraged that the expected strengths be derivedthrough testing of assemblies to model behavior moreaccurately.In terms of defining expected strength properties, thefollowing guidelines should be followed. Removal ofcoverings, including stucco, fireproofing and partitionmaterials, is generally required to facilitate thesampling and observations.• If original construction documents exist that definethe grade of wood and mechanical properties, atleast one location for each story shall be randomlyobserved from each component type (e.g., solidsawn lumber, glulam beam, plywood diaphragm)identified as having a different material grade. Theseshall be verified by sampling and testing or byobserving grade stamps and conditions.• If original construction documents definingproperties are limited or do not exist, but the date ofconstruction is known and single material use isconfirmed (e.g., all components are Douglas fir solidsawn lumber), at least three observations or samplesshould be randomly made for each component type(e.g., beam, column, shear wall) for every two floorsin the building.• If no knowledge of the structural system andmaterials used exists, at least six samples shall beremoved or observed from each element (e.g.,primary gravity- and lateral-load-resistingcomponents) and component type (e.g., solid sawnlumber, diaphragm) for every two floors ofconstruction. If it is determined from testing and/orobservation that more than one material grade exists,additional observations should be made until theextent of use for each grade in componentfabrication has been established.• In the absence of construction records definingconnector features present, at least three connectorsshall be observed for every floor of the building. Theobservations shall consist of each connector typepresent in the building (e.g., nails, bolts, straps),such that the composite strength of the connectioncan be estimated.• For an archaic systems test or other full-scale mockuptest of an assembly, at least two cyclic tests ofeach assembly shall be conducted. A third test shallbe conducted if the results of the two tests vary bymore than 20%.8.3.2.5 Default PropertiesMechanical properties for wood materials andcomponents are based on available historical data andtests on samples of components, or mock-up tests oftypical systems. In the absence of these data, or forcomparative purposes, default material strengthproperties are needed. Unlike other structural materials,default properties for wood are highly variable anddependent on factors including the species, grade,usage, age, and exposure conditions. As a minimum, itis recommended that the type and grade of wood beestablished. Historically, codes and standards includingthe National Design Specification (AF&PA, 1991a)have published allowable stresses as opposed tostrengths. These values are conservative, representingmean values from previous research. Default strengthvalues, consistent with NEHRP RecommendedProvisions (BSSC, 1995), may be calculated as theallowable stress multiplied by a 2.16 conversion factor,a capacity reduction factor of 0.8, and time effect factorof 1.6 for seismic loading. This results in anapproximate 2.8 factor to translate allowable stressvalues to yield or limit state values. The expectedstrength, Q CE , is determined based on these yield orlimit state strengths. If significant inherent damage ordeterioration is found to be present, default values maynot be used. Structural elements with significantdamage need to be replaced with new materials, or elsea significant reduction in the capacity and stiffness mustbe incorporated into the analysis.It is recommended that the results of any materialtesting performed be compared to the default values forthe particular era of building construction; shouldsignificantly reduced properties from testing bediscovered in this testing, further evaluation as to thecause shall be undertaken. Default values may not beused if they are greater than those obtained from testing.Default material strength properties may only be used inconjunction with the LSP. For all other analysisprocedures, expected strengths from specified testingand/or mock-up testing shall be used to determineanticipated performance.Default values for connectors shall be established in amanner similar to that for the members. The publishedvalues in the National Design Specification (AF&PA,FEMA 273 Seismic Rehabilitation Guidelines 8-5

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)1994) shall be increased by a factor of 2.8 to convertfrom allowable stress levels to yield or limit statevalues, Q CE , for seismic loading.Deformation at yield of nailed connectors may beassumed to be 0.02 inches for wood to wood and woodto metal connections. For wood screws the deformationmay be assumed to be 0.05 inches; for lag bolts thedeformation may be assumed at 0.10 inches. For bolts,the deformation for wood to wood connections can beassumed to be 0.2 inches; for wood to steel connections,0.15 inches. In addition the estimated deformation ofany hardware or allowance, e.g., for poor fit oroversized holes, should be summed to obtain the totalconnection deformation.8.3.3 Condition Assessment8.3.3.1 GeneralA condition assessment of the existing building and siteconditions shall be performed as part of the seismicrehabilitation process. The goal of this assessment isfourfold:1. To examine the physical condition of primary andsecondary components and the presence of anydegradation2. To verify the presence and configuration ofcomponents and their connections, and continuity ofload paths between components, elements, andsystems3. To review other conditions—such as neighboringparty walls and buildings, presence of nonstructuralcomponents, prior remodeling, and limitations forrehabilitation—that may influence buildingperformance4. To formulate a basis for selecting a knowledgefactor (see Section 8.3.4)The physical condition of existing components andelements, and their connections, must be examined forpresence of degradation. Degradation may includeenvironmental effects (e.g., decay; splitting; firedamage; biological, termite, and chemical attack) orpast/current loading effects (e.g., overload, damagefrom past earthquakes, crushing, and twisting). Naturalwood also has inherent discontinuities such as knots,checks, and splits that must be accounted for. Thecondition assessment shall also examine forconfiguration problems observed in recent earthquakes,including effects of discontinuous components,improper nailing or bolting, poor fit-up, and connectionproblems at the foundation level. Often, unfinishedareas such as attic spaces, basements, and crawl spacesprovide suitable access to wood components used andcan give a general indication of the condition of the restof the structure. Invasive inspection of criticalcomponents and connections is typically required.Connections in wood components, elements, andsystems require special consideration and evaluation.The load path for the system must be determined, andeach connection in the load path(s) must be evaluated.This includes diaphragm-to-component andcomponent-to-component connections. The strengthand deformation capacity of connections must bechecked where the connection is attached to one ormore components that are expected to experiencesignificant inelastic response. Anchorage of exteriorwalls to roof and floors for concrete and masonrybuildings, for which wood diaphragms are used for outof-planeloading, requires detailed inspection. Boltholes in relatively narrow straps sometime preclude theductile behavior of the steel strap. Twists and kinks inthe strap can also have a serious impact on itsanticipated behavior. Cross-ties across the building,which are part of the wall anchorage system, need to beinspected to confirm their presence and the connectionof each piece, to ensure that a positive load path existsto tie the building walls together.The condition assessment also affords an opportunity toreview other conditions that may influence woodelements and systems, and overall buildingperformance. Of particular importance is theidentification of other elements and components thatmay contribute to or impair the performance of thewood system in question, including infills, neighboringbuildings, and equipment attachments. Limitationsposed by existing coverings, wall and ceiling spaceinsulation, and other material shall also be defined suchthat prudent rehabilitation measures can be planned.8.3.3.2 Scope and ProceduresThe scope of a condition assessment shall include allprimary structural elements and components involvedin gravity- and lateral-load resistance. Accessibilityconstraints may necessitate the use of instruments suchas a fiberscope or video probe to reduce the amount ofdamage to covering materials and fabrics. Theknowledge and insight gained from the condition8-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)assessment is invaluable to the understanding of loadpaths and the ability of components to resist andtransfer these loads. The degree of assessmentperformed also affects the knowledge factor, κ,discussed in Section 8.3.4. General guidelines andprocedures are also contained in that section; foradditional guidance and references, see theCommentary.Direct visual inspection provides the most valuableinformation, as it can be used to quickly identify anyconfiguration issues, and allows both measurement ofcomponent dimensions, and determination whetherdegradation is present. The continuity of load paths maybe established through viewing of components andconnection condition. From visual inspection, the needfor other test methods to quantify the presence anddegree of degradation may be established.The dimensions and features of all accessiblecomponents shall be measured and compared toavailable design information. Similarly, theconfiguration and condition of all connections shall beverified, with any deformations or other anomaliesnoted. If design documents for the structure do notexist, this technique shall be followed to develop an asbuiltdrawing set.If coverings or other obstructions exist, indirect visualinspection through use of drilled holes and a fiberscopeshall be utilized (as allowed by access). If this method isnot appropriate, then local removal of coveringmaterials will be necessary. The following guidelinesshall be used.• If detailed design drawings exist, exposure of at leastthree different primary connections shall occur foreach connection type (e.g., beam-column, shearwall-diaphragm, shear wall-foundation). If nosignificant capacity-reducing deviations from thedrawings exist, the sample may be consideredrepresentative. If deviations are noted, then removalof all coverings from primary connections of thattype may be necessary, if reliance is to be placed onthe connection.• In the absence of accurate drawings, either invasivefiberscopic inspections or exposure of at least 50%of all primary connection types for inspection shalloccur. If common detailing is observed, this samplemay be considered representative. If a multitude ofdetails or conditions are observed, full exposure isrequired.The scope of this removal effort is dictated by thecomponent and element design. For example, in abraced frame, exposure of several key connections maysuffice if the physical condition is acceptable andconfiguration matches the design drawings. However,for shear walls and diaphragms it may be necessary toexpose more connection points because of varyingdesigns and the critical nature of the connections. Forencased walls and frames for which no drawings exist,it is necessary to indirectly view or expose all primaryend connections for verification.Physical condition of components and connectors mayalso support the need to use certain destructive andnondestructive test methods. Devices normally utilizedfor the detection of reinforcing steel in concrete ormasonry can be utilized to verify the extent of metalstraps and hardware located beneath the finish surfaces.Further guidelines and procedures for destructive andnondestructive tests that may be used in the conditionassessment are contained in the Commentary.8.3.3.3 Quantifying ResultsThe results of the condition assessment shall be used inthe preparation of building system models forevaluation of seismic performance. To aid in this effort,the results shall be quantified and reduced, with thefollowing specific topics addressed:• Component section properties and dimensions• Component configuration and presence of anyeccentricities• Interaction of nonstructural components and theirinvolvement in lateral-load resistanceThe acceptance criteria for existing components dependon the design professional’s knowledge of the conditionof the structural system and material properties, aspreviously noted. Certain damage—such as waterstaining, evidence of prior leakage, splitting, cracking,checking, warping, and twisting—may be allowable.The design professional must establish a case-by-caseacceptance for such damage on the basis of capacityloss or deformation constraints. Degradation atconnection points should be carefully examined;significant capacity reductions may be involved, as wellas a loss of ductility. All deviations noted betweenFEMA 273 Seismic Rehabilitation Guidelines 8-7

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)available construction records and as-built conditionsshall be accounted for and considered in the structuralanalysis.8.3.4 Knowledge (κ) FactorAs described in Section 2.7, computation of componentcapacities and allowable deformations shall involve theuse of a knowledge (κ) factor. For cases where an LSPwill be used in the analysis, two categories of κ exist.This section further describes those requirementsspecific to wood structural elements that must beaccomplished in the selection of a κ factor.If the wood structural system is exposed, significantknowledge regarding configuration and behavior maybe gained through condition assessment. In general, a κfactor of 1.0 can be utilized when a thoroughassessment is performed on the primary and secondarycomponents and load path, and the requirements ofSection 2.7 are met. Similarly, if the wood system isencased, a κ factor of 1.0 can be utilized when threesamples of each primary component connection typeare exposed and verified as being compliant withconstruction records, and fiberscopic examinations areperformed to confirm the condition and configuration ofprimary load-resisting components.If knowledge of as-built component or connectionconfiguration/condition is incomplete or nonexistent,the κ factor used in the final component evaluation shallbe reduced to 0.75. Examples of where this value shallbe applied are contained in Section 2.7 andEquation 3-18. For encased components whereconstruction documents are limited and knowledge ofconfiguration and condition is incomplete, a factor of0.75 shall be used.8.3.5 Rehabilitation IssuesUpon determining that portions of a wood buildingstructure are deficient or inadequate for theRehabilitation Objective, the next step is to definereinforcement or replacement alternatives. If areinforcement program is to be followed and attachmentto the existing framing system is proposed, it isnecessary to closely examine material factors that mayinfluence reinforcement/attachment design, including:• Degree of any degradation in the component fromsuch mechanisms as biological attack, creep, highstatic or dynamic loading, moisture, or other effects• Level of steady state stress in the components to bereinforced (and potential to temporarily remove thisstress if appropriate)• Elastic and plastic properties of existingcomponents, to preserve strain compatibility withany new reinforcement materials• Ductility, durability, and suitability of existingconnectors between components, and access forreinforcement or modification• Prerequisite efforts necessary to achieve appropriatefit-up for reinforcing components and connections• Load flow and deformation of the components atend connections (especially at foundationattachments and connections where mixedconnectors such as bolts and nails exist)• Presence of components manufactured with archaicmaterials, which may contain materialdiscontinuities and shall be examined during therehabilitation design to ensure that the selectedreinforcement is feasible8.4 Wood and Light Frame ShearWallsThe behavior of wood and light frame shear walls iscomplex and influenced by many factors, the primaryfactor being the wall sheathing. Wall sheathings can bedivided into many categories (e.g., brittle, elastic,strong, weak, good at dissipating energy, and poor atdissipating energy). In many existing buildings, thewalls were not expected to act as shear walls (e.g., awall sheathed with wood lath and plaster). Most shearwalls are designed based on values from monotonicload tests and historically accepted values. Theallowable shear per unit length used for design wasassumed to be the same for long walls, narrow walls,walls with stiff tie-downs, and walls with flexible tiedowns.Only recently have shear wall assemblies(framing, covering, anchorage) been tested using cyclicloading.Another major factor influencing the behavior of shearwalls is the aspect ratio of the wall. The UniformBuilding Code (UBC) (ICBO, 1994a) limits the aspectratio (height-to-width) to 3.5:1. After the 1994Northridge earthquake, the city of Los Angeles reduced8-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)the allowable aspect ratio to 2:1, pending additionaltests. The interaction of the floor and roof with the wall,the end conditions of the wall, and the redundancy ornumber of walls along any wall line would affect thewall behavior for walls with the same aspect ratio. Inaddition, the rigidity of the tie-downs at the wall endshas an important effect in the behavior of narrow walls.Dissimilar wall sheathing materials on opposite sides ofa wall should not be combined when calculating thecapacity of the wall. Similarly, different walls sheathedwith dissimilar materials along the same line of lateralforce resistance should be analyzed based on only onetype of sheathing. The wall sheathing with the greatestcapacity should be used for determining capacity. Thewalls should also be analyzed based on the relativerigidity and capacity of the materials to determine ifperformance of the “nonparticipating” material will beacceptable.For uplift calculations on shear-wall elements, theoverturning moment on the wall should be based on thecalculated load on the wall from the base shear and theuse of an appropriate m factor for the uplift connector.As an alternative, the uplift can be based on a lateralload equal to 1.2 times the yield capacity of the wall.However, no m factor is involved in the demand versuscapacity equation, and the uplift connector yieldcapacity should not be exceeded.Connections between elements, drag ties, struts, andother structured members should be based on thecalculated load to the connection under study, andanalyzed. As an alternative, the connection can beanalyzed for a maximum load, at the connection, of 1.2times the yield capacity of the weaker element. Theyield capacity of the connection should not be exceededand no m factor is used in the analysis.For wood and light frame shear walls, the importantlimit states are sheathing failure, connection failure, tiedownfailure, and excessive deflection. Limit statesdefine the point of life safety and, often, of structuralstability. To reduce damage or retain usabilityimmediately after an earthquake, deflection must belimited (see Section 2.5). The ultimate capacity is themaximum capacity the assembly can resist, regardlessof the deflection. See Section 8.5.11 for the effect ofopenings in diaphragms. The expected capacity, Q CE , isequal to the yield capacity of the shear wall, V y .8.4.1 Types of Light Frame Shear Walls8.4.1.1 Existing Shear WallsA. Single Layer Horizontal Lumber Sheathing orSidingTypically, 1" x horizontal sheathing or siding is applieddirectly to studs. Forces are resisted by nail couples.Horizontal boards, from 1" x 4" to 1" x 12" typically arenailed to 2" x or greater width studs with two or morenails (typically 8d or 10d) per stud. The strength andstiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)B. Diagonal Lumber SheathingTypically, 1" x 6" or 1" x 8" diagonal sheathing, applieddirectly to the studs, resists lateral forces primarily bytriangulation (i.e., direct tension and compression).Sheathing boards are installed at a 45-degree angle tostuds, with three or more nails (typically 8d) per stud,and to sill and top plates. A second layer of diagonalsheathing is sometimes added on top of the first layer, at90 degrees to the first layer (called Double DiagonalSheathing), for increased load capacity and stiffness.C. Vertical Wood Siding OnlyTypically, 1" x 8", 1" x 10", or 1" x 12" vertical boardsare nailed directly to 2" x or greater width studs andblocking with 8d to 10d galvanized nails. The lateralforces are resisted by nail couples, similarly tohorizontal siding. The strength and stiffness degradewith cyclic loading. (See Section 3.3.1.3 and Table 3-1for the appropriate C 2 value.)D. Wood Siding over Horizontal SheathingTypically, siding is nailed with 8d to 10d galvanizednails through the sheathing to the studs. Lateral forcesare resisted by nail couples for both layers. The strengthand stiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)E. Wood Siding over Diagonal SheathingTypically, siding is nailed with 8d or 10d galvanizednails to and through the sheathing into the studs.Diagonal sheathing provides most of the lateralresistance by truss-shear action.FEMA 273 Seismic Rehabilitation Guidelines 8-9

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)F. Structural Panel or Plywood Panel Sheathing orSidingTypically, 4' x 8' panels are applied vertically orhorizontally to 2" x or greater studs and nailed with 6dto 10d nails. These panels resist lateral forces by paneldiaphragm action.G. Stucco on Studs (over sheathing or wire backedbuilding paper)Typically, 7/8-inch portland cement plaster is applied onwire lath or expanded metal lath. Wire lath or expandedmetal lath is nailed to the studs with 11 gage nails or 16gage staples at 6 inches on center. This assembly resistslateral forces by panel diaphragm action. The strengthand stiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)H. Gypsum Plaster on Wood LathTypically, 1-inch gypsum plaster is keyed onto spaced1-1/4-inch wood lath that is nailed to studs with 13 gagenails. Gypsum plaster on wood lath resists lateral forcesby panel diaphragm-shear action. The strength andstiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)I. Gypsum Plaster on Gypsum LathTypically, 1/2-inch plaster is glued or keyed to 16-inchx 48-inch gypsum lath, which is nailed to studs with 13gage nails. Gypsum plaster on gypsum lath resistslateral loads by panel diaphragm action. The strengthand stiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)J. Gypsum Wallboard or DrywallTypically, 4' x 8' to 4' x 12' panels are laid-uphorizontally or vertically and nailed to studs or blockingwith 5d to 8d cooler nails at 4 to 7 inches on center.Multiple layers are utilized in some situations. Theassembly resists lateral forces by panel diaphragmshearaction. The strength and stiffness degrade withcyclic loading. (See Section 3.3.1.3 and Table 3-1 forthe appropriate C 2 value.)K. Gypsum SheathingTypically, 4' x 8' to 4' x 12' panels are laid-up horizontallyor vertically and nailed to studs or blocking withgalvanized 11 gage 7/16-inch diameter head nails at 4 to7 inches on center. Gypsum sheathing is usually installedon the exterior of structures with siding over it in order toimprove fire resistance. Lateral forces are resisted bypanel diaphragm action. The strength and stiffnessdegrades with cyclic loading. (See Section 3.3.1.3 andTable 3-1 for the appropriate C 2 value.)L. Plaster on Metal LathTypically, 1-inch gypsum plaster is applied on expandedwire lath that is nailed to the studs. Lateral forces areresisted by panel diaphragm action. The strength andstiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)M. Horizontal Lumber Sheathing with Cut-In Bracesor Diagonal BlockingThis is installed in the same manner as horizontalsheathing, except the wall is braced at the corners withcut-in (or let-in) braces or blocking. The bracing isusually installed at a 45-degree angle and nailed with 8dor 10d nails at each stud, and at the top and bottomplates. Bracing provides only nominal increase inresistance. The strength and stiffness degrade withcyclic loading. (See Section 3.3.1.3 and Table 3-1 forthe appropriate C 2 value.)N. Fiberboard or Particleboard SheathingTypically, 4' x 8' panels are applied directly to the studswith nails. The fiberboard requires nails (typically 8d)with large heads such as roofing nails. Lateral loads areresisted by panel diaphragm action. The strength andstiffness degrade with cyclic loading. (SeeSection 3.3.1.3 and Table 3-1 for the appropriate C 2value.)8.4.1.2 Shear Wall Enhancements forRehabilitationA. Structural Panel Sheathing Added to UnfinishedStud WallsWall shear capacity and stiffness can be increased byadding structural panel sheathing to one side ofunfinished stud walls, such as cripple walls or attic endwalls.B. Structural Panel Sheathing Overlay of ExistingShear WallsFor a moderate increase in shear capacity and stiffnessthat can be applied in most places in most structures, theexisting wall covering can be overlaid with structural8-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)panel sheathing; for example, plywood sheathing can beapplied over an interior wall finish. For exteriorapplications, the structural panel can be placed over theexterior finish and nailed directly through it to the studs.This rehabilitation procedure typically can be used forthe following shear walls, which are described inSection 8.4.1.1:• Single layer horizontal lumber sheathing or siding• Single layer diagonal lumber sheathing• Vertical wood siding only• Gypsum plaster or wallboard on studs (also ongypsum lath and gypsum wallboard)• Gypsum sheathing• Horizontal lumber sheathing with cut-in braces ordiagonal blocking• Fiberboard or particleboard sheathingThe enhanced shear wall is evaluated in accordancewith Section 8.4.9, discounting the original sheathingand reducing the yield capacity of the overlay materialby 20%.C. Structural Panel Sheathing Added Under ExistingWall CoveringTo obtain a significant increase in shear capacity, theexisting wall covering can be removed; structural panelsheathing, connections, and tie-downs added; and thewall covering replaced. In some cases, whereearthquake loads are large, this may be the best methodof rehabilitation. This rehabilitation procedure can beused on any of the existing shear wall assemblies.Additional framing members can be added if necessary,and the structural panels can be cut to fit existing studspacings.D. Increased AttachmentFor existing structural panel sheathed walls, additionalnailing will result in higher capacity and increasedstiffness. Other connectors—such as collector straps,splice straps, or tie-downs—are often necessary toincrease the rigidity and capacity of existing structuralpanel shear walls. Increased ductility will notnecessarily result from the additional nailing. Access tothese shear walls will often require the removal andreplacement of existing finishes.E. Rehabilitation of ConnectionsMost shear wall rehabilitation procedures require acheck of all existing connections, especially todiaphragms and foundations. Additional blockingbetween floor or roof joists at shear walls is oftenneeded on existing structures. The blocking must beconnected to the shear wall and the diaphragm toprovide a load path for lateral loads. Sheet metalframing clips can be used to provide a verifiableconnection between the wall framing, the blocking, andthe diaphragm. Framing clips are also often used forconnecting blocking or rim joists to sill plates.The framing in existing buildings is usually very dry,hard, and easily split. Care must be taken not to split theexisting framing when adding connectors. Predrillingholes for nails will reduce splitting, and framing clipsthat use small nails are less likely to split the existingframing.When existing shear walls are overlaid with structuralpanels, the connections of the structural panels to theexisting framing must be considered. Splitting canoccur in both the wood sheathing and the framing. Thelength of nails needed to achieve full capacityattachment in the existing framing must be determined.This length will vary with the thickness of the existingwall covering. Sometimes staples are used instead ofnails to prevent splitting. The overlay is stapled to thewood sheathing instead of the framing. Nails arerecommended for overlay attachment to the underlyingframing. In some cases, new blocking at structural paneljoints may also be needed.When framing members or blocking are added to astructure, the wood should be kiln-dried or wellseasonedto prevent it from shrinking away from theexisting framing or splitting.8.4.1.3 New Shear Walls Sheathed withStructural Panels or Plywood PanelSheathing or SidingNew shear walls using the existing framing or newframing are sheathed with structural panels (i.e.,plywood or oriented strand board). The thickness andgrade of these panels can vary. In most cases, the panelsare placed vertically and fastened directly to the studsand plates. This reduces the need for blocking at thejoints. All edges of panels must be blocked to obtainfull capacity. The thickness, size, and number offasteners, and aspect ratio and connections willFEMA 273 Seismic Rehabilitation Guidelines 8-11

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)determine the capacity of the new walls. Additionalinformation on the various panels available and theirapplication can be found in documents from theAmerican Plywood Association (APA), such as APA(1983).8.4.2 Light Gage Metal Frame Shear Walls8.4.2.1 Existing Light Gage Metal FrameShear WallsA. Plaster on Metal LathTypically, 1 inch of gypsum plaster is applied to metallath or expanded metal that is connected to the metalframing with wire ties.B. Gypsum WallboardTypically, 4' x 8' to 4' x 12' panels are laid-uphorizontally and screwed with No. 6 x 1-inch-long selftappingscrews to studs at 4 to 7 inches on center.C. Plywood or Structural PanelsTypically, the structural panels are applied verticallyand screwed to the studs and track with No. 8 to No. 12self-tapping screws.8.4.2.2 Light Gage Metal FrameEnhancements for RehabilitationA. Addition of Plywood Structural Panels to ExistingMetal Stud WallsAny existing covering other than plywood is removedand replaced with structural panels. Connections to thediaphragm(s) and the foundation must be checked andmay need to be strengthened.B. Existing Plywood or Structural Panels on MetalStudsAdded screws and possibly additional connections todiaphragms and foundation may be required.8.4.2.3 New Light Gage Metal Frame ShearWallsA. Plywood or Structural PanelsRefer to Section 8.4.1.3.8.4.3 Knee-Braced and MiscellaneousTimber Frames8.4.3.1 Knee-Braced FramesKnee-braced frames produce moment-resisting joints bythe addition of diagonal members between columns andbeams. The resulting “semi-rigid” frame resists lateralloads. The moment-resisting capacity of knee-bracedframes varies widely. The controlling part of theassembly is usually the connection; however, bending ofmembers can be the controlling feature of some frames.Once the capacity of the connection is determined,members can be checked and the capacity of the framecan be determined by statics. For a detailed discussion onconnections, see Section C8.3.2.2B in the Commentary.8.4.3.2 Rod-Braced FramesSimilarly to knee-braced frames, the connections ofrods to timber framing will usually govern the capacityof the rod-braced frame. Typically, the rods act only intension. Once the capacity of the connection isdetermined, the capacity of the frame can be determinedby statics. See Section 8.3.2.2B.8.4.4 Single Layer Horizontal LumberSheathing or Siding Shear Walls8.4.4.1 Stiffness for AnalysisHorizontal lumber sheathed shear walls are weak andvery flexible and have long periods of vibration. Theseshear walls are suitable only where earthquake shearloads are low and deflection control is not required. Thedeflection of these shear walls can be approximated byEquation 8-1:where:bhv yG d∆ yd a∆ y = v y h/G d + (h/b)d a (8-1)= Shear wall length, ft= Shear wall height, ft= Shear at yield, lb/ft= Shear stiffness in lb/in.= Calculated shear wall deflection at yield, in.= Elongation of anchorage at end of walldetermined by anchorage details and loadmagnitude, in.8-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)For horizontal lumber sheathed shear walls,G d = 2,000 lb/in.8.4.4.2 Strength Acceptance CriteriaHorizontal sheathing or siding has an estimated yieldcapacity of 80 pounds per linear foot. This capacity isdependent on the width of the boards, spacing of thestuds, and the size, number, and spacing of the nails.Allowable capacities are listed for variousconfigurations, together with a description of the nailcouple method, in the Western Woods Use Book(WWPA, 1983). See also ATC (1981) for a discussionof the nail couple.8.4.4.3 Deformation Acceptance CriteriaThe deformation acceptance criteria are determined bythe capacity of lateral- and gravity-load-resistingcomponents and elements to deform with limiteddamage or without failure. Excessive deflection couldresult in major damage to the structure and/or itscontents. See Table 8-1 for m factors for use in the LSPin performing design analyses.The coordinates for the normalized force-deflectioncurve used for modeling in connection with thenonlinear procedures (Figure 8-1) are shown inTable 8-2. The values in this table refer to Figure 8-1 inthe following way. Distance d is considered themaximum deflection at the point of first loss ofstrength. Distance e is the maximum deflection at astrength or capacity equal to value c. Figure 8-1 alsoshows the deformation ratios for IO, LS, and CPPerformance Levels for primary components. (SeeChapter 3 for the use of the force-deflection curve in theNSP.)Deformation acceptance criteria for use in connectionwith nonlinear procedures are given in footnotes ofTable 8-2 for primary and secondary components,respectively.Table 8-1Numerical Acceptance Factors for Linear Procedures—Wood Componentsm Factors for Linear Procedures 2PrimarySecondaryIO LS CP LS CPHeight/LengthShear WallsRatio (h/L) 1Horizontal 1" x 6" Sheathing h/L < 1.0 1.8 4.2 5.0 5.0 5.5Horizontal 1" x 10" Sheathing h/L < 1.0 1.6 3.4 4.0 4.0 5.0Horizontal Wood Siding Overh/L < 1.5 1.4 2.6 3.0 3.1 4.0Horizontal 1" x 6" SheathingHorizontal Wood Siding Overh/L < 1.5 1.3 2.3 2.6 2.8 3.0Horizontal 1" x 10" SheathingDiagonal 1" x 6" Sheathing h/L < 1.5 1.5 2.9 3.3 3.4 3.8Diagonal 1" x 8" Sheathing h/L < 1.5 1.4 2.7 3.1 3.1 3.6Horizontal Wood Siding Overh/L < 2.0 1.3 2.2 2.5 2.5 3.0Diagonal 1" x 6" SheathingHorizontal Wood Siding Overh/L < 2.0 1.3 2.0 2.3 2.5 2.8Diagonal 1" x 8" SheathingDouble Diagonal 1" x 6" Sheathing h/L < 2.0 1.2 1.8 2.0 2.3 2.5Double Diagonal 1" x 8" Sheathing h/L < 2.0 1.2 1.7 1.9 2.0 2.5Vertical 1" x 10" Sheathing h/L < 1.0 1.5 3.1 3.6 3.6 4.11. For ratios greater than the maximum listed values, the component is considered not effective in resisting lateral loads.2 Linear interpolation is permitted for intermediate value if h/L has asterisks.FEMA 273 Seismic Rehabilitation Guidelines 8-13

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Table 8-1Numerical Acceptance Factors for Linear Procedures—Wood Components (continued)m Factors for Linear Procedures 2PrimarySecondaryIO LS CP LS CPStructural Panel or Plywood Panel Sheathing or Siding h/L < 1.0* 1.7 3.8 4.5 4.5 5.5h/L > 2.0* 1.4 2.6 3.0 3.0 4.0h/L < 3.5Stucco on Studs h/L

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Table 8-1Numerical Acceptance Factors for Linear Procedures—Wood Components (continued)m Factors for Linear Procedures 2PrimarySecondaryIO LS CP LS CPComponent/ElementFrame elements subject to axial and bending stresses 1.0 2.5 3.0 2.5 4.0ConnectionsNails - 8d and larger - Wood to Wood 2.0 6.0 8.0 8.0 9.0Nails - 8d and larger - Metal to Wood 2.0 4.0 6.0 5.0 7.0Screws - Wood to Wood 1.2 2.0 2.2 2.0 2.5Screws - Metal to Wood 1.1 1.8 2.0 1.8 2.3Lag Bolts - Wood to Wood 1.4 2.5 3.0 2.5 3.3Lag Bolts - Metal to Wood 1.3 2.3 2.5 2.4 3.0Machine Bolts - Wood to Wood 1.3 3.0 3.5 3.3 3.9Machine Bolts - Metal to Wood 1.4 2.8 3.3 3.1 3.7Split Rings and Shear Plates 1.3 2.2 2.5 2.3 2.7Bolts - Wood to Concrete or Masonry 1.4 2.7 3.0 2.8 3.51. For ratios greater than the maximum listed values, the component is considered not effective in resisting lateral loads.2 Linear interpolation is permitted for intermediate value if h/L has asterisks.QQ CEFigure 8-1V uV yAreaSimplified backbone curvedIOeArea∆∆yLSCPNormalized Force versus DeformationRatio for Wood Elementsc8.4.4.4 ConnectionsThe connections between parts of the shear wallassembly and other elements of the lateral-forceresistingsystem must be investigated and analyzed. Thecapacity and ductility of these connections will oftendetermine the failure mode as well as the capacity of theassembly. Ductile connections with sufficient capacitywill give acceptable and expected performance (seeSection 8.3.2.2B).8.4.5 Diagonal Lumber Sheathing ShearWalls8.4.5.1 Stiffness for AnalysisDiagonal lumber sheathed shear walls are stiffer andstronger than horizontal sheathed shear walls. They alsoprovide greater stiffness for deflection control, andthereby greater damage control. The deflection of theseshear walls can be determined using Equation 8-1, withG d = 8,000 lb/in. for single layer diagonal siding andG d = 18,000 lb/in. for double diagonal siding.FEMA 273 Seismic Rehabilitation Guidelines 8-15

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Table 8-2Normalized Force-Deflection Curve Coordinates for Nonlinear Procedures—WoodComponentsShear Wall Type - Types of Existing Wood and Light FrameShear WallsHeight/LengthRatio h/L 1d e cHorizontal 1" x 6" Sheathing h/L < 1.0 5.0 6.0 0.3Horizontal 1" x 10" Sheathing h/L < 1.0 4.0 5.0 0.3Horizontal Wood Siding Over Horizontal 1" x 6" Sheathing h/L < 1.5 3.0 4.0 0.2Horizontal Wood Siding Over Horizontal 1" x 10" Sheathing h/L < 1.5 2.6 3.6 0.2Diagonal 1" x 6" Sheathing h/L < 1.5 3.3 4.0 0.2Diagonal 1" x 8" Sheathing h/L < 1.5 3.1 4.0 0.2Horizontal Wood Siding Over Diagonal 1" x 6" Sheathing h/L < 2.0 2.5 3.0 0.2Horizontal Wood Siding Over Diagonal 1" x 8" Sheathing h/L < 2.0 2.3 3.0 0.2Double Diagonal 1" x 6" Sheathing h/L < 2.0 2.0 2.5 0.2Double Diagonal 1" x 8" Sheathing h/L < 2.0 2.0 2.5 0.2Vertical 1" x 10" Sheathing h/L < 1.0 3.6 4.0 0.3Structural Panel or Plywood Panel Sheathing or Siding h/L < 1.0* 4.5 5.5 0.3h/L > 2.0*3.0 4.0 0.2h/L < 3.5Stucco on Studs h/L < 1.0* 3.6 4.0 0.2h/L = 2.0* 2.5 3.0 0.2Stucco over 1" x Horizontal Sheathing h/L < 2.0 3.5 4.0 0.2Gypsum Plaster on Wood Lath h/L < 2.0 4.6 5.0 0.2Gypsum Plaster on Gypsum Lath h/L < 2.0 5.0 6.0 0.2Gypsum Plaster on Metal Lath h/L < 2.0 4.4 5.0 0.2Gypsum Sheathing h/L < 2.0 5.7 6.3 0.2Gypsum Wallboard h/L < 1.0* 5.7 6.3 0.2h/L = 2.0* 4.0 5.0 0.2Horizontal 1" x 6" Sheathing With Cut-In Braces or Diagonal h/L < 1.0 4.4 5.0 0.2BlockingFiberboard or Particleboard Sheathing h/L < 1.5 3.8 4.0 0.2Length/WidthDiaphragm Type - Horizontal Wood DiaphragmsRatio (L/b) 1Single Straight Sheathing, Chorded L/b < 2.0 2.5 3.5 0.2Single Straight Sheathing, Unchorded L/b < 2.0 2.0 3.0 0.3Double Straight Sheathing, Chorded L/b < 2.0 2.5 3.5 0.21. For ratios greater than the maximum listed values, the component is considered not effective in resisting lateral loads.Notes: (a) Acceptance criteria for primary components(∆/∆ y ) IO = 1.0 + 0.2 (d - 1.0)(∆/∆ y ) LS = 1.0 + 0.8 (d - 1.0)(∆/∆ y ) CP = d(b) Acceptance criteria for secondary components(∆/∆ y ) LS = d(∆/∆ y ) CP = e(c) Linear interpolation is permitted for intermediate values if h/L or L/b has asterisks.8-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Table 8-2Normalized Force-Deflection Curve Coordinates for Nonlinear Procedures—WoodComponents (continued)d e cDouble Straight Sheathing, Unchorded L/b < 2.0 2.0 3.0 0.3Single Diagonal Sheathing, Chorded L/b < 2.0 2.5 3.5 0.2Single Diagonal Sheathing, Unchorded L/b < 2.0 2.0 3.0 0.3Straight Sheathing Over Diagonal Sheathing, Chorded L/b < 2.0 3.0 4.0 0.2Straight Sheathing Over Diagonal Sheathing, Unchorded L/b < 2.0 2.5 3.5 0.3Double Diagonal Sheathing, Chorded L/b < 2.0 3.0 4.0 0.2Double Diagonal Sheathing, Unchorded L/b < 2.0 2.5 3.5 0.2Wood Structural Panel, Blocked, Chorded L/b < 3*L/b = 4*Wood Structural Panel, Unblocked, Chorded L/b < 3*L/b = 4*Wood Structural Panel, Blocked, Unchorded L/b < 2.5*L/b = 3.5*Wood Structural Panel, Unblocked, Unchorded L/b < 2.5*L/b = 3.5*Wood Structural Panel Overlay On Sheathing, Chorded L/b < 3*L/b = 4*Wood Structural Panel Overlay On Sheathing, Unchorded L/b < 2.5*L/b = 3.5*Connection TypeNails - Wood to Wood 7.0 8.0 0.2Nails - Metal to Wood 5.5 7.0 0.2Screws - Wood to Wood 2.5 3.0 0.2Screws - Wood to Metal 2.3 2.8 0.2Lag Bolts - Wood to Wood 2.8 3.2 0.2Lag Bolts - Metal to Wood 2.5 3.0 0.2Bolts - Wood to Wood 3.0 3.5 0.2Bolts - Metal to Wood 2.8 3.3 0.21. For ratios greater than the maximum listed values, the component is considered not effective in resisting lateral loads.Notes: (a) Acceptance criteria for primary components(∆/∆ y ) IO = 1.0 + 0.2 (d - 1.0)(∆/∆ y ) LS = 1.0 + 0.8 (d - 1.0)(∆/∆ y ) CP = d(b) Acceptance criteria for secondary components(∆/∆ y ) LS = d(∆/∆ y ) CP = e(c) Linear interpolation is permitted for intermediate values if h/L or L/b has asterisks.4.03.03.02.53.02.52.52.03.02.52.52.05.04.04.03.54.03.53.53.04.03.53.53.00.30.30.30.40.30.48.4.5.2 Strength Acceptance CriteriaDiagonal sheathing has an estimated yield capacity ofapproximately 700 pounds per linear foot for singlelayer and 1300 pounds per linear foot for doublediagonal sheathing. This capacity is dependent on thewidth of the boards, the spacing of the studs, the size ofnails, the number of nails per board, and the boundaryconditions. Allowable capacities are listed for variousconfigurations in WWPA (1983).FEMA 273 Seismic Rehabilitation Guidelines 8-17

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)8.4.5.3 Deformation Acceptance CriteriaThe deformation acceptance criteria will be determinedby the capacity of lateral- and gravity-load-resistingelements to deform without failure. See Table 8-1 form factors for use in the LSP.The coordinates for the normalized force-deflectioncurve used in the nonlinear procedures are shown inTable 8-2. The values in this table refer to Figure 8-1 inthe following way. Distance d is considered themaximum deflection at the point of loss of strength.Distance e is the maximum deflection at a strength orcapacity equal to value c. (See Chapter 3 for the use ofthe force deflection curve in the NSP.)8.4.5.4 ConnectionsSee Sections 8.3.2.2B and 8.4.4.4.8.4.6 Vertical Wood Siding Shear Walls8.4.6.1 Stiffness for AnalysisVertical wood siding has a very low lateral-forceresistancecapacity and is very flexible. These shearwalls are suitable only where earthquake shear loads arevery low and deflection control is not needed. Thedeflection of these shear walls can be determined usingEquation 8-1, with G d = 1,000 lb/in.8.4.6.2 Strength Acceptance CriteriaVertical siding has a yield capacity of approximately 70pounds per linear foot. This capacity is dependent onthe width of the boards, the spacing of the studs, thespacing of blocking, and the size, number, and spacingof the nails. The nail couple method can be used tocalculate the capacity of vertical wood siding, in amanner similar to the method used for horizontal siding.8.4.6.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.6.4 ConnectionsThe load capacity of the vertical siding is low; thismakes the capacity of connections between the shearwall and the other elements of secondary concern (seeSection 8.3.2.2B).8.4.7 Wood Siding over HorizontalSheathing Shear Walls8.4.7.1 Stiffness for AnalysisDouble layer horizontal sheathed shear walls are stifferand stronger than single layer horizontal sheathed shearwalls. These shear walls are often suitable for resistingearthquake shear loads that are low to moderate inmagnitude. They also provide greater stiffness fordeflection control, and thereby greater damage control.The deflection of these shear walls can be determinedusing Equation 8-1, with G d = 4,000 lb/in.8.4.7.2 Strength Acceptance CriteriaWood siding over horizontal sheathing has a yieldcapacity of approximately 500 pounds per linear foot.This capacity is dependent on the width of the boards,the spacing of the studs, the size, number, and spacingof the nails, and the location of joints.8.4.7.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.7.4 ConnectionsSee Sections 8.3.2.2B and 8.4.4.4.8.4.8 Wood Siding over Diagonal SheathingShear Walls8.4.8.1 Stiffness for AnalysisHorizontal wood siding over diagonal sheathing willprovide stiff, strong shear walls. These shear walls areoften suitable for resisting earthquake shear loads thatare moderate in magnitude. They also provide goodstiffness for deflection control and damage control. Thedeflection of these shear walls can be approximated byusing Equation 8-1, with G d = 11,000 lb/in.8.4.8.2 Strength Acceptance CriteriaWood siding over diagonal sheathing has an estimatedyield capacity of approximately 1,100 pounds per linearfoot. This capacity is dependent on the width of theboards, the spacing of the studs, the size, number, andspacing of the nails, the location of joints, and theboundary conditions.8.4.8.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)8.4.8.4 ConnectionsSee Sections 8.3.2.2B and 8.4.4.4.8.4.9 Structural Panel or Plywood PanelSheathing Shear Walls8.4.9.1 Stiffness for AnalysisThe response of wood structural shear walls isdependent on the thickness of the wood structuralpanels, the height-to-length (h/L) ratio, the nailingpattern, and other factors. The approximate deflectionof wood structural shear walls at yield can bedetermined using Equation 8-2:where:∆ y = 8 v y h 3 /(E A b) + v y h/(G t)+ 0.75h e n + (h/b)d a (8-2)v y = Shear at yield in the direction underconsideration in lb/fth = Wall height, ftE = Modulus of wood end boundary member,psiA = Area of boundary member cross section,in. 2b = Wall width, ftG = Modulus of rigidity of plywood, psit = Effective thickness of structural panel, in.d a = Deflection at yield of tie-down anchorageor deflection at load level to anchorage atend of wall, anchorage details, and deadload, in.e n = Nail deformation, in.For 6d nails at yield: e n = .10For 8d nails at yield: e n = .06For 10d nails at yield: e n = .048.4.9.2 Strength Acceptance CriteriaShear capacities of wood structural panel shear wallsare primarily dependent on the nailing at the plywoodpanel edges, and the thickness and grade of theplywood. The yield shear capacity, V y , of woodstructural shear walls can be calculated as follows:V y = .8V u (8-3)Values of ultimate capacity , V u , of structural panelshear walls are provided in Table 8-3.If there is no ultimate load for the assembly, use:where:Q CE = V u = 6.3Zs/a (8-4)Z = Nail value from NDS (1991)s = Minimum [m-1 or (n-1)(a/h)]m = Number of nails along the bottom of one paneln = Number of nails along one side of one panela = Length of one panelh = Height of one panel8.4.9.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.9.4 ConnectionsSee Sections 8.3.2.2B and 8.4.4.4.8.4.10 Stucco on Studs, Sheathing, orFiberboard Shear Walls8.4.10.1 Stiffness for AnalysisStucco is brittle and the lateral-force-resistance capacityof stucco shear walls is low. However, the walls are stiffuntil cracking occurs. These shear walls are suitableonly where earthquake shear loads are low. Thedeflection of these shear walls can be determined usingEquation 8-1 with G d = 14,000 lb/in.8.4.10.2 Strength Acceptance CriteriaStucco has a yield capacity of approximately 350pounds per linear foot. This capacity is dependent onthe attachment of the stucco netting to the studs and theembedment of the netting in the stucco.8.4.10.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.10.4 ConnectionsThe connection between the stucco netting and theframing is of primary concern. Of secondary concern isthe connection of the stucco to the netting. Unlikeplywood, the tensile capacity of the stucco materialFEMA 273 Seismic Rehabilitation Guidelines 8-19

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Table 8-3 Ultimate Capacities of Structural Panel Shear Walls 2, 3, 5, 6MinimumMinimum NailNail Spacing at Panel Edges (in.)Nominal Panel Penetration Nail Size 4 Ultimate Capacities (lb/ft)Panel GradeThickness(inches)in Framing 4(inches)(Common orGalvanized Box) 6" 4" 3" 1 2" 1Structural 1 5/16 1 1/4 6d 700 1010 1130 12003/8 1 1/2 8d 750 1080 1220 15407/16 815 1220 1340 159015/32 880 1380 1550 162015/32 1 5/8 110d 1130 1500 1700 2000C-D, C-CSheathing,plywood panelsiding (and othergrades covered inUBC Standard23-2 or 23-3),structuralparticleboard5/16 1 1/4 6d 650 700 900 12003/8 680 800 1000 13503/8 1 1/2 8d 700 880 1200 15007/16 720 900 1300 156015/32 820 1040 1420 160015/32 1 5/8 110d 900 1400 1500 190019/32 1000 1500 1620 19501. 3x or greater framing at plywood joints.2. Panels applied directly to framing, blocked at all edges.3. Value extrapolated from cyclic testing.4. For other nail sizes or nail penetration less than indicated, adjust values based on calculated nail strength (see AF&PA, 1991).5. Values are for panels on one side. Values may be doubled for panels on both sides.6. Use 80% of values listed for yield capacity.(portland cement) rather than the connections, willoften govern failure. The connections between the shearwall and foundation and between the shear wall anddiaphragm must be investigated. See Section 8.3.2.2B.8.4.11 Gypsum Plaster on Wood Lath ShearWalls8.4.11.1 Stiffness for AnalysisGypsum plaster shear walls are similar to stucco, excepttheir strength is lower. Again, the walls are stiff untilfailure. These shear walls are suitable only whereearthquake shear loads are very low. The deflection ofthese shear walls can be determined using Equation 8-1,with G d = 8,000 lb/in.8.4.11.2 Strength Acceptance CriteriaGypsum plaster has a yield capacity of approximately400 pounds per linear foot.8.4.11.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.11.4 ConnectionsThe tensile and bearing capacity of the plaster, ratherthan the connections, will often govern failure. Therelatively low strength of this material makesconnections between parts of the shear wall assemblyand the other elements of the lateral-force-resistingsystem of secondary concern.8.4.12 Gypsum Plaster on Gypsum LathShear Walls8.4.12.1 Stiffness for AnalysisGypsum plaster on gypsum lath is similar to gypsumwallboard (see Section 8.4.13 for a discussion ofgypsum wallboard). The deflection of these shear wallscan be determined using Equation 8-1, withG d = 10,000 lb/in.8-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)8.4.12.2 Strength Acceptance CriteriaThese are similar to those for gypsum wallboard, withan approximate yield capacity of 80 pounds per linearfoot. See Section 8.4.13.8.4.12.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.12.4 ConnectionsSee Section 8.4.11.4.8.4.13 Gypsum Wallboard Shear Walls8.4.13.1 Stiffness for AnalysisGypsum wallboard has a very low lateral-forceresistancecapacity, but is relatively stiff until crackingoccurs. These shear walls are suitable only whereearthquake shear loads are very low. The deflection ofthese shear walls can be determined using Equation 8-1,with G d = 8,000 lb/in.8.4.13.2 Strength Acceptance CriteriaGypsum wallboard has a yield capacity ofapproximately 100 pounds per linear foot. This capacityis for typical 7-inch nail spacing of 1/2-inch or 5/8-inchthickpanels with 4d or 5d nails. Higher capacities canbe used if closer nail spacing, multilayers of gypsumboard, and/or the presence of blocking at all panel edgesis verified.8.4.13.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.13.4 ConnectionsSee Section 8.4.11.4.8.4.14 Gypsum Sheathing Shear Walls8.4.14.1 Stiffness for AnalysisGypsum sheathing is similar to gypsum wallboard (seeSection 8.4.13 for a detailed discussion). The deflectionof these shear walls can be determined usingEquation 8-1, with G d = 8,000 lb/in.8.4.14.2 Strength Acceptance CriteriaThese are similar to those for gypsum wallboard (seeSection 8.4.13).8.4.14.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.14.4 ConnectionsSee Section 8.4.11.4.8.4.15 Plaster on Metal Lath Shear Walls8.4.15.1 Stiffness for AnalysisPlaster on metal lath is similar to stucco but with lessstrength. Metal lath and plaster walls are stiff untilcracking occurs. These shear walls are suitable onlywhere earthquake shear loads are low. The deflection ofthese shear walls can be determined using Equation 8-1,with G d = 12,000 lb/in.8.4.15.2 Strength Acceptance CriteriaPlaster on metal lath has a yield capacity ofapproximately 150 pounds per linear foot.8.4.15.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.15.4 ConnectionsSee Section 8.3.2.2B.8.4.16 Horizontal Lumber Sheathing withCut-In Braces or Diagonal BlockingShear Walls8.4.16.1 Stiffness for AnalysisThis assembly is similar to horizontal sheathing withoutbraces, except that the cut-in braces or diagonalblocking provide higher stiffness at initial loads. Afterthe braces or blocking fail (at low loads), the behaviorof the wall is the same as with horizontal sheathingwithout braces. See Section 8.4.4 for more informationabout horizontal sheathing.8.4.16.2 Strength Acceptance CriteriaSee Section 8.4.4.8.4.16.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.16.4 ConnectionsSee Section 8.3.2.2B.FEMA 273 Seismic Rehabilitation Guidelines 8-21

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)8.4.17 Fiberboard or ParticleboardSheathing Shear Walls8.4.17.1 Stiffness for AnalysisFiberboard sheathing is very weak, lacks stiffness, andis not able to resist lateral loads. Particleboard comes intwo varieties: one is similar to structural panels, theother (nonstructural) is slightly stronger than gypsumboard but more brittle. Fiberboard sheathing is notsuitable for resisting lateral loads, and nonstructuralparticleboard should only be used to resist very lowearthquake loads. For structural particleboardsheathing, see Section 8.4.9. The deflection of shearwalls sheathed in nonstructural particleboard can bedetermined using Equation 8-1, with G d = 6,000 lb/in.8.4.17.2 Strength Acceptance CriteriaFiberboard has very low strength. For structuralparticleboard, see the structural panel section(Section 8.4.9). Nonstructural particleboard has a yieldcapacity of approximately 100 pounds per linear foot.8.4.17.3 Deformation Acceptance CriteriaSee Section 8.4.5.3.8.4.17.4 ConnectionsSee Section 8.4.11.4.8.4.18 Light Gage Metal Frame Shear Walls8.4.18.1 Plaster on Metal LathSee Section 8.4.15.8.4.18.2 Gypsum WallboardSee Section 8.4.13.8.4.18.3 Plywood or Structural PanelsSee Section 8.4.9. Refer to fastener manufacturer’s datafor allowable loads on fasteners. Yield capacity can beestimated by multiplying normal allowable load valuesfor 2.8, or for allowable load values that are listed forwind or seismic loads, multiply by 2.1 to obtainestimated yield values.8.5 Wood DiaphragmsThe behavior of horizontal wood diaphragms isinfluenced by the type of sheathing, size, and amount offasteners, presence of perimeter chord or flangemembers, and the ratio of span to depth of thediaphragm. Openings or penetrations through thediaphragm also effect the behavior and capacity of thediaphragm (see Section 8.5.11).The expected capacity of the diaphragm, Q CE , isdetermined from the yield shear capacity of the existingor enhanced diaphragm as described in Sections 8.5.2through 8.5.9. For braced or horizontal truss typesystems, the expected capacity, Q CE , is determinedfrom the member or connection yield capacity andconventional static truss analysis, as described inSection 8.5.10.8.5.1 Types of Wood Diaphragms8.5.1.1 Existing Wood DiaphragmsA. Single Straight Sheathed DiaphragmsTypically, these consist of 1" x sheathing laidperpendicular to the framing members; 2" x or 3" xsheathing may also be present. The sheathing serves thedual purpose of supporting gravity loads and resistingshear forces in the diaphragm. Most often, 1" xsheathing is nailed with 8d or 10d nails, with two ormore nails at each sheathing board. Shear forcesperpendicular to the direction of the sheathing areresisted by the nail couple. Shear forces parallel to thedirection of the sheathing are transferred through thenails in the supporting joists or framing members belowthe sheathing joints.B. Double Straight Sheathed DiaphragmsConstruction is the same as that for single straightsheathed diaphragms, except that an upper layer ofstraight sheathing is laid over the lower layer ofsheathing. The upper sheathing can be placed eitherperpendicular or parallel to the lower layer of sheathing.If the upper layer of sheathing is parallel to the lowerlayer, the board joints are usually offset sufficiently thatnails at joints in the upper layer of sheathing are driveninto a common sheathing board below, with sufficientedge distance. The upper layer of sheathing is nailed tothe framing members through the lower layer ofsheathing.C. Single Diagonally Sheathed Wood DiaphragmsTypically, 1" x sheathing is laid at an approximate 45-degree angle to the framing members. In some cases2" x sheathing may also be used. The sheathingsupports gravity loads and resists shear forces in thediaphragm. Commonly, 1" x sheathing is nailed with 8d8-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)nails, with two or more nails per board. Therecommended nailing for diagonally sheatheddiaphragms is published in the Western Woods UseBook (WWPA, 1983) and UBC (ICBO, 1994a). Theshear capacity of the diaphragm is dependent on the sizeand quantity of the nails at each sheathing board.D. Diagonal Sheathing with Straight Sheathing orFlooring AboveTypically, these consist of a lower layer of 1" x diagonalsheathing laid at a 45-degree angle to the framingmembers, with a second layer of straight sheathing orwood flooring laid on top of the diagonal sheathing at a90-degree angle to the framing members. Both layers ofsheathing support gravity loads, and resist shear forcesin the diaphragm. Sheathing boards are commonlynailed with 8d nails, with two or more nails per board.E. Double Diagonally Sheathed Wood DiaphragmsTypically, these consist of a lower layer of 1" x diagonalsheathing with a second layer of 1" x diagonalsheathing laid at a 90-degree angle to the lower layer.The sheathing supports gravity loads and resists shearforces in the diaphragm. The sheathing is commonlynailed with 8d nails, with two or more nails per board.The recommended nailing for double diagonallysheathed diaphragms is published in the WWPA (1983).F. Wood Structural Panel Sheathed DiaphragmsTypically, these consist of wood structural panels, suchas plywood or oriented strand board, placed on framingmembers and nailed in place. Different grades andthicknesses of wood structural panels are commonlyused, depending on requirements for gravity loadsupport and shear capacity. Edges at the ends of thewood structural panels are usually supported by theframing members. Edges at the sides of the panels canbe blocked or unblocked. In some cases, tongue andgroove wood structural panels are used. Nailingpatterns and nail size can vary greatly. Nail spacing iscommonly in the range of 3 to 6 inches on center at thesupported and blocked edges of the panels, and 10 to 12inches on center at the panel infield. Staples aresometimes used to attach the wood structural panels.G. Braced Horizontal DiaphragmsTypically, these consist of “X” rod bracing and woodstruts forming a horizontal truss system at the floor orroof levels of the building. The “X” bracing usuallyconsists of steel rods drawn taut by turnbuckles or nuts.The struts usually consist of wood members, which mayor may not be part of the gravity-load-bearing system ofthe floor or roof. The steel rods function as tensionmembers in the horizontal truss, while the strutsfunction as compression members. Truss chords(similar to diaphragm chords) are needed to resistbending in the horizontal truss system.8.5.1.2 Wood Diaphragms Enhanced forRehabilitationA. Wood Structural Panel Overlays on Straight orDiagonally Sheathed DiaphragmsDiaphragm shear capacity and stiffness can beincreased by overlaying new wood structural panelsover existing sheathed diaphragms. These diaphragmstypically consist of new wood structural panels placedover existing straight or diagonal sheathing and nailedor stapled to the existing framing members through theexisting sheathing. If the new overlay is nailed only tothe existing framing members—without nailing at thepanel edges perpendicular to the framing—the responseof the new overlay will be similar to that of anunblocked wood structural panel diaphragm. Nails andstaples should be of sufficient length to provide therequired embedment into framing members below thesheathing.If a stronger and stiffer diaphragm is desired, the jointsof the new wood structural panel overlay can be placedparallel to the joints of the existing sheathing, with theoverlay nailed or stapled to the existing sheathing. Theedges of the new wood structural panels should beoffset from the joints in the existing sheathing below bya sufficient distance that the new nails may be driveninto the existing sheathing without splitting thesheathing. If the new panels are nailed at all edges asdescribed above, the response of the new overlay willbe similar to that of a blocked wood structural paneldiaphragm. As an alternative, new blocking may beinstalled below all panel joints perpendicular to theexisting framing members.Because the joints of the overlay and the joints of theexisting sheathing may not be offset consistentlywithout cutting the panels, it may be advantageous toplace the wood structural panel overlay at a 45-degreeangle to the existing sheathing. If the existingdiaphragm is straight sheathed, the new overlay shouldbe placed at a 45-degree angle to the existing sheathingand joists. If the existing diaphragm is diagonallysheathed, the new wood structural panel overlay shouldbe placed perpendicular to the existing joists at a 45-FEMA 273 Seismic Rehabilitation Guidelines 8-23

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)degree angle to the diagonal sheathing. Nails should bedriven into the existing sheathing with sufficient edgedistance to prevent splitting of the existing sheathing.At boundaries, nails should be of sufficient length topenetrate through the sheathing into the framing below.New structural panel overlays shall be connected to theshear wall or vertical bracing elements to ensure theeffectiveness of the added panel.Care should be exercised when placing new woodstructural panel overlays on existing diaphragms. Thechanges in stiffness and dynamic characteristics of thediaphragm may have negative effects by causingincreased forces in other components or elements. Theincreased stiffness and the associated increase indynamic forces may not be desirable in somediaphragms for certain Performance Levels.B. Wood Structural Panel Overlays on Existing WoodStructural Panel DiaphragmsNew wood structural panel overlays may be placed overexisting wood structural panel diaphragms to strengthenand stiffen existing diaphragms. The placement of anew overlay over an existing diaphragm should followthe same construction methods and procedures as forstraight and diagonally sheathed diaphragms (seeSection 8.5.1.2A). Panel joints should be offset, or elsethe overlay should be placed at a 45-degree angle to theexisting wood structural panels.C. Increased AttachmentIn some cases, existing diaphragms may be enhancedby increasing the nailing or attachment of the existingsheathing to the supporting framing. For straightsheathed diaphragms, the increase in shear capacity willbe minimal. Double straight sheathed diaphragms withminimal nailing in the upper or both layers of sheathingmay be enhanced significantly by adding new nails orstaples to the existing diaphragm. The same is true fordiaphragms that are single diagonally sheathed, doublediagonally sheathed, or single diagonally sheathed withstraight sheathing or flooring.Plywood diaphragms can also be enhanced by increasednailing or attachment to the supporting framing and byadding blocking to the diaphragm at the plywood joints.In some cases, increased nailing at the plywood panelinfield may also be required. If the required shearcapacity and/or stiffness is greater than that which canbe provided by increased attachment, a new overlay onthe existing diaphragm may be required to provide thedesired enhancement.8.5.1.3 New Wood DiaphragmsA. Wood Structural Panel Sheathed DiaphragmsTypically, these consist of wood structural panels—suchas plywood or oriented strand board—placed, nailed, orstapled in place on existing framing members afterexisting sheathing has been removed. Different gradesand thicknesses of wood structural panels can be used,depending on the requirements for gravity load supportand diaphragm shear capacity. In most cases, the panelsare placed with the long dimension perpendicular to theframing members, and panel edges at the ends of thepanels are supported by, and nailed to, the framingmembers. Edges at the sides of the panels can beblocked or unblocked, depending on the shear capacityand stiffness required in the new diaphragm. Woodstructural panels can be placed in various patterns asshown in APA publications (APA, 1983) and variouscodes (e.g., ICBO, 1994a).B. Single Diagonally Sheathed Wood DiaphragmsSee Section 8.5.1.1C.C. Double Diagonally Sheathed Wood DiaphragmsSee Section 8.5.1.1E.D. Braced Horizontal DiaphragmsSee Section 8.5.1.1G. Because the special horizontalframing in the truss is an added structural feature, it isusually more economical to design floor or roofsheathing as a diaphragm in new construction, whicheliminates the need for the “X” bracing and strongerwood members at the compression struts. Bracedhorizontal diaphragms are more feasible wheresheathing cannot provide sufficient shear capacity, orwhere diaphragm openings reduce the shear capacity ofthe diaphragm and additional shear capacity is needed.8.5.2 Single Straight Sheathed Diaphragms8.5.2.1 Stiffness for AnalysisStraight sheathed diaphragms are characterized by highflexibility with a long period of vibration. Thesediaphragms are suitable for low shear conditions wherecontrol of diaphragm deflections is not needed to attainthe desired Performance Levels. The deflection ofstraight sheathed diaphragms can be approximatedusing Equation 8-5:∆ = v L 4 / (G d b 3 ) (8-5)8-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)where:bG dLv∆= Diaphragm width, ft= Diaphragm shear stiffness, lb/in.= Diaphragm span, ft between shear walls orcollectors= Maximum shear in the direction underconsideration, lb/ft= Calculated diaphragm deflection, in.For straight sheathed diaphragms with or withoutchords, G d = approximately 200,000 lb/in.8.5.2.2 Strength Acceptance CriteriaStraight sheathed diaphragms have a low yield capacityof approximately 120 pounds per foot for chorded andunchorded diaphragms. The yield capacity for straightsheathed diaphragms is dependent on the size, number,and spacing between the nails at each sheathing board,and the spacing of the supporting framing members.The shear capacity of straight sheathed diaphragms canbe calculated using the nail-couple method. See ATC(1981) for a discussion of calculating the shear capacityof straight sheathed diaphragms.8.5.2.3 Deformation Acceptance CriteriaDeformation acceptance criteria will largely depend onthe allowable deformations for other structural andnonstructural components and elements that arelaterally supported by the diaphragm. Allowabledeformations must also be consistent with thepermissible damage state of the diaphragm. SeeTable 8-1 for m factors for use in Equation 3-18 for theLSP.The coordinates for the normalized force-deflectioncurve for use in nonlinear procedures are shown inTable 8-2. The values in this table refer to Figure 8-1 inthe following way. Distance d (see Figure 8-1) isconsidered the maximum deflection the diaphragm canundergo and still maintain its yield strength. Distance eis the maximum deflection at a reduced strength c.8.5.2.4 ConnectionsThe load capacity of connections between diaphragmsand shear walls or other vertical elements, as well asdiaphragm chords and shear collectors, is veryimportant. These connections should have sufficientload capacity and ductility to deliver the required forceto the vertical elements without sudden brittle failure ina connection or series of connections.8.5.3 Double Straight Sheathed WoodDiaphragms8.5.3.1 Stiffness for AnalysisThe double sheathed system will provide a significantincrease in stiffness over a single straight sheatheddiaphragm, but very little test data is available on thestiffness and strength of these diaphragms. It isimportant that both layers of straight sheathing havesufficient nailing, and that the joints of the top layer areeither offset or perpendicular to the bottom layer. Theapproximate deflection of double straight sheatheddiaphragms can be calculated using Equation 8-5, withG d as follows:Double straight sheathing,chorded:Double straight sheathing,unchorded:8.5.3.2 Strength Acceptance CriteriaTypical yield shear capacity of double straight sheatheddiaphragms is approximately 600 pounds per foot forchorded diaphragms. For unchorded diaphragms, thetypical yield capacity is approximately 400 pounds perfoot. The strength and stiffness of double straightsheathed diaphragms is highly dependent on the nailingof the upper layer of sheathing. If the upper layer hasminimal nailing, the increase in strength and stiffnessover a single straight sheathed diaphragm may be slight.If the upper layer of sheathing has nailing similar to thatof the lower layer of sheathing, the increase in strengthand stiffness will be significant.8.5.3.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.3.4 ConnectionsSee Section 8.5.2.4.8.5.4 Single Diagonally Sheathed WoodDiaphragms8.5.4.1 Stiffness for AnalysisG d = 1,500,000 lb/in.G d = 700,000 lb/in.Single diagonally sheathed diaphragms are significantlystiffer than straight sheathed diaphragms, but are stillFEMA 273 Seismic Rehabilitation Guidelines 8-25

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)quite flexible. The deflection of single diagonallysheathed diaphragms can be calculated usingEquation 8-5, with G d as follows:Single diagonalsheathing, chorded:Single diagonal sheathing,unchorded:8.5.4.2 Strength Acceptance CriteriaDiagonally sheathed diaphragms are usually capable ofresisting moderate shear loads. Typical yield shearcapacity for diagonally sheathed wood diaphragms withchords is approximately 600 pounds per foot. Typicalyield capacity for unchorded diaphragms isapproximately 70% of the value for chordeddiaphragms, or 420 pounds per foot. The shear capacityof diagonally sheathed diaphragms can be calculatedbased on the shear capacity of the nails in each of thesheathing boards. Because the diagonal sheathingboards function in tension and compression to resistshear forces in the diaphragm, and the boards are placedat a 45-degree angle to the chords at the ends of thediaphragm, the component of the force in the sheathingboards that is perpendicular to the axis of the end chordswill create a bending force in the end chords. If theshear in diagonally sheathed diaphragms is limited toapproximately 300 pounds per foot or less, bendingforces in the end chords is usually neglected. If shearforces exceed 300 pounds per foot, the end chordsshould be designed or reinforced to resist bendingforces from the sheathing. See ATC (1981) for methodsof calculating the shear capacity of diagonally sheatheddiaphragms.8.5.4.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.4.4 ConnectionsSee Section 8.5.2.4.8.5.5 Diagonal Sheathing with StraightSheathing or Flooring Above WoodDiaphragms8.5.5.1 Stiffness for AnalysisG d = 500,000 lb/in.G d = 400,000 lb/in.Straight sheathing or flooring over diagonal sheathingwill provide a significant increase in stiffness oversingle sheathed diaphragms. The approximatedeflection of diagonally sheathed diaphragms withstraight sheathing or flooring above can be calculatedusing Equation 8-5, with G d as follows:Diagonal sheathing withstraight sheathing, chorded:Diagonal sheathing withstraight sheathing,unchorded:The increased stiffness of these diaphragms may makethem suitable where Life Safety or ImmediateOccupancy Performance Levels are desired.8.5.5.2 Strength Acceptance CriteriaShear capacity is dependent on the nailing of thediaphragm. Typical yield capacity for these diaphragmsis approximately 900 pounds per foot for chordeddiaphragms and approximately 625 pounds per foot forunchorded diaphragms. The strength and stiffness ofdiagonally sheathed diaphragms with straight sheathingabove is highly dependent on the nailing of both layersof sheathing. Both layers of sheathing should have atleast two 8d common nails at each support.8.5.5.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.5.4 ConnectionsSee Section 8.5.2.4.8.5.6 Double Diagonally Sheathed WoodDiaphragms8.5.6.1 Stiffness for AnalysisG d = 1,800,000 lb/in.G d = 900,000 lb/in.Double diagonally sheathed diaphragms have greaterstiffness than diaphragms with single diagonalsheathing. The response of these diaphragms is similarto the response of diagonally sheathed diaphragms withstraight sheathing overlays. The approximate deflectionof double diagonally sheathed diaphragms can becalculated using Equation 8-5, with G d as follows:Double diagonal sheathing,chorded:Double diagonal sheathing,unchorded:G d = 1,800,000 lb/in.G d = 900,000 lb/in.8-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)The increased stiffness of these diaphragms may makethem suitable where Life Safety or ImmediateOccupancy Performance Levels are desired.8.5.6.2 Strength Acceptance CriteriaShear capacity is dependent on the nailing of thediaphragm, but these diaphragms are usually suitablefor moderate to high shear loads, and have a typicalyield capacity of approximately 900 pounds per foot forchorded diaphragms and 625 pounds per foot forunchorded diaphragms. Yield shear capacities aresimilar to those of diagonally sheathed diaphragms withstraight sheathing overlays. The sheathing boards inboth layers of sheathing should be nailed with at leasttwo 8d common nails. The presence of a double layer ofdiagonal sheathing will eliminate the bending forcesthat single diagonally sheathed diaphragms impose onthe chords at the ends of the diaphragm. As a result, thebending capacity of the end chords does not have aneffect on the shear capacity and stiffness of thediaphragm.8.5.6.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.6.4 ConnectionsSee Section 8.5.2.4.8.5.7 Wood Structural Panel SheathedDiaphragms8.5.7.1 Stiffness for AnalysisThe response of wood structural panel sheatheddiaphragms is dependent on the thickness of the woodstructural panels, nailing pattern, and presence ofchords in the diaphragm, as well as other factors. Thedeflection of blocked and chorded wood structuralpanel diaphragms with constant nailing across thediaphragm length can be determined usingEquation 8-6:where:∆ y = 5 v y L 3 / (8EAb) + v y L / (4Gt)+ 0.188 L e n + Σ(∆ c X) / (2b) (8-6)A = Area of chord cross section, in. 2b = Diaphragm width, ftE = Modulus of elasticity of diaphragmchords, psie n = Nail deformation at yield load per nail,based on maximum shear per foot v ydivided by the number of nails per footFor 8d nails, e n = .06For 10d nails, e n = .04G = Modulus of rigidity of wood structuralpanel, psiL = Diaphragm span between shear walls orcollectors, ftt = Effective thickness of plywood forshear, in.v y = Yield shear in the direction underconsideration, lb/ft∆ y = Calculated diaphragm deflection atyield, in.Σ (∆ cX) = Sum of individual chord-splice slipvalues on both sides of the diaphragm,each multiplied by its distance to thenearest supportThe deflection of blocked and chorded wood structuralpanel diaphragms with variable nailing across thediaphragm length can be determined usingEquation 8-7:∆ y = 5 v y L 3 / (8EAb) + v y L / (4Gt) +0.376 L e n + Σ(∆ c X) / (2b) (8-7)The deflection for unblocked diaphragms may becalculated using Equation 8-5, with diaphragm shearstiffness G d as follows:Unblocked, chordeddiaphragms:Unblocked, unchordeddiaphragms:G d = 800,000 lb/in.G d = 400,000 lb/in.8.5.7.2 Strength Acceptance CriteriaShear capacities of wood structural panel diaphragmsare primarily dependent on the nailing at the plywoodpanel edges, and the thickness and grade of the plywoodin the diaphragm. The yield shear capacity, V y , of woodFEMA 273 Seismic Rehabilitation Guidelines 8-27

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)structural panel diaphragms with chords can becalculated as follows:If test data are available: V y = 0.8 x ultimatediaphragm shear value V uIf test data are not V y = 2.1 x allowableavailable:diaphragm shear value (seeICBO, [1994a] and APA[1983])For unchorded diaphragms, multiply the yield shearcapacity for chorded diaphragms calculated above by70%. Unchorded diaphragms with L/b > 3.5 are notconsidered to be effective for resisting lateral forces.8.5.7.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.7.4 ConnectionsSee Section 8.5.2.4.8.5.8 Wood Structural Panel Overlays onStraight or Diagonally SheathedDiaphragms8.5.8.1 Stiffness for AnalysisThe stiffness of existing straight sheathed diaphragmscan be increased significantly by placing a newplywood overlay over the existing diaphragm. Thestiffness of existing diagonally sheathed diaphragmsand plywood diaphragms will be increased, but not inproportion to the stiffness increase for straight sheatheddiaphragms. Placement of the new wood structuralpanel overlay should be consistent withSection 8.5.1.2A. Depending on the nailing of the newoverlay, the response of the diaphragm may be similarto that of a blocked or an unblocked diaphragm. Theapproximate deflection of wood structural paneloverlays on straight or diagonally sheathed diaphragmscan be calculated using Equation 8-5, with G d asfollows:Unblocked, chordeddiaphragm:Unblocked, unchordeddiaphragm:G d = 900,000 lb/in.G d = 500,000 lb/in.Blocked, chorded diaphragm: G d = 1,800,000 lb/in.Blocked, unchordeddiaphragm:G d = 700,000 lb/in.The increased stiffness of these diaphragms may makethem suitable where Life Safety or ImmediateOccupancy Performance Levels are desired.8.5.8.2 Strength Acceptance CriteriaTypical yield capacity for diaphragms with a plywoodoverlay over existing straight and diagonal sheathing isapproximately 450 pounds per foot for unblockedchorded diaphragms and approximately 300 pounds perfoot for unblocked unchorded diaphragms. The yieldcapacity of blocked and chorded wood structural paneloverlays over existing sheathing is approximately 65%of the ultimate shear capacity, or 2 times the allowableshear capacity of a comparable wood structural paneldiaphragm without the existing sheathing below. Theyield capacity of blocked and unchorded woodstructural panel overlays over existing sheathing isapproximately 50% of the ultimate shear capacity, or1.5 times the allowable shear capacity of a comparablewood structural panel diaphragm without the existingsheathing below.8.5.8.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.8.4 ConnectionsSee Section 8.5.2.4.8.5.9 Wood Structural Panel Overlays onExisting Wood Structural PanelDiaphragms8.5.9.1 Stiffness for AnalysisDiaphragm deflection shall be calculated according toEquation 8-6. Since two layers of plywood are present,the effective thickness of plywood t will be based ontwo layers of plywood. The nail slip e n portion of theequation shall be adjusted for the increased nailing withtwo layers of plywood. Nail slip in the outer layer ofplywood shall be increased 25% to account for theincreased slip. It is important that nails in the upperlayer of plywood have sufficient embedment in theframing to resist the required force and limit slip to therequired level.8-28 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)8.5.9.2 Strength Acceptance CriteriaYield shear capacity for chorded diaphragms shall becalculated based on the combined two layers ofplywood, using the methodology in Section 8.5.7.2. Theyield shear capacity of the overlay should be limited to75% of the values calculated using these procedures.8.5.9.3 Deformation Acceptance CriteriaSee Section 8.5.2.3.8.5.9.4 ConnectionsSee Section 8.5.2.4.8.5.10 Braced Horizontal Diaphragms8.5.10.1 Stiffness for AnalysisThe stiffness and deflection of braced horizontaldiaphragms can be determined using typical analysistechniques for trusses.8.5.10.2 Strength Acceptance CriteriaThe strength of the horizontal truss system can bedetermined using typical analysis techniques fortrusses, and is dependent on the strength of theindividual components and connections in the trusssystem. In many cases the capacity of the connectionsbetween truss components will be the limiting factor inthe strength of the horizontal truss system.8.5.10.3 Deformation Acceptance CriteriaDeformation acceptance criteria will largely depend onthe allowable deformations for other structural andnonstructural components and elements that arelaterally supported by the diaphragm. Allowabledeformations must also be consistent with the desireddamage state of the diaphragm. The individual mfactors for the components and connections in thehorizontal truss system listed in Table 8-1 will need tobe applied in the truss analysis for the desiredPerformance Level.8.5.10.4 ConnectionsThe load capacity of connections between the membersof the horizontal truss and shear walls or other verticalelements is very important. These connections musthave sufficient load capacity and ductility to deliver therequired force to the vertical elements without suddenbrittle failure in a connection or series of connections.See Section 8.3.2.2B.8.5.11 Effects of Chords and Openings inWood DiaphragmsThe presence of any but small openings in wooddiaphragms will cause a reduction in the stiffness andyield capacity of the diaphragm, due to a reduced lengthof diaphragm available to resist lateral forces. Specialanalysis techniques and detailing are required at theopenings. The presence or addition of chord membersaround the openings will reduce the loss in stiffness ofthe diaphragm and limit damage in the area of theopenings. See ATC (1981) and APA (1983) for adiscussion of the effects of openings in wooddiaphragms.The presence of chords at the perimeter of a diaphragmwill significantly reduce the diaphragm deflection dueto bending, and increase the stiffness of the diaphragmover that of an unchorded diaphragm. However, theincrease in stiffness due to chords in a single straightsheathed diaphragm is minimal, due to the flexiblenature of these diaphragms.8.6 Wood Foundations8.6.1 Wood PilingWood piles are generally used with a concrete pile cap,and simply key into the base of the concrete cap. Thepiles are usually treated with preservatives; they shouldbe checked to determine whether deterioration hasoccurred and to verify the type of treatment. Piles areeither friction- or end-bearing piles resisting onlyvertical loads. Piles are generally not able to resist upliftloads because of the manner in which they are attachedto the pile cap. The piles may be subjected to lateralloads from seismic loading, which are resisted bybending of the piles. The analysis of pile bending isgenerally based on a pinned connection at the top of thepile, and fixity of the pile at some depth established bythe geotechnical engineer. Maximum stress in the pilesshould be based on 2.8 time the values given in theNational Design Specification for Wood Construction,Part VI (AF&PA, 1991a). Deflection of piles underseismic load can be calculated based on the assumedpoint of fixity. However, it should be evaluated withconsideration for the approximate nature of the originalassumption of the depth to point of fixity. Wherebattered piles are present, the lateral loads can beresisted by the horizontal component of the axial load.FEMA 273 Seismic Rehabilitation Guidelines 8-29

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)For Immediate Occupancy, an m factor of 1.25 shouldbe used in the analysis of the pile bending or axial force;for Life Safety, a factor of 2.5 can be used; and a factorof 3.0 can be used for Collapse Prevention.8.6.2 Wood FootingsWood grillage footings, sleepers, skids, and pressuretreatedall-wood foundations are sometime encounteredin existing structures. These footings should bethoroughly inspected for indications of deterioration,and replaced with reinforced concrete footings wherepossible. The seismic resistance for these types offootings is generally very low; they are essentiallydependent on friction between the wood and soil fortheir performance.8.6.3 Pole StructuresPole structures resist lateral loads by acting ascantilevers fixed in the ground, with the lateral loadconsidered to be applied perpendicular to the pole axis.It is possible to design pole structures to have momentresistingcapacity at floor and roof levels by the use ofknee braces or trusses. Pole structures are frequentlyfound on sloping sites. The varying unbraced lengths ofthe poles generally affect the stiffness and performanceof the structure, and can result in unbalanced loads tothe various poles along with significant torsionaldistortion, which must be investigated and evaluated.Added horizontal and diagonal braces can be used toreduce the flexibility of tall poles or reduce the torsionaleccentricity of the structure.8.6.3.1 Materials and Component PropertiesThe strength of the components, elements, andconnections of a pole structure are the same as for aconventional structure. See Section 8.3 forrecommendations.8.6.3.2 Deformation Acceptance CriteriaDeformation characteristics are the same as for amember or frame that is subject to combined flexuraland axial loads; pole structures are analyzed usingconventional procedures.8.6.3.3 Factors for the Linear StaticProcedureIt is recommended that an m factor of 1.2 be used for acantilevered pole structure for the ImmediateOccupancy Performance Level, a value of 3.0 for LifeSafety, and 3.5 for Collapse Prevention. Whereconcentrically braced diagonals are used or added toenhance the capacity of the structure, an m factor of 1.0should be used for Immediate Occupancy, a value of 2.5for Life Safety, and 3.0 for Collapse Prevention.8.7 DefinitionsAssembly: A collection of structural members and/orcomponents connected in such a manner that loadapplied to any one component will affect the stressconditions of adjacent parallel components.Aspect ratio: Ratio of height to width for verticaldiaphragms, and width to depth for horizontaldiaphragms.Balloon framing: Continuous stud framing from sillto roof, with intervening floor joists nailed to studs andsupported by a let-in ribbon. (See platform framing.)Boundary component (boundary member): Amember at the perimeter (edge or opening) of a shearwall or horizontal diaphragm that provides tensile and/or compressive strength.Composite panel: A structural panel comprising thinwood strands or wafers bonded together with exterioradhesive.Chord:Collector:See diaphragm chord.See drag strut.Condition of service: The environment to which thestructure will be subjected. Moisture conditions are themost significant issue; however, temperature can have asignificant effect on some assemblies.Connection: A link between components or elementsthat transmits actions from one component or elementto another component or element. Categorized by typeof action (moment, shear, or axial), connection links arefrequently nonductile.Cripple wall: Short wall between foundation andfirst floor framing.Cripple studs: Short studs between header and topplate at opening in wall framing or studs between basesill and sill of opening.8-30 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)Decay: Decomposition of wood caused by action ofwood-destroying fungi. The term “dry rot” is usedinterchangeably with decay.Decking: Solid sawn lumber or glued laminateddecking, nominally two to four inches thick and fourinches and wider. Decking may be tongue-and-grooveor connected at longitudinal joints with nails or metalclips.Design resistance: Resistance (force or moment asappropriate) provided by member or connection; theproduct of adjusted resistance, the resistance factor,confidence factor, and time effect factor.Diaphragm: A horizontal (or nearly horizontal)structural element used to distribute inertial lateralforces to vertical elements of the lateral-force-resistingsystem.Diaphragm chord: A diaphragm componentprovided to resist tension or compression at the edges ofthe diaphragm.Diaphragm ratio:Diaphragm strut:See aspect ratio.See drag strut.Dimensioned lumber: Lumber from nominal twothrough four inches thick and nominal two or moreinches wide.Dowel bearing strength: The maximumcompression strength of wood or wood-based productswhen subjected to bearing by a steel dowel or bolt ofspecific diameter.Dowel type fasteners: Includes bolts, lag screws,wood screws, nails, and spikes.Drag strut: A component parallel to the applied loadthat collects and transfers diaphragm shear forces to thevertical lateral-force-resisting components or elements,or distributes forces within a diaphragm. Also calledcollector, diaphragm strut, or tie.Dressed size: The dimensions of lumber aftersurfacing with a planing machine. Usually 1/2 to 3/4inch less than nominal size.Dry service: Structures wherein the maximumequilibrium moisture content does not exceed 19%.Edge distance: The distance from the edge of themember to the center of the nearest fastener. When amember is loaded perpendicular to the grain, the loadededge shall be defined as the edge in the direction towardwhich the fastener is acting.Gauge or row spacing: The center-to-center distancebetween fastener rows or gauge lines.Glulam beam:beam.Shortened term for glued-laminatedGrade: The classification of lumber in regard tostrength and utility, in accordance with the grading rulesof an approved agency.Grading rules: Systematic and standardized criteriafor rating the quality of wood products.Gypsum wallboard or drywall: An interior wallsurface sheathing material sometimes considered forresisting lateral forces.Hold-down: Hardware used to anchor the verticalchord forces to the foundation or framing of thestructure in order to resist overturning of the wall.King stud: Full height stud or studs adjacent toopenings that provide out-of-plane stability to cripplestuds at openings.Light framing: Repetitive framing with smalluniformly spaced members.Load duration: The period of continuous applicationof a given load, or the cumulative period of intermittentapplications of load. (See time effect factor.)Load/slip constant: The ratio of the applied load to aconnection and the resulting lateral deformation of theconnection in the direction of the applied load.Load sharing: The load redistribution mechanismamong parallel components constrained to deflecttogether.LRFD (Load and Resistance Factor Design): Amethod of proportioning structural components(members, connectors, connecting elements, andassemblages) using load and resistance factors such thatno applicable limit state is exceeded when the structureFEMA 273 Seismic Rehabilitation Guidelines 8-31

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)is subjected to all design load and resistance factorcombinations.Lumber: The product of the sawmill and planingmill, usually not further manufactured other than bysawing, resawing, passing lengthwise through astandard planing machine, crosscutting to length, andmatching.Lumber size: Lumber is typically referred to by sizeclassifications. Additionally, lumber is specified bymanufacturing classification. Rough lumber anddressed lumber are two of the routinely usedmanufacturing classifications.Mat-formed panel: A structural panel designationrepresenting panels manufactured in a mat-formedprocess, such as oriented strand board and waferboard.Moisture content: The weight of the water in woodexpressed as a percentage of the weight of the ovendriedwood.Nominal size: The approximate rough-sawncommercial size by which lumber products are knownand sold in the market. Actual rough-sawn sizes varyfrom the nominal. Reference to standards or grade rulesis required to determine nominal to actual finished sizerelationships, which have changed over time.Oriented strandboard: A structural panelcomprising thin elongated wood strands with surfacelayers arranged in the long panel direction and corelayers arranged in the cross panel direction.Panel:A sheet-type wood product.Panel rigidity or stiffness: The in-plane shearrigidity of a panel, the product of panel thickness andmodulus of rigidity.Panel shear:thickness.Shear stress acting through the panelParticleboard: A panel manufactured from smallpieces of wood, hemp, and flax, bonded with syntheticor organic binders, and pressed into flat sheets.Pile: A deep structural component transferring theweight of a building to the foundation soils and resistingvertical and lateral loads; constructed of concrete, steel,or wood; usually driven into soft or loose soils.Pitch or spacing: The longitudinal center-to-centerdistance between any two consecutive holes or fastenersin a row.Planar shear: The shear that occurs in a planeparallel to the surface of a panel, which has the abilityto cause the panel to fail along the plies in a plywoodpanel or in a random layer in a nonveneer or compositepanel.Platform framing: Construction method in whichstud walls are constructed one floor at a time, with afloor or roof joist bearing on top of the wall framing ateach level.Ply: A single sheet of veneer, or several strips laidwith adjoining edges that form one veneer lamina in aglued plywood panel.Plywood: A structural panel comprising plies ofwood veneer arranged in cross-aligned layers. The pliesare bonded with an adhesive that cures upon applicationof heat and pressure.Pole: A round timber of any size or length, usuallyused with the larger end in the ground.Pole structure: A structure framed with generallyround continuous poles that provide the primary verticalframe and lateral-load-resisting system.Preservative: A chemical that, when suitably appliedto wood, makes the wood resistant to attack by fungi,insects, marine borers, or weather conditions.Pressure-preservative treated wood: Woodproducts pressure-treated by an approved process andpreservative.Primary (strong) panel axis: The direction thatcoincides with the length of the panel.Punched metal plate: A light steel plate fasteninghaving punched teeth of various shapes andconfigurations that are pressed into wood members toeffect transfer shear. Used with structural lumberassemblies.Required member resistance: Load effect (force,moment, stress, action as appropriate) acting on anelement or connection, determined by structural8-32 Seismic Rehabilitation Guidelines FEMA 273

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)analysis from the factored loads and the critical loadcombinations.Resistance: The capacity of a structure, component,or connection to resist the effects of loads. It isdetermined by computations using specified materialstrengths, dimensions, and formulas derived fromaccepted principles of structural mechanics, or by fieldor laboratory tests of scaled models, allowing formodeling effects and differences between laboratoryand field conditions.Resistance factor: A reduction factor applied tomember resistance that accounts for unavoidabledeviations of the actual strength from the nominalvalue, and the manner and consequences of failure.Row of fasteners: Two or more fasteners alignedwith the direction of load.Rough lumber: Lumber as it comes from the sawprior to any dressing operation.Subdiaphragm: A portion of a larger diaphragmused to distribute loads between members.Seasoned lumber: Lumber that has been dried.Seasoning takes place by open-air drying within thelimits of moisture contents attainable by this method, orby controlled air drying (i.e., kiln drying).Sheathing: Lumber or panel products that areattached to parallel framing members, typically formingwall, floor, ceiling, or roof surfaces.Shrinkage: Reduction in the dimensions of wood dueto a decrease of moisture content.Structural-use panel: A wood-based panel productbonded with an exterior adhesive, generally 4' x 8' orlarger in size. Included under this designation areplywood, oriented strand board, waferboard, andcomposite panels. These panel products meet therequirements of PS 1-95 (NIST, 1995) or PS 2-92(NIST, 1992) and are intended for structural use inresidential, commercial, and industrial applications.Stud: Wood member used as vertical framingmember in interior or exterior walls of a building,usually 2" x 4" or 2" x 6" sizes, and precision endtrimmed.Tie:See drag strut.Tie-down: Hardware used to anchor the verticalchord forces to the foundation or framing of thestructure in order to resist overturning of the wall.Timbers: Lumber of nominal five or more inches insmaller cross-section dimension.Time effect factor: A factor applied to adjustedresistance to account for effects of duration of load.(See load duration.)Waferboard: A nonveneered structural panelmanufactured from two- to three-inch flakes or wafersbonded together with a phenolic resin and pressed intosheet panels.8.8 SymbolsThis list may exclude symbols appearing once only,when defined at that appearance.EGG dhh/LLL/bVV ybbe nvv y∆∆ yYoung’s modulus of elasticity of chord membersModulus of rigidity of wood structural panelModulus of rigidity of diaphragmHeight of wallAspect ratioLength of wall or floor/roof diaphragmDiaphragm ratioShear to element or componentShear to element at yieldDepth of floor/roof horizontal diaphragmDiaphragm width, ftNail deformation at yield load levelShear per footShear per foot at yieldDeflection of diaphragm or bracing element, in.Deflection of diaphragm or bracing element atyieldFEMA 273 Seismic Rehabilitation Guidelines 8-33

Chapter 8: Wood and Light Metal Framing(Systematic Rehabilitation)8.9 ReferencesAF&PA, 1991a, National Design Specification forWood Construction, American Forest & PaperAssociation, Washington, D.C.AF&PA, 1991b, Design Values for Wood Construction,supplement to the 1991 Edition National DesignSpecification, American Forest & Paper Association,Washington, D.C.AF&PA, 1994, National Design Specification for WoodConstruction, American Forest & Paper Association,Washington, D.C.APA, 1983, Research Report #138, American PlywoodAssociation, Tacoma, Washington.APA, 1988, Plywood Design Specification Supplement,American Plywood Association, Tacoma, Washington.APA, 1990, Research Report #154, American PlywoodAssociation, Tacoma, Washington.ASCE, 1991, Guideline for Structural ConditionAssessment of Existing Buildings, ANSI/ASCE 11-90,American Society of Civil Engineers, New York, NewYork.ASTM, 1980, Conducting Strength Test of Panels forBuilding Construction, Report No. ASTM E-72,American Society for Testing Materials, Philadelphia,Pennsylvania.ASTM, 1992, Standard Methods for EstablishingStructural Grades and Related Allowable Properties forVisually Graded Lumber, Report No. ASTM D 245,American Society for Testing and Materials,Philadelphia, Pennsylvania.ATC, 1981, Guidelines for the Design of HorizontalWood Diaphragms, Report No. ATC-7, AppliedTechnology Council, Redwood City, California.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos.FEMA 222A and 223A), Washington, D.C.Dolan, J. D., et al., 1994, Sequential PhasedDisplacement Tests to Determine Short-Term Durationof-LoadPerformance of Nail and Bolt Connections,Volume I: Summary Report, Virginia PolytechnicInstitute and State University, Blacksburg, Virginia.ICBO, 1994a, Uniform Building Code, InternationalConference of Building Officials, Whittier, California.ICBO, 1994b, Uniform Code for BuildingConservation, International Conference of BuildingOfficials, Whittier, California.NELMA, 1991, National Grading Rules forNortheastern Lumber, Northeastern LumberManufacturers Association, Cumberland Center, Maine.NIST, 1992, U.S. Product Standard PS 2-92,Performance Standard for Wood-Based Structural UsePanels, National Institute of Standards and Technology,Washington, D.C.NIST, 1995, U.S. Product Standard PS 1-95,Construction & Industrial Plywood with Typical APATrademarks, National Institute of Standards andTechnology, Washington, D.C.NIST, 1986, Voluntary Product Standard PS 20-70,American Softwood Lumber Standard, NationalInstitute of Standards and Technology, Washington,D.C.SPIB, 1991, Standard Grading Rules for Southern PineLumber, Southern Pine Inspection Bureau, Pensacola,Florida.WWPA, 1983, Western Woods Use Book, WesternWood Products Association, Portland, Oregon.WWPA, 1991, Western Lumber Grading Rules, WesternWood Products Association, Portland, Oregon.8-34 Seismic Rehabilitation Guidelines FEMA 273

9.Seismic Isolation and Energy Dissipation(Systematic Rehabilitation)Chapter 9 includes detailed guidelines for buildingrehabilitation with seismic (and base) isolation andpassive energy dissipation systems, and limitedguidance for other systems such as active energydissipation devices.The basic form and formulation of guidelines forseismic isolation and energy dissipation systems havebeen established and coordinated with theRehabilitation Objectives, Performance Levels, andseismic ground shaking hazard criteria of Chapter 2 andthe linear and nonlinear procedures of Chapter 3.Criteria for modeling the stiffness, strength, anddeformation capacities of conventional structuralcomponents of buildings with seismic isolation orenergy dissipation systems are given in Chapters 5through 8 and Chapter 10.9.1 IntroductionThis chapter provides guidelines for the application ofspecial seismic protective systems to buildingrehabilitation. Specific guidance is provided for seismic(base) isolation systems in Section 9.2 and for passiveenergy dissipation systems in Section 9.3. Section 9.4provides additional, limited guidance for other specialseismic systems, including active control systems,hybrid active and passive systems, and tuned mass andliquid dampers.Special seismic protective systems should be evaluatedas possible rehabilitation strategies based on theRehabilitation Objectives established for the building.Prior to implementation of the guidelines of thischapter, the user should establish the following criteriaas presented in Chapter 2:• The Rehabilitation Objective for the building– Performance Level– Seismic Ground Shaking HazardSeismic isolation and energy dissipation systems includea wide variety of concepts and devices. In most cases,these systems and devices will be implemented withsome additional conventional strengthening of theSeismic Isolation and Energy Dissipation asRehabilitation StrategiesSeismic isolation and energy dissipation systems areviable design strategies that have already been usedfor seismic rehabilitation of a number of buildings.Other special seismic protective systems—includingactive control, hybrid combinations of active andpassive energy devices, and tuned mass and liquiddampers—may also provide practical solutions in thenear future. These systems are similar in that theyenhance performance during an earthquake bymodifying the building’s response characteristics.Seismic isolation and energy dissipation systems willnot be appropriate design strategies for mostbuildings, particularly buildings that have onlyLimited Rehabilitation Objectives. In general, thesesystems will be most applicable to the rehabilitationof buildings whose owners desire superior earthquakeperformance and can afford the special costsassociated with the design, fabrication, andinstallation of seismic isolators and/or energydissipation devices. These costs are typically offset bythe reduced need for stiffening and strengtheningmeasures that would otherwise be required to meetRehabilitation Objectives.Seismic isolation and energy dissipation systems arerelatively new and sophisticated concepts that requiremore extensive design and detailed analysis than domost conventional rehabilitation schemes. Similarly,design (peer) review is required for all rehabilitationschemes that use either seismic isolation or energydissipation systems.structure; in all cases they will require evaluation ofexisting building elements. As such, this chaptersupplements the guidelines of other chapters of thisdocument with additional criteria and methods ofanalysis that are appropriate for buildings rehabilitatedwith seismic isolators and/or energy dissipation devices.Seismic isolation is increasingly becoming consideredfor historic buildings that are free-standing and have abasement or bottom space of no particular historicsignificance. In selecting such a solution, specialconsideration should be given to the possibility thatFEMA 273 Seismic Rehabilitation Guidelines 9-1

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)historic or archaeological resources may be present at thesite. If that is determined, the guidance of the StateHistoric Preservation Officer should be obtained in atimely manner. Isolation is also often considered foressential facilities, to protect valuable contents, and onbuildings with a complete, but insufficiently stronglateral-force-resisting system.9.2 Seismic Isolation SystemsThis section specifies analysis methods and designcriteria for seismic isolation systems that are based onthe Rehabilitation Objectives, Performance Levels, andSeismic Ground Shaking Hazard criteria of Chapter 2.The methods described in this section augment theanalysis requirements of Chapter 3. The analysismethods and other criteria of this section are basedlargely on the 1994 NEHRP Provisions (BSSC, 1995)for new buildings, augmented with changes proposedby Technical Subcommittee 12 of the ProvisionsUpdate Committee of the Building Seismic SafetyCouncil for the 1997 NEHRP Provisions (BSSC, 1997).9.2.1 BackgroundBuildings rehabilitated with a seismic isolation systemmay be thought of as composed of three distinctsegments: the structure above the isolation system, theisolation system itself, and the foundation and otherstructural elements below the isolation system.The isolation system includes wind-restraint and tiedownsystems, if such systems are required by theseGuidelines. The isolation system also includessupplemental energy dissipation devices, if suchdevices are used to transmit force between the structureabove the isolation system and the structure below theisolation system.This section provides guidance primarily for the design,analysis, and testing of the isolation system and fordetermination of seismic load on structural elementsand nonstructural components. Criteria forrehabilitation of structural elements other than theisolation system, and criteria for rehabilitation ofnonstructural components, should follow the applicableguidelines of other chapters of this document, usingloads and deformations determined by the procedures ofthis section.Seismic Isolation Performance ObjectivesSeismic isolation has typically been used as aRehabilitation Strategy that enhances the performanceof the building above that afforded by conventionalstiffening and strengthening schemes. Seismicisolation rehabilitation projects have targetedperformance at least equal to, and commonlyexceeding, the Basic Safety Objective of theseGuidelines, effectively achieving ImmediateOccupancy or better performance.A number of buildings rehabilitated with seismicisolators have been historic. For these projects,seismic isolation reduced the extent and intrusion ofseismic modifications on the historical fabric of thebuilding that would otherwise be required to meetdesired Performance Levels.See the Commentary for detailed discussions on thedevelopment of isolation provisions for new buildings(Section C9.2.1.1) and the design philosophy on whichthe provisions are based (Section C9.2.1.2). TheCommentary also provides an overview of seismicisolation rehabilitation projects (Section C9.2.1.3) andgoals (Section C9.2.1.4).9.2.2 Mechanical Properties and Modelingof Seismic Isolation Systems9.2.2.1 GeneralA seismic isolation system is the collection of allindividual seismic isolators (and separate wind restraintand tie-down devices, if such devices are used to meetthe requirements of these Guidelines). Seismic isolationsystems may be composed entirely of one type ofseismic isolator, a combination of different types ofseismic isolators, or a combination of seismic isolatorsacting in parallel with energy dissipation devices (i.e., ahybrid system).Seismic isolators are classified as either elastomeric,sliding, or other isolators. Elastomeric isolators aretypically made of layers of rubber separated by steelshims. Elastomeric isolators may be any one of thefollowing: high-damping rubber bearings (HDR), lowdampingrubber bearings (RB) or low-damping rubberbearings with a lead core (LRB). Sliding isolators maybe flat assemblies or have a curved surface, such as thefriction-pendulum system (FPS). Rolling systems may9-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)be characterized as a subset of sliding systems. Rollingisolators may be flat assemblies or have a curved orconical surface, such as the ball and cone system(BNC). Other isolators are not discussed.This section provides guidance for modeling ofelastomeric isolators and sliding isolators. Guidance formodeling of energy dissipation devices may be found inSection 9.3. Information on hybrid systems is providedin the Commentary (Section C9.2.2.2C)9.2.2.2 Mechanical Properties of SeismicIsolatorsA. Elastomeric IsolatorsThe mechanical characteristics of elastomeric isolatorsshould be known in sufficient detail to establish forcedeformationresponse properties and their dependence, ifany, on axial-shear interaction, bilateral deformation,load history (including the effects of “scragging” ofvirgin elastomeric isolators; that is, the process ofsubjecting an elastomeric bearing to one or more cyclesof large amplitude displacement), temperature, and otherenvironmental loads and aging effects (over the designlife of the isolator).For the purpose of mathematical modeling of isolators,mechanical characteristics may be based on analysisand available material test properties, but verification ofisolator properties used for design should be based ontests of isolator prototypes, as described inSection 9.2.9.B. Sliding IsolatorsThe mechanical characteristics of sliding isolatorsshould be known in sufficient detail to establish forcedeformationresponse properties and their dependence,if any, on contact pressure, rate of loading (velocity),bilateral deformation, temperature, contamination, andother environmental loads and aging effects (over thedesign life of the isolator).For the purpose of mathematical modeling of isolators,mechanical characteristics may be based on analysisand available material test properties, but verification ofisolator properties used for design should be based ontests of isolator prototypes, as described inSection 9.2.9.9.2.2.3 Modeling of IsolatorsA. GeneralIf the mechanical characteristics of a seismic isolatorare dependent on design parameters such as axial load(due to gravity, earthquake overturning effects, andvertical earthquake shaking), rate of loading (velocity),bilateral deformation, temperature, or aging, thenupper- and lower-bound values of stiffness anddamping should be used to determine the range andsensitivity of response to design parameters.B. Linear ModelsLinear procedures use effective stiffness, k eff , andeffective damping, β eff , to characterize nonlinearproperties of isolators. The restoring force of an isolatoris calculated as the product of effective stiffness, k eff ,and response displacement, D:F = k eff D(9-1)The effective stiffness, k eff , of an isolator is calculatedfrom test data using Equation 9-12. Similarly, the areaenclosed by the force-displacement hysteresis loop isused to calculate the effective damping, β eff , of anisolator using Equation 9-13. Both effective stiffnessand effective damping are, in general, amplitudedependentand should be evaluated at all responsedisplacements of design interest.C. Nonlinear ModelsNonlinear procedures should explicitly model thenonlinear force-deflection properties of isolators.Damping should be modeled explicitly by inelastic(hysteretic) response of isolators. Additional viscousdamping should not be included in the model unlesssupported by rate-dependent tests of isolators.9.2.2.4 Isolation System and SuperstructureModelingA. GeneralMathematical models of the isolated building—including the isolation system, the lateral-forceresistingsystem and other structural components andelements, and connections between the isolation systemand the structure above and below the isolationsystem—should conform to the requirements ofChapters 2 and 3 and the guidelines given below.FEMA 273 Seismic Rehabilitation Guidelines 9-3

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)B. Isolation System ModelThe isolation system should be modeled usingdeformational characteristics developed and verified bytest in accordance with the requirements ofSection 9.2.9.The isolation system should be modeled with sufficientdetail to:1. Account for the spatial distribution of isolator units2. Calculate translation, in both horizontal directions,and torsion of the structure above the isolationinterface, considering the most disadvantageouslocation of mass eccentricity3. Assess overturning/uplift forces on individualisolators4. Account for the effects of vertical load, bilateralload, and/or the rate of loading, if the forcedeflection properties of the isolation system aredependent on one or more of these factors5. Assess forces due to P-∆ momentsC. Superstructure ModelThe maximum displacement of each floor, and the totaldesign displacement and total maximum displacementacross the isolation system, should be calculated using amodel of the isolated building that incorporates theforce-deflection characteristics of nonlinearcomponents, and elements of the isolation system andthe superstructure.Isolation systems with nonlinear components include,but are not limited to, systems that do not meet thecriteria of Section 9.2.3.3A Item (2).Lateral-force-resisting systems with nonlinearcomponents and elements include, but are not limitedto, systems described by both of the following criteria.1. For all deformation-controlled actions,Equation 3-18 is satisfied using a value of m equal to1.0.2. For all force-controlled actions, Equation 3-19 issatisfied.Design forces and displacements in primarycomponents of the lateral-force-resisting system may becalculated using a linearly elastic model of the isolatedstructure, provided the following criteria are met.1. Pseudo-elastic properties assumed for nonlinearisolation system components are based on themaximum effective stiffness of the isolation system.2. The lateral-force-resisting system remainsessentially linearly elastic for the earthquakedemand level of interest.9.2.3 General Criteria for Seismic IsolationDesign9.2.3.1 GeneralCriteria for the seismic isolation of buildings aredivided into two sections:1. Rehabilitation of the building2. Design, analysis, and testing of the isolation systemA. Basis For DesignSeismic Rehabilitation Objectives of the buildingshould be consistent with those set forth in Chapter 2.The design, analysis, and testing of the isolation systemshould be based on the guidelines of this chapter.B. Stability of the Isolation SystemThe stability of the vertical-load-carrying componentsof the isolation system should be verified by analysisand test, as required, for a lateral displacement equal tothe total maximum displacement, or for the maximumdisplacement allowed by displacement-restraintdevices, if such devices are part of the isolation system.C. Configuration RequirementsThe regularity of the isolated building should bedesignated as being either regular or irregular on thebasis of the structural configuration of the structureabove the isolation system.9.2.3.2 Ground Shaking CriteriaGround shaking criteria are required for the designearthquake, which is user-specified and may be chosenequal to the BSE-1, and for the Maximum ConsideredEarthquake (MCE), equal to the BSE-2, as described inChapter 2.9-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)A. User-Specified Design EarthquakeFor the design earthquake, the following groundshaking criteria should be established:1. Short period spectral response accelerationparameter, S DS , and spectral response accelerationparameter at 1.0 second, S D12. Five-percent-damped response spectrum of thedesign earthquake (when a response spectrum isrequired for linear procedures by Section 9.2.3.3A,or to define acceleration time histories)3. At least three acceleration time histories compatiblewith the design earthquake spectrum (whenacceleration time histories are required for nonlinearprocedures by Section 9.2.3.3B)B. Maximum EarthquakeFor the BSE-2, the following ground shaking criteriashould be established:1. Short period spectral response accelerationparameter, S MS , and spectral response accelerationparameter at 1.0 second, S M12. Five-percent-damped site-specific responsespectrum of the BSE-2 (when a response spectrum isrequired for linear procedures by Section 9.2.3.3A,or to define acceleration time histories)3. At least three acceleration time histories compatiblewith the BSE-2 spectrum (when acceleration timehistories are required for nonlinear procedures bySection 9.2.3.3B)9.2.3.3 Selection of Analysis ProcedureA. Linear ProceduresLinear procedures may be used for design ofseismically isolated buildings, provided the followingcriteria are met.1. The building is located on Soil Profile Type A, B, C,or D; or E (if S 1 ≥ 0.6 for BSE-2).2. The isolation system meets all of the followingcriteria:a. The effective stiffness of the isolation system atthe design displacement is greater than one-thirdof the effective stiffness at 20% of the designdisplacement.b. The isolation system is capable of producing arestoring force as specified in Section 9.2.7.2D.c. The isolation system has force-deflectionproperties that are essentially independent of therate of loading.d. The isolation system has force-deflectionproperties that are independent of vertical loadand bilateral load.e. The isolation system does not limit BSE-2displacement to less than S M1 /S D1 times the totaldesign displacement.3. The structure above the isolation system remainsessentially linearly elastic for the BSE-2.Response spectrum analysis should be used for designof seismically-isolated buildings that meet any of thefollowing criteria.• The building is over 65 feet (19.8 meters) in height.• The effective period of the structure, T M , is greaterthan three seconds.• The effective period of the isolated structure, T D , isless than or equal to three times the elastic, fixedbaseperiod of the structure above the isolationsystem.• The structure above the isolation system is irregularin configuration.B. Nonlinear ProceduresNonlinear procedures should be used for design ofseismic-isolated buildings for which the followingconditions apply.1. The structure above the isolation system is nonlinearfor the BSE-2.2. The building is located on Soil Profile Type E (ifS 1 > 0.6 for BSE-2) or Soil Profile Type F.3. The isolation system does not meet all of the criteriaof Section 9.2.3.3A, Item (2).FEMA 273 Seismic Rehabilitation Guidelines 9-5

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)Nonlinear acceleration time history analysis is requiredfor the design of seismically isolated buildings forwhich conditions (1) and (2) apply.9.2.4 Linear Procedures9.2.4.1 GeneralExcept as provided in Section 9.2.5, every seismicallyisolated building, or portion thereof, should be designedand constructed to resist the earthquake displacementsand forces specified by this section.9.2.4.2 Deformation Characteristics of theIsolation SystemThe deformation characteristics of the isolation systemshould be based on properly substantiated testsperformed in accordance with Section 9.2.9.The deformation characteristics of the isolation systemshould explicitly include the effects of the windrestraintand tie-down systems, and supplementalenergy-dissipation devices, if such a systems anddevices are used to meet the design requirements ofthese guidelines.9.2.4.3 Minimum Lateral DisplacementsA. Design DisplacementThe isolation system should be designed andconstructed to withstand, as a minimum, lateralearthquake displacements that act in the direction ofeach of the main horizontal axes of the structure inaccordance with the equation:D D=g-------- S D1 T D4π 2 ----------------B D1B. Effective Period at the Design Displacement(9-2)The effective period, T D , of the isolated building at thedesign displacement should be determined using thedeformational characteristics of the isolation system inaccordance with the equation:T D=2πW-----------------K Dmin g(9-3)C. Maximum DisplacementThe maximum displacement of the isolation system,D M , in the most critical direction of horizontal responseshould be calculated in accordance with the equation:D M=D. Effective Period at the Maximum Displacement(9-4)The effective period, T M , of the isolated building at themaximum displacement should be determined using thedeformational characteristics of the isolation system inaccordance with the equation:T M=E. Total Displacementg-------- S M1 T M4π 2 -----------------B M12πW------------------K Mmin g(9-5)The total design displacement, D TD , and the totalmaximum displacement, D TM , of components of theisolation system should include additional displacementdue to actual and accidental torsion calculatedconsidering the spatial distribution of the effectivestiffness of the isolation system at the designdisplacement and the most disadvantageous location ofmass eccentricity.The total design displacement, D TD , and the totalmaximum displacement, D TM , of components of anisolation system with a uniform spatial distribution ofeffective stiffness at the design displacement should betaken as not less than that prescribed by the equations:D TD = D D 1 + y----------------12eb 2 + d 2D TM = D M 1 + y----------------12eb 2 + d 2(9-6)(9-7)The total maximum displacement, D TM , may be takenas less than the value prescribed by Equation 9-7, butnot less than 1.1 times D M , provided the isolationsystem is shown by calculation to be configured toresist torsion accordingly.9-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)9.2.4.4 Minimum Lateral ForcesA. Isolation System and Structural Components andElements at or below the Isolation SystemThe isolation system, the foundation, and all otherstructural components and elements below the isolationsystem should be designed and constructed to withstanda minimum lateral seismic force, V b , prescribed by theequation:V b=K Dmax D D(9-8)B. Structural Components and Elements above theIsolation SystemThe components and elements above the isolationsystem should be designed and constructed to resist aminimum lateral seismic force, V s , taken as equal to thevalue of V b , prescribed by Equation 9-8.C. Limits on V sThe value of V s should be taken as not less than thefollowing:1. The base shear corresponding to the design windload2. The lateral seismic force required to fully activatethe isolation system factored by 1.5 (e.g., the yieldlevel of a softening system, the ultimate capacity ofa sacrificial wind-restraint system, or the breakawayfriction level of a sliding system factored by1.5)D. Vertical Distribution of ForceThe total force should be distributed over the height ofthe structure above the isolation interface as follows:F x =V s w x h-------------------- xn∑ w i h ii 1(9-9)At each level designated as x, the force F x should beapplied over the area of the building in accordance withthe weight, w x , distribution at that level, h x . Response ofstructural components and elements should becalculated as the effect of the force F x applied at theappropriate levels above the base.9.2.4.5 Response Spectrum AnalysisA. Earthquake InputThe design earthquake spectrum should be used tocalculate the total design displacement of the isolationsystem and the lateral forces and displacements of theisolated building. The BSE-2 spectrum should be usedto calculate the total maximum displacement of theisolation system.B. Modal DampingResponse Spectrum Analysis should be performed,using a damping value for isolated modes equal to theeffective damping of the isolation system, or 30% ofcritical, whichever is less. The damping value assignedto higher modes of response should be consistent withthe material type and stress level of the superstructure.C. Combination of Earthquake DirectionsResponse Spectrum Analysis used to determine the totaldesign displacement and total maximum displacementshould include simultaneous excitation of the model by100% of the most critical direction of ground motion,and not less than 30% of the ground motion in theorthogonal axis. The maximum displacement of theisolation system should be calculated as the vector sumof the two orthogonal displacements.D. Scaling of ResultsIf the total design displacement determined byResponse Spectrum Analysis is found to be less than thevalue of D TD prescribed by Equation 9-6, or if the totalmaximum displacement determined by responsespectrum analysis is found to be less than the value ofD TM prescribed by Equation 9-7, then all responseparameters, including components actions anddeformations, should be adjusted upward proportionallyto the D TD value, or the D TM value, and used for design.9.2.4.6 Design Forces and DeformationsComponents and elements of the building should bedesigned for forces and displacements estimated bylinear procedures using the acceptance criteria ofSection 3.4.2.2, except that deformation-controlledcomponents and elements should be designed using acomponent demand modifier no greater than 1.5.9.2.5 Nonlinear ProceduresIsolated buildings evaluated using nonlinear proceduresshould be represented by three-dimensional models thatFEMA 273 Seismic Rehabilitation Guidelines 9-7

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)incorporate both the nonlinear characteristics of theisolation system and the structure above the isolationsystem.nonlinear procedure guidelines of Section 3.3.4, exceptthat results should be scaled for design based on thecriteria given in the following section.9.2.5.1 Nonlinear Static Procedurecontrol node that is located at the center of mass of thefirst floor above the isolation interface.C. Lateral Load PatternThe pattern of applied lateral load should beproportional to the distribution of the product ofbuilding mass and the deflected shape of the isolatedmode of response at target displacement.9.2.5.2 Nonlinear Dynamic ProcedureA. GeneralThe Nonlinear Dynamic Procedure (NDP) forseismically isolated buildings should be based on theB. Scaling of ResultsD′ DA. GeneralIf the design displacement determined by Time-HistoryThe Nonlinear Static Procedure (NSP) for seismically Analysis is found to be less than the value ofa fixed base as prescribed by Equation 3-10. The target9.2.6.2 Forces and Displacementsdisplacements, D′ D and D′ M , should be evaluated at aA. Components and Elements at or above theisolated buildings should be based on the nonlinearprocedure guidelines of Section 3.3.3, except that theprescribed by Equation 9-10, or if the maximumdisplacement determined by Response Spectrumtarget displacement and pattern of applied lateral load Analysis is found to be less than the value of D′should be based on the criteria given in the followingMsections.prescribed by Equation 9-11, then all responseparameters, including component actions andB. Target Displacementdeformations, should be adjusted upward proportionallyto the D′In each principal direction, the building model shouldD value or the D′ M value, and used for design.be pushed to the design earthquake target displacement,D′ D , and to the BSE-2 target displacement, D′ M , as9.2.5.3 Design Forces and Deformationsdefined by the following equations:Components and elements of the building should bedesigned for the forces and deformations estimated byD′ Dnonlinear procedures using the acceptance criteria ofD= ---------------------------- D(9-10) Section 3.4.3.2.⎛T e ⎞ 21 + ⎜------⎟9.2.6 Nonstructural Components⎝T D ⎠9.2.6.1 GeneralD Parts or portions of a seismically isolated building,D′ M = ---------------------------- M(9-11) permanent nonstructural components and the⎛ T attachments to them, and the attachments for permanente ⎞ 21 + ⎜------⎟equipment supported by a building should be designed⎝T M ⎠to resist seismic forces and displacements as given inthis section and the applicable requirements ofwhere T e is the effective period of the superstructure on Chapter 11.Isolation InterfaceComponents and elements of seismically isolatedbuildings and nonstructural components, or portionsthereof, that are at or above the isolation interface,should be designed to resist a total lateral seismic forceequal to the maximum dynamic response of the elementor component under consideration.EXCEPTION: Elements of seismically isolatedstructures and nonstructural components, or portionsthereof, may be designed to resist total lateral seismicforce as required for conventional fixed-base buildingsby Chapter 11.9-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)B. Components and Elements That Cross theIsolation InterfaceElements of seismically isolated buildings andnonstructural components, or portions thereof, thatcross the isolation interface should be designed towithstand the total maximum (horizontal) displacementand maximum vertical displacement of the isolationsystem at the total maximum (horizontal) displacement.Components and elements that cross the isolationinterface should not restrict displacement of the isolatedbuilding or otherwise compromise the RehabilitationObjectives of the building.C. Components and Elements Below the IsolationInterfaceComponents and elements of seismically isolatedbuildings and nonstructural components, or portionsthereof, that are below the isolation interface should bedesigned and constructed in accordance with therequirements of Chapter 11.9.2.7 Detailed System Requirements9.2.7.1 GeneralThe isolation system and the structural system shouldcomply with the general requirements of Chapter 2 andthe requirements of Chapters 4 through 8. In addition,the isolation system and the structural system shouldcomply with the detailed system requirements of thissection.9.2.7.2 Isolation SystemA. Environmental ConditionsIn addition to the requirements for vertical and lateralloads induced by wind and earthquake, the isolationsystem should be designed with consideration given toother environmental conditions, including aging effects,creep, fatigue, operating temperature, and exposure tomoisture or damaging substances.B. Wind ForcesIsolated buildings should resist design wind loads at alllevels above the isolation interface in accordance withthe applicable wind design provisions. At the isolationinterface, a wind-restraint system should be provided tolimit lateral displacement in the isolation system to avalue equal to that required between floors of thestructure above the isolation interface.C. Fire ResistanceFire resistance rating for the isolation system should beconsistent with the requirements of columns, walls, orother such elements of the building.D. Lateral Restoring ForceThe isolation system should be configured to produceeither a restoring force such that the lateral force at thetotal design displacement is at least 0.025W greater thanthe lateral force at 50% of the total design displacement,or a restoring force of not less than 0.05W at alldisplacements greater than 50% of the total designdisplacement.EXCEPTION: The isolation system need not beconfigured to produce a restoring force, as requiredabove, provided the isolation system is capable ofremaining stable under full vertical load andaccommodating a total maximum displacement equal tothe greater of either 3.0 times the total designdisplacement or 36 S M1 inches.E. Displacement RestraintThe isolation system may be configured to include adisplacement restraint that limits lateral displacementdue to the BSE-2 to less than S M1 / S D1 times the totaldesign displacement, provided that the seismicallyisolated building is designed in accordance with thefollowing criteria when more stringent than therequirements of Section 9.2.3.1. BSE-2 response is calculated in accordance with thedynamic analysis requirements of Section 9.2.5,explicitly considering the nonlinear characteristicsof the isolation system and the structure above theisolation system.2. The ultimate capacity of the isolation system, andstructural components and elements below theisolation system, should exceed the force anddisplacement demands of the BSE-2.3. The structure above the isolation system is checkedfor stability and ductility demand of the BSE-2.4. The displacement restraint does not becomeeffective at a displacement less than 0.75 times thetotal design displacement, unless it is demonstratedby analysis that earlier engagement does not result inunsatisfactory performance.FEMA 273 Seismic Rehabilitation Guidelines 9-9

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)F. Vertical Load StabilityEach component of the isolation system should bedesigned to be stable under the full maximum verticalload, 1.2Q D + Q L + |Q E |, and the minimum verticalload, 0.8Q D - |Q E |, at a horizontal displacement equal tothe total maximum displacement. The earthquakevertical load on an individual isolator unit, Q E , shouldbe based on peak building response due to the BSE-2.G. OverturningThe factor of safety against global structuraloverturning at the isolation interface should be not lessthan 1.0 for required load combinations. All gravity andseismic loading conditions should be investigated.Seismic forces for overturning calculations should bebased on the BSE-2, and the vertical restoring forceshould be based on the building’s weight, W, above theisolation interface.Local uplift of individual components and elements ispermitted, provided the resulting deflections do notcause overstress or instability of the isolator units orother building components and elements. A tie-downsystem may be used to limit local uplift of individualcomponents and elements, provided that the seismicallyisolated building is designed in accordance with thefollowing criteria when more stringent than therequirements of Section 9.2.3.1. BSE-2 response is calculated in accordance with thedynamic analysis requirements of Section 9.2.5,explicitly considering the nonlinear characteristicsof the isolation system and the structure above theisolation system.2. The ultimate capacity of the tie-down system shouldexceed the force and displacement demands of theBSE-2.3. The isolation system is both designed to be stableand shown by test to be stable (Section 9.2.9.2F) forBSE-2 loads that include additional vertical load dueto the tie-down system.H. Inspection and ReplacementAccess for inspection and replacement of allcomponents and elements of the isolation system shouldbe provided.I. Manufacturing Quality ControlA manufacturing quality control testing program forisolator units should be established by the engineerresponsible for the structural design.9.2.7.3 Structural SystemA. Horizontal Distribution of ForceA horizontal diaphragm or other structural componentsand elements should provide continuity above theisolation interface. The diaphragm or other structuralcomponents and elements should have adequatestrength and ductility to transmit forces (due tononuniform ground motion) from one part of thebuilding to another, and have sufficient stiffness toeffect rigid diaphragm response above the isolationinterface.B. Building SeparationsMinimum separations between the isolated building andsurrounding retaining walls or other fixed obstructionsshould be not less than the total maximumdisplacement.9.2.8 Design and Construction Review9.2.8.1 GeneralA review of the design of the isolation system andrelated test programs should be performed by anindependent engineering team, including personslicensed in the appropriate disciplines, and experiencedin seismic analysis methods and the theory andapplication of seismic isolation.9.2.8.2 Isolation SystemIsolation system design and construction review shouldinclude, but not be limited to, the following:1. Site-specific seismic criteria, including site-specificspectra and ground motion time history, and all otherdesign criteria developed specifically for the project2. Preliminary design, including the determination ofthe total design and total maximum displacement ofthe isolation system, and the lateral force designlevel3. Isolation system prototype testing (Section 9.2.9)4. Final design of the isolated building and supportinganalyses9-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)5. Isolation system quality control testing(Section 9.2.7.2I)9.2.9 Isolation System Testing and DesignProperties9.2.9.1 GeneralThe deformation characteristics and damping values ofthe isolation system used in the design and analysis ofseismically isolated structures should be based on thefollowing tests of a selected sample of the componentsprior to construction.The isolation system components to be tested shouldinclude isolators, and components of the wind restraintsystem and supplemental energy dissipation devices ifsuch components and devices are used in the design.The tests specified in this section establish designproperties of the isolation system, and should not beconsidered as satisfying the manufacturing qualitycontrol testing requirements of Section 9.2.7.2I.9.2.9.2 Prototype TestsA. GeneralPrototype tests should be performed separately on twofull-size specimens of each type and size of isolator ofthe isolation system. The test specimens should includecomponents of the wind restraint system, as well asindividual isolators, if such components are used in thedesign. Supplementary energy dissipation devicesshould be tested in accordance with Section 9.3.8criteria. Specimens tested should not be used forconstruction unless approved by the engineerresponsible for the structural design.B. RecordFor each cycle of tests, the force-deflection andhysteretic behavior of the test specimen should berecorded.C. Sequence and CyclesThe following sequence of tests should be performedfor the prescribed number of cycles at a vertical loadequal to the average Q D + 0.5Q L on all isolators of acommon type and size:1. Twenty fully reversed cycles of loading at a lateralforce corresponding to the wind design force2. Three fully reversed cycles of loading at each of thefollowing displacements: 0.25D D , 0.50D D , 1.0D D ,and 1.0D M3. Three fully reversed cycles at the total maximumdisplacement, 1.0D TM4. 30S D1 /S DS B D , but not less than 10, fully reversedcycles of loading at the design displacement, 1.0D DIf an isolator is also a vertical-load-carrying element,then Item 2 of the sequence of cyclic tests specifiedabove should be performed for two additional verticalload cases:1. 1.2Q D + 0.5Q L + |Q E |2. 0.8Q D - |Q E |where D, L, and E refer to dead, live, and earthquakeloads. Q D and Q L are as defined in Section 3.2.8. Thevertical test load on an individual isolator unit shouldinclude the load increment Q E due to earthquakeoverturning, and should be equal to or greater than thepeak earthquake vertical force response correspondingto the test displacement being evaluated. In these tests,the combined vertical load should be taken as thetypical or average downward force on all isolators of acommon type and size.D. Isolators Dependent on Loading RatesIf the force-deflection properties of the isolators aredependent on the rate of loading, then each set of testsspecified in Section 9.2.9.2C should be performeddynamically at a frequency equal to the inverse of theeffective period, T D , of the isolated structure.EXCEPTION: If reduced-scale prototype specimensare used to quantify rate-dependent properties ofisolators, the reduced-scale prototype specimens shouldbe of the same type and material and be manufacturedwith the same processes and quality as full-scaleprototypes, and should be tested at a frequency thatrepresents full-scale prototype loading rates.The force-deflection properties of an isolator should beconsidered to be dependent on the rate of loading ifthere is greater than a plus or minus 10% difference inthe effective stiffness at the design displacement(1) when tested at a frequency equal to the inverse ofFEMA 273 Seismic Rehabilitation Guidelines 9-11

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)the effective period of the isolated structure and(2) when tested at any frequency in the range of 0.1 to2.0 times the inverse of the effective period of theisolated structure.E. Isolators Dependent on Bilateral LoadIf the force-deflection properties of the isolators aredependent on bilateral load, then the tests specified inSections 9.2.9.2C and 9.2.9.2D should be augmented toinclude bilateral load at the following increments of thetotal design displacement: 0.25 and 1.0; 0.50 and 1.0;0.75 and 1.0; and 1.0 and 1.0.EXCEPTION: If reduced-scale prototype specimensare used to quantify bilateral-load-dependent properties,then such scaled specimens should be of the same typeand material, and manufactured with the sameprocesses and quality as full-scale prototypes.The force-deflection properties of an isolator should beconsidered to be dependent on bilateral load, if thebilateral and unilateral force-deflection properties havegreater than a plus or minus 15% difference in effectivestiffness at the design displacement.F. Maximum and Minimum Vertical LoadIsolators that carry vertical load should be staticallytested for the maximum and minimum vertical load, atthe total maximum displacement. In these tests, thecombined vertical loads of 1.2Q D + 1.0Q L + |Q E | shouldbe taken as the maximum vertical force, and thecombined vertical load of 0. 8Q D - |Q E | should be takenas the minimum vertical force, on any one isolator of acommon type and size. The earthquake vertical load onan individual isolator, Q E , should be based on peakbuilding response due to the BSE-2.G. Sacrificial Wind-Restraint SystemsIf a sacrificial wind-restraint system is part of theisolation system, then the ultimate capacity should beestablished by test.H. Testing Similar UnitsPrototype tests are not required if an isolator unit is:1. Of similar dimensional characteristics2. Of the same type and materials, and3. Fabricated using identical manufacturing and qualitycontrol procedures9.2.9.3 Determination of Force-DeflectionCharacteristicsThe force-deflection characteristics of the isolationsystem should be based on the cyclic load testing ofisolator prototypes specified in Section 9.2.9.2C.As required, the effective stiffness of an isolator unit,k eff , should be calculated for each cycle of deformationby the equation:F + + F –k eff = ------------------------∆ + + ∆ –(9-12)where F + and F – are the positive and negative forces atpositive and negative test displacements, ∆ + and ∆ – ,respectively.As required, the effective damping of an isolator unit,β eff , should be calculated for each cycle of deformationby the equation:2β eff = --πE----------------------------------------Loopk eff ( ∆ + + ∆ – ) 2(9-13)where the energy dissipated per cycle of loading, E Loop ,and the effective stiffness, k eff , are based on testdisplacements, ∆ + and ∆ – .9.2.9.4 System AdequacyThe performance of the test specimens should beassessed as adequate if the following conditions aresatisfied.1. The force-deflection plots of all tests specified inSection 9.2.9.2 have a nonnegative incrementalforce-carrying capacity.2. For each increment of test displacement specified inSection 9.2.9.2C, Item (2), and for each vertical loadcase specified in Section 9.2.9.2C the followingcriteria are met.a. There is no greater than a plus or minus 15%difference between the effective stiffness at each9-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)of the three cycles of test and the average valueof effective stiffness for each test specimen.b. There is no greater than a 15% difference in theaverage value of effective stiffness of the two testspecimens of a common type and size of theisolator unit over the required three cycles of test.3. For each specimen there is no greater than a plusor minus 20% change in the initial effective stiffnessof each test specimen over the 30S D1 /S DS B D , but notless than 10, cycles of the test specified inSection 9.2.9.2C, Item (3).4. For each specimen there is no greater than a 20%decrease in the initial effective damping over the30S D1 /S DS B D , but not less than 10, cycles of the testspecified in Section 9.2.9.2C, Item (4).5. All specimens of vertical-load-carrying elements ofthe isolation system remain stable at the totalmaximum displacement for static load as prescribedin Section 9.2.9.2F.+–∑ F M max + ∑ F M maxK Mmax = ---------------------------------------------------------2D M+–∑ F M min + ∑ F M minK Mmin = --------------------------------------------------------2D M(9-16)(9-17)For isolators that are found by the tests ofSections 9.2.9.2C, 9.2.9.2D, and 9.2.9.2E to have forcedeflectioncharacteristics that vary with vertical load,rate of loading, or bilateral load, respectively, the valuesof K Dmax and K Mmax should be increased and the valuesof K Dmin and K Mmin should be decreased, as necessary,to bound the effects of the measured variation ineffective stiffness.B. Effective DampingAt the design displacement, the effective damping ofthe isolation system, β D , should be based on the cyclictests of Section 9.2.9.2 and calculated by the formula:6. The effective stiffness and effective damping of testspecimens fall within the limits specified by theengineer responsible for structural design.1β D = -----2π∑ E D-----------------------2K Dmax D D(9-18)9.2.9.5 Design Properties of the IsolationSystemA. Maximum and Minimum Effective StiffnessAt the design displacement, the maximum andminimum effective stiffness of the isolation system,K Dmax and K Dmin , should be based on the cyclic tests ofSection 9.2.9.2 and calculated by the formulas:+–∑ F D max + ∑ F D maxK Dmax = --------------------------------------------------------2D D(9-14)In Equation 9-18, the total energy dissipated in theisolation system per displacement cycle, ΣE D , should betaken as the sum of the energy dissipated per cycle in allisolators measured at test displacements, ∆ + and ∆ – ,that are equal in magnitude to the design displacement,D D .At the maximum displacement, the effective dampingof the isolation system, β M , should be based on thecyclic tests of Section 9.2.9.2 and calculated by theformula:+–∑ F D min + ∑ F D minK Dmin = -------------------------------------------------------2D D(9-15)1β M = -----2π∑ E M------------------------2K Mmax D M(9-19)At the maximum displacement, the maximum andminimum effective stiffness of the isolation systemshould be based on cyclic tests of Section 9.2.9.2 andcalculated by the formulas:In Equation 9-19, the total energy dissipated in theisolation system per displacement cycle, ΣE M , shouldbe taken as the sum of the energy dissipated per cycle inall isolators measured at test displacements, ∆ + and ∆ – ,FEMA 273 Seismic Rehabilitation Guidelines 9-13

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)that are equal in magnitude to the maximumdisplacement, D M .9.3 Passive Energy DissipationSystemsThis section specifies analysis methods and designcriteria for energy dissipation systems that are based onthe Rehabilitation Objectives, Performance Levels, andSeismic Ground Shaking Hazard criteria of Chapter 2.9.3.1 General RequirementsThis section provides guidelines for the implementationof passive energy dissipation devices in the seismicrehabilitation of buildings. In addition to therequirements provided herein, every rehabilitatedbuilding incorporating energy dissipation devicesshould be designed in accordance with the applicableprovisions of the remainder of the Guidelines unlessmodified by the requirements of this section.The energy dissipation devices should be designed withconsideration given to other environmental conditionsincluding wind, aging effects, creep, fatigue, ambienttemperature, operating temperature, and exposure tomoisture or damaging substances.The building height limitations should not exceed thevalues for the structural system into which the energydissipation devices are implemented.The mathematical model of a rehabilitated buildingshould include the plan and vertical distribution of theenergy dissipation devices. Analysis of themathematical model should account for the dependenceof the devices on excitation frequency, ambient andoperating temperature, velocity, sustained loads, andbilateral loads. Multiple analyses of the building may benecessary to capture the effects of varying mechanicalcharacteristics of the devices.Energy dissipation devices shall be capable ofsustaining larger displacements (and velocities forvelocity-dependent devices) than the maxima calculatedin the BSE-2. The increase in displacement (andvelocity) capacity is dependent on the level ofredundancy in the supplemental damping system asfollows:1. If four or more energy dissipation devices areprovided in a given story of a building, in oneEnergy Dissipation Performance LevelsPassive energy dissipation is an emerging technologythat enhances the performance of the building byadding damping (and in some cases stiffness) to thebuilding. The primary use of energy dissipationdevices is to reduce earthquake displacement of thestructure. Energy dissipation devices will also reduceforce in the structure—provided the structure isresponding elastically—but would not be expected toreduce force in structures that are responding beyondyield.For most applications, energy dissipation provides analternative approach to conventional stiffening andstrengthening schemes, and would be expected toachieve comparable Performance Levels. In general,these devices would be expected to be goodcandidates for projects that have a Performance Levelof Life Safety, or perhaps Immediate Occupancy, butwould be expected to have only limited applicabilityto projects with a Performance Level of CollapsePrevention.Other objectives may also influence the decision touse energy dissipation devices, since these devicescan also be useful for control of building response dueto small earthquakes, wind, or mechanical loads.principal direction of the building, with a minimumof two devices located on each side of the center ofstiffness of the story in the direction underconsideration, all energy dissipation devices shall becapable of sustaining displacements equal to 130%of the maximum calculated displacement in thedevice in the BSE-2. A velocity-dependent device(see Section 9.3.3) shall also be capable ofsustaining the force associated with a velocity equalto 130% of the maximum calculated velocity for thatdevice in the BSE-2.2. If fewer than four energy dissipation devices areprovided in a given story of a building, in oneprincipal direction of the building, or fewer than twodevices are located on each side of the center ofstiffness of the story in the direction underconsideration, all energy dissipation devices shall becapable of sustaining displacements equal to 200%of the maximum calculated displacement in thedevice in the BSE-2. A velocity-dependent deviceshall also be capable of sustaining the force9-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)associated with a velocity equal to 200% of themaximum calculated velocity for that device in theBSE-2.The components and connections transferring forcesbetween the energy dissipation devices shall bedesigned to remain linearly elastic for the forcesdescribed in items 1 or 2 above—dependent upon thedegree of redundancy in the supplemental dampingsystem.9.3.2 Implementation of Energy DissipationDevicesThe following subsections of Section 9.3 provideguidance to the design professional to aid in theimplementation of energy dissipation devices.Guidelines and criteria for analysis procedures andcomponent acceptance can be found in other chapters ofthe Guidelines.Restrictions on the use of linear procedures areestablished in Chapter 2. These restrictions also applyto the linear procedures of Section 9.3.4. Restrictions onthe use of nonlinear procedures, established inChapter 2, also apply to the nonlinear procedures ofSection 9.3.5. Example applications of linear andnonlinear procedures are provided in the Commentary,Section C9.3.9 (there is no corresponding section in theGuidelines).9.3.3 Modeling of Energy DissipationDevicesEnergy dissipation devices are classified in this sectionas either displacement-dependent, velocity-dependent,or other. Displacement-dependent devices may exhibiteither rigid-plastic (friction devices), bilinear (metallicyielding devices), or trilinear hysteresis. The responseof displacement-dependent devices should beindependent of velocity and/or frequency of excitation.Velocity-dependent devices include solid and fluidviscoelastic devices, and fluid viscous devices. Thethird classification (other) includes all devices thatcannot be classified as either displacement- or velocitydependent.Examples of “other” devices include shapememoryalloys (superelastic effect), friction-springassemblies with recentering capability, and fluidrestoring force-damping devices.Models of the energy dissipation system should includethe stiffness of structural components that are part of theload path between energy dissipation devices and theground, if the flexibility of these components issignificant enough to affect the performance of theenergy dissipation system. Structural componentswhose flexibility could affect the performance of theenergy dissipation system include components of thefoundation, braces that work in series with the energydissipation devices, and connections between bracesand the energy dissipation devices.Energy dissipation devices should be modeled asdescribed in the following subsections, unless moreadvanced methods or phenomenological models areused.9.3.3.1 Displacement-Dependent DevicesThe force-displacement response of a displacementdependentdevice is primarily a function of the relativedisplacement between each end of the device. Theresponse of such a device is substantially independentof the relative velocity between each end of the device,and/or frequency of excitation.Displacement-dependent devices should be modeled insufficient detail so as to capture their forcedisplacementresponse adequately, and theirdependence, if any, on axial-shear-flexure interaction,or bilateral deformation response.For the purposes of evaluating the response of adisplacement-dependent device from testing data, theforce in a displacement-dependent device may beexpressed as:F=k eff Dwhere the effective stiffness k eff of the device iscalculated as:F + + F –k eff = -------------------------D + + D –and where forces in the device, F + and F – , areevaluated at displacements D + and D – , respectively.9.3.3.2 Velocity-Dependent Devices(9-20)(9-21)The force-displacement response of a velocitydependentdevice is primarily a function of the relativevelocity between each end of the device.FEMA 273 Seismic Rehabilitation Guidelines 9-15

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)A. Solid Viscoelastic DevicesThe cyclic response of viscoelastic solids is generallydependent on the frequency and amplitude of themotion, and the operating temperature (includingtemperature rise due to excitation).Solid viscoelastic devices may be modeled using aspring and dashpot in parallel (Kelvin model). Thespring and dashpot constants selected should adequatelycapture the frequency and temperature dependence ofthe device consistent with fundamental frequency of therehabilitated building (f 1 ), and the operatingtemperature range. If the cyclic response of aviscoelastic solid device cannot be adequately capturedby single estimates of the spring and dashpot constants,the response of the rehabilitated building should beestimated by multiple analyses of the building frame,using limited values for the spring and dashpotconstants.The force in a viscoelastic device may be expressed as:F = k eff D + CD·(9-22)where C is the damping coefficient for the viscoelasticdevice, D is the relative displacement between each endof the device, D·is the relative velocity between eachend of the device, and k eff is the effective stiffness of thedevice calculated as:F + + F –k eff = -------------------------D + + D – = K′where K′ is the so-called storage stiffness.(9-23)B. Fluid Viscoelastic DevicesThe cyclic response of viscoelastic fluid devices isgenerally dependent on the frequency and amplitude ofthe motion, and the operating temperature (includingtemperature rise due to excitation).Fluid viscoelastic devices may be modeled using aspring and dashpot in series (Maxwell model). Thespring and dashpot constants selected should adequatelycapture the frequency and temperature dependence ofthe device consistent with fundamental frequency of therehabilitated building (f 1 ), and the operatingtemperature range. If the cyclic response of aviscoelastic fluid device cannot be adequately capturedby single estimates of the spring and dashpot constants,the response of the rehabilitated building should beestimated by multiple analyses of the building frame,using limiting values for the spring and dashpotconstants.C. Fluid Viscous DevicesThe cyclic response of a fluid viscous device isdependent on the velocity of motion; may be dependenton the frequency and amplitude of the motion; and isgenerally dependent on the operating temperature(including temperature rise due to excitation). Fluidviscous devices may exhibit some stiffness at highfrequencies of cyclic loading. Linear fluid viscousdampers exhibiting stiffness in the frequency range 0.5f 1 to 2.0 f 1 should be modeled as a fluid viscoelasticdevice.In the absence of stiffness in the frequency range 0.5 f 1to 2.0 f 1 , the force in the fluid viscous device may beexpressed as:The damping coefficient for the device should becalculated as:αF = C 0 D· sgn ( D·)(9-25)CW= --------------------- D=2πω 1 D ave------K″ω 1(9-24)where K″ is the loss stiffness, the angular frequency ω 1is equal to 2πf 1 , D ave is the average of the absolutevalues of displacements D + and D – , and W D is the areaenclosed by one complete cycle of the forcedisplacementresponse of the device.where C 0 is the damping coefficient for the device, α isthe velocity exponent for the device, D·is the relativevelocity between each end of the device, and sgn is thesignum function that, in this case, defines the sign of therelative velocity term.9.3.3.3 Other Types of DevicesEnergy dissipation devices not classified as eitherdisplacement-dependent or velocity-dependent shouldbe modeled using either established principles ofmechanics or phenomenological models. Such models9-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)should accurately describe the force-velocitydisplacementresponse of the device under all sources ofloading (e.g., gravity, seismic, thermal).9.3.4 Linear ProceduresLinear procedures are only permitted if it can bedemonstrated that the framing system exclusive of theenergy dissipation devices remains essentially linearlyelastic for the level of earthquake demand of interestafter the effects of added damping are considered.Further, the effective damping afforded by the energydissipation shall not exceed 30% of critical in thefundamental mode. Other limits on the use of linearprocedures are presented below.The secant stiffness of each energy dissipation device,calculated at the maximum displacement in the device,shall be included in the mathematical model of therehabilitated building. For the purpose of evaluating theregularity of a building, the energy dissipation devicesshall be included in the mathematical model.9.3.4.1 Linear Static ProcedureA. Displacement-Dependent DevicesThe Linear Static Procedure (LSP) may be used toimplement displacement-dependent energy dissipationdevices, provided that the following requirements aresatisfied:1. The ratio of the maximum resistance in each story, inthe direction under consideration, to the story sheardemand calculated using Equations 3-7 and 3-8,shall range between 80% and 120% of the averagevalue of the ratio for all stories. The maximum storyresistance shall include the contributions from allcomponents, elements, and energy dissipationdevices.2. The maximum resistance of all energy dissipationdevices in a story, in the direction underconsideration, shall not exceed 50% of the resistanceof the remainder of the framing where saidresistance is calculated at the displacementsanticipated in the BSE-2. Aging and environmentaleffects shall be considered in calculating themaximum resistance of the energy dissipationdevices.The pseudo lateral load of Equation 3-6 should bereduced by the damping modification factors ofTable 2-15 to account for the energy dissipation(damping) afforded by the energy dissipationdevices. The calculation of the damping effectshould be estimated as:β eff(9-26)where β is the damping in the framing system and isset equal to 0.05 unless modified in Section 2.6.1.5,W j is work done by device j in one complete cyclecorresponding to floor displacements δ i , thesummation extends over all devices j, and W k is themaximum strain energy in the frame, determinedusing Equation 9-27:(9-27)where F i is the inertia force at floor level i and thesummation extends over all floor levels.B. Velocity-Dependent DevicesThe LSP may be used to implement velocity-dependentenergy dissipation devices provided that the followingrequirements are satisfied:• The maximum resistance of all energy dissipationdevices in a story, in the direction underconsideration, shall not exceed 50% of the resistanceof the remainder of the framing where saidresistance is calculated at the displacementsanticipated in the BSE-2. Aging and environmentaleffects shall be considered in calculating themaximum resistance of the energy dissipationdevices.The pseudo lateral load of Equation 3-6 should bereduced by the damping modification factors ofTable 2-15 to account for the energy dissipation(damping) afforded by the energy dissipation devices.The calculation of the damping effect should beestimated as:∑W j= β + ------------- j4πW k1W k = -- F2∑i δ iiβ eff =∑jW jβ + -------------4πW k(9-28)FEMA 273 Seismic Rehabilitation Guidelines 9-17

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)where β is the damping in the structural frame and is setequal to 0.05 unless modified in Section 2.6.1.5, W j iswork done by device j in one complete cyclecorresponding to floor displacements δ i , the summationextends over all devices j, and W k is the maximumstrain energy in the frame, determined usingEquation 9-27.The work done by linear viscous device j in onecomplete cycle of loading may be calculated as:2π 2 2W j = --------CT jδ rj(9-29)where T is the fundamental period of the rehabilitatedbuilding including the stiffness of the velocitydependentdevices, C j is the damping constant fordevice j, and δ rj is the relative displacement between theends of device j along the axis of device j. Analternative equation for calculating the effectivedamping of Equation 9-28 is:β eff=T∑jC j cos 2 2θ jφ rjβ + ------------------------------------------w iπ ⎛----⎞ 2∑ φi⎝ g ⎠(9-30)where θ j is the angle of inclination of device j to thehorizontal, φ rj is the first mode relative displacementbetween the ends of device j in the horizontal direction,w i is the reactive weight of floor level i, φ i is the firstmode displacement at floor level i, and other terms areas defined above. Equation 9-30 applies to linearviscous devices only.The design actions for components of the rehabilitatedbuilding should be calculated in three distinct stages ofdeformation as follows. The maximum action should beused for design.1. At the stage of maximum drift. The lateral forcesat each level of the building should be calculatedusing Equations 3-7 and 3-8, where V is themodified equivalent base shear.i2. At the stage of maximum velocity and zero drift.The viscous component of force in each energydissipation device should be calculated byEquations 9-22 or 9-25, where the relative velocityD·is given by 2π f 1 D, where D is the relativedisplacement between the ends of the devicecalculated at the stage of maximum drift. Thecalculated viscous forces should be applied to themathematical model of the building at the points ofattachment of the devices and in directionsconsistent with the deformed shape of the building atmaximum drift. The horizontal inertia forces at eachfloor level of the building should be appliedconcurrently with the viscous forces so that thehorizontal displacement of each floor level is zero.3. At the stage of maximum floor acceleration.Design actions in components of the rehabilitatedbuilding should be determined as the sum of [actionsdetermined at the stage of maximum drift] times[CF 1 ] and [actions determined at the stage ofmaximum velocity] times [CF 2 ], whereCF 1 = cos[ tan – 1 ( 2β eff )]CF 2 = sin[ tan – 1 ( 2β eff )](9-31)(9-32)in which β eff is defined by either Equation 9-28 orEquation 9-30.9.3.4.2 Linear Dynamic ProcedureThe Linear Dynamic Procedures (LDP) ofSection 3.3.2.2 should be followed unless explicitlymodified by this section.The response spectrum method of the LDP may by usedwhen the effective damping in the fundamental mode ofthe rehabilitated building, in each principal direction,does not exceed 30% of critical.A. Displacement-Dependent DevicesApplication of the LDP for the analysis of rehabilitatedbuildings incorporating displacement-dependentdevices is subject to the restrictions set forth inSection 9.3.4.1A.For analysis by the Response Spectrum Method, the5%-damped response spectrum may be modified toaccount for the damping afforded by the displacementdependentenergy dissipation devices. The 5%-damped9-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)acceleration spectrum should be reduced by the modaldependentdamping modification factor, B, either B s orB 1 , for periods in the vicinity of the mode underconsideration; note that the value of B will be differentfor each mode of vibration. The damping modificationfactor in each significant mode should be determinedusing Table 2-15 and the calculated effective dampingin that mode. The effective damping should bedetermined using a procedure similar to that describedin Section 9.3.4.1A.If the maximum base shear force calculated by dynamicanalysis is less than 80% of the modified equivalentbase shear of Section 9.3.4.1, component and elementactions and deformations shall be proportionallyincreased to correspond to 80% of the modifiedequivalent base shear.B. Velocity-Dependent DevicesFor analysis by the Response Spectrum Method, the5%-damped response spectrum may be modified toaccount for the damping afforded by the velocitydependentenergy dissipation devices. The 5%-dampedacceleration spectrum should be reduced by the modaldependentdamping modification factor, B, either B s orB 1 , for periods in the vicinity of the mode underconsideration; note that the value of B will be differentfor each mode of vibration. The damping modificationfactor in each significant mode should be determinedusing Table 2-15 and the calculated effective dampingin that mode.The effective damping in the m-th mode of vibration( β ) eff – m shall be calculated as:W mjβ eff – m = βjm + -----------------4πW mk(9-33)where β m is the m-th mode damping in the buildingframe, W mj is work done by device j in one completecycle corresponding to modal floor displacements δ mi ,and W mk is the maximum strain energy in the frame inthe m-th mode, determined using Equation 9-34:∑1W mk = -- F2∑miδ mii(9-34)where F mi is the m-th mode horizontal inertia force atfloor level i and δ mi is the m-th mode horizontaldisplacement at floor level i. The work done by linearviscous device j in one complete cycle of loading in them-th mode may be calculated as:2π 2 2W mj = --------CT jδ mrj m(9-35)where T m is the m-th mode period of the rehabilitatedbuilding including the stiffness of the velocitydependentdevices, C j is the damping constant fordevice j, and δ mrj is the m-th mode relativedisplacement between the ends of device j along theaxis of device j.Direct application of the Response Spectrum Methodwill result in member actions at maximum drift.Member actions at maximum velocity and maximumacceleration in each significant mode should bedetermined using the procedure described inSection 9.3.4.1B. The combination factors CF 1 and CF 2should be determined from Equations 9-31 and 9-32using β eff –m for the m-th mode.If the maximum base shear force calculated by dynamicanalysis is less than 80% of the modified equivalentbase shear of Section 9.3.4.2, component and elementactions and deformations shall be proportionallyincreased to correspond to 80% of the modifiedequivalent base shear.9.3.5 Nonlinear ProceduresSubject to the limits set forth in Chapter 2, the nonlinearprocedures of Section 3.3.3 may be used to implementpassive energy dissipation devices without restriction.9.3.5.1 Nonlinear Static ProcedureThe Nonlinear Static Procedure (NSP) of Section 3.3.3should be followed unless explicitly modified by thissection.The nonlinear mathematical model of the rehabilitatedbuilding should explicitly include the nonlinear forcevelocity-displacementcharacteristics of the energydissipation devices, and the mechanical characteristicsof the components supporting the devices. Stiffnesscharacteristics should be consistent with thedeformations corresponding to the target displacementFEMA 273 Seismic Rehabilitation Guidelines 9-19

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)and a frequency equal to the inverse of period T e asdefined in Section 3.3.3.2.Benefits of Adding EnergyDissipation DevicesThe benefit of adding displacement-dependent energydissipation devices is recognized in the Guidelines bythe increase in building stiffness afforded by suchdevices, and the reduction in target displacementassociated with the reduction in T e . The alternativeNonlinear Static Procedure, denoted in theCommentary as Method 2, uses a different strategy tocalculate the target displacement and explicitlyrecognizes the added damping provided by the energydissipation devices.The benefits of adding velocity-dependent energydissipation devices are recognized by the increases instiffness and equivalent viscous damping in thebuilding frame. For most velocity-dependent devices,the primary benefit will be due to the added viscousdamping. Higher-mode damping forces in the energydissipation devices must be evaluated regardless ofthe Nonlinear Static Procedure used—refer to theCommentary for additional information.The nonlinear mathematical model of the rehabilitatedbuilding shall include the nonlinear force-velocitydisplacementcharacteristics of the energy dissipationdevices, and the mechanical characteristics of thecomponents supporting the devices. Energy dissipationdevices with stiffness and damping characteristics thatare dependent on excitation frequency and/ortemperature shall be modeled with characteristicsconsistent with (1) the deformations expected at thetarget displacement, and (2) a frequency equal to theinverse of the effective period.Equation 3-11 should be used to calculate the targetdisplacement. For velocity-dependent energydissipation devices, the spectral acceleration inEquation 3-11 should be reduced to account for thedamping afforded by the viscous dampers.A. Displacement-Dependent DevicesEquation 3-11 should be used to calculate the targetdisplacement. The stiffness characteristics of the energydissipation devices should be included in themathematical model.B. Velocity-Dependent DevicesThe target displacement of Equation 3-11 should bereduced to account for the damping added by thevelocity-dependent energy dissipation devices. Thecalculation of the damping effect should be estimatedas:β eff(9-36)where β is the damping in the structural frame and is setequal to 0.05 unless modified in Section 2.6.1.5, W j iswork done by device j in one complete cyclecorresponding to floor displacements δ i , the summationextends over all devices j, and W k is the maximumstrain energy in the frame, determined usingEquation 9-27.The work done by device j in one complete cycle ofloading may be calculated as:(9-37)where T s is the secant fundamental period of therehabilitated building including the stiffness of thevelocity-dependent devices (if any), calculated usingEquation 3-10 but replacing the effective stiffness (K e )with the secant stiffness (K s ) at the target displacement(see Figure 9-1); C j is the damping constant for device j;and δ rj is the relative displacement between the ends ofdevice j along the axis of device j at a roof displacementcorresponding to the target displacement.The acceptance criteria of Section 3.4.3 apply tobuildings incorporating energy dissipation devices. Theuse of Equation 9-36 will generally capture themaximum displacement of the building. Checking fordisplacement-controlled actions should usedeformations corresponding to the target displacement.Checking for force-controlled actions should usecomponent actions calculated for three limit states:maximum drift, maximum velocity, and maximumacceleration. Maximum actions shall be used for design.Higher-mode effects should be explicitly evaluated.∑W j= β + ------------- j4πW k2π 2 2W j = --------CT jδ j s9-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)Base shearV y0.6V yK eFigure 9-19.3.5.2 Nonlinear Dynamic ProcedureNonlinear Time History Analysis should be undertakenas in the requirements of Section 3.3.4.2, except asmodified by this section. The mathematical modelshould account for both the plan and vertical spatialdistribution of the energy dissipation devices in therehabilitated building. If the energy dissipation devicesare dependent on excitation frequency, operatingtemperature (including temperature rise due toexcitation), deformation (or strain), velocity, sustainedloads, and bilateral loads, such dependence should beaccounted for in the analysis.The viscous forces in velocity-dependent energydissipation devices should be included in the calculationof design actions and deformations. Substitution ofviscous effects in energy dissipation devices by globalstructural damping for nonlinear Time History Analysisis not permitted.9.3.6 Detailed Systems Requirements9.3.6.1 GeneralK iK sδ y δ tRoof displacementCalculation of Secant Stiffness, K sThe energy dissipation system and the remainder of thelateral-force-resisting system should comply with all ofthe requirements of the Guidelines.9.3.6.2 Operating TemperatureThe force-displacement response of an energydissipation device will generally be dependent onambient temperature and temperature rise due to cyclicor earthquake excitation. The analysis of a rehabilitatedbuilding should account for likely variations in theforce-displacement response of the energy dissipationdevices to bound the seismic response of the buildingduring the design earthquake, and develop limits fordefining the acceptable response of the prototype(Section 9.3.8) and production (Section 9.3.6.6)devices.9.3.6.3 Environmental ConditionsIn addition to the requirements for vertical and lateralloads induced by wind and earthquake actions, theenergy dissipation devices should be designed withconsideration given to other environmental conditions,including aging effects, creep, fatigue, ambienttemperature, and exposure to moisture and damagingsubstances.9.3.6.4 Wind ForcesThe fatigue life of energy dissipation devices, orcomponents thereof (e.g., seals in a fluid viscousdevice), should be investigated and shown to beadequate for the design life of the devices. Devicessubject to failure by low-cycle fatigue should resistwind forces in the linearly elastic range.9.3.6.5 Inspection and ReplacementAccess for inspection and replacement of the energydissipation devices should be provided.9.3.6.6 Manufacturing Quality ControlA quality control plan for manufacturing energydissipation devices should be established by theengineer of record. This plan should includedescriptions of the manufacturing processes, inspectionprocedures, and testing necessary to ensure qualitydevice production.9.3.6.7 MaintenanceThe engineer of record should establish a maintenanceand testing schedule for energy dissipation devices toensure reliable response of said devices over the designlife of the damper hardware. The degree of maintenanceand testing should reflect the established in-servicehistory of the devices.FEMA 273 Seismic Rehabilitation Guidelines 9-21

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)9.3.7 Design and Construction Review9.3.7.1 GeneralDesign and construction review of all rehabilitatedbuildings incorporating energy dissipation devicesshould be performed in accordance with therequirements of Section 2.12, unless modified by therequirements of this section. Design review of theenergy dissipation system and related test programsshould be performed by an independent engineeringreview panel, including persons licensed in theappropriate disciplines, and experienced in seismicanalysis including the theory and application of energydissipation methods.The design review should include, but should notnecessarily be limited to the following:• Preliminary design including sizing of the devices• Prototype testing (Section 9.3.8.2)• Final design of the rehabilitated building andsupporting analyses• Manufacturing quality control program for theenergy dissipation devices9.3.8 Required Tests of Energy DissipationDevices9.3.8.1 GeneralThe force-displacement relations and damping valuesassumed in the design of the passive energy dissipationsystem should be confirmed by the following tests of aselected sample of devices prior to production ofdevices for construction. Alternatively, if these testsprecede the design phase of a project, the results of thistesting program should be used for the design.The tests specified in this section are intended to: (1)confirm the force-displacement properties of thepassive energy dissipation devices assumed for design,and (2) demonstrate the robustness of individualdevices to extreme seismic excitation. These testsshould not be considered as satisfying themanufacturing quality control (production) plan ofSection 9.3.6.6.The engineer of record should provide explicitacceptance criteria for the effective stiffness anddamping values established by the prototype tests.These criteria should reflect the values assumed indesign, account for likely variations in materialproperties, and provide limiting response values outsideof which devices will be rejected.The engineer of record should provide explicitacceptance criteria for the effective stiffness anddamping values established by the production tests ofSection 9.3.6.6. The results of the prototype tests shouldform the basis of the acceptance criteria for theproduction tests, unless an alternate basis is establishedby the engineer of record in the specification. Suchacceptance criteria should recognize the influence ofloading history on the response of individual devices byrequiring production testing of devices prior toprototype testing.The fabrication and quality control procedures used forall prototype and production devices should beidentical. These procedures should be approved by theengineer of record prior to the fabrication of prototypedevices.9.3.8.2 Prototype TestsA. GeneralThe following prototype tests should be performedseparately on two full-size devices of each type and sizeused in the design. If approved by the engineer ofrecord, representative sizes of each type of device maybe selected for prototype testing, rather than each typeand size, provided that the fabrication and qualitycontrol procedures are identical for each type and sizeof devices used in the rehabilitated building.Test specimens should not be used for constructionunless approved in writing by the engineer of record.B. Data RecordingThe force-deflection relationship for each cycle of eachtest should be electronically recorded.C. Sequence and Cycles of TestingEnergy dissipation devices should not form part of thegravity-load-resisting system, but may be required tosupport some gravity load. For the following minimumtest sequence, each energy dissipation device should beloaded to simulate the gravity loads on the device asinstalled in the building, and the extreme ambienttemperatures anticipated.9-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)1. Each device should be loaded with the number ofcycles expected in the design wind storm, but notless than 2000 fully-reversed cycles of load(displacement-dependent and viscoelastic devices)or displacement (viscous devices) at amplitudesexpected in the design wind storm, at a frequencyequal to the inverse of the fundamental period of therehabilitated building.EXCEPTION: Devices not subject to wind-inducedforces or displacements need not be subjected tothese tests.2. Each device should be loaded with 20 fully reversedcycles at the displacement in the energy dissipationdevice corresponding to the BSE-2, at a frequencyequal to the inverse of the fundamental period of therehabilitated building.EXCEPTION: Energy dissipation devices may betested by other methods than those noted above,provided that: (1) equivalency between the proposedmethod and cyclic testing can be demonstrated; (2) theproposed method captures the dependence of the energydissipation device response to ambient temperature,frequency of loading, and (3) temperature rise duringtesting; and the proposed method is approved by theengineer of record.D. Devices Dependent on Velocity and/or Frequencyof ExcitationIf the force-deformation properties of the energydissipation devices at any displacement less than orequal to the total design displacement change by morethan 15% for changes in testing frequency from 0.5 f 1 to2.0 f 1 , the preceding tests should be performed atfrequencies equal to 0.5 f 1 , f 1 , and 2.0 f 1 .EXCEPTION: If reduced-scale prototypes are used toquantify the rate-dependent properties of energydissipation devices, the reduced-scale prototypes shouldbe of the same type and materials—and manufacturedwith the same processes and quality controlprocedures—as full-scale prototypes, and tested at asimilitude-scaled frequency that represents the fullscaleloading rates.E. Devices Dependent on Bilateral DisplacementIf the energy dissipation devices are subjected tosubstantial bilateral deformation, the preceding testsshould be made at both zero bilateral displacement, andpeak lateral displacement in the BSE-2.EXCEPTION: If reduced-scale prototypes are used toquantify the bilateral displacement properties of theenergy dissipation devices, the reduced-scale prototypesshould be of the same type and materials, andmanufactured with the same processes and qualitycontrol procedures, as full-scale prototypes, and testedat similitude-scaled displacements that represent thefull-scale displacements.F. Testing Similar DevicesEnergy dissipation devices that are (1) of similar size,and identical materials, internal construction, and staticand dynamic internal pressures (if any), and (2)fabricated with identical internal processes andmanufacturing quality control procedures, that havebeen previously tested by an independent laboratory, inthe manner described above, may not need be tested,provided that:1. All pertinent testing data are made available to, andapproved by the engineer of record.2. The manufacturer can substantiate the similarity ofthe previously tested devices to the satisfaction ofthe engineer of record.3. The submission of data from a previous testingprogram is approved in writing by the engineer ofrecord.9.3.8.3 Determination of Force-DisplacementCharacteristicsThe force-displacement characteristics of an energydissipation device should be based on the cyclic loadand displacement tests of prototype devices specified inSection 9.3.8.2.As required, the effective stiffness (k eff ) of an energydissipation device with stiffness should be calculatedfor each cycle of deformation as follows:F – + F +k eff = ------------------------∆ – + ∆ +(9-38)where forces F + and F – are calculated at displacements∆ + and ∆ – , respectively. The effective stiffness of anFEMA 273 Seismic Rehabilitation Guidelines 9-23

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)energy dissipation device should be established at thetest displacements in Section 9.3.8.2C.The equivalent viscous damping of an energydissipation device (β eff ) exhibiting stiffness should becalculated for each cycle of deformation as:W D1β eff = ----- -------------------2π 2k eff∆ ave(9-39)where k eff is established in Equation 9-38, and W D is thearea enclosed by one complete cycle of the forcedisplacementresponse for a single energy dissipationdevice at a prototype test displacement (∆ ave ) equal tothe average of the absolute values of displacements ∆ +and ∆ – .9.3.8.4 System AdequacyThe performance of a prototype device may be assessedas adequate if all of the following conditions aresatisfied:1. The force-displacement curves for the tests inSection 9.3.8.2C have nonnegative incrementalforce-carrying capacities.EXCEPTION: Energy dissipation devices thatexhibit velocity-dependent behavior need notcomply with this requirement.2. Within each test of Section 9.3.8.2C, the effectivestiffness (k eff ) of a prototype energy dissipationdevice for any one cycle does not differ by morethan plus or minus 15% from the average effectivestiffness as calculated from all cycles in that test.EXCEPTIONS: (1) The 15% limit may beincreased by the engineer of record in thespecification, provided that the increased limit hasbeen demonstrated by analysis to not have adeleterious effect on the response of the rehabilitatedbuilding. (2) Fluid viscous energy dissipationdevices, and other devices that do not have effectivestiffness, need not comply with this requirement.3. Within each test of Section 9.3.8.2C, the maximumforce and minimum force at zero displacement for aprototype device for any one cycle does not differ bymore than plus or minus 15% from the averagemaximum and minimum forces as calculated fromall cycles in that test.EXCEPTION: The 15% limit may be increased bythe engineer of record in the specification, providedthat the increased limit has been demonstrated byanalysis to not have a deleterious effect on theresponse of the rehabilitated building.4. Within each test of Section 9.3.8.2C, the area of thehysteresis loop (W D ) of a prototype energydissipation device for any one cycle does not differby more than plus or minus 15% from the averagearea of the hysteresis curve as calculated from allcycles in that test.EXCEPTION: The 15% limit may be increased bythe engineer of record in the specification, providedthat the increased limit has been demonstrated byanalysis to not have a deleterious effect on theresponse of the rehabilitated building.5. For displacement-dependent devices, the averageeffective stiffness, average maximum and minimumforce at zero displacement, and average area of thehysteresis loop (W D ), calculated for each test in thesequence described in Section 9.3.8.2C, shall fallwithin the limits set by the engineer-of-record in thespecification. The area of the hysteresis loop at theend of cyclic testing should not differ by more thanplus or minus 15% from the average area of the 20test cycles.6. For velocity-dependent devices, the averagemaximum and minimum force at zero displacement,effective stiffness (for viscoelastic devices only),and average area of the hysteresis loop (W D ),calculated for each test in the sequence described inSection 9.3.8.2C, shall fall within the limits set bythe engineer of record in the specification.9.4 Other Response Control SystemsResponse control strategies other than base isolation(Section 9.2) and passive energy dissipation(Section 9.3) systems have been proposed. Dynamicvibration absorption and active control systems are twosuch response control strategies. Although bothdynamic vibration absorption and active controlsystems have been implemented to control the windinducedvibration of buildings, the technology is not9-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)sufficiently mature, and the necessary hardware is notsufficiently robust, to warrant the preparation of generalguidelines for the implementation of other responsecontrol systems. However, Commentary Section C9.4provides a more detailed discussion of two othersystems: dynamic vibration absorbers and active controlsystems.The analysis and design of other response controlsystems should be reviewed by an independentengineering review panel per the requirements ofSection 9.3.7. This review panel should include personsexpert in the theory and application of the responsecontrol strategies being considered, and should beimpaneled by the owner prior to the development of thepreliminary design.9.5 DefinitionsBSE-1: Basic Safety Earthquake-1, which is thelesser of the ground shaking at a site for a 10%/50 yearearthquake or two thirds of the MCE earthquake at thesite.BSE-2: Basic Safety Earthquake-2, which is theground shaking at a site for an MCE earthquake.Design displacement: The design earthquakedisplacement of an isolation or energy dissipationsystem, or elements thereof, excluding additionaldisplacement due to actual and accidental torsion.Design earthquake: A user-specified earthquake forthe design of an isolated building, having groundshaking criteria described in Chapter 2.Displacement-dependent energy dissipationdevices: Devices having mechanical properties suchthat the force in the device is related to the relativedisplacement in the device.Displacement restraint system: Collection ofstructural components and elements that limit lateraldisplacement of seismically-isolated buildings duringthe BSE-2.Effective damping: The value of equivalent viscousdamping corresponding to the energy dissipated by thebuilding, or element thereof, during a cycle of response.Effective stiffness: The value of the lateral force inthe building, or an element thereof, divided by thecorresponding lateral displacement.Energy dissipation device (EDD): Non-gravityload-supportingelement designed to dissipate energy ina stable manner during repeated cycles of earthquakedemand.Energy dissipation system (EDS): Completecollection of all energy dissipation devices, theirsupporting framing, and connections.Isolation interface: The boundary between the upperportion of the structure (superstructure), which isisolated, and the lower portion of the structure, whichmoves rigidly with the ground.Isolation system: The collection of structuralelements that includes all individual isolator units, allstructural elements that transfer force between elementsof the isolation system, and all connections to otherstructural elements. The isolation system also includesthe wind-restraint system, if such a system is used tomeet the design requirements of this section.Isolator unit: A horizontally flexible and verticallystiff structural element of the isolation system thatpermits large lateral deformations under seismic load.An isolator unit may be used either as part of or inaddition to the weight-supporting system of thebuilding.Maximum displacement: The maximum earthquakedisplacement of an isolation or energy dissipationsystem, or elements thereof, excluding additionaldisplacement due to actual or accidental torsion.Tie-down system: The collection of structuralconnections, components, and elements that providerestraint against uplift of the structure above theisolation system.Total design displacement: The BSE-1 displacementof an isolation or energy dissipation system, or elementsthereof, including additional displacement due to actualand accidental torsion.Total maximum displacement: The maximumearthquake displacement of an isolation or energydissipation system, or elements thereof, includingFEMA 273 Seismic Rehabilitation Guidelines 9-25

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)additional displacement due to actual and accidental D M Maximum displacement, in in. (mm), attorsion.the center of rigidity of the isolationsystem in the direction underVelocity-dependent energy dissipation devices:consideration, as prescribed byDevices having mechanical characteristics such that theEquation 9-4force in the device is dependent on the relative velocityD′ Maximum displacement, in in. (mm), atin the device.Mthe center of rigidity of the isolationsystem in the direction underWind-restraint system: The collection of structuralconsideration, as prescribed byelements that provides restraint of the seismic-isolatedEquation 9-11structure for wind loads. The wind-restraint system mayDbe either an integral part of isolator units or a separate TD Total design displacement, in in. (mm), ofan element of the isolation system,device.including both translational displacementat the center of rigidity and the component9.6 Symbolsof torsional displacement in the directionunder consideration, as specified byThis list may not contain symbols defined at their firstEquation 9-6use if not used thereafter.D TM Total maximum displacement, in in. (mm),of an element of the isolation system,B D1 Numerical coefficient taken equal to theincluding both translational displacementvalue of β 1 , as set forth in Table 2-15, atat the center of rigidity and the componenteffective damping equal to the value of β Dof torsional displacement in the directionB M1 Numerical coefficient taken equal to theunder consideration, as specified byvalue of β 1 , as set forth in Table 2-15, atEquation 9-7effective damping equal to the value of β M E Loop Energy dissipated, in kip-inches (kN-mm),C or C j Damping coefficientin an isolator unit during a full cycle ofreversible load over a test displacementCF i State combination factors for use withvelocity-dependent energy dissipationrange from ∆ + to ∆ - , as measured by thedevicesarea enclosed by the loop of the forcedeflectioncurveD Target spectral displacementF Force in an energy dissipation unitD Displacement of an energy dissipation unitF – Negative force, in k, in an isolator orD ave Average displacement of an energydissipation unit, equal to (|D + | + |D – energy dissipation unit during a single|)/2cycle of prototype testing at aD – Maximum negative displacement of andisplacement amplitude of ∆ –energy dissipation unitF + Positive force, in k, in an isolator or energyD + Maximum positive displacement of andissipation unit during a single cycle ofenergy dissipation unitprototype testing at a displacementD· Relative velocity of an energy dissipationamplitude of ∆ +unitK Dmax Maximum effective stiffness, in k/in., ofD D Design displacement, in in. (mm), at thethe isolation system at the designcenter of rigidity of the isolation system indisplacement in the horizontal directionthe direction under consideration, asunder consideration, as prescribed byprescribed by Equation 9-2Equation 9-14D′ BSE-1 displacement, in in. (mm), at theD K Dmin Minimum effective stiffness, in k/in., ofcenter of rigidity of the isolation system inthe isolation system at the designthe direction under consideration, asdisplacement in the horizontal directionprescribed by Equation 9-10under consideration, as prescribed byEquation 9-159-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)K′K″K MmaxK MminS D1S DSS M1S MST DT eT MT sV *V bStorage stiffnessLoss stiffnessMaximum effective stiffness, in k/in., ofthe isolation system at the maximumdisplacement in the horizontal directionunder consideration, as prescribed byEquation 9-16Minimum effective stiffness, in k/in., ofthe isolation system at the maximumdisplacement in the horizontal directionunder consideration, as prescribed byEquation 9-17One-second, 5%-damped spectralacceleration for the design earthquake, asset forth in Chapter 2Short-period, 5%-damped spectralacceleration for the design earthquake, asset forth in Chapter 2One-second, 5%-damped spectralacceleration, as set forth in Chapter 2 forthe BSE-2Short-period, 5%-damped spectralacceleration, as set forth in Chapter 2 forthe BSE-2Effective period, in seconds, of theseismic-isolated structure at the designdisplacement in the direction underconsideration, as prescribed byEquation 9-3Effective fundamental-mode period, inseconds, of the building in the directionunder considerationEffective period, in seconds, of theseismic-isolated structure at the maximumdisplacement in the direction underconsideration, as prescribed byEquation 9-5Secant fundamental period of arehabilitated building calculated usingEquation 3-10 but replacing the effectivestiffness (K e ) with the secant stiffness (K s )at the target displacementModified equivalent base shearThe total lateral seismic design force orshear on elements of the isolation systemor elements below the isolation system, asprescribed by Equation 9-8V sV tWW Dbdef 1gk effmqy∆ aveThe total lateral seismic design force orshear on elements above the isolationsystem, as prescribed by Section 9.2.4.4BTotal base shear determined by Time-History AnalysisThe total seismic dead load. For design ofthe isolation system, W is the total seismicdead load weight of the structure above theisolation interfaceEnergy dissipated, in in.-k, in a building orelement thereof during a full cycle ofdisplacementThe shortest plan dimension of therehabilitated building, in ft (mm),measured perpendicular to dThe longest plan dimension of therehabilitated building, in ft (mm)Actual eccentricity, ft (mm), measured inplan between the center of mass of thestructure above the isolation interface andthe center of rigidity of the isolationsystem, plus accidental eccentricity, ft(mm), taken as 5% of the maximumbuilding dimension perpendicular to thedirection of force under considerationFundamental frequency of the buildingAcceleration of gravity (386.1 in/sec. 2 , or9,800 mm/sec. 2 for SI units)Effective stiffness of an isolator unit, asprescribed by Equation 9-12, or an energydissipation unit, as prescribed byEquation 9-38Mass (k-sec 2 /in.)Coefficient, less than one, equal to theratio of actual hysteresis loop area toidealized bilinear hysteresis loop areaThe distance, in ft (mm), between thecenter of rigidity of the isolation systemrigidity and the element of interest,measured perpendicular to the direction ofseismic loading under considerationAverage displacement of an energydissipation unit during a cycle of prototypetesting, equal to (|∆ + | + |∆ – |)/2FEMA 273 Seismic Rehabilitation Guidelines 9-27

Chapter 9: Seismic Isolation andEnergy Dissipation (Systematic Rehabilitation)∆ +∆ –ΣE DΣE M+Σ F D+Σ F D–Σ F D–Σ F D+Σ F M+Σ F M–Σ F M–Σ F Mββ bmaxminmaxminmaxminmaxminPositive displacement amplitude, in in.(mm), of an isolator or energy dissipationunit during a cycle of prototype testingNegative displacement amplitude, in in.(mm), of an isolator or energy dissipationunit during a cycle of prototype testingTotal energy dissipated, in in.-k, in theisolation system during a full cycle ofresponse at the design displacement, D DTotal energy dissipated, in in.-k, in theisolation system during a full cycle ofresponse at the maximum displacement,D MSum, for all isolator units, of the maximumabsolute value of force, k, at a positivedisplacement equal to D DSum, for all isolator units, of the minimumabsolute value of force, k, at a positivedisplacement equal to D DSum, for all isolator units, of the maximumabsolute value of force, k, at a negativedisplacement equal to D DSum, for all isolator units, of the minimumabsolute value of force, k, at a negativedisplacement equal to D DSum, for all isolator units, of the maximumabsolute value of force, k, at a positivedisplacement equal to D MSum, for all isolator units, of the minimumabsolute value of force, k, at a positivedisplacement equal to D MSum, for all isolator units, of the maximumabsolute value of force, k, at a negativedisplacement equal to D MSum, for all isolator units, of the minimumabsolute value of force, k, at a negativedisplacement equal to D MDamping inherent in the building frame(typically equal to 0.05)Equivalent viscous damping of a bilinearsystemβ effβ Dβ Mδ iθ jφ iφ rjω 1 2πf 1Effective damping of isolator unit, asprescribed by Equation 9-13, or an energydissipation unit, as prescribed byEquation 9-39; also used for the effectivedamping of the building, as prescribed byEquations 9-26, 9-30, and 9-36Effective damping of the isolation systemat the design displacement, as prescribedby Equation 9-18Effective damping of the isolation systemat the maximum displacement, asprescribed by Equation 9-19Floor displacementAngle of inclination of energy dissipationdeviceModal displacement of floor iRelative modal displacement in horizontaldirection of energy dissipation device j9.7 ReferencesBSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos.FEMA 222A and 223A), Washington, D.C.BSSC, 1997, NEHRP Recommended Provisions forSeismic Regulations for New Buildings and otherStructures, 1997 Edition, Part 1: Provisions and Part 2:Commentary, prepared by the Building Seismic SafetyCouncil for the Federal Emergency ManagementAgency (Report Nos. FEMA 302 and 303),Washington, D.C.Secretary of the Interior, 1993, Standards andGuidelines for Archaeology and Historic Preservation,published in the Federal Register, Vol. 48, No. 190,pp. 44716–44742.9-28 Seismic Rehabilitation Guidelines FEMA 273

10.Simplified Rehabilitation10.1 ScopeThis chapter presents the Simplified RehabilitationMethod, which is intended primarily for use on aselected group of simple buildings being rehabilitated tothe Life Safety Performance Level for the level ofground motion specified in FEMA 178, NEHRPHandbook for the Seismic Evaluation of ExistingBuildings (BSSC, 1992a). In an area of low or moderateseismicity, designing for this level of ground motionmay not be sufficient to provide Life SafetyPerformance if a large infrequent earthquake occurs.The technique described in this chapter is one of the tworehabilitation methods defined in Chapter 2. It is to beused only by a design professional, and only in amanner consistent with the Guidelines. Considerationmust be given to all aspects of the rehabilitationprocess, including the development of appropriate asbuiltinformation, proper design of rehabilitationtechniques, and specification of appropriate levels ofquality assurance. Systematic Rehabilitation is the otherrehabilitation method defined in Chapter 2.The term “Simplified Rehabilitation” is intended toreflect a level of analysis and design that (1) isappropriate for small, regular buildings, and buildingsthat do not require advanced analytical procedures, and(2) does not achieve the Basic Safety Objective (BSO).FEMA 178 (BSSC, 1992a), the NEHRP Handbook forthe Seismic Evaluation of Existing Buildings, anationally applicable evaluation method, is the basis forthe Simplified Rehabilitation Method. FEMA 178 isbased on the historic behavior of buildings in pastearthquakes and the success of current code provisionsin achieving the Life Safety Performance Level. It isorganized around a set of common construction stylescalled model buildings. The performance of certaincommon building types that meet specific limitations onheight and regularity can be substantially improved bysimply eliminating all of the deficiencies found usingFEMA 178. See Section C10.1 in the Commentary forfurther information on FEMA 178 and otherintroductory comments. FEMA 178 is currently underrevision (October, 1997) and the revised version will beavailable soon. These Guidelines refer frequently toFEMA 178 as a pointer to the FEMA 178 references.Since the preliminary version of FEMA 178 wascompleted in the late 1980s, new information hasbecome available, which will be added to FEMA 178 inthe updated edition of the document now underway.This information has been included in the SimplifiedRehabilitation Method, presented as amendments toFEMA 178 (BSSC, 1992a), and includes additionalModel Building Types and eight new evaluationstatements for new potential deficiencies. They arepresented in the same format and style as used inFEMA 178. The set of common Model Building Typeshas been expanded to separate those buildings with stiffand flexible diaphragms, and to account for the uniquebehavior of multistory, multi-unit, wood-framestructures. While near-fault effects are also beingproposed to amend FEMA 178, they are not expected toaffect the buildings eligible for SimplifiedRehabilitation and therefore need not be considered.The evaluation statements and procedures contained inFEMA 178 apply best to low-rise and, in some cases,mid-rise buildings of regular configuration and welldefinedbuilding type. Table 10-1 identifies thosebuildings for which the Simplified RehabilitationMethod can be used to achieve the Life SafetyPerformance Level for ground motions specified inFEMA 178 (BSSC, 1992a). It is required, however, thatthe building deficiencies be corrected by strengtheningand/or modifying the existing components of thebuilding using the same basic style of construction.Buildings that have configuration irregularities, asdefined in the NEHRP Recommended Provisions forRegulations for New Buildings (BSSC, 1995), may usethis Simplified Rehabilitation Method to achieve theLife Safety Performance Level only if the resultingrehabilitation work eliminates all significant verticaland horizontal irregularities and results in a buildingwith a complete seismic lateral-force-resisting loadpath.The Simplified Rehabilitation Method may be used toachieve Limited Rehabilitation Objectives for anybuilding not listed in Table 10-1. (Note that Table 10-1,the remaining Tables 10-2 to 10-22, and Figure 10-1 areat the end of this chapter.)The Simplified Rehabilitation Method may yield a moreconservative result than the Systematic Method. This isdue to the variety of simplifying assumptions. Becauseof the small size and simplicity of the buildings that areFEMA 273 Seismic Rehabilitation Guidelines 10-1

Chapter 10: Simplified Rehabilitationeligible for the Simplified Method of achieving the LifeSafety Performance Level, the economic consequencesof this conservatism are likely to be insignificant. Itmust be understood, however, that a simple comparisonof the design-based shear in FEMA 178 with Chapter 3of the Guidelines will lead to the opposite conclusion.The equivalent lateral forces used in these twodocuments have entirely different definitions and bases.The FEMA 178 (BSSC, 1992a) values, which are basedon the traditional techniques used in building codes,have been developed on a different basis than inChapter 3 of the Guidelines, and have been taken fromthe 1988 NEHRP Provisions. The Chapter 3 valuescalculated from a “pseudo lateral load,” are defined fora component-based analysis and do not include thesame reduction factors. As shown in Figure 10-1, whilethe base and story shear values may vary byapproximately six times, the ratios of demand/capacityvary only slightly.Implementing a rehabilitation scheme that mitigates allof a building's FEMA 178 (BSSC, 1992a) deficienciesusing the Simplified Rehabilitation Method does not inand of itself achieve the Basic Safety Objective or anyEnhanced Rehabilitation Objective as defined inChapter 2, since the rehabilitated building may not meetthe Collapse Prevention Performance Level for BSE-2.If the goal is to attain the Basic Safety Objective asdescribed in Chapter 2 or other EnhancedRehabilitation Objectives, this can be accomplished byusing the Systematic Rehabilitation Method defined inChapter 2.10.2 Procedural StepsThe application of the Simplified RehabilitationMethod first requires a complete FEMA 178 (BSSC,1992a) evaluation of a building, which results in a listof deficiencies. These deficiencies are then ranked, andcommon and simple rehabilitation procedures areapplied to correct them. Once a full rehabilitationscheme has been devised, the building is reevaluatedusing FEMA 178 to verify that it fully meets therequirements. A more complete statement of thisprocedure follows. The procedures are applicable onlyto buildings that meet the qualification criteria shown inTable 10-1.1. Identify the model building type. Each is describedin Table 10-2 and in more detail in FEMA 178(BSSC, 1992a). The building must be one of thecommon building types and satisfy the criteriadescribed in Table 10-1.2. Identify and rank all potential deficiencies for thebuilding from Tables 10-3 through 10-21. The itemsin these tables are ordered roughly from highestpriority at the top to lowest at the bottom, thoughthis can vary widely in individual cases. Develop asbuiltinformation as required in GuidelinesSection 2.7. Use the procedures in FEMA 178—andalso those listed in Guidelines Section 10.4 for theeight new potential deficiencies—in order toevaluate fully each potential deficiency and developa list of actual deficiencies in priority order forcorrection. If necessary, refer to Section C10.5 of theCommentary for a complete list of FEMA 178deficiencies and their relationship to the deficiencylist used here. Table 10-22 provides a crossreferencebetween all FEMA 178 (BSSC, 1992a)deficiencies and those of this chapter.3. Develop strengthening details to mitigate thedeficiencies using the same basic style and materialsof construction. Refer to Section 10.3 and theCommentary for rehabilitation strategies associatedwith each identified deficiency. In most cases, theresulting rehabilitated building must be one of theModel Building Types. For example, addingconcrete shear walls to concrete shear wall buildingsor adding a complete system of concrete shear wallsto a concrete frame building meets this requirement.Some exceptions include using steel bracing tostrengthen wood or URM construction. For largebuildings, it is advisable to explore severalrehabilitation strategies and compare alternativeways of eliminating deficiencies.4. Design the proposed rehabilitation based on theFEMA 178 (BSSC, 1992a) criteria, including itsAppendix C, such that all deficiencies areeliminated.5. Once rehabilitation techniques have been developedfor all deficiencies, perform a complete evaluationof the building in its proposed rehabilitated state,following the FEMA 178 (BSSC, 1992a)procedures. This step should confirm that thestrengthening of any one element or system has notmerely shifted the deficiency to another.6. To achieve the BSO, consider the rehabilitatedstructure’s potential performance using the10-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationSystematic Rehabilitation Method. Determinewhether the total strength of the building issufficient, and judge whether the building canexperience the predicted maximum displacementwithout partial or complete collapse.7. Identify and develop strengthening details for thearchitectural, mechanical, and electricalcomponents. Refer to the procedures in Chapter 11for the evaluation and rehabilitation of nonstructuralelements related to the Life Safety PerformanceLevel, given the BSE-1 earthquake.8. Develop the needed construction documents,including drawings and specifications, and includean appropriate quality assurance program as definedin Chapter 2. If only partial rehabilitation isintended, it is recommended that the deficiencies becorrected in priority order and in a way that willfacilitate fulfillment of the requirements of a higherobjective at a later date. Care must be taken toensure that a partial rehabilitation effort does notmake the building’s overall performance worse, suchas by unintentionally channeling failure to a morecritical element.10.3 Suggested Corrective Measuresfor DeficienciesTables 10-3 to 10-21 list the potential deficiencies forthe various Model Building Types. Each of these maybe shown to be a deficiency that needs correction duringa rehabilitation effort. (See Commentary Section C10.5for a complete list of evaluation statements foridentifying potential deficiencies, both those inFEMA 178 (BSSC, 1992a) and the Amendments toFEMA 178 in these Guidelines, Section 10.4. Thefollowing sections describe suggested correctivemeasures for each deficiency. They are organized intodeficiency groups similar to those used in FEMA 178,and are intended to assist the thinking of the designprofessional. Other appropriate solutions may be used.The Commentary provides further discussion of theranking of the deficiencies.10.3.1 Building Systems10.3.1.1 Load PathLoad path discontinuities can be mitigated by addingelements to complete the load path. This may requireadding new well-founded shear walls or frames to fill inthe gaps in existing shear walls or frames that are notcarried continuously all the way down to thefoundation. Alternatively, it may require the addition ofelements throughout the building to pick up loads fromdiaphragms that have no path into existing verticalelements. (FEMA 178 [BSSC, 1992a], Section 3.1.)10.3.1.2 RedundancyThe most prudent rehabilitation strategy for a buildingwithout redundancy is to add new lateral-force-resistingelements in locations where the failure of a singleelement will cause an instability in the building. Theadded lateral-force-resisting elements should be of thesame stiffness as the elements they are supplementing.It is not generally satisfactory just to strengthen anonredundant element (such as by adding cover platesto a slender brace), because its failure would still resultin an instability. (FEMA 178 [BSSC, 1992a],Section 3.2.)10.3.1.3 Vertical IrregularitiesNew vertical lateral-force-resisting elements can beprovided to eliminate the vertical irregularity. For weakstories, soft stories, and vertical discontinuities, newelements of the same type can be added as needed.Mass and geometric discontinuities must be evaluatedand strengthened based on Systematic Rehabilitation, ifrequired by Chapter 2. (FEMA 178 [BSSC, 1992a],Sections 3.3.1 through 3.3.5.)10.3.1.4 Plan IrregularitiesThe effects of plan irregularities that create torsion canbe eliminated with the addition of lateral-force-resistingbracing elements that will support all major diaphragmsegments in a balanced manner. While it is possible insome cases to allow the irregularity to remain andinstead strengthen those structural elements that areoverstressed by its existence, this may requiresubstantial additional analysis, does not directly addressthe problem, and requires use of the SystematicRehabilitation Method. (FEMA 178 [BSSC, 1992a],Section 3.3.6.)10.3.1.5 Adjacent BuildingsStiffening elements (typically braced frames or shearwalls) can be added to one or both buildings to reducethe expected drifts to acceptable levels. With separatestructures in a single building complex, it may bepossible to tie them together structurally to force themto respond as a single structure. The relative stiffnessesFEMA 273 Seismic Rehabilitation Guidelines 10-3

Chapter 10: Simplified Rehabilitationof each and the resulting force interactions must bedetermined to ensure that additional deficiencies are notcreated. Pounding can also be eliminated bydemolishing a portion of one building to increase theseparation. (FEMA 178 [BSSC, 1992a], Section 3.4.)10.3.1.6 Lateral Load Path at Pile CapsTypically, deficiencies in the load path at the pile capsare not a life safety concern. However, if the designprofessional has determined that there is strongpossibility of a life safety hazard due to this deficiency,piles and pile caps may be modified, supplemented,repaired, or in the most severe condition, replaced intheir entirety. Alternatively, the building system may berehabilitated such that the pile caps are protected.10.3.1.7 Deflection CompatibilityVertical lateral-force-resisting elements can be added todecrease the drift demands on the columns, or theductility of the columns can be increased. Jacketing thecolumns with steel or concrete is one approach toincrease their ductility.10.3.2 Moment Frames10.3.2.1 Steel Moment FramesA. DriftThe most direct mitigation approach is to add properlyplaced and distributed stiffening elements—such asnew moment frames, braced frames, or shear walls—that can reduce the inter-story drifts to acceptablelevels. Alternatively, the addition of energy dissipationdevices to the system may reduce the drift, though theseare outside the scope of Simplified Rehabilitation.(FEMA 178 [BSSC, 1992a], Section 4.2.1.)B. FramesNoncompact members can be eliminated by addingappropriate steel plates. Eliminating or properlyreinforcing large member penetrations will develop thedemanded strength and deformations. Lateral bracing inthe form of new steel elements can be added to reducemember unbraced lengths to within the limitsprescribed. Stiffening elements (e.g., braced frames,shear walls, or additional moment frames) can be addedthroughout the building to reduce the expected framedemands. (FEMA 178 [BSSC, 1992a], Sections 4.2.2,4.2.3, and 4.2.9.)C. Strong Column-Weak BeamSteel plates can be added to increase the strength of thesteel columns to beyond that of the beams, to eliminatethis issue. Stiffening elements (e.g., braced frames,shear walls, or additional moment frames) can be addedthroughout the building to reduce the expected framedemands. (FEMA 178 [BSSC, 1992a], Section 4.2.8.)D. ConnectionsAdding a stiffer lateral-force-resisting system (e.g.,braced frames or shear walls) can reduce the expectedrotation demands. Connections can be modified byadding flange cover plates, vertical ribs, haunches, orbrackets, or removing beam flange material to initiateyielding away from the connection location (e.g., via apattern of drilled holes or the cutting out of flangematerial). Partial penetration splices, which maybecome more vulnerable for conditions where thebeam-column connections are modified to be moreductile, can be modified by adding plates and/or welds.Adding continuity plates alone is not likely to enhancethe connection performance significantly. (FEMA 178[BSSC, 1992a], Sections 4.2.4, 4.2.5, 4.2.6, and 4.2.7.)Moment-resisting connection capacity can be increasedby adding cover plates or haunches, or using othertechniques as stipulated in the SAC Interim Guidelines,FEMA 267 (SAC, 1995).10.3.2.2 Concrete Moment FramesA. Frame and Nonductile Detail ConcernsAdding properly placed and distributed stiffeningelements such as shear walls will fully supplement themoment frame system with a new lateral-force-resistingsystem. For eccentric joints, columns and/or beams maybe jacketed to reduce the effective eccentricity. Jacketsmay also be provided for shear-critical columns.It must be verified that this new system sufficientlyreduces the frame shears and inter-story drifts toacceptable levels. (FEMA 178 [BSSC, 1992a],Sections 4.3.1–4.3.15.)B. Precast Moment FramesPrecast concrete frames without shear walls may not beaddressed under the Simplified Rehabilitation Method(see Table 10-1). Where shear walls are present, theprecast connections must be strengthened sufficiently tomeet the FEMA 178 (BSSC, 1992a) requirements.10-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationThe development of a competent load path is extremelycritical in these buildings. If the connections havesufficient strength so that yielding will first occur in themembers rather than in the connections, the buildingshould be evaluated as a shear wall system (Type C2).(FEMA 178 [BSSC, 1992a] Section 4.4.1.)10.3.2.3 Frames Not Part of theLateral-Force-Resisting SystemA. Complete FramesComplete frames, of steel or concrete, form a completevertical-load-carrying system.Incomplete frames are essentially bearing wall systems.The wall must be strengthened to resist the combinedgravity/seismic loads or new columns added tocomplete the gravity load path. (FEMA 178 [BSSC,1992a], Section 4.5.1.)B. Short Captive ColumnsColumns may be jacketed with steel or concrete suchthat they can resist the expected forces and drifts.Alternatively, the expected story drifts can be reducedthroughout the building by infilling openings or addingshear walls. (Section 10.4.2.2.)10.3.3 Shear Walls10.3.3.1 Cast-in-Place Concrete Shear WallsA. Shearing StressNew shear walls can be provided and/or the existingwalls can be strengthened to satisfy seismic demandcriteria. New and strengthened walls must form acomplete, balanced, and properly detailed lateral-forceresistingsystem for the building. Special care is neededto ensure that the connection of the new walls to theexisting diaphragm is appropriate and of sufficientstrength such that yielding will first occur in the wall.All shear walls must have sufficient shear andoverturning resistance to meet the FEMA 178 loadcriteria. (FEMA 178 [BSSC, 1992a], Section 5.1.1.)B. OverturningLengthening or adding shear walls can reduceoverturning demands; increasing the length of footingswill capture additional building dead load. (FEMA 178[BSSC, 1992a], Section 5.1.2.)C. Coupling BeamsTo eliminate the need to rely on the coupling beam, thewalls may be strengthened as required. The beamshould be jacketed only as a means of controllingdebris. If possible, the opening that defines the couplingbeam should be infilled. (FEMA 178 [BSSC, 1992a],Section 5.1.3.)D. Boundary Component DetailingSplices may be improved by welding bars together afterexposing them. The shear transfer mechanism can beimproved by adding steel studs and jacketing theboundary components. (FEMA 178 [BSSC, 1992a],Sections 5.1.4 through 5.1.6.)E. Wall ReinforcementThe shear walls can be strengthened by infillingopenings, or by thickening the walls (see FEMA 172[BSSC, 1992b], Section 3.2.1.2). (FEMA 178 [BSSC,1992a], Sections 5.1.7 and 5.1.8.)10.3.3.2 Precast Concrete Shear WallsA. Panel-to-Panel ConnectionsAppropriate Simplified Rehabilitation solutions areoutlined in FEMA 172, Section 3.2.2.3. (FEMA 178[BSSC, 1992a], Section 5.2.1.)Inter-panel connections with inadequate capacity can bestrengthened by adding steel plates across the joint, orby providing a continuous wall by exposing thereinforcing steel in the adjacent units, providing tiesbetween the panels and patching with concrete.Providing steel plates across the joint is typically themost cost-effective approach, although care must betaken to ensure adequate anchor bolt capacity byproviding adequate edge distances (see FEMA 172,Section 3.2.2).B. Wall OpeningsInfilling openings or adding shear walls in the plane ofthe open bays can reduce demand on the connectionsand eliminate frame action. (FEMA 178 [BSSC,1992a], Section 5.2.2.)C. CollectorsUpgrading the concrete section and/or the connections(e.g., exposing the existing connection, addingconfinement ties, increasing embedment) can increasestrength and/or ductility. Alternative load paths forlateral forces can be provided, and shear walls added toFEMA 273 Seismic Rehabilitation Guidelines 10-5

Chapter 10: Simplified Rehabilitationreduce demand on the existing collectors. (FEMA 178[BSSC, 1992a], Section 5.2.3.)10.3.3.3 Masonry Shear WallsA. Reinforcing in Masonry WallsNondestructive methods should be used to locatereinforcement, and selective demolition used ifnecessary to determine the size and spacing of thereinforcing. If it cannot be verified that the wall isreinforced in accordance with the minimumrequirements, then the wall should be assumed to beunreinforced, and therefore must be supplemented withnew walls, or the procedures for unreinforced masonryshould be followed. (FEMA 178 [BSSC, 1992a],Section 5.3.2.)B. Shearing StressTo meet the lateral force requirements of FEMA 178(BSSC, 1992a), new walls can be provided, or theexisting walls strengthened as needed. New andstrengthened walls must form a complete, balanced, andproperly detailed lateral-force-resisting system for thebuilding. Special care is needed to ensure that theconnection of the new walls to the existing diaphragm isappropriate and of sufficient strength to deliver theactual lateral loads or force yielding in the wall. Allshear walls must have sufficient shear and overturningresistance.C. Reinforcing at OpeningsThe presence and location of reinforcing steel atopenings may be established using nondestructive ordestructive methods at selected locations to verify thesize and location of the reinforcing, or using bothmethods. Reinforcing must be provided at all openingsas required to meet the FEMA 178 criteria. Steel platemay be bolted to the surface of the section as long as thebolts are sufficient to yield the steel plate. (FEMA 178[BSSC, 1992a], Section 5.3.3.)D. Unreinforced Masonry Shear WallsOpenings in the lateral-force-resisting walls should beinfilled as needed to meet the FEMA 178 (BSSC,1992a) stress check. If supplemental strengthening isrequired, it should be designed using the SystematicRehabilitation Method as defined in Chapter 2. Wallsthat do not meet the masonry lay-up requirementsshould not be considered as lateral-force-resistingelements and shall be specially supported forout-of-plane loads. (FEMA 178 [BSSC, 1992a],Sections 5.4.1, 5.4.2.)E. Proportions of Solid WallsWalls with insufficient thickness should be strengthenedeither by increasing the thickness of the wall or byadding a well-detailed strong back system. Thethickened wall must be detailed in a manner that fullyinterconnects the wall over its full height. The strongback system must be designed for strength, connectedto the structure in a manner that: (1) develops the fullyield strength of the strong back, and (2) connects to thediaphragm in a manner that distributes the load into thediaphragm and has sufficient stiffness to ensure that theelements will perform in a compatible and acceptablemanner. The stiffness of the bracing should limit theout-of-plane deflections to acceptable levels such as L/600 to L/900 (FEMA 178 [BSSC, 1992a],Sections 5.5.1, 5.5.2.)F. Infill WallsThe partial infill wall should be isolated from theboundary columns to avoid a “short column” effect,except when it can be shown that the column isadequate. In sizing the gap between the wall and thecolumns, the anticipated inter-story drift must beconsidered. The wall must be positively restrainedagainst out-of-plane failure by either bracing the top ofthe wall, or installing vertical girts. These bracingelements must not violate the isolation of the framefrom the infill. (FEMA 178 [BSSC, 1992a], Sections5.5.3, 4.1.1.)10.3.3.4 Shear Walls in Wood Frame BuildingsA. Shear StressWalls may be added or existing openings filled.Alternatively, the existing walls and connections can bestrengthened. The walls should be distributed across thebuilding in a balanced manner to reduce the shear stressfor each wall. Replacing heavy materials such as tileroofing with lighter materials will also reduce shearstress. (FEMA 178 [BSSC, 1992a], Section 5.6.1.)B. OpeningsLocal shear transfer stresses can be reduced bydistributing the forces from the diaphragm. Chords and/or collector members can be provided to collect anddistribute shear from the diaphragm to the shear wall orbracing (see FEMA 172, Figure 3.7.1.3). Alternatively,the opening can be closed off by adding a new wall with10-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified Rehabilitationplywood sheathing. (FEMA 178 [BSSC, 1992a],Section 5.6.2.)C. Wall DetailingIf the walls are not bolted to the foundation or thebolting is inadequate, bolts can be installed through thesill plates at regular intervals (see FEMA 172 [BSSC,1992b], Figure 3.8.1.2a). If the crawl space is not deepenough for vertical holes to be drilled through the sillplate, the installation of connection plates or angles maybe a more practical alternative (see FEMA 172,Figure 3.8.1.2b). Sheathing and additional nailing canbe added where walls lack proper nailing orconnections. Where the existing connections areinadequate, adding clips or straps will deliver lateralloads to the walls and to the foundation sill plate.(FEMA 178 [BSSC, 1992a], Section 5.6.3.)D. Cripple WallsWhere bracing is inadequate, new plywood sheathingcan be added to the cripple wall studs. The top edge ofthe plywood is nailed to the floor framing and thebottom edge is nailed into the sill plate (see FEMA 172,Figure 3.8.1.3). Verify that the cripple wall does notchange height along its length (stepped top offoundation). If it does, the shorter portion of the cripplewall will carry the majority of the shear and significanttorsion will occur in the foundation. Added plywoodsheathing must have adequate strength and stiffness toreduce torsion to an acceptable level. Also, it should beverified that the sill plate is properly anchored to thefoundation. If anchor bolts are lacking or insufficient,additional anchor bolts should be installed. Blockingand/or framing clips may be needed to connect thecripple wall bracing to the floor diaphragm or the sillplate. (FEMA 178 [BSSC, 1992a], Section 5.6.4.)E. Narrow Wood Shear WallsWhere narrow shear walls lack capacity, they should bereplaced with shear walls with a height-to-width aspectratio of two to one or less. These replacement wallsmust have sufficient strength, including beingadequately connected to the diaphragm and sufficientlyanchored to the foundation for shear and overturningforces. (Guidelines Section 10.4.3.1.)F. Stucco Shear WallsFor strengthening or repair, the stucco should beremoved, a plywood shear wall added, and new stuccoapplied. The plywood should be the manufacturer’srecommended thickness for the installation of stucco.The new stucco should be installed in accordance withbuilding code requirements for waterproofing. Wallsshould be sufficiently anchored to the diaphragm andfoundation. (Guidelines Section 10.4.3.2.)G. Gypsum Wallboard or Plaster Shear WallsPlaster and gypsum wallboard can be removed andreplaced with structural panel shear wall as required,and the new shear walls covered with gypsumwallboard. (Guidelines Section 10.4.3.3.)10.3.4 Steel Braced Frames10.3.4.1 System ConcernsIf the strength of the braced frames is inadequate, morebraced bays or shear wall panels can be added. Theresulting lateral-force-resisting system must form awell-balanced system of braced frames that do not failat their joints, and are properly connected to the floordiaphragms, and whose failure mode is yielding ofbraces rather than overturning. (FEMA 178 [BSSC,1992a], Section 6.1.1.)10.3.4.2 Stiffness of DiagonalsDiagonals with inadequate stiffness should bestrengthened using the supplemental steel plates, orreplaced with a larger and/or different type of section.Global stiffness can be increased by the addition ofbraced bays or shear wall panels. (FEMA 178 [BSSC,1992a], Sections 6.1.2 and 6.1.3.)10.3.4.3 Chevron or K-BracingColumns or horizontal girts can be added as needed tosupport the tension brace when the compression bracebuckles, or the bracing can be revised to another systemthroughout the building. The beam elements can bestrengthened with cover plates to provide them with thecapacity to fully develop the unbalanced forces createdby tension brace yielding. (FEMA 178 [BSSC, 1992a],Section 6.1.4.)10.3.4.4 Braced Frame ConnectionsColumn splices or other braced frame connections canbe strengthened by adding plates and welds to ensurethat they are strong enough to develop the connectedmembers. Connection eccentricities that reducemember capacities can be eliminated, or the memberscan be strengthened to the required level by the additionof properly placed plates. Demands on the existingelements can be reduced by adding braced bays or shearFEMA 273 Seismic Rehabilitation Guidelines 10-7

Chapter 10: Simplified Rehabilitationwall panels. (FEMA 178 [BSSC, 1992a], Sections6.1.5, 6.1.6, and 6.1.7.)10.3.5 Diaphragms10.3.5.1 Re-entrant CornersNew chords with sufficient strength to resist therequired force can be added at the re-entrant corner. If avertical lateral-force-resisting element exists at the reentrantcorner, a new shear collector element should beplaced at the diaphragm, connected to the verticalelement, to reduce tensile and compressive forces at there-entrant corner. The same basic materials used in thediaphragm being strengthened should be used for thechord. (FEMA 178 [BSSC, 1992a], Section 7.1.1.)10.3.5.2 CrosstiesNew crossties and wall connections can be added toresist the required out-of-plane wall forces anddistribute these forces through the diaphragm. Newstrap plates and/or rod connections can be used toconnect existing framing members together so theyfunction as a crosstie in the diaphragm. (FEMA 178[BSSC, 1992a], Section 7.1.2.)10.3.5.3 Diaphragm OpeningsNew drag struts or diaphragm chords can be addedaround the perimeter of existing openings to distributetension and compression forces along the diaphragm.The existing sheathing should be nailed to the new dragstruts or diaphragm chords. In some cases it may also benecessary to: (1) increase the shear capacity of thediaphragm adjacent to the opening by overlaying theexisting diaphragm with a wood structural panel, or (2)decrease the demand on the diaphragm by adding newvertical elements near the opening. (FEMA 178 [BSSC,1992a], Sections 7.1.3 through 7.1.6.)10.3.5.4 Diaphragm Stiffness/StrengthA. Board SheathingWhen the diaphragm does not have at least two nailsthrough each board into each of the supportingmembers, and the lateral drift and/or shear demands onthe diaphragm are not excessive, the shear capacity andstiffness of the diaphragm can be increased by addingnails at the sheathing boards. This method of upgrade ismost often suitable in areas of low seismicity. In othercases, a new wood structural panel should be placedover the existing straight sheathing, and the joints of thewood structural panels placed so they are near thecenter of the sheathing boards or at a 45-degree angle tothe joints between sheathing boards (see FEMA 172[BSSC, 1992b], Section 3.5.1.2; ATC, [1981]; andFEMA 178 [BSSC, 1992a], Section 7.2.1).B. Unblocked DiaphragmsThe shear capacity of unblocked diaphragms can beimproved by adding new wood blocking and nailing atthe unsupported panel edges. Placing a new woodstructural panel over the existing diaphragm willincrease the shear capacity. Both of these methods willrequire the partial or total removal of existing flooringor roofing to place and nail the new overlay or nail theexisting panels to the new blocking. Strengthening ofthe diaphragm is usually not necessary at the centralarea of the diaphragm where shear is low. In certaincases when the design loads are low, it may be possibleto increase the shear capacity of unblocked diaphragmswith sheet metal plates stapled on the underside of theexisting wood panels. These plates and staples must bedesigned for all related shear and torsion caused by thedetails related to their installation. (FEMA 178 [BSSC,1992a], Section 7.2.3.)C. SpansNew vertical elements can be added to reduce thediaphragm span. The reduction of the diaphragm spanwill also reduce the lateral deflection and shear demandin the diaphragm. However, adding new verticalelements will result in a different distribution of sheardemands. Additional blocking, nailing, or otherrehabilitation measures may need to be provided atthese areas. (FEMA 172, Section 3.4 and FEMA 178[BSSC, 1992a], Section 7.2.2.)D. Span-to-Depth RatioNew vertical elements can be added to reduce thediaphragm span-to-depth ratio. The reduction of thediaphragm span-to-depth ratio will also reduce thelateral deflection and shear demand in the diaphragm.(Typical construction details and methods are discussedin FEMA 172, Section 3.4.) (FEMA 178 [BSSC,1992a], Section 7.2.4.)E. Diaphragm ContinuityThe diaphragm discontinuity should in all cases beeliminated by adding new vertical elements at thediaphragm offset or the expansion joint (seeFEMA 172, Section 3.4). In some cases, special detailsmay be used to transfer shear across an expansionjoint—while still allowing the expansion joint to10-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified Rehabilitationfunction—thus eliminating a diaphragm discontinuity.(FEMA 178 [BSSC, 1992a], Section 7.2.5.)F. Chord ContinuityIf members such as edge joists, blocking, or wall topplates have the capacity to function as chords but lackconnection, adding nailed or bolted continuity spliceswill provide a continuous diaphragm chord. Newcontinuous steel or wood chord members can be addedto the existing diaphragm where existing members lacksufficient capacity or no chord exists. New chordmembers can be placed at either the underside ortopside of the diaphragm. In some cases, new verticalelements can be added to reduce the diaphragm spanand stresses on any existing chord members (seeFEMA 172, Section 3.5.1.3, and ATC-7). New chordconnections should not be detailed such that they are theweakest element in the chord. (FEMA 178 [BSSC,1992a], Section 7.2.6.)10.3.6 Connections10.3.6.1 Diaphragm/Wall Shear TransferCollector members, splice plates, and shear transferdevices can be added as required to deliver collectorforces to the shear wall. Adding shear connectors fromdiaphragm to wall and/or to collectors will transfershear. (See FEMA 172, Section 3.7 for WoodDiaphragms, 3.7.2 for concrete diaphragms, 3.7.3 forpoured gypsum, and 3.7.4 for metal deck diaphragms.)(FEMA 178 [BSSC, 1992a], Sections 8.3.1 and 8.3.3.)10.3.6.2 Diaphragm/Frame Shear TransferAdding collectors and connecting the framing willtransfer loads to the collectors. Connections can beprovided along the collector length and at the collectorto-frameconnection to withstand the calculated forces(see FEMA 172, Sections 3.7.5 and 3.7.6). (FEMA 178[BSSC, 1992a], Sections 8.3.2 and 8.3.3.)10.3.6.3 Anchorage for Normal ForcesTo account for inadequacies identified by FEMA 178and in Section C10.3.6.3 of the Commentary, wallanchors can be added. Complications that may resultfrom inadequate anchorage include cross-grain tensionin wood ledgers, or failure of the diaphragm-to-wallconnection, due to: (1) insufficient strength, number, orstability of anchors; (2) inadequate embedment ofanchors; (3) inadequate development of anchors andstraps into the diaphragm; and (4) deformation ofanchors and their fasteners that permit diaphragmboundary connection pullout, or cross-grain tension inwood ledgers.Existing anchors should be tested to determine loadcapacity and deformation potential including fastenerslip, according to the requirements in Appendix C ofFEMA 178 (BSSC, 1992a). Special attention should begiven to the testing procedure to maintain a high levelof quality control. Additional anchors should beprovided as needed to supplement those that fail thetest, as well as those needed to meet the FEMA 178criteria. The quality of the rehabilitation dependsgreatly on the quality of the performed tests.(FEMA 178 [BSSC, 1992a], Sections 8.2.1 to 8.2.6;Guidelines Section 10.4.4.1.)10.3.6.4 Girder-Wall ConnectionsThe existing reinforcing must be exposed, and theconnection modified as necessary. For out-of-planeloads, the number of column ties can be increased byjacketing the pilaster, or alternatively, by developing asecond load path for the out-of-plane forces. Bearinglength conditions can be addressed by adding bearingextensions. Frame action in welded connections can bemitigated by adding shear walls. (FEMA 178 [BSSC,1992a], Sections 8.5.1 through 8.5.3.)10.3.6.5 Precast ConnectionsThe connections of chords, ties, and collectors can beupgraded to increase strength and/or ductility, providingalternative load paths for lateral forces. Upgrading canbe achieved by such methods as adding confinementties or increasing embedment. Shear walls can be addedto reduce the demand on connections. (FEMA 178[BSSC, 1992a], Section 4.4.2.)10.3.6.6 Wall Panels and CladdingIt may be possible to improve the connection betweenthe panels and the framing. If architectural oroccupancy conditions warrant, the cladding can bereplaced with a new system. The building can bestiffened with the addition of shear walls or bracedframes, to reduce the drifts in the cladding elements.(FEMA 178 [BSSC, 1992a], Section 8.6.2.)10.3.6.7 Light Gage Metal, Plastic, orCementitious Roof PanelsIt may be possible to improve the connection betweenthe roof and the framing. If architectural or occupancyconditions warrant, the roof diaphragm can be replacedFEMA 273 Seismic Rehabilitation Guidelines 10-9

Chapter 10: Simplified Rehabilitationwith a new one. Alternatively, a new diaphragm may beadded, using rod braces or plywood above or below theexisting roof, which remains in place. (FEMA 178[BSSC, 1992a], Section 8.6.1.)10.3.6.8 Mezzanine ConnectionsDiagonal braces, moment frames, or shear walls can beadded at or near the perimeter of the mezzanine wherebracing elements are missing, so that a complete andbalanced lateral-force-resisting system is provided thatmeets the requirements of FEMA 178.10.3.7 Foundations and Geologic Hazards10.3.7.1 Anchorage to FoundationsFor wood walls, expansion anchors or epoxy anchorscan be installed by drilling through the wood sill to theconcrete foundation at an appropriate spacing of four tosix feet on center. Similarly, steel columns and woodposts can be anchored to concrete slabs or footings,using expansion anchors and clip angles. If the concreteor masonry walls and columns lack dowels, a concretecurb can be installed adjacent to the wall or column bydrilling dowels and installing anchors into the wall thatlap with dowels installed in the slab or footing.However, this curb can cause significant architecturalproblems. Alternatively, steel angles may be used withdrilled anchors. The anchorage of shear wall boundarycomponents can be challenging due to very highconcentrated forces. (FEMA 178 [BSSC, 1992a],Sections 8.4.1 through 8.4.7.)10.3.7.2 Condition of FoundationsAll deteriorated and otherwise damaged foundationsshould be strengthened and repaired using the samematerials and style of construction. Some conditions ofmaterial deterioration can be mitigated in the field,including patching of spalled concrete. Pest infestationor dry rot of wood piles can be very difficult to correct,and often require full replacement. The deterioration ofthese elements may have implications that extendbeyond seismic safety and must be considered in therehabilitation. (FEMA 178 [BSSC, 1992a],Sections 9.1.1 through 9.1.2.)10.3.7.3 OverturningExisting foundations can be strengthened as needed toresist overturning forces. Spread footings can beenlarged or additional piles, rock anchors, or piersadded to deep foundations. It may also be possible touse grade beams or new wall elements to spread outoverturning loads over a greater distance. Adding newlateral-load-resisting elements will reduce overturningeffects of existing elements. (FEMA 178 [BSSC,1992a], Section 9.2.1.)10.3.7.4 Lateral LoadsAs with overturning effects, the correction of lateralload deficiencies in the foundations of existingbuildings is expensive and may not be justified by morerealistic analysis procedures. For this reason,Systematic Rehabilitation is recommended for thesecases. (FEMA 178 [BSSC, 1992a], Sections 9.2.1through 9.2.5.)10.3.7.5 Geologic Site HazardsSite hazards other than ground shaking should beconsidered. Rehabilitation of structures subject to lifesafety hazards from ground failures is impractical,unless site hazards can be mitigated to the point whereacceptable performance can be achieved. Not all groundfailures need necessarily be considered as life safetyhazards. For example, in many cases liquefactionbeneath a building does not pose a life safety hazard;however, related lateral spreading can result in collapseof buildings with inadequate foundation strength. Forthis reason, the liquefaction potential and the relatedconsequences should be thoroughly investigated forsites that do not satisfy the FEMA 178 statement.Further information on the evaluation of site hazards isprovided in Chapter 4 of these Guidelines. (FEMA 178[BSSC, 1992a], Sections 9.3.1 through 9.3.3.)10.3.8 Evaluation of Materials andConditions10.3.8.1 GeneralProper evaluation of the existing conditions andconfiguration of the existing building structure is animportant aspect of Simplified Rehabilitation. AsSimplified Rehabilitation is often concerned withspecific deficiencies in a particular structural system,the evaluation can either be focused on affectedstructural elements and components, or becomprehensive and inclusive of the complete structure.If the degree of existing damage or deficiencies in astructure has not been established, the evaluation shallconsist of a comprehensive inspection of gravity- andlateral-load-resisting systems that includes thefollowing steps.1. Verify existing data (e.g., accuracy of drawings).10-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified Rehabilitation2. Develop other needed data (e.g., measure and sketchbuilding if necessary).3. Verify the vertical and lateral systems.4. Check the condition of the building.5. Look for special conditions and anomalies.6. Address the evaluation statements and goals duringthe inspection.7. Perform material tests that are justified through aweighing of the cost of destructive testing and thecost of corrective work.10.3.8.2 Condition of WoodAn inspection should be conducted to grade the existingwood and verify physical condition, using techniquesfrom Section 10.3.8.1. Any damage or deterioration andits source must be identified. Wood that is significantlydamaged due to splitting, decay, aging, or otherphenomena must be removed and replaced. Localizedproblems can be eliminated by adding newappropriately sized reinforcing components extendingbeyond the damaged area and connecting to undamagedportions. Additional connectors between componentsshould be provided to correct any discontinuous loadpaths. It is necessary to verify that any new reinforcingcomponents or connectors will not be exposed tosimilar deterioration or damage. (FEMA 178 [BSSC,1992a], Section 3.5.1.)10.3.8.3 Overdriven FastenersWhere visual inspection determines that extensiveoverdriving of fasteners exists in greater than 20% ofthe installed connectors, the fasteners and shear panelscan generally be repaired through addition of a newsame-sized fastener for every two overdriven fasteners.To avoid splitting because of closely spaced nails, itmay be necessary to predrill to 90% of the nail shankdiameter for installation of new nails. For otherconditions, such as cases where the addition of newconnectors is not possible or where component damageis suspected, further investigation shall be conductedusing the guidance of Section 10.3.8.1. (FEMA 178[BSSC, 1992a], Section 3.5.2.)10.3.8.4 Condition of SteelShould visual inspection or testing conducted perSection 10.3.8.1 reveal the presence of steel componentor connection deterioration, further evaluation isneeded. The source of the damage shall be identifiedand mitigative action shall be taken to preserve theremaining structure. In areas of significantdeterioration, restoration of the material cross sectioncan be performed by the addition of plates or otherreinforcing. When sizing reinforcements, the designprofessional shall consider the effects of existingstresses in the original structure, load transfer, andstrain compatibility. The demands on the deterioratedsteel elements and components may also be reducedthrough careful addition of bracing or shear wall panels.(FEMA 178 [BSSC, 1992a], Section 3.5.3.)10.3.8.5 Condition of ConcreteShould visual inspections or testing conducted perSection 10.3.8.1 reveal the presence of concretecomponent or reinforcing steel deterioration, furtherevaluation is needed. The source of the damage shall beidentified and mitigative action shall be taken topreserve the remaining structure. Existing deterioratedmaterial, including reinforcing steel, shall be removedto the limits defined by testing; reinforcing steel in goodcondition shall be cleaned and left in place for splicingpurposes as appropriate. Cracks in otherwise soundmaterial shall be evaluated to determine cause, andrepaired as necessary using techniques appropriate tothe source and activity level. (FEMA 178 [BSSC,1992a], Sections 3.5.4 through 3.5.8.)10.3.8.6 Post-Tensioning AnchorsPrestressed concrete systems may be adversely affectedby cyclic deformations produced by earthquake motion.One rehabilitation process that may be considered is toadd stiffness to the system. Another concern for thesesystems is the adverse effects of tendon corrosion. Athorough visual inspection of prestressed systems shallbe performed to verify absence of concrete cracking orspalling, staining from embedded tendon corrosion, orother signs of damage along the tendon spans and atanchorage zones. If degradation is observed orsuspected, more detailed evaluations will be required,as indicated in Chapter 6. Rehabilitation of thesesystems, except for local anchorage repair, should be inaccordance with the Systematic Rehabilitationprovisions in the balance of these Guidelines.Professionals with special prestressed concreteconstruction expertise should also be consulted forfurther interpretation of damage. (FEMA 178 [BSSC,1992a], Section 3.5.5.)FEMA 273 Seismic Rehabilitation Guidelines 10-11

Chapter 10: Simplified Rehabilitation10.3.8.7 Quality of MasonryShould visual inspections or testing conducted perSection 10.3.8.1 reveal the presence of masonrycomponent or construction deterioration, furtherevaluation is needed. Certain damage, such as degradedmortar joints or simple cracking, may be rehabilitatedthrough repointing or rebuild. If the wall is repointed,care should be taken to ensure that the new mortar iscompatible with the existing masonry units and mortar,and that suitable wetting is performed. The strength ofthe new mortar is critical to load-carrying capacity andseismic performance. Significant degradation should betreated as specified in Chapter 7 of these Guidelines.(FEMA 178 [BSSC, 1992a], Sections A4, 3.5.9, 3.5.10,and 3.5.11.)10.4 Amendments to FEMA 178Since the development and publication of FEMA 178(BSSC, 1992a), significant earthquakes have occurred:the 1989 Loma Prieta earthquake in the San FranciscoBay area, the 1994 Northridge earthquake in the LosAngeles area, and the 1995 Hyogoken-Nanbuearthquake in the Kobe, Japan area. While each onegenerally validated the fundamental assumptionsunderlying the procedures, each also offered newinsights into the potential weaknesses of certain lateralforce-resistingsystems.In the process of developing the Guidelines andCommentary, eight new potential deficiencies wereidentified and are developed below. They are presentedin the same style as in FEMA 178 (BSSC, 1992a). Eachis presented as a statement to be answered “True” or“False,” which permits rapid screening andidentification of potential weak links. Each statement isfollowed by a paragraph of commentary written toidentify the concern clearly. A suggested procedure forevaluating the potential weak link concludes eachsection, and should be carried out if the statement isfound to be false. Completion of the procedure permitseach potential deficiency to be properly evaluated andthe actual deficiencies identified.These eight new potential deficiencies should beconsidered as additions to the general list of buildingdeficiencies (pages A3 to A16 of FEMA 178 [BSSC,1992a]) and applied to the individual model buildingsas indicated in Tables 10-3 through 10-20.10.4.1 New Potential Deficiencies Related toBuilding Systems10.4.1.1 Lateral Load Path at Pile CapsEvaluation Statement: Pile caps are capable oftransferring lateral and overturning forces between thestructure and individual piles in the pile group.Common problems with pile caps include a lack of topreinforcing in the pile cap. A loss of bond of pile andcolumn reinforcing can occur when top cracks formduring load reversals.Procedure: Calculate the moment and shear capacity ofthe pile cap to transfer uplift and lateral forces based onthe forces in FEMA 178 (BSSC, 1992a), from the pointof application to each pile.10.4.1.2 Deflection CompatibilityEvaluation Statement: Column and beam assembliesthat are not part of the lateral-force-resisting system(i.e., gravity-load-resisting frames) are capable ofaccommodating imposed building drifts, includingamplified drift caused by diaphragm deflections,without loss of their vertical-load-carrying capacity.Frame components, especially columns, that are notspecifically designed to participate in the lateral systemwill still undergo displacements associated with theoverall seismic story drifts. If the columns are locatedfar from the lateral-force-resisting elements, the addeddeflections due to semi-rigid floor diaphragms willincrease the drifts. Stiff columns, designed forpotentially high gravity loads, may develop significantbending moments due to the imposed drifts. Themoment-axial force interaction may lead to brittlefailures in nonductile columns, which could causebuilding collapse.Procedure: Calculate expected drifts on the columns inframes that are not part of the lateral-force-resistingsystem, using procedures described in FEMA 178(BSSC, 1992a), Section 2.4.4. Use cracked/transformedsections for all lateral-force-resisting concrete elements.Calculate additional drift from diaphragms bydetermining the deflection of the diaphragm at forcesequal to those prescribed in FEMA 178, Chapter 2, forelements of structures. Evaluate the capacity of the nonlateral-force-resistingcolumn and beam assemblies toundergo the combined drift, considering moment-axialinteraction and column shear.10-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified Rehabilitation10.4.2 New Potential Deficiencies Related toMoment Frames10.4.2.1 Moment-Resisting ConnectionsEvaluation Statement: All moment connections areable to develop the strength of the adjoining membersor panel zones.Connection failure is generally not ductile behavior. Itis more desirable to have all inelastic action occur in themembers rather than in the connections. The momentresistingbeam-column connection should provide forthe development of the lesser of (1) the plastic girderstrength in flexure or (2) the moment corresponding tothe development of the panel zone shear strength,considering the effects of strain hardening and materialoverstrength. The deficiency is in the strength of theconnections.Procedure: At the time of this writing, this problem isthe subject of the FEMA-funded effort carried out bythe SAC Joint Venture, which is composed of theStructural Engineers Association of California(SEAOC), the Applied Technology Council (ATC), andCalifornia Universities for Research in EarthquakeEngineering (CUREe), and which has produced interimguidance on the evaluation, repair, and rehabilitation ofsteel moment frames (SAC, 1995).Using the latest guidelines, demonstrate by test orcalculation that the connection meets the expectedinelastic rotation demand on the joint, and that inelasticaction is not concentrated in the vicinity of welds at thecolumn face.10.4.2.2 Short Captive ColumnsEvaluation Statement: There are no columns withheight-to-depth ratios less than 75% of the nominalheight-to-depth ratios of the typical columns at thatlevel.Short captive columns (which are usually not designedas part of the primary lateral-load-resisting system) tendto attract shear forces because of their high stiffnessrelative to other lateral-force-resisting vertical elementsat that story level. Significant damage has beenobserved in parking structure columns adjacent toramping slabs, even in structures with shear walls.Captive column behavior may also be found inbuildings with clerestory windows, in buildings wherecolumns are partially braced by masonry or concretenonstructural construction, and in buildings withimproperly designed mezzanines.Procedure: Calculate the anticipated story drift, anddetermine the shear (V e ) demand in the short columncaused by the drift (V e = 2M/L). Compare V e with themember nominal shear capacity (V n ) calculated inaccordance with ACI (1989) Chapter 21. The ratioV e /V n should be less than or equal to 1.0.10.4.3 New Potential Deficiencies Related toShear Walls10.4.3.1 Narrow Wood Shear WallsEvaluation Statement: Narrow wood shear walls withan aspect ratio greater than two to one do not resistforces developed in the building.Most of the deformation of the narrow shear wallsoccurs at the base, and consists of sliding of the sillplate and stretching of hold-down attachments. Splittingof the end studs at the attachment of hold-downs is alsoa common failure. Narrow shear walls are relativelyflexible and thus tend to take less shear than would beanticipated when compared to wider shear walls. Thisresults in greater loading of the shear walls with lowerheight-to-width ratios and less load in the narrow walls.Procedure: Determine the shear capacity of the walland related overturning demand. Verify that shear andoverturning can be transferred to the foundation withinallowable stresses calculated in accordance withFEMA 178 (BSSC, 1992a).10.4.3.2 Stucco (Exterior Plaster) Shear WallsEvaluation Statement: Multistory buildings do notrely on exterior stucco walls as the primary lateralforce-resistingsystem.Exterior stucco plaster walls are often used(intentionally and unintentionally) for resisting lateralearthquake loads. Stucco is relatively stiff and brittle,with low shear resistance value. Differential foundationmovement and earthquake shaking cause cracking ofthe stucco and loss of lateral strength. The cracking canrange from minor to severe. Sometimes the stuccodelaminates from the framing and the lateral-forceresistingsystem is lost. Multistory buildings shall notrely on stucco walls as the primary lateral-forceresistingsystem since there is not enough availablestrength.FEMA 273 Seismic Rehabilitation Guidelines 10-13

Chapter 10: Simplified RehabilitationProcedure: Inspect stucco clad buildings to determineif there is a lateral system such as plywood or diagonalsheathing in at least all but the top floor. Where exteriorplaster is utilized and there is a supplemental system,verify that the wire reinforcing is attached directly tothe wall framing and the wire is completely embeddedin the plaster material. Verify that lateral loads do notexceed 100 pounds per linear foot.10.4.3.3 Gypsum Wallboard or Plaster ShearWallsEvaluation Statement: Interior plaster or gypsumwallboard is not being used for shear walls on buildingsover one story in height.Gypsum wallboard or gypsum plaster sheathing tends tobe easily damaged by differential foundation movementor earthquake shaking. Most residential buildings havenumerous walls constructed with plaster or gypsumwallboard. Though the capacity of these walls is low,the amount of wall is often high. As a result, plaster andgypsum wallboard walls may provide adequateresistance to moderate earthquake shaking. Theproblem that can occur is incompatibility with otherlateral-forcing-resisting elements. For example, narrowplywood shear walls are more flexible than long stiffplaster walls; as a result, the plaster or gypsum wallswill take all the load until they fail and then theplywood walls will start to resist the lateral loads.Plaster or gypsum wallboard walls should not be usedfor shear walls except for one-story buildings or on thetop story of multistory buildings.Procedure: Determine the walls with plaster or gypsumsheathing that would be required to resist lateralearthquake forces (i.e., earthquake loads would have topass through these walls), due to the location of thewalls in the building. Verify that all walls have beenproperly constructed with nailing required byFEMA 222A (BSSC, 1995), and that loads are withinallowable limits. Remove gypsum wallboard andplaster as required, and replace with panel shear walls.Cover the new shear walls with gypsum wallboard andplaster.10.4.4 New Potential Deficiencies Related toConnections10.4.4.1 Stiffness of Wall AnchorsEvaluation Statement: Anchors of heavy concrete ormasonry walls to wood structural elements are installedtaut and are stiff enough to prevent movement betweenthe wall and roof. If bolts are used, the bolt holes in boththe connector and framing are a maximum of1/16" larger than the bolt diameter.The small separation that can occur between the walland roof sheathing, due to anchors that are not taut,requires movement before taking hold and can result inan out-of-plane failure of the ledger support. Bolts inoversized holes can also cause slippage and separationbetween the wall and framing.Procedure: Field check that no anchor has a twist,kink, or offset, and that no anchor is otherwise installedsuch that some separation must occur prior to its takinghold, and that such movement will lead to aperpendicular-to-grain bending failure in the woodledger. Remove a representative sample of bolts andverify that the holes are not oversized. For oversizedholes, replace bolts and fill gaps with epoxy or othersuitable filler.10.5 FEMA 178 Deficiency StatementsNo guidelines are provided for this section. See theCommentary for a complete list—augmented with theeight new deficiency statements from Section 10.4,above—presented in a logical, combined order.10.6 DefinitionsBoundary component (boundary member): Amember at the perimeter (edge or opening) of a shearwall or horizontal diaphragm that provides tensile and/or compressive strength.Column (or beam) jacketing: A method in which aconcrete column or beam is covered with a steel orconcrete “jacket” in order to strengthen and/or repairthe member by confining the concrete.Coupling beam: Flexural member that ties orcouples adjacent shear walls acting in the same plane. Acoupling beam is designed to yield and dissipateinelastic energy, and, when properly detailed andproportioned, has a significant effect on the overallstiffness of the coupled wall.Crosstie: A beam or girder that spans across thewidth of the diaphragm, accumulates the wall loads, andtransfers them, over the full depth of the diaphragms,10-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified Rehabilitationinto the next bay and onto the nearest shear wall orframe.Diaphragm chord: A diaphragm componentprovided to resist tension or compression at the edges ofthe diaphragm.Drag strut: A component parallel to the applied loadthat collects and transfers diaphragm shear forces to thevertical lateral-force-resisting components or elements,or distributes forces within a diaphragm.Flexible diaphragm: A diaphragm consisting of oneof the following systems: plywood sheathing, spacedtimber sheathing, straight timber sheathing, diagonaltimber sheathing, metal deck without concrete fill,corrugated transit panels, or steel rod bracing or othersteel bracing using light members such as angles or splittees.Inter-story drift: The relative horizontaldisplacement of two adjacent floors in a building. Interstorydrift can also be expressed as a percentage of thestory height separating the two adjacent floors.Load path: A path that seismic forces pass through tothe foundation of the structure and, ultimately, to thesoil. Typically, the load travels from the diaphragmthrough connections to the vertical lateral-forceresistingelements, and then proceeds to the foundationby way of additional connections.Model Building Type: Fifteen common buildingtypes used to categorize expected deficiencies,reasonable rehabilitation methods, and estimated costs.See Table 10-2 for descriptions of Model BuildingTypes.Narrow wood shear wall: Wood shear walls with anaspect ratio (height to width) greater than two to one.These walls are relatively flexible and thus tend to beincompatible with other building components, therebytaking less shear than would be anticipated whencompared to wider walls.Noncompact member: A steel section incompression whose width-to-thickness ratio does notmeet the limiting values for compactness, as shown inTable B5.1 of AISC (1986).Overturning: Action resulting when the momentproduced at the base of vertical lateral-force-resistingelements is larger than the resistance provided by thefoundation’s uplift resistance and building weight.Panel zone: Area of a column at the beam-to-columnconnection delineated by beam and column flanges.Plan irregularity: Horizontal irregularity in thelayout of vertical lateral-force-resisting elements,producing a misalignment between the center of massand center of rigidity that typically results in significanttorsional demands on the structure.Pounding: Two adjacent buildings coming intocontact during earthquake excitation because they aretoo close together and/or exhibit different dynamicdeflection characteristics.Re-entrant corner: Plan irregularity in a diaphragm,such as an extending wing, plan inset, or E-, T-, X-, orL-shaped configuration, where large tensile andcompressive forces can develop.Redundancy: Quality of having alternative paths inthe structure by which the lateral forces are resisted,allowing the structure to remain stable following thefailure of any single element.Rehabilitation Objective: A statement of the desiredlimits of damage or loss for a given seismic demand,usually selected by the owner, engineer, and/or relevantpublic agencies. (See Chapter 2.)Repointing: A method of repairing a cracked ordeteriorating mortar joint in masonry. The damaged ordeteriorated mortar is removed and the joint is refilledwith new mortar.Short captive column: Columns with height-todepthratios less than 75% of the nominal height-todepthratios of the typical columns at that level. Thesecolumns, which may not be designed as part of theprimary lateral-load-resisting system, tend to attractshear forces because of their high stiffness relative toadjacent elements.Simplified Rehabilitation Method: An approach,applicable to some types of buildings and RehabilitationObjectives, in which analyses of the entire building’sresponse to earthquake hazards are not required.Stiff diaphragm: A diaphragm consisting of one ofthe following systems: monolithic reinforced concreteFEMA 273 Seismic Rehabilitation Guidelines 10-15

Chapter 10: Simplified Rehabilitationslabs, precast concrete slabs or planks bonded togetherby a reinforced topping slab or by welded inserts,concrete-filled metal deck, or masonry arches with orwithout concrete fill or topping.Strong column-weak beam: A connection requiredto localize damage and control drift; the capacity of thecolumn in any moment frame joint must be greater thanthat of the beams, to ensure inelastic action in thebeams.Strong back system: A secondary system, such as aframe, commonly used to provide out-of-plane supportfor an unreinforced or under-reinforced masonry wall.Systematic Rehabilitation Method: An approach torehabilitation in which complete analysis of thebuilding’s response to earthquake shaking is performed.Vertical irregularity: A discontinuity of strength,stiffness, geometry, or mass in one story with respect toadjacent stories.10.7 SymbolsLMV eV nLength of the columnMoment expected in the column at maximumexpected driftShear demand in the column caused by the driftNominal shear capacity of a column10.8 ReferencesACI, 1989, Building Code Requirements for ReinforcedConcrete, ACI 318-89, American Concrete Institute,Detroit, Michigan.BSSC, 1992a, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 178), Washington, D.C.BSSC, 1992b, NEHRP Handbook of Techniques for theSeismic Rehabilitation of Existing Buildings, developedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 172), Washington, D.C.BSSC, 1995, NEHRP Recommended Provisions forSeismic Regulations for New Buildings, 1994 Edition,Part 1: Provisions and Part 2: Commentary, preparedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report Nos.FEMA 222A and 223A), Washington, D.C.Masonry Standards Joint Committee (MSJC), 1995,Building Code Requirements for Masonry Structures,ACI 530-95/ASCE 5-95/TMS 402-95, AmericanConcrete Institute, Detroit, Michigan; AmericanSociety of Civil Engineers, New York, New York; andthe Masonry Society, Boulder, Colorado.SAC, 1995, Interim Guidelines: Evaluation, Repair,Modification and Design of Steel Moment Frames,Report No. SAC-95-02, developed by the SAC JointVenture (the Structural Engineers Association ofCalifornia [SEAOC], the Applied Technology Council[ATC], and California Universities for Research inEarthquake Engineering [CUREe]) for the FederalEmergency Management Agency (Report No. FEMA267), Washington, D.C.ATC, 1981, Guidelines for the Design of HorizontalWood Diaphragms, Report No. ATC-7, AppliedTechnology Council, Redwood City, California.10-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationSix-Story Reinforced Concrete Shear Wall Building in Low-Seismicity RegionLow-seismicity region (NEHRP Region 1,BSSC [1988] )120-foot square in plan with 8-inch-thick reinforcedconcrete exterior walls and 9-inch-thick reinforcedconcrete floor slabsLife Safety Performance Level10%/50 year ground motion (Guidelines)Soil Type: S2 (FEMA 178) or class C (Guidelines)m = 2.5 (Table 6-19) Low Axial and Low ShearDemand, no confined boundaryGuidelines Shear Capacity = V n * m66 ftMaximum pier shearBase shear Demand Capacity DemaFEMA 178 300 kips 33 psi 276 psi 0.12Guidelines 2117 kips 230 psi 690 psi 0.21Legend:GuidelinesFEMA 178FEMA 178 ElasticDisplacementFEMA 178 UnreducedShear120 ft 0.15 0.30 1000 2000Wall elevation Displacement, ∆ (in.) Shear, V (kips)Three-Story Reinforced Concrete Shear Wall Building in High-Seismicity RegionHigh-seismicity region (NEHRP Region 7,BSSC [1988] )120-foot square in plan with 8-inch-thick reinforcedconcrete exterior walls and 9-inch-thick reinforcedconcrete floor slabsLife Safety Performance Level10%/50 year ground motion (Guidelines)Soil Type: S2 (FEMA 178) or class C (Guidelines)36 ftMaximum pier shearBase shear Demand Capacity DemaFEMA 178 1157 kips 126 psi 276 psi 0.46Guidelines 6091 kips 659 psi 829 psi 0.79Legend:GuidelinesFEMA 178FEMA 178 ElasticDisplacementFEMA 178 UnreducedShear120 ft 0.10 0.20 2000 4000 6000Wall elevation Displacement, ∆ (in.) Shear, V (kips)Figure 10-1Comparison of FEMA 178 (BSSC, 1992a) and Guidelines Acceptance CriteriaFEMA 273 Seismic Rehabilitation Guidelines 10-17

Chapter 10: Simplified RehabilitationTable 10-1Limitations on Use of Simplified Rehabilitation MethodMaximum Building Height in Stories bySeismic Zone 1 for Use of SimplifiedRehabilitation MethodModel Building Type 2Low Moderate HighWood FrameLight (W1) 3 3 2Multistory Multi-Unit Residential (W1A) 3 3 2Commercial and Industrial (W2) 3 3 2Steel Moment FrameStiff Diaphragm (S1) 6 4 3Flexible Diaphragm (S1A) 4 4 3Steel Braced FrameStiff Diaphragm (S2) 6 4 3Flexible Diaphragm (S2A) 3 3 3Steel Light Frame (S3) 2 2 2Steel Frame with Concrete Shear Walls (S4) 6 4 3Steel Frame with Infill Masonry Shear WallsStiff Diaphragm (S5) 3 3Flexible Diaphragm (S5A) 3 3Concrete Moment Frame (C1) 3Concrete Shear WallsStiff Diaphragm (C2) 6 4 3Flexible Diaphragm (C2A) 3 3 3Concrete Frame with Infill Masonry Shear WallsStiff Diaphragm (C3) 3Flexible Diaphragm (C3A) 3Precast/Tilt-up Concrete Shear WallsFlexible Diaphragm (PC1) 3 2 2Stiff Diaphragm (PC1A) 3 2 2Precast Concrete FrameWith Shear Walls (PC2) 3 2Without Shear Walls (PC2A)Reinforced Masonry Bearing WallsFlexible Diaphragm (RM1) 3 3 3Stiff Diaphragm (RM2) 6 4 3= Use of Simplified Rehabilitation Method not appropriate.1. Seismic Zones are defined in Chapter 2 of the Guidelines.2. Buildings with different types of flexible diaphragms may be considered to have flexible diaphragms.Multistory buildings having stiff diaphragms at all levels except the roof may be considered as having stiff diaphragms.Buildings having both flexible and stiff diaphragms, or having diaphragm systems that are neither flexible nor stiff, in accordance with this chapter, shallbe rehabilitated using the Systematic Method.10-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-1Limitations on Use of Simplified Rehabilitation Method (continued)Model Building Type 2Maximum Building Height in Stories bySeismic Zone 1 for Use of SimplifiedRehabilitation MethodLow Moderate HighUnreinforced Masonry Bearing WallsFlexible Diaphragm (URM) 3 3 2Stiff Diaphragm (URMA) 3 3 2= Use of Simplified Rehabilitation Method not appropriate.1. Seismic Zones are defined in Chapter 2 of the Guidelines.2. Buildings with different types of flexible diaphragms may be considered to have flexible diaphragms.Multistory buildings having stiff diaphragms at all levels except the roof may be considered as having stiff diaphragms.Buildings having both flexible and stiff diaphragms, or having diaphragm systems that are neither flexible nor stiff, in accordance with this chapter, shallbe rehabilitated using the Systematic Method.FEMA 273 Seismic Rehabilitation Guidelines 10-19

Chapter 10: Simplified RehabilitationTable 10-2Description of Model Building TypesModel Building Type designations are similar to those used in FEMA 178 (BSSC, 1992a). Designations ending in the letter Aindicate types added to the FEMA 178 list. Refer to FEMA 178 for additional information.Building Type 1—Wood, Light FrameType W1: These building are typically single- or multiple-family dwellings of one or more stories. The essential structuralcharacter of this type is repetitive framing by wood joists on wood studs. Loads are light and spans are small.These buildings may have relatively heavy chimneys and may be partially or fully covered with veneer. Most ofthese buildings are not engineered; however, they usually have the components of a lateral-force-resistingsystem even though it may be incomplete. Lateral loads are transferred by diaphragms to shear walls. Thediaphragms are roof panels and floors. Shear walls are exterior walls sheathed with plank siding, stucco,plywood, gypsum board, particleboard, or fiberboard. Interior partitions are sheathed with plaster or gypsumboard.Type W1A: Similar to W1 buildings, but are typically multistory multi-unit residential structures, often with open front garagesat the first story.Building Type 2—Wood, Commercial and IndustrialType W2: These buildings usually are commercial or industrial buildings with a floor area of 5,000 square feet or more andfew, if any, interior walls. The essential structural character is framing by beams on columns. The beams may beglulam beams, steel beams, or trusses. Lateral forces usually are resisted by wood diaphragms and exteriorwalls sheathed with plywood, stucco, plaster, or other paneling. The walls may have rod bracing. Large openingsfor stores and garages often require post-and-beam framing. Lateral force resistance on those lines can beachieved with steel rigid frames or diagonal bracing.Building Type 3—Steel Moment FrameType S1: These buildings have a frame of steel columns and beams. In some cases, the beam-column connections havevery small moment-resisting capacity, but in other cases some of the beams and columns are fully developed asmoment frames to resist lateral forces. Usually the structure is concealed on the outside by exterior walls, whichcan be of almost any material (curtain walls, brick masonry, or precast concrete panels), and on the inside byceilings and column furring. Lateral loads are transferred by diaphragms to moment-resisting frames. Thediaphragms are typically concrete or metal deck with concrete fill, and are considered stiff with respect to theframes. The frames develop their stiffness by full or partial moment connections. The frames can be locatedalmost anywhere in the building. Usually the columns have their strong directions oriented so that some columnsact primarily in one direction while the others act in the other direction, and the frames consist of lines of strongcolumns and their intervening beams. Steel moment frame buildings are typically more flexible than shear wallbuildings. This low stiffness can result in large inter-story drifts that may lead to extensive nonstructural damage.Type S1A: Similar to Type S1, except that diaphragms are typically wood, tile arch, or bare metal deck and are consideredflexible with respect to the frames. Concrete or metal deck with concrete fill diaphragms may be consideredflexible if the span-to-depth ratio between lines of moment frames is high. Steel frame with wood or tile floors ismore common in older styles of construction.Building Type 4—Steel Braced FrameType S2: These buildings are similar to Type 3 (S1) buildings, except that the vertical components of the lateral-forceresistingsystem are braced frames rather than moment frames.Type S2A: These buildings are similar to Type 3 (S1A) buildings, except that the vertical components of the lateral-forceresistingsystem are braced frames rather than moment frames.Building Type 5—Steel Light FrameType S3: These buildings are pre-engineered and prefabricated with transverse rigid frames. The roof and walls consist oflightweight panels. The frames are designed for maximum efficiency, often with tapered beam and columnsections built up of light plates. The frames are built in segments and assembled in the field with bolted joints.Lateral loads in the transverse direction are resisted by the rigid frames with loads distributed to them by shearelements. Loads in the longitudinal direction are resisted entirely by shear elements. The shear elements can beeither the roof and wall sheathing panels, an independent system of tension-only rod bracing, or a combinationof panels and bracing.10-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-2Description of Model Building Types (continued)Model Building Type designations are similar to those used in FEMA 178 (BSSC, 1992a). Designations ending in the letter Aindicate types added to the FEMA 178 list. Refer to FEMA 178 for additional information.Building Type 6—Steel Frame with Concrete Shear WallsType S4: The shear walls in these buildings are cast-in-place concrete and may be bearing walls. The steel frame isdesigned for vertical loads only. Lateral loads are transferred by diaphragms—typically of cast-in-placeconcrete—to the shear walls. The steel frame may provide a secondary lateral-force-resisting system dependingon the stiffness of the frame and the moment capacity of the beam-column connections. In modern “dual”systems, the steel moment frames are designed to work together with the concrete shear walls in proportion totheir relative rigidities. In this case, the walls would be evaluated under this building type and the frames wouldbe evaluated under Building Type 3, Steel Moment Frame.Building Type 7—Steel Frame with Infill Masonry Shear WallsType S5: This is one of the older types of building. The infill walls usually are offset from the exterior frame members, wraparound them, and present a smooth masonry exterior with no indication of the frame. Solidly infilled masonrypanels act as a diagonal compression strut between the intersections of the moment frame. If the walls do notfully engage the frame members (i.e., lie in the same plane), the diagonal compression struts will not develop.The peak strength of the diagonal strut is determined by the diagonal tensile stress capacity of the masonrypanel. The post-cracking strength is determined by an analysis of a moment frame that is partially restrained bythe cracked infill. The analysis should be based on published research and should treat the system as acomposite of a frame and the infill. An analysis that attempts to treat the system as a frame and shear wall is notcapable of assuring compatibility. Diaphragms are typically concrete or tile arch with short spans between infillwalls, and are considered stiff with respect to the walls.Type S5A: Similar to Type S5, except that diaphragms either are wood, or contain concrete or tile floors with a high span-todepthratio between infill walls, and are considered flexible with respect to the walls.Building Type 8—Concrete Moment FrameType C1: These buildings are similar to Type 3 (S1) buildings, except that the frames are of concrete. Some older concreteframes may be proportioned and detailed such that brittle failure can occur. There is a large variety of framesystems. Buildings in zones of low seismicity or older buildings in zones of high seismicity can have framebeams that have broad shallow cross sections or are simply the column strips of flat slabs. Modern frames inzones of high seismicity are detailed for ductile behavior and the beams and columns have definitely regulatedproportions. Diaphragms are typically concrete.Building Type 9—Concrete Shear WallsType C2: The vertical components of the lateral-force-resisting system in these buildings are concrete shear walls that areusually bearing walls. In older buildings, the walls often are quite extensive and the wall stresses are low, butreinforcing is light. When remodeling calls for enlarging the windows, the strength of the modified walls becomesa critical concern. In newer buildings, the shear walls often are limited in extent, thus generating concerns aboutboundary members and overturning forces. Diaphragms are typically cast-in-place concrete slabs with or withoutbeams and are considered stiff with respect to the walls.Type C2A: Similar to Type C2, except that diaphragms are typically wood and are considered flexible with respect to theframes. This is typically evident in older styles of construction. Concrete diaphragms may be considered flexibleif the span-to-depth ratio between shear walls is high. This is common in parking structures and in buildings withnarrow aspect ratios.Building Type 10—Concrete Frame with Infill Masonry Shear WallsType C3: These buildings are similar to Type 7 (S5) buildings except that the frame is of reinforced concrete. The analysisof this building is similar to that recommended for Type 7 (S5), except that the shear strength of the concretecolumns, after cracking of the infill, may limit the semiductile behavior of the system. Research that is specific toconfinement of the infill by reinforced concrete frames should be used for the analysis.Type C3A: Similar to Type C3, except that diaphragms either are wood, or contain concrete or tile floors with a high span-todepthratios between infill walls, and are considered flexible with respect to the walls.FEMA 273 Seismic Rehabilitation Guidelines 10-21

Chapter 10: Simplified RehabilitationTable 10-2Description of Model Building Types (continued)Model Building Type designations are similar to those used in FEMA 178 (BSSC, 1992a). Designations ending in the letter Aindicate types added to the FEMA 178 list. Refer to FEMA 178 for additional information.Building Type 11—Precast/Tilt-up Concrete Shear WallsType PC1: These buildings have a wood or metal deck roof diaphragm, which often is very large, that distributes lateralforces to precast concrete shear walls and is considered flexible with respect to the walls. They may also haveprecast concrete diaphragms if the span-to-depth ratio between walls is very high or there is no topping slab.The walls are thin but relatively heavy, while the roofs are relatively light. Older buildings often have inadequateconnections for anchorage of the walls to the roof for out-of-plane forces, and the panel connections often arebrittle. Tilt-up buildings often have more than one story. Walls may have numerous openings for doors andwindows of such size that the wall looks more like a frame than a shear wall.Type PC1A: Similar to Type PC1, except that diaphragms are precast or cast-in-place concrete with small span-to-depthratios, and are considered stiff with respect to the walls.Building Type 12—Precast Concrete FrameType PC2: These buildings contain floor and roof diaphragms typically composed of precast concrete elements with orwithout cast-in-place concrete topping slabs. The diaphragms are supported by precast concrete girders andcolumns. The girders often bear on column corbels. Closure strips between precast floor elements and beamcolumnjoints usually are cast-in-place concrete. Welded steel inserts often are used to interconnect precastelements. Lateral loads are resisted by precast or cast-in-place concrete shear walls. Buildings with precastframes and concrete shear walls should perform well if the details used to connect the structural elements havesufficient strength and displacement capacity; however, in some cases the connection details between theprecast elements have negligible ductility.Type PC2A: Similar to Type PC2, except that lateral loads are resisted by the concrete frames directly without the presenceof shear walls. This type of construction is not permitted in regions of high seismicity.Building Type 13—Reinforced Masonry Bearing Walls with Flexible DiaphragmsType RM1: These buildings have bearing and shear walls of reinforced brick or concrete-block masonry, which are thevertical elements in the lateral-force-resisting system. The floors and roofs are framed either with wood joistsand beams with plywood or straight or diagonal sheathing, or with steel beams with metal deck with or without aconcrete fill. Wood floor framing is supported by interior wood posts or steel columns; steel beams are supportedby steel columns.Building Type 14—Reinforced Masonry Bearing Walls with Stiff DiaphragmsType RM2: These buildings have walls similar to those of Type 13 (RM1) buildings, but the roof and floors are composed ofprecast concrete elements such as planks or T-beams, and the precast roof and floor elements are supported oninterior beams and columns of steel or concrete (cast-in-place or precast). The precast horizontal elements oftenhave a cast-in-place topping.Building Type 15—Unreinforced Masonry Bearing WallsType URM: These buildings include structural elements that vary depending on the building's age and, to a lesser extent, itsgeographic location. In buildings built before 1900, the majority of floor and roof construction consists of woodsheathing supported by wood subframing. In buildings built after 1950, unreinforced masonry building with woodfloors usually have plywood rather than board sheathing. The diaphragms are considered flexible with respect tothe walls. The perimeter walls, and possibly some interior walls, are unreinforced masonry. The walls may ormay not be anchored to the diaphragms. Ties between the walls and diaphragms are more common for thebearing walls than for walls that are parallel to the floor framing. Roof ties usually are less common and moreerratically spaced than those at the floor levels. Interior partitions that interconnect the floors and roof can havethe effect of reducing diaphragm displacements.Type URMA: Similar to Type URM, except that the floors are cast-in-place concrete supported by the unreinforced masonrywalls and/or steel or concrete interior framing. This is more common in older, large, multistory buildings. Inregions of lower seismicity, buildings of this type constructed more recently can include floor and roof framingthat consists of metal deck and concrete fill supported by steel framing elements. The diaphragms areconsidered stiff with respect to the walls.10-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-3W1: Wood Light FrameTypical DeficienciesLoad PathRedundancyVertical IrregularitiesShear Walls in Wood Frame BuildingsShear StressOpeningsWall DetailingCripple WallsNarrow Wood Shear WallsStucco Shear WallsGypsum Wallboard or Plaster Shear WallsDiaphragm OpeningsDiaphragm Stiffness/StrengthSpansDiaphragm ContinuityAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsCondition of WoodTable 10-4W1A: Multistory, Multi-Unit, WoodFrame ConstructionTypical DeficienciesLoad PathRedundancyVertical IrregularitiesShear Walls in Wood Frame BuildingsShear StressOpeningsWall DetailingCripple WallsNarrow Wood Shear WallsStucco Shear WallsGypsum Wallboard or Plaster Shear WallsDiaphragm OpeningsDiaphragm Stiffness/StrengthSpansDiaphragm ContinuityAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsCondition of WoodTable 10-5W2: Wood, Commercial, andIndustrialTypical DeficienciesLoad PathRedundancyVertical IrregularitiesShear Walls in Wood Frame BuildingsShear StressOpeningsWall DetailingCripple WallsNarrow Wood Shear WallsStucco Shear WallsGypsum Wallboard or Plaster Shear WallsDiaphragm OpeningsDiaphragm Stiffness/StrengthSheathingUnblocked DiaphragmsSpansSpan-to-Depth RatioDiaphragm ContinuityChord ContinuityAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsCondition of WoodTable 10-6Typical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesAdjacent BuildingsLateral Load Path at Pile CapsSteel Moment FramesDrift CheckFrame ConcernsStrong Column-Weak BeamConnectionsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Frame Shear TransferAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of SteelS1 and S1A: Steel Moment Frameswith Stiff or Flexible DiaphragmsFEMA 273 Seismic Rehabilitation Guidelines 10-23

Chapter 10: Simplified RehabilitationTable 10-7S2 and S2A: Steel Braced Frameswith Stiff or Flexible DiaphragmsTable 10-9S4: Steel Frames with ConcreteShear WallsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesLateral Load Path at Pile CapsStress LevelStiffness of DiagonalsChevron or K-BracingBraced Frame ConnectionsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Frame Shear TransferAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of SteelTable 10-8S3: Steel Light FramesTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesLateral Load Path at Pile CapsCast-in-Place Concrete Shear WallsShear StressOverturningCoupling BeamsBoundary Component DetailingWall ReinforcementRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Wall Shear TransferAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of SteelCondition of ConcreteTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesSteel Moment FramesFrame ConcernsMasonry Shear WallsInfill WallsSteel Braced FramesStress LevelBraced Frame ConnectionsRe-entrant CornersDiaphragm OpeningsDiaphragm/Frame Shear TransferWall Panels and CladdingLight Gage Metal, Plastic, or Cementitious Roof PanelsAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsCondition of Steel10-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-10S5, S5A: Steel Frames with InfillMasonry Shear Walls and Stiff orFlexible DiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesLateral Load Path at Pile CapsFrames Not Part of the Lateral Force Resisting SystemComplete FramesMasonry Shear WallsReinforcing in Masonry WallsShear StressReinforcing at OpeningsUnreinforced Masonry Shear WallsProportions, Solid WallsInfill WallsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthSpan/Depth RatioDiaphragm/Wall Shear TransferAnchorage for Normal ForcesAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of SteelQuality of MasonryTable 10-11C1: Concrete Moment FramesTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesAdjacent BuildingsLateral Load Path at Pile CapsDeflection CompatibilityConcrete Moment FramesQuick Checks, Frame and Nonductile Detail ConcernsPrecast Moment Frame ConcernsFrames Not Part of the Lateral Force Resisting SystemShort “Captive” ColumnsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Frame Shear TransferPrecast ConnectionsAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteFEMA 273 Seismic Rehabilitation Guidelines 10-25

Chapter 10: Simplified RehabilitationTable 10-12C2, C2A: Concrete Shear Walls withStiff or Flexible DiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesLateral Load Path at Pile CapsDeflection CompatibilityFrames Not Part of the Lateral Force Resisting SystemShort "Captive" ColumnsCast-in-Place Concrete Shear WallsShear StressOverturningCoupling BeamsBoundary Component DetailingWall ReinforcementRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthSheathingDiaphragm/Wall Shear TransferAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteTable 10-13C3, C3A: Concrete Frames with InfillMasonry Shear Walls and Stiff orFlexible DiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesLateral Load Path at Pile CapsDeflection CompatibilityFrames Not Part of the Lateral Force Resisting SystemComplete FramesMasonry Shear WallsReinforcing in Masonry WallsShear StressReinforcing at OpeningsUnreinforced Masonry Shear WallsProportions, Solid WallsInfill WallsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthSpan/Depth RatioDiaphragm/Wall Shear TransferAnchorage for Normal ForcesAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteQuality of Masonry10-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-14PC1: Precast/Tilt-up Concrete ShearWalls with Flexible DiaphragmsTable 10-15PC1A: Precast/Tilt-up ConcreteShear Walls with Stiff DiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesDeflection CompatibilityPrecast Concrete Shear WallsPanel-to-Panel ConnectionsWall OpeningsCollectorsRe-entrant CornersCrosstiesDiaphragm OpeningsDiaphragm Stiffness/StrengthSheathingUnblocked DiaphragmsSpan/Depth RatioChord ContinuityDiaphragm/Wall Shear TransferAnchorage for Normal ForcesGirder/Wall ConnectionsStiffness of Wall AnchorsAnchorage to FoundationsCondition of FoundationOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesPrecast Concrete Shear WallsPanel-to-Panel ConnectionsWall OpeningsCollectorsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Wall Shear TransferAnchorage for Normal ForcesGirder/Wall ConnectionsAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteFEMA 273 Seismic Rehabilitation Guidelines 10-27

Chapter 10: Simplified RehabilitationTable 10-16PC2: Precast Concrete Frames withShear WallsTable 10-17PC2A: Precast Concrete FramesWithout Shear WallsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesLateral Load Path at Pile CapsDeflection CompatibilityConcrete Moment FramesPrecast Moment Frame ConcernsCast-in-Place Concrete Shear WallsShear StressOverturningCoupling BeamsBoundary Component DetailingWall ReinforcementRe-entrant CornersCrosstiesDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Wall Shear TransferAnchorage for Normal ForcesGirder/Wall ConnectionsPrecast ConnectionsAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesAdjacent BuildingsLateral Load Path at Pile CapsDeflection CompatibilityConcrete Moment FramesPrecast Moment Frame ConcernsFrames Not Part of the Lateral Force Resisting SystemShort Captive ColumnsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Frame Shear TransferPrecast ConnectionsAnchorage to FoundationsCondition of FoundationsOverturningLateral LoadsGeologic Site HazardsCondition of ConcreteTable 10-18RM1: Reinforced Masonry BearingWall Buildings with FlexibleDiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesMasonry Shear WallsReinforcing in Masonry WallsShear StressReinforcing at OpeningsRe-entrant CornersCrosstiesDiaphragm OpeningsDiaphragm Stiffness/StrengthSheathingUnblocked DiaphragmsSpan/Depth RatioDiaphragm/Wall Shear TransferAnchorage for Normal ForcesStiffness of Wall AnchorsAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsQuality of Masonry10-28 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-19Typical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesMasonry Shear WallsReinforcing in Masonry WallsShear StressReinforcing at OpeningsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Wall Shear TransferAnchorage for Normal ForcesAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsQuality of MasonryRM2: Reinforced Masonry BearingWall Buildings with Stiff DiaphragmsTable 10-21URMA: Unreinforced MasonryBearing Walls Buildings with StiffDiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesAdjacent BuildingsMasonry Shear WallsUnreinforced Masonry Shear WallsProperties, Solid WallsRe-entrant CornersDiaphragm OpeningsDiaphragm Stiffness/StrengthDiaphragm/Wall Shear TransferAnchorage for Normal ForcesAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsQuality of MasonryTable 10-20URM: Unreinforced MasonryBearing Wall Buildings with FlexibleDiaphragmsTypical DeficienciesLoad PathRedundancyVertical IrregularitiesPlan IrregularitiesAdjacent BuildingsMasonry Shear WallsUnreinforced Masonry Shear WallsProperties, Solid WallsRe-entrant CornersCrosstiesDiaphragm OpeningsDiaphragm Stiffness/StrengthSheathingUnblocked DiaphragmsSpan/Depth RatioDiaphragm/Wall Shear TransferAnchorage for Normal ForcesStiffness of Wall AnchorsAnchorage to FoundationsCondition of FoundationsGeologic Site HazardsQuality of MasonryFEMA 273 Seismic Rehabilitation Guidelines 10-29

Chapter 10: Simplified RehabilitationTable 10-22Cross Reference Between the Guidelines and FEMA 178 (BSSC, 1992a) Deficiency ReferenceNumbersFEMA 178GuidelinesSection Section Heading Section Section Heading3.1 Load Path 10.3.1.1 Load Path3.2 Redundancy 10.3.1.2 Redundancy3.3 Configuration3.3.1 Weak Story 10.3.1.3 Vertical Irregularities3.3.2 Soft Story 10.3.1.3 Vertical Irregularities3.3.3 Geometry 10.3.1.3 Vertical Irregularities3.3.4 Mass 10.3.1.3 Vertical Irregularities3.3.5 Vertical Discontinuities 10.3.1.3 Vertical Irregularities3.3.6 Torsion 10.3.1.4 Plan Irregularities3.4 Adjacent Buildings 10.3.1.5 Adjacent Buildings3.5 Evaluation of Materials and Conditions3.5.1 Deterioration of Wood 10.3.8.2 Condition of Wood3.5.2 Overdriven Nails 10.3.8.3 Overdriven Fasteners3.5.3 Deterioration of Steel 10.3.8.4 Condition of Steel3.5.4 Deterioration of Concrete 10.3.8.5 Condition of Concrete3.5.5 Post-Tensioning Anchors 10.3.8.6 Post-Tensioning Anchors3.5.6 Concrete Wall Cracks 10.3.8.5 Condition of Concrete3.5.7 Cracks in Boundary Columns 10.3.8.5 Condition of Concrete3.5.8 Precast Concrete Walls 10.3.8.5 Condition of Concrete3.5.9 Masonry Joints 10.3.8.7 Quality of Masonry3.5.10 Masonry Units 10.3.8.7 Quality of Masonry3.5.11 Cracks in Infill Walls 10.3.8.7 Quality of Masonry4.1 Frames with Infill Walls4.1.4 Interfering Walls 10.3.3.3F Infill Walls4.2 Steel Moment Frames4.2.1 Drift Check 10.3.2.1A Drift4.2.2 Compact Members 10.3.2.1B Frames4.2.3 Beam Penetration 10.3.2.1B Frames4.2.4 Moment Connections 10.3.2.1D Connections4.2.5 Column Splices 10.3.2.1B Frames4.2.6 Joint Webs 10.3.2.1D Connections4.2.7 Girder Flange Continuity Plates 10.3.2.1D Connections4.2.8 Strong Column-Weak Beam 10.3.2.1C Strong Column-Weak Beam4.2.9 Out-of-Plane Bracing 10.3.2.1B Frames4.3 Concrete Moment Frames4.3.1 Shearing Stress Check 10.3.2.2A Frame and Nonductile Detail Concerns4.3.2 Drift Check 10.3.2.2A Frame and Nonductile Detail Concerns10-30 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-22Cross Reference Between the Guidelines and FEMA 178 (BSSC, 1992a) Deficiency ReferenceNumbers (continued)FEMA 178GuidelinesSection Section Heading Section Section Heading4.3.3 Prestressed Frame Elements 10.3.2.2A Frame and Nonductile Detail Concerns4.3.4 Joint Eccentricity 10.3.2.2A Frame and Nonductile Detail Concerns4.3.5 No Shear Failures 10.3.2.2A Frame and Nonductile Detail Concerns4.3.6 Strong Column-Weak Beam 10.3.2.2A Frame and Nonductile Detail Concerns4.3.7 Stirrup and Tie Hooks 10.3.2.2A Frame and Nonductile Detail Concerns4.3.8 Column-Tie Spacing 10.3.2.2A Frame and Nonductile Detail Concerns4.3.9 Column-Bar Splices 10.3.2.2A Frame and Nonductile Detail Concerns4.3.10 Beam Bars 10.3.2.2A Frame and Nonductile Detail Concerns4.3.11 Beam-Bar Splices 10.3.2.2A Frame and Nonductile Detail Concerns4.3.12 Stirrup Spacing 10.3.2.2A Frame and Nonductile Detail Concerns4.3.13 Beam Truss Bars 10.3.2.2A Frame and Nonductile Detail Concerns4.3.14 Joint Reinforcing 10.3.2.2A Frame and Nonductile Detail Concerns4.3.15 Flat Slab Frames 10.3.2.2A Frame and Nonductile Detail Concerns4.4 Precast Moment Frames4.4.1 Precast Frames 10.3.2.2B Precast Moment Frames4.4.2 Precast Connections 10.3.6.5 Precast Connections4.5 Frames Not Part of the Lateral-Force-ResistingSystem4.5.1 Complete Frames 10.3.2.3A Complete Frames5.1 Concrete Shear Walls5.1.1 Shearing Stress Check 10.3.3.1A Shearing Stress5.1.2 Overturning 10.3.3.1B Overturning5.1.3 Coupling Beams 10.3.3.1C Coupling Beams5.1.4 Column Splices 10.3.3.1D Boundary Component Detailing5.1.5 Wall Connection 10.3.3.1D Boundary Component Detailing5.1.6 Confinement Reinforcing 10.3.3.1D Boundary Component Detailing5.1.7 Reinforcing Steel 10.3.3.1E Wall Reinforcement5.1.8 Reinforcing at Openings 10.3.3.1E Wall Reinforcement5.2 Precast Concrete Shear Walls5.2.1 Panel-to-Panel Connections 10.3.3.2A Panel-to-Panel Connections5.2.2 Wall Openings 10.3.3.2B Wall Openings5.2.3 Collectors 10.3.3.2C Collectors5.3 Reinforced Masonry Shear Walls5.3.1 Shearing Stress Check 10.3.3.3B Shearing Stress5.3.2 Reinforcing 10.3.3.3A Reinforcing in Masonry Walls5.3.3 Reinforcing at Openings 10.3.3.3C Reinforcing at OpeningsFEMA 273 Seismic Rehabilitation Guidelines 10-31

Chapter 10: Simplified RehabilitationTable 10-22Cross Reference Between the Guidelines and FEMA 178 (BSSC, 1992a) Deficiency ReferenceNumbers (continued)FEMA 178GuidelinesSection Section Heading Section Section Heading5.4 Unreinforced Masonry Shear Walls5.4.1 Shearing Stress Check 10.3.3.3D Unreinforced Masonry Shear Walls5.4.2 Masonry Lay-up 10.3.3.3D Unreinforced Masonry Shear Walls5.5 Unreinforced Masonry Infill Walls in Frames5.5.1 Proportions 10.3.3.3E Proportions of Solid Walls5.5.2 Solid Walls 10.3.3.3E Proportions of Solid Walls5.5.3 Cavity Walls 10.3.3.3F Infill Walls5.5.4 Wall Connections 10.3.3.3F Infill Walls5.6 Walls in Wood Frame Buildings5.6.1 Shearing Stress Check 10.3.3.4A Shear Stress5.6.2 Openings 10.3.3.4B Openings5.6.3 Wall Requirements 10.3.3.4C Wall Detailing5.6.4 Cripple Walls 10.3.3.4D Cripple Walls6.1 Concentrically Braced Frames6.1.1 Stress Check 10.3.4.1 System Concerns6.1.2 Stiffness of Diagonals 10.3.4.2 Stiffness of Diagonals6.1.3 Tension-Only Braces 10.3.4.2 Stiffness of Diagonals6.1.4 Chevron Bracing 10.3.4.3 Chevron or K-Bracing6.1.5 Concentric Joints 10.3.4.4 Braced Frame Connections6.1.6 Connection Strength 10.3.4.4 Braced Frame Connections6.1.7 Column Splices 10.3.4.4 Braced Frame Connections7.1 Diaphragms7.1.1 Plan Irregularities 10.3.5.1 Re-entrant Corners7.1.2 Cross Ties 10.3.5.2 Crossties7.1.3 Reinforcing at Openings 10.3.5.3 Diaphragm Openings7.1.4 Openings at Shear Walls 10.3.5.3 Diaphragm Openings7.1.5 Openings at Braced Frames 10.3.5.3 Diaphragm Openings7.1.6 Openings at Exterior Masonry Shear Walls 10.3.5.3 Diaphragm Openings7.2 Wood Diaphragms7.2.1 Sheathing 10.3.5.4A Board Sheathing7.2.2 Spans 10.3.5.4C Spans7.2.3 Unblocked Diaphragms 10.3.5.4B Unblocked Diaphragms7.2.4 Span/Depth Ratio 10.3.5.4D Span-to-Depth Ratio7.2.5 Diaphragm Continuity 10.3.5.4E Diaphragm Continuity7.2.6 Chord Continuity 10.3.5.4F Chord Continuity8.2 Anchorage for Normal Forces8.2.1 Wood Ledgers 10.3.6.3 Anchorage for Normal Forces10-32 Seismic Rehabilitation Guidelines FEMA 273

Chapter 10: Simplified RehabilitationTable 10-22Cross Reference Between the Guidelines and FEMA 178 (BSSC, 1992a) Deficiency ReferenceNumbers (continued)FEMA 178GuidelinesSection Section Heading Section Section Heading8.2.2 Wall Anchorage 10.3.6.3 Anchorage for Normal Forces8.2.3 Masonry Wall Anchors 10.3.6.3 Anchorage for Normal Forces8.2.4 Anchor Spacing 10.3.6.3 Anchorage for Normal Forces8.2.5 Tilt-up Walls 10.3.6.3 Anchorage for Normal Forces8.2.6 Panel-Roof Connection 10.3.6.3 Anchorage for Normal Forces8.3 Shear Transfer8.3.1 Transfer to Shear Walls 10.3.6.1 Diaphragm/Wall Shear Transfer8.3.2 Transfer to Steel Frames 10.3.6.2 Diaphragm/Frame Shear Transfer8.3.3 Topping Slab to Walls and Frames 10.3.6.1 Diaphragm/Wall Shear Transfer10.3.6.2 Diaphragm/Frame Shear Transfer8.4 Vertical Components to Foundations8.4.1 Steel Columns 10.3.7.1 Anchorage to Foundations8.4.2 Concrete Columns 10.3.7.1 Anchorage to Foundations8.4.3 Wood Posts 10.3.7.1 Anchorage to Foundations8.4.4 Wall Reinforcing 10.3.7.1 Anchorage to Foundations8.4.5 Shear-Wall-Boundary Columns 10.3.7.1 Anchorage to Foundations8.4.6 Wall Panels 10.3.7.1 Anchorage to Foundations8.4.7 Wood Sills 10.3.7.1 Anchorage to Foundations8.5 Interconnection of Elements8.5.1 Girders 10.3.6.4 Girder-Wall Connections8.5.2 Corbel Bearing 10.3.6.4 Girder-Wall Connections8.5.3 Corbel Connections 10.3.6.4 Girder-Wall Connections8.6 Roof Decking8.6.1 Light-Gage Metal Roof Panels 10.3.6.7 Light Gage Metal, Plastic, orCementitious Roof Panels8.6.2 Wall Panels 10.3.6.6 Wall Panels and Cladding9.1 Condition of Foundations9.1.1 Foundation Performance 10.3.7.2 Condition of Foundations9.1.2 Deterioration 10.3.7.2 Condition of Foundations9.2 Capacity of Foundations9.2.1 Overturning 10.3.7.3 Overturning9.2.2 Ties Between Foundation Elements 10.3.7.4 Lateral Loads9.2.3 Lateral Force on Deep Foundations 10.3.7.4 Lateral Loads9.2.4 Pole Buildings 10.3.7.4 Lateral Loads9.2.5 Sloping Sites 10.3.7.4 Lateral LoadsFEMA 273 Seismic Rehabilitation Guidelines 10-33

Chapter 10: Simplified RehabilitationTable 10-22Cross Reference Between the Guidelines and FEMA 178 (BSSC, 1992a) Deficiency ReferenceNumbers (continued)FEMA 178GuidelinesSection Section Heading Section Section Heading9.3 Geologic Site Hazards9.3.1 Liquefaction 10.3.7.5 Geologic Site Hazards9.3.2 Slope Failure 10.3.7.5 Geologic Site Hazards9.3.3 Surface Fault Rupture 10.3.7.5 Geologic Site Hazards10-34 Seismic Rehabilitation Guidelines FEMA 273

11.Architectural, Mechanical, and Electrical Components(Simplified and Systematic Rehabilitation)11.1 ScopeThis chapter establishes rehabilitation criteria forarchitectural, mechanical, and electrical componentsand systems that are permanently installed in buildings,or are an integral part of a building system, includingtheir supports and attachments. These components arecollectively referred to as “nonstructural components.”Contents introduced into buildings by owners oroccupants are not within the scope of the Guidelines.Guidance for rehabilitating existing nonstructuralcomponents is included within this chapter, while newnonstructural components shall conform to thematerials, detailing, and construction requirements forsimilar elements in new buildings.Nonstructural components in historic buildings may behighly significant, especially if they are original to thebuilding or innovative for their age. Guidance for theirseismic rehabilitation should be sought from the StateHistoric Preservation Officer or other historicpreservation specialist, and from specializedpublications. Equally important are other nonseismicconsiderations, such as accessibility for the disabled,fire protection, and hazardous materials considerations(especially asbestos-containing nonstructuralmaterials). The variety of such nonseismic factors is sogreat as to make it impossible to treat them in detail inthis document.The assessment process necessary to make a finaldetermination of which nonstructural components are tobe rehabilitated is not part of the Guidelines, but thesubject is touched on briefly in Section 11.3, and theCommentary to this chapter provides an outline of anassessment procedure.The core of this chapter is contained in Table 11-1,which provides:• A list of nonstructural components subject to LifeSafety requirements of these Guidelines• Rehabilitation requirements related to Seismic Zoneand Life Safety Performance Level• Identification of the required Analysis Procedure(analytical or prescriptive)Section 11.4 provides general requirements anddiscussion of Rehabilitation Objectives, PerformanceLevels, and Performance Ranges as they pertain tononstructural components. Criteria for means of egressare not specifically included in these Guidelines; anextensive discussion in the Commentary reviews theissues involved if this topic is selected forconsideration.Section 11.5 offers a brief discussion of structuralnonstructuralinteraction, and Section 11.6 providesgeneral requirements for acceptance criteria foracceleration-sensitive and deformation-sensitivecomponents, and those sensitive to both kinds ofresponse.Section 11.7 provides sets of equations for a simple“default” force analysis, as well as an extended analysismethod that considers a number of additional factors.Another set of equations sets out the AnalyticalProcedures for determining drift ratios and relativedisplacements. The general requirements forprescriptive procedures are also set out.Section 11.8 notes the general ways in whichnonstructural rehabilitation is carried out, with a moreextended discussion in the Commentary.Sections 11.9, 11.10, and 11.11 provide therehabilitation criteria for each component categoryidentified in Table 11-1. For each component thefollowing information is given:• Definition and scope• Component behavior and rehabilitation concepts• Acceptance criteria• Evaluation requirementsMethods of rehabilitation are discussed in more detail inthe Commentary for each component.11.2 Procedural StepsOnce the general philosophy of Section 11.1 isunderstood, its use can be reduced to the followingsteps, conducted within the framework of aFEMA 273 Seismic Rehabilitation Guidelines 11-1

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)nonstructural hazard mitigation plan (discussed in theCommentary, Section C11.3.2).1. Determine the Performance Level or Range desired.2. Refer to Table 11-1 to determine for eachnonstructural component the applicability of LifeSafety or Immediate Occupancy requirementsrelated to seismic zone, and required method ofanalysis.3. Refer to Sections 11.9, 11.10, and 11.11 foracceptance criteria for each nonstructuralcomponent.Table 11-1Nonstructural Components: Applicability of Life Safety and Immediate OccupancyRequirements and Methods of AnalysisCOMPONENTHighSeismicityModerateSeismicityLowSeismicityLS IO LS IO LS IOAnalysisMethodA. ARCHITECTURAL1. Exterior SkinAdhered Veneer Yes Yes Yes Yes No Yes F/DAnchored Veneer Yes Yes Yes Yes No Yes F/DGlass Blocks Yes Yes Yes Yes No Yes F/DPrefabricated Panels Yes Yes Yes Yes Yes Yes F/DGlazing Systems Yes Yes Yes Yes Yes Yes F/D2. PartitionsHeavy Yes Yes Yes Yes No Yes F/DLight No Yes No Yes No Yes F/D3. Interior VeneersStone, Including Marble Yes Yes Yes Yes No Yes F/DCeramic Tile Yes Yes No Yes No Yes F/D4. Ceilingsa. Directly Applied to Structure No 13 Yes No 13 Yes No Yes Fb. Dropped. Furred, Gypsum Board No Yes No Yes No Yes Fc. Suspended Lath and Plaster Yes Yes Yes Yes No Yes Fd. Suspended Integrated Ceiling No 11 Yes No 11 Yes No 11 Yes PR5. Parapets and Appendages Yes Yes Yes Yes Yes Yes F 16. Canopies and Marquees Yes Yes Yes Yes Yes Yes F7. Chimneys and Stacks Yes Yes Yes Yes No Yes F 28. Stairs Yes Yes Yes Yes Yes Yes *Notes and definitions provided on page 11-411-2 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Table 11-1Nonstructural Components: Applicability of Life Safety and Immediate OccupancyRequirements and Methods of Analysis (continued)COMPONENTHighSeismicityModerateSeismicityLowSeismicityLS IO LS IO LS IOAnalysisMethodB. MECHANICAL EQUIPMENT1. Mechanical EquipmentBoilers and Furnaces Yes Yes Yes Yes Yes Yes FGeneral Mfg. and Process Machinery No 3 Yes No Yes No Yes FHVAC Equipment, Vibration-Isolated No 3 Yes No Yes No Yes FHVAC Equipment, Non-Vibration-Isolated No 3 Yes No Yes No Yes FHVAC Equipment, Mounted In-Line with No 3 Yes No Yes No Yes PRDuctwork2. Storage Vessels and Water HeatersStructurally Supported Vessels (Category 1) No 3 Yes No Yes No Yes Note 4Flat Bottom Vessels (Category 2) No 3 Yes No Yes No Yes Note 53. Pressure Piping Yes Yes No Yes No Yes Note 54. Fire Suppression Piping Yes Yes No Yes No Yes PR5. Fluid Piping, not Fire SuppressionHazardous Materials Yes Yes Yes Yes Yes Yes PR/F/DNonhazardous Materials No Yes No Yes No Yes PR/F/D6. Ductwork No 6 Yes No 6 Yes No Yes PRC. ELECTRICAL AND COMMUNICATIONS1. Electrical and CommunicationsNo 7 Yes No 7 Yes No Yes FEquipment2. Electrical and CommunicationsNo 8 Yes No 8 Yes No Yes PRDistribution Equipment3. Light FixturesRecessed No No No No No NoSurface Mounted No No No No No NoIntegrated Ceiling Yes Yes Yes Yes No Yes PRPendant No 9 Yes No 9 Yes No Yes FNotes and definitions provided on page 11-4FEMA 273 Seismic Rehabilitation Guidelines 11-3

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Table 11-1Nonstructural Components: Applicability of Life Safety and Immediate OccupancyRequirements and Methods of Analysis (continued)COMPONENTHighSeismicityModerateSeismicityLowSeismicityLS IO LS IO LS IOAnalysisMethodD. FURNISHINGS AND INTERIOR EQUIPMENT1. Storage Racks Yes 10 Yes Yes 10 Yes No Yes F2. Bookcases Yes Yes Yes Yes No Yes F3. Computer Access Floors No Yes No Yes No Yes PR/FD4. Hazardous Materials Storage Yes Yes No 12 Yes No 12 Yes PR5. Computer and Communication Racks No Yes No Yes No Yes PR/F/D6. Elevators Yes Yes Yes Yes No Yes F/D7. Conveyors No Yes No Yes No Yes F/D1. Unreinforced masonry parapets not over 4 ft in height may be rehabilitated to Prescriptive Design Concept.2. Residential masonry chimneys may be rehabilitated to Prescriptive Design Concept.3. Rehabilitation required to Life Safety Performance Level when:Equipment type A or B, or vessel, 6 ft or over in heightEquipment type CEquipment forming part of an emergency power systemGas-fired equipment in occupied or unoccupied space4. Residential water heaters with capacity less than 100 gal may be rehabilitated by Prescriptive Procedure. Other vessels to meet force provisions ofSection 11.7.3 or 11.7.4.5. Vessels or piping systems may be rehabilitated according to Prescriptive Standards. Large systems or vessels shall meet force provisions of Section 11.7.3or 11.7.4; piping also shall meet drift provisions of Section 11.7.5.6. Rehabilitation required when ductwork conveys hazardous materials, exceeds 6 sq. ft in cross-sectional area, or is suspended more than 12 in. from topof duct to supporting structure.7. Rehabilitation required to Life Safety Performance Level when:Equipment is 6 ft or over in heightEquipment weighs over 20 lbs.Equipment forms part of an emergency power and/or communication system8. Rehabilitation required to Life Safety Performance Level when equipment forms part of an emergency lighting, power, and/or communication system9. Rehabilitation required to Life Safety Performance Level when fixture weight per support exceeds 20 lbs.10. Rehabilitation not required for storage racks in essentially unoccupied space.11. Rehabilitation required to Life Safety Performance Level when panels exceed 2 lb/sq. ft and for Enhanced Rehabilitation Objectives.12. Rehabilitation required where material is in close proximity to occupancy, and leakage can cause immediate life safety threat.13. Rehabilitation required to achieve Life Safety Performance Level for poorly attached large areas (over 10 sq. ft) of plaster ceilings on metal or wood lath.Key:LS Life Safety Performance LevelIO Immediate Occupancy Performance LevelPR Prescriptive Procedure acceptableF Analytical Procedure: force analysis, Section 11.7.3 or 11.7.4F/D Analytical Procedure: force and relative displacement analysis, Sections 11.7.4 and 11.7.5* Rehabilitate as required for individual components4. Use equations in Section 11.7 to conduct anynecessary analysis.5. Develop any necessary design solutions to meet theforce requirements, the deformation criteria, and anyprescriptive requirements. For capacities of11-4 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)nonstructural components and their connectionsrefer to Chapters 5 through 8, or derive capacityvalues in a manner consistent with those chapters.11.3 Historical and ComponentEvaluation Considerations11.3.1 Historical PerspectivePrior to the 1961 Uniform Building Code and the 1964Alaska earthquake, architectural components andmechanical and electrical systems for buildings hadtypically been designed with little, if any, regard tostability when subjected to seismic forces. By the timeof the 1971 San Fernando earthquake, it became quiteclear that damage to nonstructural elements could resultin serious casualties, severe building functionalimpairment, and major economic losses even when thestructural damage was not significant (Lagorio, 1990).The architectural, mechanical, and electricalcomponents and systems of a historic building may bevery significant, especially if they are original to thebuilding, very old, or innovative. An assessment of theirsignificance by an appropriate professional—such as anarchitectural historian, historical preservation architect,or historian of engineering and technology—may benecessary. Historic buildings may also have materials,such as lead pipes and asbestos, that may or may notpose a hazard depending on their location, condition,use or abandonment, containment, and/or disturbanceduring the rehabilitation.Readers are referred to the Commentary to this sectionfor further discussion and a chronology of theintroduction of nonstructural considerations intoseismic codes.11.3.2 Component EvaluationProcedures for detailed assessment to decide whichexisting nonstructural components should berehabilitated are not part of these Guidelines. However,there is a brief discussion under “Evaluation Needs” ineach component section. To achieve the Basic SafetyObjective (BSO), nonstructural components as listed inTable 11-1 must meet the Life Safety PerformanceLevel for specified ground motion, as defined inChapter 2. In other cases—such as when the LimitedSafety Performance Range applies—there may be morelatitude in the selection of components forrehabilitation. A suggested procedure for the detailedevaluation of existing nonstructural components—withcost-effectiveness and a ranking of importance inmind—is outlined in the Commentary, Section C11.3.2.11.4 Rehabilitation Objectives,Performance Levels, andPerformance RangesThe nonstructural Rehabilitation Objective may be thesame as for the structural rehabilitation, or may differ,except for the BSO, in which case structural andnonstructural requirements specified in the Guidelinesmust be met.These Guidelines are also intended to be applicable tothe situation where nonstructural—but not structural—components are to be rehabilitated. Rehabilitation thatis restricted to the nonstructural components willtypically fall within the Limited Safety PerformanceRange, unless the structure is already determined tomeet a specified Rehabilitation Objective.To qualify for any Rehabilitation Objective higher thanLimited Safety, consideration of structural behavior isnecessary even if only nonstructural components are tobe rehabilitated, to properly take into account loads onnonstructural components generated by inertial forcesor imposed deformations.11.4.1 Performance Levels for NonstructuralComponentsFour Nonstructural Performance Levels and threeStructural Performance Levels are described inChapter 2 of the Guidelines. For nonstructuralcomponents, the Collapse Prevention PerformanceLevel does not, in general, apply, since mostnonstructural damage resulting from a building at theCollapse Prevention damage state is regarded asacceptable. (Rehabilitation of parapets and heavyappendages is required, however, for conformance withthe Collapse Prevention Building Performance Level.)The four defined Performance Levels applying tononstructural components are:• Hazards Reduced Performance Level. Thisrepresents a post-earthquake damage level in whichextensive damage has occurred to nonstructuralcomponents but large or heavy items—such asparapets, cladding, plaster ceilings, or storageFEMA 273 Seismic Rehabilitation Guidelines 11-5

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)racks—posing a falling hazard to many people areprevented from falling.• Life Safety Performance Level. This PerformanceLevel is intended primarily to prevent nonstructuralfalling hazards that can directly cause injury.Excluded from the Life Safety Performance Levelare specific criteria relating to post-earthquakenonstructural performance, such as egress, alarmand communications systems, fire protectionsystems, and other functional issues. The issue ofegress protection, although not specificallyaddressed, is substantially taken care of byrehabilitation of relevant nonstructural componentsto the Life Safety Performance Level.Acceptance Criteria for the Life Safety PerformanceLevel are provided in the sections on eachnonstructural equipment category.Post-earthquake functional concerns are addressedwithin the Damage Control Performance Range andby the Immediate Occupancy Performance Level.• Immediate Occupancy Performance Level. Toattain this Performance Level, conformance withrequirements for the Life Safety Performance Levelmust be met, together with the requirements forImmediate Occupancy where applicable.Acceptance criteria for the Immediate OccupancyPerformance Level are provided only in the sections oneach nonstructural component category.• Operational Performance Level. A theoreticalBuilding Performance Level beyond ImmediateOccupancy, this level depends on the continuingfunctioning of all utilities and systems, and, often, ofother sensitive equipment. Specific criteria fornonstructural components for this PerformanceLevel are not provided in these Guidelines becausethe critical components and systems are buildingspecific,and operational capability may bedependent on equipment over which the design teamhas no authority.The procedure for attaining an Operational PerformanceLevel is to use the criteria for Immediate Occupancyand develop additional criteria based on a detailedevaluation of the specific building relative to theoperational functions to be maintained.Tables 2-6 through 2-8 summarize nonstructuraldamage states in relation to Performance Levels.11.4.2 Performance Ranges forNonstructural ComponentsIncluding the Hazards Reduced Performance Level,below the Life Safety Nonstructural PerformanceLevel, there are nonstructural rehabilitation damagestates that will fall below or above the Life SafetyLevel. For example, it is possible to exceed the LifeSafety Level but fall short of Immediate Occupancy, orexceed Immediate Occupancy but not meet OperationalPerformance Level requirements. Performance inexcess of the Operational Performance Level is alsoconceivable, though unlikely. While the ranges may beconceptually referred to as Enhanced or Limited(relative to Life Safety), such ranges are not formallydefined by the Guidelines for nonstructuralcomponents, nor are requirements specified.11.4.3 Regional Seismicity and NonstructuralComponentsRequirements for the rehabilitation of nonstructuralcomponents relating to the three Seismic Zones—High,Moderate, and Low—are shown in Table 11-1 andnoted in each section, where applicable. In general, inregions of low seismicity, certain nonstructuralcomponents have no rehabilitation requirements withrespect to the Life Safety Performance Level.Rehabilitation of these components, particularly whererehabilitation is simple, may nevertheless be desirablefor damage control and property loss reduction.11.4.4 Means of Egress: Escape and RescueEmergency post-earthquake access into and out ofbuildings is one of the aspects of nonstructuralperformance that may be selected for consideration inthe Damage Control Performance Range. Because theDamage Control Performance Range is not specificallydefined by requirements in the Guidelines, emergencyescape and rescue criteria are not included within theGuidelines.Preservation of egress is accomplished primarily byensuring that the most hazardous nonstructural elementsare replaced or rehabilitated. The items listed inTable 11-1 for achieving the Life Safety PerformanceLevel show that typical requirements for maintainingegress will, in effect, be accomplished if the egressrelatedcomponents are addressed. These would include11-6 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)the following items listed in FEMA 178, NEHRPHandbook for the Seismic Evaluation of ExistingBuildings (pp. 91–92, and pp. A-20) (BSSC, 1992b).• Walls around stairs, elevator enclosures, andcorridors are not hollow clay tile or unreinforcedmasonry.• Stair enclosures do not contain any piping orequipment except as required for life safety.• Veneers, cornices, and other ornamentation abovebuilding exits are well anchored to the structuralsystem.• Parapets and canopies are anchored and braced toprevent collapse and blockage of building exits.Beyond this, the following list describes someconditions that might be commonly recognized asrepresenting major obstruction; the building should beinspected to see whether these, or any similar hazardousconditions exist; if so, their replacement orrehabilitation should be included in the rehabilitationplan.• Partitions taller than six feet and weighing more thanfive pounds per square foot, if collapse of the entirepartition—rather than cracking—is the expectedmode of failure, and if egress would be impeded• Ceilings, soffits, or any ceiling or decorative ceilingcomponent weighing more than two pounds persquare foot, if it is expected that large areas (piecesmeasuring ten square feet or larger) would fall• Potential for falling ceiling-located light fixtures orpiping; diffusers and ductwork, speakers and alarms,and other objects located higher than 42 inches offthe floor• Potential for falling debris weighing more than 100pounds that, if it fell in an earthquake, wouldobstruct a required exit door or other component,such as a rescue window or fire escape• Potential for jammed doors or windows required aspart of an exit path—including doors to individualoffices, rest rooms, and other occupied spacesOf these, the first four are also taken care of in the LifeSafety Performance Level requirement. The lastcondition is very difficult to remove with any assurance,except for low levels of shaking in which structural driftand deformation will be minimal, and the need forescape and rescue correspondingly slight.Refer to the Commentary for this section for furtherdiscussion of egress, escape, and rescue issues.11.5 Structural-NonstructuralInteraction11.5.1 Response ModificationIn cases where a nonstructural component directlymodifies the strength or stiffness of the buildingstructural elements, or its mass affects the buildingloads, its characteristics should be considered in thestructural analysis of the building. Particular careshould be taken to identify masonry infill that couldreduce the effective length of adjacent columns.11.5.2 Base IsolationNonstructural components that cross the isolationinterface in a base-isolated structure should be designedto accommodate the total maximum displacement of theisolator.11.6 Acceptance Criteria forAcceleration-Sensitive andDeformation-SensitiveComponents11.6.1 Acceleration-Sensitive ComponentsAcceleration-sensitive components shall meet the forcerequirements derived from equations in Section 11.7.Acceleration-sensitive components are discussed,where necessary, in each component section(Sections 11.9, 11.10, and 11.11). The guiding principlefor deciding whether a component requires a forceanalysis, as defined in Section 11.7, is that analysis ofinertial loads generated within the component isnecessary to properly consider the component’s seismicbehavior. The steps for application of accelerationsensitiveacceptance criteria are as follows:1. Determination of the Rehabilitation Objective andassociated Performance Level (see Table 11-1 forFEMA 273 Seismic Rehabilitation Guidelines 11-7

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)the applicability of requirements keyed to the LifeSafety Performance Level)2. Determination of the seismicity—Low, Moderate, orHigh—as defined in Section 2.6.33. Application of design forces to the existing ormodified component (Section 11.7), if theAnalytical Procedure is required by Table 11-1; or, ifthe Prescriptive Procedure is acceptable according toTable 11-1, comparison of the existing componentwith required characteristics as defined in areference or standard4. Verification that the component can meet theacceptance criteria for the applicable PerformanceLevel (see each specific component section,Sections 11.9, 11.10, and 11.11).11.6.2 Deformation-Sensitive ComponentsDeformation-sensitive components shall meet thegeneral acceptance criteria of this section, as well asadditional requirements listed for specific components.The steps for application of deformation-sensitiveacceptance criteria are:1. Determination of the Rehabilitation Objective andassociated Performance Level (see Table 11-1 forthe applicability of requirements keyed toPerformance Level) shall be made.2. Determination of the seismicity—Low, Moderate, orHigh—as defined in Section 2.6.3, shall be made.3. Determination of the deformation and associateddrift ratio of the structural component(s) to whichthe deformation-sensitive nonstructural componentis attached (see structural Analysis Procedures ofpreceding sections) shall be made.4. Analysis shall be made of the nonstructuralcomponent’s response to the deformation of thestructure, including a consideration of the transfer ofloads through the particular connection details of thenonstructural component, or comparison of theexisting component with required characteristics asdefined in a reference or standard, if the PrescriptiveProcedure is acceptable according to Table 11-1.5. Verification shall be made that the component canmeet the acceptance criteria for the applicablePerformance Level (see each specific componentsection, Sections 11.9, 11.10 and 11.11). In lieu ofapplication of the specific acceptance criteria listedfor each component, the following requirementsmay be used:Life Safety Performance Level. The component meetsdeformation-sensitive acceptance criteria if the driftratio at that story level is 0.01 or less. (This alternativewill require consideration of glazing or othercomponents that can hazardously fail at lesser driftratios—depending on installation details—orcomponents that can undergo greater distortion withouthazardous failure resulting—for example, typicalgypsum board partitions. This alternative may beappropriate only where the Prescriptive Procedure isallowed [though calculations are required here becausethe structure’s drift must be known].)Use of Drift Ratio Values as Acceptance Criteria. Thedata on drift ratio values related to damage states islimited, and the use of single median drift ratio valuesas acceptance criteria must cover a broad range ofactual conditions. It is therefore suggested that thelimiting drift values shown in this chapter be used as aguide for evaluating the probability of a given damagestate for a subject building, but not be used as absoluteacceptance criteria. At higher Performance Levels it islikely that the criteria for nonstructural deformationsensitivecomponents may control the structuralrehabilitation design. These criteria should be regardedas a flag for the careful evaluation of structuralnonstructuralinteraction and consequent damage states,rather than the required imposition of an absoluteacceptance criterion that might require costly redesignof the structural rehabilitation. For further discussion,see the Commentary for this section.11.6.3 Acceleration- and Deformation-Sensitive ComponentsSome components are both acceleration- anddeformation-sensitive. They must be analyzed forconformance to acceptance criteria for both forms ofresponse.11.7 Analytical and PrescriptiveProcedures11.7.1 Application of Analytical andPrescriptive ProceduresThere are two nonstructural rehabilitation procedures:11-8 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)• Prescriptive Procedure• Analytical ProcedureThere are three analysis methods for calculating forceswithin the Analytical Procedure.• Equation 11-1, a simple conservative defaultequation, may be used.• Equations 11-2 and 11-3 offer more completeequivalent lateral force equations. In addition,Equations 11-4 and 11-5 should be used when driftis a consideration.• Results from any structural analysis method allowedfor the building’s rehabilitation may be used, as longas Performance Level criteria, response modificationfactors, and other considerations are treatedconsistently. The Analytical Procedure is alwaysacceptable; the Prescriptive Procedure is acceptablefor the combinations of seismicity, PerformanceLevel, and component listed in Table 11-1.11.7.2 Prescriptive ProcedureA Prescriptive Procedure consists of publishedstandards and references that describe the designconcepts and construction features that must be presentfor a given nonstructural component to be seismicallyprotected. No engineering calculations are required in aPrescriptive Procedure, although in some cases anengineering review of the design and installation isrequired.Where a Prescriptive Procedure is allowed, the specificprescriptive references are given in the section on theindividual component, Sections 11.9, 11.10, and 11.11.11.7.3 Analytical Procedure: DefaultEquationSeismic forces shall be determined in accordance withEquation 11-1:F p = 1.6 S XS I p W p (11-1)whereF p = Seismic design force applied horizontally atthe component’s center of gravity anddistributed relative to the component’s massdistributionS XS = Spectral response acceleration at shortperiods for any hazard levelI p = Component performance factor that is either1.0 for Life Safety Performance Level or 1.5for Immediate Occupancy PerformanceLevel= Component operating weightW p11.7.4 Analytical Procedure: GeneralEquationAlternatively, seismic forces shall be determined inaccordance with Equations 11-2 and 11-3Note: F p calculated from Equation 11-2 need notexceed F p calculated from Equation 11-1.wherea p0.4a p S XS I p W ⎛ 2x1 + ----- ⎞p ⎝ h ⎠F p = --------------------------------------------------------R p(11-2)F p (minimum) = 0.3 S XS I p W p (11-3)= Component amplification factor, related torigidity of component that varies from 1.00 to2.50 (select appropriate value fromTable 11-2)F p = Seismic design force applied horizontally atthe component’s center of gravity anddistributed relative to the component’s massdistributionS XS = Spectral response acceleration at shortperiods for any hazard levelh = Average roof elevation of structure, relativeto grade elevationI p = Component performance factor that is either1.0 for Life Safety Performance Level or 1.5for Immediate Occupancy Performance LevelFEMA 273 Seismic Rehabilitation Guidelines 11-9

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)R p = Component response modification factor,related to ductility of anchorage that variesfrom 1.25 to 6.0 (select appropriate valuefrom Table 11-2)W p = Component operating weightx = Elevation in structure of component relativeto grade elevation11.7.5 Drift Ratios and RelativeDisplacementsDrift ratios (D r ) shall be determined in accordance withthe following equations:For two connection points on the same building orstructural system, useD r = (δ xA - δ yA ) / (X – Y) (11-4)Relative displacements (D p ) shall be determined inaccordance with the following equation:For relative displacement of two connection points onseparate buildings or structural systems, usewhereD p = | δ xA | + | δ xB | (11-5)D p = Relative seismic displacement that thecomponent must be designed to accommodateD r = Drift ratioX = Height of upper support attachment at level xas measured from gradeY = Height of lower support attachment at level yas measured from gradeδ xA = Deflection at building level x of Building A,determined by analysis as defined inChapter 3δ yA = Deflection at building level y of Building A,determined by analysis as defined inChapter 3δ xB = Deflection at building level x of Building B,determined by analysis as defined inChapter 3The effects of seismic relative displacements shall beconsidered in combination with displacements causedby other loads, as appropriate.11.7.6 Other ProceduresOther procedures are available that requiredetermination of the maximum acceleration of thebuilding at each component support and the maximumrelative displacements between supports common to anindividual component.Linear Procedures can be used to calculate themaximum acceleration of each component support andthe inter-story drifts of the building, taking into accountthe location of the component in the building.Consideration of the flexibility of the component, andthe possible amplification of the building roof and flooraccelerations and displacements in the component,would require the development of roof and floorresponse spectra or acceleration time histories at thenonstructural support locations, derived from thedynamic response of the structure.Relative displacements between component supportsare difficult to calculate, even with the use ofacceleration time histories, because the maximumdisplacement of each component support at differentlevels in the building might not occur at the same timeduring the building response.Guidelines for these dynamic analyses for nonstructuralcomponents are given in Chapter 6 of Seismic DesignGuidelines for Essential Buildings, a supplement toSeismic Design of Buildings (Department of the Army,Navy, and Air Force, 1986).These other analytical procedures are considered toocomplex for the rehabilitation of nonessential buildingnonstructural components for Immediate Occupancyand Life Safety Performance Levels.Recent research (Drake and Bachman, 1995) has shownthat the Analytical Procedures in Sections 11.7.3 and11.7.4, which are based on the 1997 NEHRP Provisionsfor New Buildings (BSSC, 1997) Analytical Procedures,provide a reasonable upper bound for the seismic forceson nonstructural components.Therefore, the other complex analytical proceduresoutlined above to develop roof and floor spectra are notrequired to evaluate and rehabilitate the typicalnonstructural components discussed in this chapter. Useof the Analytical Procedures in Sections 11.7.3 and11.7.4 is recommended.11-10 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Table 11-2Nonstructural Component Amplification and Response Modification FactorsCOMPONENT a p1R p2A. ARCHITECTURAL1. Exterior SkinAdhered Veneer 1 4Anchored Veneer 1 3 3Glass Block 1 2Prefabricated Panels 1 3 3Glazing Systems 1 22. PartitionsHeavy 1 1.5Light 1 33. Interior VeneersStone, Including Marble 1 1.5Ceramic Tile 1 1.54. Ceilingsa. Directly Applied to Structure 1 1.5b. Dropped, Furred Gypsum Board 1 1.5c. Suspended Lath and Plaster 1 1.5d. Suspended Integrated Ceiling 1 1.55. Parapets and Appendages 2.5 1.256. Canopies and Marquees 2.5 1.57. Chimneys and Stacks 2.5 1.258. Stairs 1 3B. MECHANICAL EQUIPMENT1. Mechanical EquipmentBoilers and Furnaces 1 3General Mfg. and Process Machinery 1 3HVAC Equipment, Vibration-Isolated 2.5 3HVAC Equipment, Non-Vibration-Isolated 1 3HVAC Equipment, Mounted In-Line with Ductwork 1 32. Storage Vessels and Water HeatersVessels on Legs (Category 1) 2.5 1.5Flat Bottom Vessels (Category 2) 2.5 33. High-Pressure Piping 2.5 44. Fire Suppression Piping 2.5 4FEMA 273 Seismic Rehabilitation Guidelines 11-11

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Table 11-2Nonstructural Component Amplification and Response Modification Factors (continued)COMPONENT1a p2R p5. Fluid Piping, not Fire SuppressionHazardous Materials 2.5 1Nonhazardous Materials 2.5 46 Ductwork 1 3C. ELECTRICAL AND COMMUNICATIONS EQUIPMENT1. Electrical and Communications Equipment 1 32. Electrical and Communications Distribution Equipment 2.5 53. Light FixturesRecessed 1 1.5Surface Mounted 1 1.5Integrated Ceiling 1 1.5Pendant 1 1.5D. FURNISHINGS AND INTERIOR EQUIPMENT1. Storage Racks 4 2.5 42. Bookcases 1 33. Computer Access Floors 1 34. Hazardous Materials Storage 2.5 15. Computer and Communications Racks 2.5 66. Elevators 1 37. Conveyors 2.5 31. A lower value for a p may be justified by detailed dynamic analysis. The value for a p shall be not less than 1. The value of a p = 1 is for equipment generallyregarded as rigid and rigidly attached. The value of a p = 2.5 is for equipment generally regarded as flexible and flexibly attached. See the definitions(Section 11.12) for explanations of “Component, rigid” and “Component, flexible.” Where flexible diaphragms provide lateral support for walls andpartitions, the value of a p shall be increased to 2.0 for the center one-half of the span.2. R p = 1.5 for anchorage design where component anchorage is provided by expansion anchor bolts, shallow chemical anchors, or shallow (nonductile)cast-in-place anchors, or where the component is constructed of nonductile materials. Shallow anchors are those with an embedment length-to-boltdiameter ratio of less than eight.3. Applies when attachment is ductile material and design, otherwise 1.5.4. Storage racks over six feet in height shall be designed in accordance with the provisions of Section 11.11.1.11-12 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)11.8 Rehabilitation ConceptsNonstructural rehabilitation is accomplished throughreplacement, strengthening, repair, bracing, orattachment. These methods are discussed in more depthin the Commentary to this section.11.9 Architectural Components:Definition, Behavior, andAcceptance Criteria11.9.1 Exterior Wall Elements11.9.1.1 Adhered VeneerA. Definition and ScopeAdhered veneer includes thin exterior finish materialssecured to a backing material by adhesives. Thebacking may be masonry, concrete, cement plaster, or astructural framework material. The four main categoriesof adhered veneer are:1. Tile, masonry, stone, terra cotta, or other similarmaterials not over one inch thick2. Glass mosaic units not over 2" x 2" x 3/8" thick3. Ceramic tile4. Exterior plaster (stucco)B. Component Behavior and Rehabilitation ConceptsAdhered veneers are predominantly deformationsensitive;deformation of the substrate leads to crackingor separation of the veneer from its backing. Poorlyadhered veneers may be dislodged by directacceleration.Calculation of the drift of the structure to which thenonstructural component is attached is necessary toestablish conformance with drift acceptance criteriarelated to Performance Level. Nonconformancerequires limiting drift, special detailing to isolatesubstrate from structure to permit drift, or replacementwith drift-tolerant material. Poorly adhered veneersshould be replaced.C. Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design of the attachment to the backing to meet theout-of-plane force provisions of Section 11.7.3 or 11.7.4and to meet the relative displacement in-plane driftprovisions of Section 11.7.5. The limiting in-plane driftratio is 0.03.Immediate Occupancy Performance Level.Compliance is provided by design of the attachment tothe backing to meet the out-of-plane force provisions ofSection 11.7.3 or 11.7.4 and to meet the relativedisplacement in-plane drift provisions of Section 11.7.5.The limiting in-plane drift ratio is 0.01.D. Evaluation RequirementsAdhered veneer must be evaluated by visualobservation, as well as tapping to discern looseness orcracking that may be present. If found, this may indicateeither defective bonding to the substrate or excessiveflexibility of the supporting structure.11.9.1.2 Anchored VeneerA. Definition and ScopeAnchored veneer includes masonry or stone units thatare attached to the supporting structure by mechanicalmeans. The three main categories of anchored veneerare:1. Masonry and stone units not over five inchesnominal thickness2. Stone units from five inches to ten inches nominalthickness3. Stone slab units not over two inches nominalthicknessThe provisions of this section apply to units that aremore than 48 inches above the ground or adjacentexterior area.B. Component Behavior and Rehabilitation ConceptsAnchored veneer is both acceleration- and deformationsensitive.Heavy units may be dislodged by directacceleration, which distorts or fractures the mechanicalconnections. Deformation of the supporting structure,particularly if it is a frame, may similarly affect theconnections, and the units may be displaced ordislodged by racking.Drift analysis is necessary to establish conformancewith drift acceptance criteria related to PerformanceLevel. Nonconformance requires limiting structuralFEMA 273 Seismic Rehabilitation Guidelines 11-13

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)drift, or special detailing to isolate substrate fromstructure to permit drift. Defective connections must bereplaced.C. Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design of the attachment to the backing to meet theout-of-plane force provisions of Section 11.7.3 or 11.7.4and relative displacement to meet the relativedisplacement in-plane drift provisions of Section 11.7.5.The limiting drift ratio is 0.02.Immediate Occupancy Performance Level.Compliance is provided by design of the attachment tothe backing to meet the out-of-plane force provisions ofSection 11.7.3 or 11.7.4 and to meet the relativedisplacement in-plane drift provisions of Section 11.7.5.The limiting drift ratio is 0.01.D. Evaluation RequirementsThe stone units must have adequate stability, jointdetailing, and maintenance to prevent moisturepenetration from weather that could destroy theanchors. The anchors must be visually evaluated and,based on the engineer’s judgment, tested to establishcapacity to sustain design forces and deformations.11.9.1.3 Glass Block Units and OtherNonstructural MasonryA. Definition and ScopeThis category includes glass block, and other units thatare self-supporting for static vertical loads, heldtogether by mortar, and structurally detached from thesurrounding structure.B. Component Behavior and Rehabilitation ConceptsThese units are both acceleration- and deformationsensitive;failure in-plane generally occurs bydeformation in the surrounding structure that results inunit cracking and displacement along the cracks.Failure out-of-plane takes the form of dislodgment orcollapse caused by direct acceleration.For small wall areas (less than 144 square feet or 15 feetin any dimension), rehabilitation can be accomplishedby restoration, using the Prescriptive Procedure basedon the Uniform Building Code, 1994, Section 2110(ICBO, 1994). For larger areas, the AnalyticalProcedure must be used to establish forces and driftsagainst which the design must be measured.Nonconformance with deformation criteria requireslimiting structural drift, or special detailing to isolatethe glass block wall from the surrounding structure topermit drift. Sufficient reinforcing must be provided todeal with out-of-plane forces. Large walls may need tobe subdivided by additional structural supports intosmaller areas that can meet the drift or force acceptancecriteria.C. Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design of the glass block wall and its enclosingframing, to meet both the in-plane and out-of-planeforce provisions of Section 11.7.3 or 11.7.4 and to meetthe relative displacement in-plane drift provisions ofSection 11.7.5. The limiting drift ratio is 0.02.Immediate Occupancy Performance Level .Compliance is provided by design of the glass blockwall and its enclosing framing, to meet both the inplaneand out-of-plane force provisions ofSection 11.7.3 or 11.7.4 and to meet the relativedisplacement in-plane drift provisions of Section 11.7.5.The limiting drift ratio is 0.01.D. Evaluation RequirementsThe Prescriptive Procedure referred to above will serveas the criteria against which the wall must be evaluated.11.9.1.4 Prefabricated PanelsA. Definition and ScopeThis category consists of prefabricated panels that areinstalled with adequate structural strength withinthemselves and their connections to resist wind,seismic, and other forces. These panels are generallyattached around their perimeters to the primarystructural system. The three typical types ofprefabricated panels are the following:1. Precast concrete, and concrete panels with facing(generally stone) laminated or mechanicallyattached2. Laminated metal-faced insulated panels3. Steel strong-back panels, with insulated, waterresistantfacing, or mechanically attached metal orstone facing11-14 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)B. Component Behavior and Rehabilitation ConceptsPrefabricated panels are both acceleration- anddeformation-sensitive. Lightweight units may bedamaged by racking; heavy units may be dislodged bydirect acceleration, which distorts or fractures themechanical connections. Excessive deformation of thesupporting structure—most likely if it is a frame—mayresult in the units imposing external racking forces onone another, and distorting or fracturing theirconnections, with consequent displacement ordislodgment.Drift analysis is necessary to establish conformancewith drift acceptance criteria related to PerformanceLevel. Nonconformance requires limiting structuraldrift, or special detailing to isolate panels from structureto permit drift; this generally requires panel removal.Defective connections must be replaced.C. Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design of the panel and connections to meet the inplaneand out-of-plane force provisions ofSection 11.7.3 or 11.7.4 and to meet the relativedisplacement in-plane drift provisions of Section 11.7.5.The limiting drift ratio is 0.02.Immediate Occupancy Performance Level.Compliance is provided by design of the panel andconnections to meet the in-plane and out-of-plane forceprovisions of Section 11.7.3 or 11.7.4 and to meet therelative displacement in-plane drift provisions ofSection 11.7.5. The limiting drift ratio is 0.01.D. Evaluation RequirementsThe attachment of prefabricated panels to the structuremust be evaluated for in- and out-of-plane forces andfor in-plane displacement. Connections must bevisually inspected and, based on the engineer’sjudgment, testing to establish capacity to sustain designforces and loads.11.9.1.5 Glazing SystemsA. Definition and ScopeGlazing systems consist of assemblies of walls that aremade up from structural subframes attached to the mainstructure. The subframes may be assembled in the field,or prefabricated in sections and assembled in the field.Five typical categories of glazing system are:1. Stick curtainwall systems, assembled on site2. Unitized curtain wall systems, assembled fromprefabricated units3. Sloped glazing and skylights—may be prefabricatedunits or assembled on site4. “Storefront” type glazing, assembled on site5. Structural glazing in which the glass is attached toits supporting framework on two or four sides withadhesive silicone without mechanical restraintWithin each of these categories, there are three basictypes of glazed openings:1. Marine glazing (mostly factory built), in which theglass is clasped in a “U” rubber or vinyl gasket andthen surrounded by a screwed-together aluminumframe (i.e., sliding doors and windows)2. “Wet” glazing, in which the glass is held into theframe with silicone or other sealant compound or isattached to the frame with silicone as in structuralglazing3. “Dry” glazing, in which the glass is held into theframe with either putty, a rubber/vinyl bead, orwood/metal stopsB. Component Behavior and Rehabilitation ConceptsGlazing systems are predominantly deformationsensitive,but may also become displaced or detachedby large acceleration forces. Failures predominantlystem from the third method of glazing (“dry” glazing),and generally occur by the glass shattering due to inplanedisplacements, or glass falling out of itssupporting frame due to out-of-plane forces, oftencombined with loss of edge blocks and sealant stripscaused by racking.Drift analysis is necessary to establish conformancewith drift acceptance criteria related to PerformanceLevel. Nonconformance requires limiting structuraldrift, or special detailing to isolate the glazing systemfrom the structure to permit drift; this would requireremoval of the glazing system and replacement with analternative design. Glazing with insufficient edge bite orinsufficient resilience and clearance from the metalframing must be reglazed.FEMA 273 Seismic Rehabilitation Guidelines 11-15

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)C. Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design of the glazing system and its supportingstructure to meet the force provisions of Section 11.7.3or 11.7.4 for out-of-plane forces, and to meet therelative displacement in-plane drift provisions ofSection 11.7.5. The limiting drift ratio is 0.02.Immediate Occupancy Performance Level.Compliance is provided by design of the glazing systemand its supporting structure to meet the force provisionsof Section 11.7.3 or 11.7.4 for out-of-plane forces, andto meet the relative displacement in-plane driftprovisions of Section 11.7.5. The limiting drift ratio is0.01.D. Evaluation RequirementsGlazed walls must be evaluated visually to determinethe details of glass support, mullion configuration,sealant (wet or dry), and connectors.11.9.2 Partitions11.9.2.1 Definition and ScopePartitions are vertical non-load-bearing interiorelements that provide space division. They may spanlaterally from floor to underside of floor or roof above,with connections at the top that may or may not allowfor isolation from in-plane drift. Other partitions extendonly up to a hung ceiling, and may or may not havelateral bracing above that level to structural support, ormay be freestanding.Heavy partitions are constructed of masonry materialssuch as hollow clay tile or concrete block, or areassemblies that weigh five pounds per square foot ormore.Light partitions are constructed of metal or wood studssurfaced with lath and plaster, gypsum board, wood, orother facing materials, and weigh less than five poundsper square foot.Glazed partitions that span from floor to ceiling or tothe underside of floor or roof above are subject to therequirements of Section 11.9.1.5.Modular office furnishings that include movablepartitions are considered as contents rather thanpartitions, and as such are not within the Guidelines’scope.Heavy partitions—whether infill or freestanding—constructed of masonry materials, such as hollow claytile or concrete block, are subject to the requirements ofChapter 7.11.9.2.2 Component Behavior andRehabilitation ConceptsPartitions are both acceleration- and deformationsensitive.Partitions attached to the structural floorsboth above and below, and loaded in-plane, canexperience shear cracking, distortion and fracture of thepartition framing, and detachment of the surface finish,because of structural deformations. Similar partitionsloaded out-of-plane can experience flexural cracking,failure of connections to structure, and collapse. Thehigh incidence of unsupported block partitions in lowand moderate seismic zones represents a significantcollapse threat.Partitions subject to deformations from the structure canbe protected by providing a continuous gap between thepartition and the surrounding structure, combined withattachment that provides for in-plane movement butout-of-plane restraint. Lightweight partitions that arenot part of a fire-resistive system are regarded asreplaceable. Refer to the Commentary for discussion onrehabilitation of lightweight partitions used as firewalls.11.9.2.3 Acceptance CriteriaLife Safety Performance LevelHeavy Partitions. Compliance is provided by design ofthe partitions to meet the out-of-plane force provisionsof Section 11.7.3 or 11.7.4 and to meet the in-planerelative displacement provisions of Section 11.7.5. Thelimiting drift ratio is 0.01.Light Partitions. No requirements.Immediate Occupancy Performance LevelHeavy Partitions. Compliance is provided by design ofthe partitions to meet the out-of-plane force provisionsof Section 11.7.3 or 11.7.4 and to meet the in-planerelative displacement drift provisions of Section 11.7.5.The limiting drift ratio is 0.005.Light Partitions. Compliance is provided by design ofthe partitions to meet the out-of-plane force provisionsof Section 11.7.3 or 11.7.4 and to meet the in-planerelative displacement drift provisions of Section 11.7.5.The limiting drift ratio is 0.01.11-16 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)11.9.2.4 Evaluation RequirementsPartitions must be evaluated to ascertain the type ofmaterial. For concrete block partitions, presence ofreinforcing and connection conditions at edges areimportant. For light partitions, bracing, or anchoring ofthe top of the partitions, is important.11.9.3 Interior Veneers11.9.3.1 Definition and ScopeInterior veneers are thin decorative-finish materialsapplied to interior walls and partitions. Theseprovisions apply to veneers mounted four feet or moreabove the floor.11.9.3.2 Component Behavior andRehabilitation ConceptsInterior veneers typically experience in-plane crackingand detachment, but may also be displaced or detachedout-of-plane by direct acceleration. Interior partitionsloaded out-of-plane and supported on flexible backupsupport systems can experience cracking anddetachment.Drift analysis is necessary to establish conformancewith drift acceptance criteria related to PerformanceLevel. Nonconformance requires limiting structuraldrift, or special detailing to isolate the veneer supportsystem from the structure to permit drift; this generallyrequires disassembly of the support system and veneerreplacement. Inadequately adhered veneer must bereplaced.11.9.3.3 Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design of the attachment to the backing to meet theout-of-plane force provisions of Section 11.7.3 or11.7.4, and to meet the in-plane relative displacementdrift provisions of Section 11.7.5. The limiting driftratio is 0.02.Immediate Occupancy Performance Level.Compliance is provided by design of the attachment tothe backing to meet the out-of-plane force provisions ofSection 11.7.3 or 11.7.4, and to meet the in-planerelative displacement drift provisions of Section 11.7.5.The limiting drift ratio is 0.01.11.9.3.4 Evaluation RequirementsThe backup wall or other support and the attachment tothat support must be considered, as well as thecondition of the veneer itself.11.9.4 Ceilings11.9.4.1 Definition and ScopeCeilings are horizontal and sloping assemblies ofmaterials attached to or suspended from the buildingstructure, or separately supported. Ceilings in anexterior location are referred to as soffits; theseprovisions also apply to them. Ceilings are mainly ofthe following types:Category a. Surface-applied or furred with materialssuch as wood or metal furring acoustical tile, gypsumboard, plaster, or metal panel ceiling materials, whichare applied directly to wood joists, concrete slabs, orsteel decking with mechanical fasteners or adhesivesCategory b. Short dropped gypsum board sectionsattached to wood or metal furring supported by carriermembersCategory c. Suspended metal lath and plasterCategory d. Suspended acoustical board insertedwithin T-bars, together with lighting fixtures andmechanical items, to form an integrated ceiling systemSome older buildings have heavy decorative ceilings ofmolded plaster, which may be directly attached to thestructure or suspended; these are typically Category a orCategory c ceilings.11.9.4.2 Component Behavior andRehabilitation ConceptsCeiling systems are both acceleration- and deformationsensitive.Surface-applied or furred ceilings areprimarily influenced by the performance of theirsupports. Rehabilitation of the ceiling takes the form ofensuring good attachment and adhesion. Metal lath andplaster ceilings depend on their attachment and bracingfor large ceiling areas. Analysis is necessary to establishthe acceleration forces and deformations that must beaccommodated. Suspended integrated ceilings arehighly susceptible to damage, if not braced, withdistortion of grid and loss of panels; however, this is notregarded as a life safety threat with lightweight panels(less than two pounds per square foot).FEMA 273 Seismic Rehabilitation Guidelines 11-17

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Rehabilitation takes the form of bracing, attachment,and edge details to prescriptive design standards such asthe CISCA recommendations appropriate to the seismiczone (CISCA, 1990, 1991).11.9.4.3 Acceptance CriteriaLife Safety Performance Level. There are norequirements for ceiling Categories a, b, and d, exceptas noted in the footnotes to Table 11-1. Whererehabilitation is required for ceiling Categories a and b,strengthening to meet force provisions of Section 11.7.3or 11.7.4 provides compliance. For ceiling Category c,rehabilitation must also comply with relativedisplacement provisions of Section 11.7.5. Whererehabilitation is required for ceiling Category d,rehabilitation by the Prescriptive Procedure providescompliance.Immediate Occupancy Performance Level. For ceilingCategories a and b, strengthening to meet forceprovisions of Section 11.7.3 or 11.7.4 providescompliance. For ceiling Category c, rehabilitation mustalso comply with relative displacement provisions ofSection 11.7.5. For ceiling Category d, rehabilitation bythe Prescriptive Procedure provides compliance.11.9.4.4 Evaluation RequirementsThe condition of the ceiling finish material and itsattachment to the ceiling support system, the attachmentand bracing of the ceiling support system to thestructure, and the potential seismic impacts of othernonstructural systems on the ceiling system must beevaluated.11.9.5 Parapets and Appendages11.9.5.1 Definition and ScopeParapets and appendages include exterior nonstructuralfeatures that project above or away from a building.They include sculpture and ornament in addition toconcrete, masonry, or terra cotta parapets. Thefollowing parapets and appendages are within the scopeof these requirements:• Unreinforced masonry parapets more than one and ahalf times as high as they are thick• Reinforced masonry parapets more than three timesas high as they are wide• Cornices or ledges constructed of stone, terra cotta,or brick, unless supported by steel or reinforcedconcrete structure• Other appendages, such as flagpoles and signs, thatare similar to the above in size, weight, or potentialconsequence of failure11.9.5.2 Component Behavior andRehabilitation ConceptsParapets and appendages are acceleration-sensitive inthe out-of-plane direction. Materials or components thatare not properly braced may become disengaged andtopple; the results are among the most seismicallyserious consequences of any nonstructural components.Prescriptive design strategies for masonry parapets notexceeding four feet in height consist of bracing inaccordance with the concepts shown in FEMA 74(FEMA, 1994) and FEMA 172 (BSSC, 1992a), withdetailing to conform to accepted engineering practice.Braces for parapets should be spaced at a maximum ofeight feet on center, and, when the parapet constructionis discontinuous, a continuous backing element shouldbe provided. Where there is no adequate connection,roof construction should be tied to parapet walls at theroof level. Other parapets and appendages should beanalyzed for acceleration forces, and braced andconnected according to accepted engineering principles.11.9.5.3 Acceptance CriteriaLife Safety Performance Level. Compliance is providedby strengthening and bracing to a prescriptive conceptwith engineering evaluation or design to meet the forceprovisions of Section 11.7.3 or 11.7.4.Immediate Occupancy Performance Level.Compliance is similar to that for the Life SafetyPerformance Level.11.9.5.4 Evaluation RequirementsEvaluation of masonry parapets should consider thecondition of mortar and masonry, connection tosupports, type and stability of the supporting structure,and horizontal continuity of the parapet coping.11-18 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)11.9.6 Canopies and Marquees11.9.6.1 Definition and ScopeCanopies are projections from an exterior wall toprovide weather protection. They may be extensions ofthe horizontal building structure, or independentstructures that are sometimes also tied to the building.Marquees are free-standing structures, oftenconstructed of metal and glass, providing weatherprotection. Canopies and marquees included within thescope of this document are those that project over exitsor exterior walkways, and those with sufficient mass togenerate significant seismic forces. Specificallyexcluded are canvas or other fabric projections.11.9.6.2 Component Behavior andRehabilitation ConceptsCanopies and marquees are acceleration-sensitive.Their variety of design is so great that they must beindependently analyzed and evaluated for their abilityto withstand seismic forces. Rehabilitation may take theform of improving attachment to the building structure,strengthening, bracing, or a combination of measures.11.9.6.3 Acceptance CriteriaLife Safety Performance Level. Compliance is providedby design to meet the force provisions of Section 11.7.3or 11.7.4. Consider both horizontal and verticalaccelerations.Immediate Occupancy Performance Level.Compliance is similar to that for the Life SafetyPerformance Level.11.9.6.4 Evaluation RequirementsEvaluation should consider buckling in bracing,connection to supports, and type and stability of thesupporting structure.11.9.7 Chimneys and Stacks11.9.7.1 Definition and ScopeChimneys and stacks that are cantilevered abovebuilding roofs are included within the scope of thisdocument. Light metal residential chimneys, whetherenclosed within other structures or not, are not included.11.9.7.2 Component Behavior andRehabilitation ConceptsChimneys and stacks are acceleration-sensitive, andmay fail through flexure, shear, or overturning. Theymay also disengage from adjoining floor or roofstructures and damage them, and their collapse oroverturning may also damage adjoining structures.Rehabilitation may take the form of strengthening and/or bracing and material repair. Residential chimneysmay be braced in accordance with the concepts shownin FEMA 74 (FEMA, 1994).11.9.7.3 Acceptance CriteriaLife Safety Performance Level. Compliance is providedby strengthening and bracing to a prescriptive conceptwith engineering evaluation or design to meet the forceprovisions of Section 11.7.3 or 11.7.4.Immediate Occupancy Performance Level.Compliance is similar to that for the Life SafetyPerformance Level.11.9.7.4 Evaluation RequirementsEvaluation of masonry chimneys should consider thecondition of mortar and masonry, connection toadjacent structure, and type and stability of foundations.Concrete should be evaluated for spalling and exposedreinforcement; steel should be evaluated for corrosion.11.9.8 Stairs and Stair Enclosures11.9.8.1 Definition and ScopeStairs included within the scope of this document aredefined as the treads, risers, and landings that make uppassageways between floors, as well as the surroundingshafts, doors, windows, and fire-resistant assembliesthat constitute the stair enclosure.11.9.8.2 Component Behavior andRehabilitation Concepts.Stairs include a variety of separate components that canbe either acceleration- or deformation-sensitive. Thestairs themselves may be independent of the structure,or integral with the structure. If integral, they shouldform part of the overall structural evaluation andanalysis, with particular attention paid to the possibilityof response modification due to localized stiffness. Ifindependent, the stairs must be evaluated for normalstair loads and their ability to withstand directFEMA 273 Seismic Rehabilitation Guidelines 11-19

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)acceleration or loads transmitted from the structurethrough connections.Stair enclosure materials may fall and render the stairsunusable due to debris.Rehabilitation of integral or independent stairs may takethe form of necessary structural strengthening orbracing, or the introduction of connection details toeliminate or reduce interaction between stairs and thebuilding structure.Rehabilitation of enclosing walls or glazing shouldfollow the requirements of the relevant sections of thisdocument.11.9.8.3 Acceptance CriteriaLife Safety Performance Level. Stairs shall meet theforce provisions of Section 11.7.3 or 11.7.4 and relativedisplacement provisions of Section 11.7.5. Otherelements of the stair assemblage shall meet the LifeSafety acceptance criteria for applicable sections of thischapter.Immediate Occupancy Performance Level. Stairs shallmeet the force provisions of Section 11.7.3 or 11.7.4and relative displacement provisions of Section 11.7.5.Other elements of the stair assemblage shall meet theapplicable Immediate Occupancy acceptance criteriafor applicable sections of this chapter.11.9.8.4 Evaluation RequirementsEvaluation of individual stair elements should considerthe materials and condition of stair members and theirconnections to supports, and the types and stability ofsupporting and adjacent walls, windows, and otherportions of the stair shaft system.11.10 Mechanical, Electrical, andPlumbing Components:Definition, Behavior, andAcceptance Criteria11.10.1 Mechanical Equipment11.10.1.1 Definition and ScopeEquipment that is used for the operation of the building,and is therefore an integral part of it, is included withinthe scope of the Guidelines. Included are:1. All equipment weighing over 400 pounds2. Unanchored equipment weighing over 100 poundsthat does not have a factor of safety againstoverturning of 1.5 or greater when design loads, asrequired by the Guidelines, are applied3. Equipment weighing over 20 pounds that is attachedto ceiling, wall, or other support more than four feetabove the floor4. Building operation equipment not included in one ofthe three categories aboveThese categories of equipment include, but are notlimited to:• Boilers and furnaces• Conveyors (nonpersonnel)• HVAC system equipment, vibration-isolated• HVAC system equipment, non-vibration-isolated• HVAC system equipment mounted in-line withductworkEquipment such as manufacturing or processingequipment related to the occupant’s business, should beevaluated separately for the effects that failure due to aseismic event could have on the operation of thebuilding.11.10.1.2 Component Behavior andRehabilitation ConceptsMechanical equipment is acceleration-sensitive. Failureof these components consists of sliding, tilting, oroverturning of floor- or roof-mounted equipment off itsbase, and possible loss of attachment (with consequentfalling) for equipment attached to a vertical structure orsuspended, and failure of piping or electrical wiringconnected to the equipment.Construction of mechanical equipment to nationallyrecognized codes and standards, such as those approvedby the American National Standards Institute, providesadequate strength to accommodate all normal and upsetoperating loads.11-20 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Basic rehabilitation consists of securely anchoringfloor-mounted equipment by bolting, with detailingappropriate to the base construction of the equipment.Seismic forces can be established by analysis using thedefault Equation 11-1. Equipment weighing over 400pounds and located on the third floor or above (or on anequivalent-height roof) should be analyzed usingEquations 11-2 and 11-3.Existing attachments for attached or suspendedequipment must be evaluated for seismic load capacity,and strengthened or braced as necessary. Attachmentsthat provide secure anchoring eliminate or reduce thelikelihood of piping or electrical distribution failure.11.10.1.3 Acceptance CriteriaLife Safety Performance Level. Equipment anchorageshould meet the force provisions of Section 11.7.3 or11.7.4.Immediate Occupancy Performance Level.Compliance criteria are similar to those for the LifeSafety Performance Level.11.10.1.4 Evaluation RequirementsEquipment must be analyzed to establish accelerationinducedforces, and visually evaluated for the existenceof satisfactory supports, hold-downs, and bracing.Existing concrete anchors may have to be tested byapplying torque to the nuts to confirm that adequatestrength is present.11.10.2 Storage Vessels and Water Heaters11.10.2.1 Definition and ScopeThis section includes all vessels that contain fluids usedfor building operation. The vessel may be fabricated ofmaterials such as steel or other metals, or fiberglass, orit may be a glass-lined tank. These requirements mayalso be applied, with judgment, to vessels that containsolids that act as a fluid, and vessels containing fluidsnot involved in the operation of the building.Vessels are classified into two categories:Category 1. Vessels with structural support of contents,in which the shell is supported by legs or a skirtCategory 2. Flat bottom vessels in which the weight ofthe contents is supported by the floor, roof, or astructural platform11.10.2.2 Component Behavior andRehabilitation ConceptsTanks and vessels are acceleration-sensitive. Category 1vessels fail by stretching of anchor bolts, buckling anddisconnection of supports, and consequent tilting oroverturning of the vessel. A Category 2 vessel may bedisplaced from its foundation, or its shell may fail byyielding near the bottom, creating a visible bulge, orpossible leakage. Displacement of both types of vesselmay cause rupturing of connecting piping and leakage.Category 1 residential water heaters with a capacity nogreater than 100 gallons may be rehabilitated byprescriptive design methods, such as concepts shown inFEMA 74 (FEMA, 1994) or FEMA 172 (BSSC,1992a). Category 1 vessels with a capacity less than1000 gallons should be designed to meet the forceprovisions of Section 11.7.3 or 11.7.4, and bracingstrengthened or added as necessary. Other Category 1and Category 2 vessels should be evaluated against arecognized standard, such as API STD 650-93 or API-90 by the American Petroleum Institute (API, 1993), forvessels containing petroleum products or otherchemicals, or AINSI/AWWA D100-96 (AWS D5 2-96)by the American Water Works Association (AWWA,1996), for water vessels.11.10.2.3 Acceptance CriteriaLife Safety Performance Level.Category 1 equipment. Refer to Table 11-1 forapplicability. Design and support to meet the forceprovisions of Section 11.7.3 or 11.7.4 will providecompliance.Category 2 equipment. Design in accordance with arecognized prescriptive standard and to meet forceprovisions of Section 11.7.3 or 11.7.4 providescompliance.Immediate Occupancy Performance Level.Compliance criteria are similar to those for Life Safety.11.10.2.4 Evaluation RequirementsAll equipment must be visually evaluated to determinethe existence of the necessary hold-downs, supports,and bracing. Existing concrete anchors may have to beFEMA 273 Seismic Rehabilitation Guidelines 11-21

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)tested by applying torque to the nuts to confirm thatadequate strength is present.11.10.3 Pressure Piping11.10.3.1 Definition and ScopeThis section includes all piping that carries fluidswhich, in their vapor stage, exhibit a pressure of 15 psi,gauge, or higher, except fire suppression piping.11.10.3.2 Component Behavior andRehabilitation ConceptsPiping is predominantly acceleration-sensitive, butpiping that runs between floors or seismic joints may bedeformation-sensitive. The most common failure isjoint failure, caused by inadequate support or bracing.Rehabilitation is accomplished by prescriptive designapproaches to support and bracing. The prescriptiverequirements of NFPA-13 (NFPA, 1996) should beused. Piping systems should be evaluated forcompliance with the latest edition of ASME/ANSIB31.9 and other B31 standards where applicable. Forlarge critical piping systems, the building official orresponsible engineer must establish forces and evaluatesupports.11.10.3.3 Acceptance CriteriaLife Safety Performance Level. Design in accordancewith a recognized prescriptive standard, and to meetforce provisions of Section 11.7.3 or 11.7.4 anddisplacement provisions of Section 11.7.5, will providecompliance.Immediate Occupancy Performance Level.Compliance criteria are similar to those for Life Safety.11.10.3.4 Evaluation RequirementsHigh-pressure piping shall be tested in accordance withthe ASME/ANSI standards mentioned above. Inaddition to other tests, lines shall be hydrostaticallytested to 150% of the maximum anticipated pressure ofthe system.11.10.4 Fire Suppression Piping11.10.4.1 Definition and ScopeFire suppression piping includes fire sprinkler pipingconsisting of main risers and laterals weighing, loaded,in the range of 30 to 100 pounds per lineal foot, withbranches of decreasing size down to approximately twopounds per foot.11.10.4.2 Component Behavior andRehabilitation ConceptsPiping is predominantly acceleration-sensitive, butpiping that runs between floors or seismic joints may bedeformation-sensitive. The most common failure isjoint failure, caused by inadequate support or bracing,or by sprinkler heads impacting adjoining materials.Rehabilitation is accomplished by prescriptive designapproaches to support and bracing. The prescriptiverequirements of NFPA-13 (NFPA, 1996) should beused.11.10.4.3 Acceptance CriteriaLife Safety Performance Level. Design in accordancewith a recognized prescriptive standard to meet forceprovisions of Section 11.7.3 or 11.7.4. providescompliance.Immediate Occupancy Performance Level.Compliance criteria are similar to those for Life Safety.11.10.4.4 Evaluation RequirementsFire suppression piping must be evaluated for adequatesupport, flexibility, protection at seismic movementjoints, and freedom from impact from adjoiningmaterials at the sprinkler heads. The support andbracing of bends of the main risers and laterals, as wellas maintenance of adequate flexibility to preventbuckling, are especially important.11.10.5 Fluid Piping other than FireSuppression11.10.5.1 Definition and ScopeThis section includes all piping, other than pressurepiping or fire suppression lines, that transfers fluidsunder pressure by gravity, or is open to the atmosphere.This includes drainage and ventilation piping; hot, cold,and chilled water piping; and piping carrying liquids, aswell as fuel gas lines, used in industrial, medical,laboratory, and other occupancies. There are twocategories of fluids, based on potential damage orhazard to personnel:Category 1. Hazardous materials and flammableliquids that would pose an immediate life safety danger11-22 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)if exposed, because of inherent properties of thecontained material, as defined in NFPA 325-94, 49-94,491M-91, and 704-90.Category 2. Materials that, in case of line rupture,would cause property damage, but pose no immediatelife safety danger.11.10.5.2 Component Behavior andRehabilitation ConceptsPiping is predominantly acceleration-sensitive, butpiping that runs between floors or expansion or seismicjoints may be deformation-sensitive. The most commonfailure is joint failure, caused by inadequate support orbracing.Category 1 piping rehabilitation is accomplished bystrengthening support and bracing, using theprescriptive methods of SP-58 (MSS, 1993); the pipingsystems themselves should be designed to meet theforce provisions of Sections 11.7.3 or 11.7.4 andrelative displacement provisions of Section 11.7.5. Theeffects of temperature differences, dynamic fluid forces,and piping contents should be taken into account.Category 2 piping rehabilitation is accomplished bystrengthening support and bracing, using theprescriptive methods of SP-58 (MSS, 1993) as long asthe piping falls within the size limitations of thoseguidelines. Piping that exceeds the limitations of thoseguidelines shall be designed to meet the forceprovisions of Section 11.7.3 or 11.7.4 and relativedisplacement provisions of Section 11.7.5.11.10.5.3 Acceptance CriteriaLife Safety Performance LevelCategory 1 piping systems. Design to meetprescriptive standards, the force provisions ofSection 11.7.3 or 11.7.4, and the relative displacementprovisions of Section 11.7.5, provides compliance.Category 2 piping systems. Design to meetprescriptive standards provides compliance.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for Life Safety. Prescriptivestandards should be met for essential facilities.11.10.5.4 Evaluation RequirementsPiping must be evaluated for adequate support,flexibility, and protection at seismic movement joints.The support and bracing of bends in the main risers andlaterals, as well as maintenance of adequate flexibilityto prevent buckling, are especially important. Pipingmust be protected by adequate insulation fromdetrimental heat effects.11.10.6 Ductwork11.10.6.1 Definition and ScopeThis section includes HVAC and special exhaustductwork systems. Seismic restraints are not requiredfor ductwork that is not conveying hazardous materials,and that meets either of the following conditions.• HVAC ducts are suspended from hangers 12 inchesor less in length from the top of the duct to thesupporting structure. The hangers should bedesigned and placed in such a way as to avoidsignificant bending of the hangers.• HVAC ducts have a cross-sectional area of less thansix square feet.11.10.6.2 Component Behavior andRehabilitation ConceptsDucts are predominantly acceleration-sensitive, butwhen ductwork runs between floors or across expansionor seismic joints it may be deformation-sensitive.Damage is caused by failure of supports or lack ofbracing that causes deformation or rupture of the ductsat joints, leading to leakage from the system.Rehabilitation consists of strengthening supports andstrengthening or adding bracing. Prescriptive designmethods may be used, per SMACNA DuctConstruction Standards (SMACNA, 1980, 1985).11.10.6.3 Acceptance CriteriaLife Safety Performance Level. Design to meetprescriptive standards provides compliance.Immediate Occupancy Performance Level.Compliance criteria are similar to those for Life Safety.Prescriptive standards should be for essential facilities.FEMA 273 Seismic Rehabilitation Guidelines 11-23

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)11.10.6.4 Evaluation RequirementsThese components must be evaluated by visual meansto ascertain their compliance with the conditionsdefined in Section 11.10.6.1.11.10.7 Electrical and CommunicationsEquipment11.10.7.1 Definition and ScopeThis section includes all electrical and communicationequipment, including panel boards, battery racks, motorcontrol centers, switch gear, and other fixedcomponents located in electrical rooms or elsewhere inthe building.The following equipment is subject to these Guidelines:1. All equipment weighing over 400 pounds2. Unanchored equipment weighing over 100 poundsthat does not have a factor of safety againstoverturning of 1.5 or greater when design loads, asrequired by the Guidelines, are applied3. Equipment weighing over 20 pounds that is attachedto ceiling, wall, or other support more than four feetabove the floor4. Building operation equipment not falling into one ofthe three categories above11.10.7.2 Component Behavior andRehabilitation ConceptsElectrical equipment is acceleration-sensitive. Failureof these components consists of sliding, tilting, oroverturning of floor- or roof-mounted equipment off itsbase, and possible loss of attachment (with consequentfalling) for equipment attached to a vertical structure orsuspended, and failure of electrical wiring connected tothe equipment.Construction of electrical equipment to nationallyrecognized codes and standards, such as those approvedby ANSI, provides adequate strength to accommodateall normal and upset operating loads.Basic rehabilitation consists of securely anchoringfloor-mounted equipment by bolting, with detailingappropriate to the base construction of the equipment.11.10.7.3 Acceptance CriteriaLife Safety Performance Level. Design to meet theforce provisions of Section 11.7.3 or 11.7.4 providescompliance.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for Life Safety.11.10.7.4 Evaluation RequirementsEquipment should be visually evaluated to determine itscategory, and the existence of the necessary holddowns,supports, and braces. Larger equipmentrequiring the Analytical Procedure must be analyzed todetermine forces, and visually evaluated. Concreteanchors may have to be tested by applying torque to thenuts to confirm that adequate strength is present.11.10.8 Electrical and CommunicationsDistribution Components11.10.8.1 Definition and ScopeThis includes all electrical and communicationstransmission lines, conduit, and cables, and theirsupports.11.10.8.2 Component Behavior andRehabilitation ConceptsElectrical distribution equipment is predominantlyacceleration-sensitive, but wiring or conduit that runsbetween floors or expansion or seismic joints may bedeformation-sensitive. Failure occurs most commonlyby inadequate support or bracing, deformation of theattached structure, or impact from adjoining materials.Rehabilitation is accomplished by strengtheningsupport and bracing using the prescriptive methodscontained in SMACNA standards (SMACNA, 1980,1985).11.10.8.3 Acceptance CriteriaLife Safety Performance Level. Design to meetprescriptive standards provides compliance.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for Life Safety. Prescriptivestandards should be for essential facilities.11-24 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)11.10.8.4 Evaluation RequirementsComponents should be visually evaluated to determinethe existence of necessary supports and bracing.11.10.9 Light Fixtures11.10.9.1 Definition and ScopeThis section includes lighting fixtures in the followingcategories:Category 1. Recessed in ceilingsCategory 2. Surface mounted to ceilings or wallsCategory 3. Supported within a suspended ceilingsystem (integrated ceiling)Category 4. Suspended from ceilings or structure(pendant or chain)11.10.9.2 Component Behavior andRehabilitation ConceptsFailure of Category 1 and 2 components occurs throughfailure of attachment of the light fixture and/or failureof the supporting ceiling or wall. Failure of Category 3components occurs through loss of support from theT-bar system, and by distortion caused by deformationof the supporting structure or deformation of the ceilinggrid system, allowing the fixture to fall. Failure ofCategory 4 components is caused by excessiveswinging that results in the pendant or chain supportbreaking on impact with adjacent materials, or thesupport being pulled out of the ceiling.Rehabilitation of Category 1 and 2 componentsinvolves attachment repair or fixture replacement inassociation with necessary rehabilitation of thesupporting ceiling or wall. Rehabilitation of Category 3components involves the addition of independentsupport for the fixture from the structure or substructurein accordance with FEMA 74 design concepts (FEMA,1994). Rehabilitation of Category 4 componentsinvolves strengthening of attachment and ensuringfreedom to swing without impacting adjoiningmaterials.11.10.9.3 Acceptance CriteriaLife Safety Performance LevelCategories 1 and 2. There are no specific acceptancecriteria, but secure connection to ceiling or wall must beassured.Category 3. Systems bracing and support to meetprescriptive requirements provides compliance.Category 4. Fixtures weighing over 20 pounds shouldhave adequate articulating or ductile connections to thebuilding, and be free to swing without impactingadjoining materials.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for Life Safety. Prescriptivestandards should be met for essential facilities.11.10.9.4 Evaluation RequirementsFixtures must be visually evaluated to determine theadequacy of supports and, for Category 3 fixtures, theexistence of adequate independent support.11.11 Furnishings and InteriorEquipment: Definition, Behavior,and Acceptance Criteria11.11.1 Storage Racks11.11.1.1 Definition and ScopeStorage racks include systems, usually constructed ofmetal, for the purpose of holding materials eitherpermanently or temporarily. Storage racks are generallypurchased as proprietary systems installed by a tenantand are often not under the direct control of the buildingowner. Thus, they are usually not part of theconstruction contract, and often have no foundation orfoundation attachment. However, they are oftenpermanently installed, and their size and loaded weightmake them an important hazard to either life, property,or the surrounding structure. Storage racks in excess offour feet in height located in occupied locations shall beconsidered when the Life Safety Performance Level isselected.11.11.1.2 Component Behavior andRehabilitation ConceptsStorage racks are acceleration-sensitive, and may failinternally—through inadequate bracing or momentresistingcapacity—or externally, by overturning causedby absence or failure of foundation attachments.FEMA 273 Seismic Rehabilitation Guidelines 11-25

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Rehabilitation is usually accomplished by the additionof bracing to the rear and side panels of racks and/or byimproving the connection of the rack columns to thesupporting slab. In rare instances, foundationimprovements may be required to remedy insufficientbearing or uplift load capacity.Seismic forces can be established by analysis inaccordance with Section 11.7.3 or 11.7.4. However,special attention should be paid to the evaluation andanalysis of large, heavily loaded rack systems becauseof their heavy loading and lightweight structuralmembers.11.11.1.3 Acceptance CriteriaLife Safety Performance Level. Design to meet theforce provisions of Section 11.7.3 or 11.7.4 providescompliance.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for Life Safety.11.11.1.4 Evaluation RequirementsEvaluation should consider buckling or racking failureof rack elements, connection to support structures, andtype and stability of supporting structure.11.11.2 Bookcases11.11.2.1 Definition and ScopeBookcases, constructed of wood or metal, in excess offour feet high should be considered.11.11.2.2 Component Behavior andRehabilitation ConceptsBookcases are acceleration-sensitive, and may deformor overturn due to inadequate bracing or attachment tofloors or adjacent walls, columns, or other structuralmembers. Rehabilitation is usually accomplished by theaddition of metal cross bracing to the rear of thebookcase to improve its internal resistance to rackingforces, and by bracing the bookcase both in- and out-ofplaneto the adjacent structure or walls to preventoverturning and racking.11.11.2.3 Acceptance CriteriaLife Safety Performance Level. Design to meet theforce provisions of Section 11.7.3 or 11.7.4 providescompliance.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for Life Safety.11.11.2.4 Evaluation RequirementsEvaluation should consider the loading, type, andcondition of bookcases, their connection to supportstructures, and type and stability of supportingstructure.11.11.3 Computer Access Floors11.11.3.1 Definition and ScopeComputer access floors are panelized, elevated floorsystems designed to facilitate access to wiring, fiberoptics, and other services associated with computersand other electronic components. Access floors vary inheight but generally are less than three feet above thesupporting structural floor. The systems includestructural legs, horizontal panel supports, and panels.11.11.3.2 Component Behavior andRehabilitation ConceptsThese components are both acceleration- anddeformation-sensitive. They may displace laterally orbuckle vertically under seismic loads. Rehabilitation ofaccess floors usually includes a combination ofimproved attachment of computer and communicationracks through the access floor panels to the supportingsteel structure or to the underlying floor system, whileimproving the lateral-load-carrying capacity of the steelstanchion system by installing braces or improving theconnection of the stanchion base to the supporting floor,or both.Rehabilitation should be designed in accordance withconcepts described in FEMA 74 (FEMA, 1994). Theweight of the floor system, as well as supportedequipment, should be included in the analysis.11.11.3.3 Acceptance CriteriaLife Safety Performance Level. Not applicable.Immediate Occupancy Performance Level. Design tomeet force provisions of Section 11.7.3 or 11.7.4provides compliance, together with design to approvedprescriptive standards.11.11.3.4 Evaluation RequirementsEvaluation should consider buckling and racking ofaccess floor supports, and connection to the support11-26 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)structure. The effects of mounted equipment, includingpossible future equipment, should also be considered.11.11.4 Hazardous Materials Storage11.11.4.1 Definition and ScopeFor the purpose of this section, hazardous materialsstorage shall be defined as permanently installedcontainers—either freestanding, on supports, or storedon countertops or shelves—that hold materials definedto be hazardous by the National Institute forOccupational Safety and Health, including thefollowing types:• Propane gas tanks• Compressed gas vessels• Dry or liquid chemical storage containersLarge nonbuilding structures, such as large tanks foundin heavy industry or power plants, floating-roof oilstorage tanks, and large (greater than ten feet long)propane tanks at propane manufacture or distributionplants are not within the scope of these Guidelines.11.11.4.2 Component Behavior andRehabilitation ConceptsThese components are acceleration-sensitive; upset ofthe storage container may release the hazardousmaterial. Failure occurs because of buckling andoverturning of supports and/or inadequate bracing.Rehabilitation consists of strengthening and increasingor adding bracing designed according to conceptsdescribed in FEMA 74 (FEMA, 1994) and FEMA 172(BSSC, 1992a).11.11.4.3 Acceptance CriteriaLife Safety Performance Level. Design to approvedprescriptive concepts provides compliance.Immediate Occupancy Performance Level. Acceptancecriteria are similar to those for the Life SafetyPerformance Level. Prescriptive standards should bemet for essential facilities.11.11.4.4 Evaluation RequirementsEvaluation should consider the location and types ofhazardous materials, container materials, manner ofbracing, internal lateral resistance, and the effect ofhazardous material spills.11.11.5 Computer and Communication Racks11.11.5.1 Definition and ScopeComputer and communication racks are large, freestandingrack systems designed to support computerand other electronic equipment. Racks may besupported on either structural or access floors and mayor may not be attached directly to these supports. Theequipment itself is not included in this definition. Allcomputer and communication racks are included withinthe scope of this section.11.11.5.2 Component Behavior andRehabilitation ConceptsThese components are acceleration-sensitive, and mayfail internally—through inadequate bracing or momentresistingcapacity—or externally, by overturning causedby absence or failure of floor attachments.Rehabilitation is usually accomplished by the additionof bracing to the rear and side panels of the racks, and/or by improving the connection of the rack to thesupporting floor using concepts shown in FEMA 74(FEMA, 1994) or FEMA 172 (BSSC, 1992a).11.11.5.3 Acceptance CriteriaLife Safety Performance Level. Not applicable.Immediate Occupancy Performance Level. Design tomeet force provisions of Section 11.7.3 or 11.7.4provides compliance, together with design to approvedprescriptive standards.11.11.5.4 Evaluation RequirementsEvaluation should consider buckling or racking failureof rack elements, their connection to support structures,and type and stability of the supporting structure. Theeffect of rack failure on equipment should also beconsidered.11.11.6 Elevators11.11.6.1 Definition and ScopeElevators include cabs and shafts, as well as allequipment and equipment rooms associated withelevator operation, such as hoists, counterweights,cables, and controllers.FEMA 273 Seismic Rehabilitation Guidelines 11-27

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)11.11.6.2 Component Behavior andRehabilitation ConceptsMost elements of elevators are acceleration-sensitive,and can become dislodged or derailed. Shafts andhoistway rails, which rise through a number of floors,may also be deformation-sensitive. Shaft walls and theconstruction of machinery room walls are often notengineered and must be considered in a way similar tothat for other partitions. Shaft walls that are ofunreinforced masonry or hollow tile must be consideredwith special care, since failure of these elementsviolates Life Safety Performance Level criteria.Elevator machinery may be subject to the same damageas other heavy floor-mounted equipment. Electricalpower loss renders elevators inoperable.Rehabilitation measures include a variety of techniquestaken from specific component sections for partitions,controllers, and machinery. Rehabilitation specific toelevator operation can include seismic shutoffs, cablerestrainers, and counterweight retainers; such measuresshould be in accordance with ASME A17.1 (ASME,1996).11.11.6.3 Acceptance CriteriaLife Safety Performance Level. Design to meet forceprovisions of Section 11.7.3 or 11.7.4 providescompliance, together with design to approvedprescriptive standards.Immediate Occupancy Performance Level.Rehabilitation criteria are similar to those for LifeSafety.11.11.6.4 Evaluation RequirementsEvaluation should consider the construction of elevatorshafts consistent with the requirements of applicablesections of the Guidelines. The possibility ofdisplacement or derailment of hoistway counterweightsand cables should be considered, as should theanchorage of elevator machinery.11.11.7 Conveyors11.11.7.1 Definition and ScopeConveyors are defined as material conveyors only forthe purposes of this section, including all machineryand controllers necessary to operation.11.11.7.2 Component Behavior andRehabilitation ConceptsConveyors are both acceleration- and deformationsensitive.Conveyor machinery may be subject to thesame damage as other heavy floor-mounted equipment.In addition, deformation of adjoining building materialsmay render the conveyor inoperable. Electrical powerloss renders the conveyor inoperable.Rehabilitation of the conveyor involves PrescriptiveProcedures using special skills provided by theconveyor manufacturer.11.11.7.3 Acceptance CriteriaLife Safety Performance Level. Not applicable.Immediate Occupancy Performance Level. Design tomeet force provisions of Section 11.7.3 or 11.7.4 anddisplacement provisions of Section 11.7.5, togetherwith special prescriptive concepts, providescompliance.11.11.7.4 Evaluation RequirementsEvaluation should consider the stability of machineryconsistent with the requirements of applicable sectionsof these Guidelines.11.12 DefinitionsAcceleration-sensitive nonstructural component:A nonstructural component sensitive to and subject todamage from inertial loading. Once inertial loads aregenerated within the component, the deformation of thecomponent may be significant; this is separate from theissue of deformation imposed on the component bystructural deflections (see deformation-sensitivenonstructural components).Component, flexible: A component, including itsattachments, having a fundamental period greater than0.06 seconds.Component, rigid: A component, including itsattachments, having a fundamental period less than orequal to 0.06 seconds.Contents: Movable items within the buildingintroduced by the owner or occupants.11-28 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Deformation-sensitive nonstructural component:A nonstructural component sensitive to deformationimposed on it by the drift or deformation of thestructure, including deflection or deformation ofdiaphragms.Flexible connections: Connections betweencomponents that permit rotational and/or translationalmovement without degradation of performance.Examples include universal joints, bellows expansionjoints, and flexible metal hose.Nonstructural component: An architectural,mechanical, plumbing, or electrical component, or itemof interior equipment and furnishing, permanentlyinstalled in the building, as listed in Table 11-1.Storage racks: Industrial pallet racks, movable shelfracks, and stacker racks made of cold-formed or hotrolledstructural members. Does not include other typesof racks such as drive-in and drive-through racks,cantilever wall-hung racks, portable racks, or racksmade of materials other than steel.11.13 SymbolsD pRelative seismic displacement that thecomponent must be designed to accommodateD r Drift ratioF p Seismic design force applied horizontally at thecomponent’s center of gravity and distributedaccording to the component’s mass distributionR p Component response modification factor, relatedto ductility of anchorage that varies from 1.25 to6.0 (select appropriate value from Table 11-2)S XS Spectral response acceleration at short periodsfor any hazard level and any damping, gW p Component operating weightX Height of upper support attachment at level x asmeasured from gradeY Height of lower support attachment at level y asmeasured from gradea phComponent amplification factor, related torigidity of component, that varies from 1.00 to2.50 (select appropriate value from Table 11-2)Average roof elevation of structure, relative tograde elevationxElevation in structure of component relative tograde elevationδ xA Deflection at building level x of Building A,determined by an elastic analysis as defined inChapter 3δ yA Deflection at building level y of Building A,determined by an elastic analysis as defined inChapter 3δ xB Deflection at building level x of Building B,determined by an elastic analysis as defined inChapter 311.14 ReferencesAPI, 1993, Welded Steel Tanks for Oil Storage, APISTD 650, American Petroleum Institute, Washington,D.C.ASME, 1996, Safety Code for Elevators andEscalators, ASME A17.1, American Society ofMechanical Engineers, New York, New York.ASME, 1995, Boiler and Pressure Vessel Code,including addenda through 1993, American Society ofMechanical Engineers, New York, New York.ASME, latest edition, Code for Pressure Piping,ASME B31, American Society of MechanicalEngineers, New York, New York.ASME, latest edition, Power Piping, ASME B31.1,American Society of Mechanical Engineers, New York,New York.ASME, latest edition, Chemical Plant and RefineryPiping, ASME B31.3, American Society of MechanicalEngineers, New York, New York.ASME, latest edition, Liquid Transportation Systemsfor Hydrocarbons, Liquid Petroleum Gas, AnhydrousAmmonia, and Alcohols, ASME B31.4, AmericanSociety of Mechanical Engineers, New York, NewYork.ASME, latest edition, Refrigeration Plant, ASME/ANSI B31.5, American Society of MechanicalEngineers, New York, New York.ASME, latest edition, Gas Transmission andDistribution Piping Systems, ASME B31.8, AmericanFEMA 273 Seismic Rehabilitation Guidelines 11-29

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)Society of Mechanical Engineers, New York, NewYork.ASME, latest edition, Building Services Piping, ASMEB31.9, American Society of Mechanical Engineers,New York, New York.ASME, latest edition, Slurry Transportation Systems,ASME B31.11, American Society of MechanicalEngineers, New York, New York.AWWA, 1996, Welded Steel Tanks for Water Storage,ANSI/AWWA D100-96, American Water WorksAssociation, Denver, Colorado.Ayres, J. M., and Sun, T. Y., 1973a, “NonstructuralDamage to Buildings,” The Great Alaska Earthquake of1964, Engineering, National Academy of Sciences,Washington, D.C.Ayres, J. M., and Sun, T. Y., 1973b, “NonstructuralDamage,” The San Fernando, California Earthquake ofFebruary 9, 1971, National Oceanic and AtmosphericAdministration, Washington, D.C., Vol. 1B.Ayres, J. M., 1993, “History of Earthquake ResistiveDesign Building Mechanical Systems,” ASHRAETransactions: Symposia, CH-93-1-1, American Societyof Heating, Refrigerating and Air-ConditioningEngineers, Inc., Atlanta, Georgia.BSSC, 1992a, NEHRP Handbook of Techniques for theSeismic Rehabilitation of Existing Buildings, developedby the Building Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 172), Washington, D.C.BSSC, 1992b, NEHRP Handbook for the SeismicEvaluation of Existing Buildings, developed by theBuilding Seismic Safety Council for the FederalEmergency Management Agency (Report No.FEMA 178), Washington, D.C.BSSC, 1997, NEHRP Recommended Provisions forSeismic Regulations for New Buildings and OtherStructures, Part 1: Provisions and Part 2: Commentary,prepared by the Building Seismic Safety Council for theFederal Emergency Management Agency (Report Nos.FEMA 302 and 303), Washington, D.C.CISCA, 1990, Recommendations for Direct-HungAcoustical and Lay-in Panel Ceilings, SeismicZones 3-4, Ceilings and Interior Systems ConstructionAssociation, Deerfield, Illinois.CISCA, 1991, Recommendations for Direct-HungAcoustical and Lay-in Panel Ceilings, SeismicZones 0-2, Ceilings and Interior Systems ConstructionAssociation, Deerfield, Illinois.Department of the Army, Navy, and Air Force, 1986,Seismic Design Guidelines for Essential Buildings,supplement to Seismic Design of Buildings, Air ForceAFM 88-3, Army TM5-809-10.1, Navy NAVFAC P-355.1, Chapter 13.1, Department of the Army, Navy,and Air Force, Washington, D.C.Drake, R. M., and Bachman, R. E., 1995,“Interpretation of Instrumental Building Seismic Dataand Implications for Building Codes,” Proceedings,SEAOC Annual Convention, Structural EngineeringAssociation of California, Sacramento, California.FEMA, 1994, Reducing the Risks of NonstructuralEarthquake Damage, A Practical Guide, FederalEmergency Management Agency (Report No.FEMA 74), Washington, D.C.ICBO, 1994, Uniform Building Code, InternationalConference of Building Officials, Whittier, California.IEEE, 1987, Recommended Practice for SeismicQualification of Class 1E Equipment for Nuclear PowerGenerating Stations, Standard 344, Institute ofElectrical and Electronic Engineers, New York, NewYork.Lagorio, H. J., 1990, Earthquakes, An Architect’s Guideto Nonstructural Seismic Hazards, John Wiley & Sons,Inc., New York, New York.MSS, 1993, Pipe Hangers and Supports: Materials,Design and Manufacture, SP-58, ManufacturersStandardization Society of the Valve and FittingIndustry, Vienna, Virginia.NFPA, 1996, Standard for the Installation of SprinklerSystems, NFPA-13, National Fire ProtectionAssociation, Quincy, Massachusetts.NFPA, latest edition, NFPA-11, NFPA-12, NFPA-12A,NFPA-12B, NFPA-14, NFPA-16, NFPA-16A, NFPA-17, NFPA-17A, National Fire Protection Association,Quincy, Massachusetts.11-30 Seismic Rehabilitation Guidelines FEMA 273

Chapter 11: Architectural, Mechanical, and ElectricalComponents (Simplified and Systematic Rehabilitation)RMI, 1990, Specification for the Design, Testing, andUtilization of Industrial Steel Storage Racks, RackManufacturers Institute, Charlotte, North Carolina.Sheet Metal Industry Fund of Los Angeles, 1976,Guidelines for Seismic Restraints of MechanicalSystems, Los Angeles, California.SMACNA, 1980, Rectangular Industrial DuctConstruction Standards, Sheet Metal and AirConditioning Contractors National Association,Chantilly, Virginia.SMACNA, 1985, HVAC Duct Construction Standards,Metal and Flexible, Sheet Metal and Air ConditioningContractors National Association, Chantilly, Virginia.SMACNA, 1991, Seismic Restraint Manual Guidelinesfor Mechanical Equipment, and Appendix E-1993Addendum, Sheet Metal and Air ConditioningContractors National Association, Chantilly, Virginia.SMACNA, 1992, Guidelines for Seismic Restraint ofMechanical Systems and Plumbing Piping Systems,Sheet Metal Industry Fund of Los Angeles andPlumbing and Piping Industry Council, Sheet Metal andAir Conditioning Contractors National Association,Chantilly, Virginia.FEMA 273 Seismic Rehabilitation Guidelines 11-31

A.GlossaryAAcceleration-sensitive nonstructural component: Anonstructural component sensitive to and subject todamage from inertial loading. Once inertial loads aregenerated within the component, the deformation of thecomponent may be significant; this is separate from theissue of deformation imposed on the component bystructural deflections (see deformation-sensitivenonstructural components). 11-28Acceptance criteria: Permissible values of suchproperties as drift, component strength demand, andinelastic deformation, used to determine theacceptability of a component’s projected behavior at agiven Performance Level. 2-46Action: Sometimes called a generalized force, mostcommonly a single force or moment. However, an actionmay also be a combination of forces and moments, adistributed loading, or any combination of forces andmoments. Actions always produce or causedisplacements or deformations; for example, a bendingmoment action causes flexural deformation in a beam;an axial force action in a column causes axialdeformation in the column; a torsional moment action ona building causes torsional deformations(displacements) in the building. 2-4 6Allowable bearing capacity: Foundation load or stresscommonly used in working-stress design (oftencontrolled by long-term settlement rather than soilstrength). 4-19Aspect ratio: Ratio of height to width for verticaldiaphragms, and width to depth for horizontaldiaphragms. 8-30Assembly: A collection of structural members and/orcomponents connected in a such manner that loadapplied to any one component will affect the stressconditions of adjacent components. 8-30BBalloon framing: Continuous stud framing from sill toroof, with intervening floor joists nailed to studs andsupported by a let-in ribbon. (See platform framing.) 8-30Base: The level at which earthquake effects areconsidered to be imparted to the building. 3-17Beam: A structural member whose primary function isto carry loads transverse to its longitudinal axis, usuallya horizontal member in a seismic frame system. 5-40Bearing wall: A wall that supports gravity loads of atleast 200 pounds per linear foot from floors and/orroofs. 7-23Bed joint: The horizontal layer of mortar on which amasonry unit is laid. 7-23Boundary component (boundary member): Amember at the perimeter (edge or opening) of a shearwall or horizontal diaphragm that provides tensile and/orcompressive strength. 8-30Boundary members: Portions along wall anddiaphragm edges strengthened by longitudinal andtransverse reinforcement and/or structural steelmembers. A-1Braced frame: An essentially vertical truss system ofconcentric or eccentric type that resists lateral forces. 5-40BSE-1: Basic Safety Earthquake-1, which is the lesserof the ground shaking at a site for a 10%/50 yearearthquake or two-thirds of the Maximum ConsideredEarthquake (MCE) at the site. 2-46BSE-2: Basic Safety Earthquake-2, which is the groundshaking at a site for an MCE. 2-46BSO: Basic Safety Objective, a RehabilitationObjective in which the Life Safety Performance Level isreached for the BSE-1 demand and the CollpasePrevention Performance Level is reached for the BSE-2.Building Performance Level: A limiting damage state,considering structural and nonstructural buildingcomponents, used in the definition of RehabilitationObjectives. 2-46CCapacity: The permissible strength or deformation for acomponent action. 2-46Cavity wall: A masonry wall with an air space betweenwythes. Wythes are usually joined by wirereinforcement, or steel ties. Also known as anoncomposite wall. 7-23Chevron bracing: See V-braced frame. A-1FEMA 273 Seismic Rehabilitation Guidelines A-1

Appendix A: GlossaryChord: See diaphragm chord. 8-3Clay tile masonry: Masonry constructed with hollowunits made of clay tile. Typically, units are laid withcells running horizontally, and are thus ungrouted. Insome cases, units are placed with cells runningvertically, and may or may not be grouted. 7-23Clay-unit masonry: Masonry constructed with solid,cored, or hollow units made of clay. Hollow clay unitsmay be ungrouted, or grouted. 7-23Coefficient of variation: For a sample of data, the ratioof the standard deviation for the sample to the men valuefor the sample. 2-46Collar joint: Vertical longitudinal joint between wythesof masonry or between masonry wythe and back-upconstruction that may be filled with mortar or grout. 7-23Collector: See drag strut. 8-31Column (or beam) jacketing: A method in which aconcrete column or beam is covered with a steel orconcrete “jacket” in order to strengthen and/or repair themember by confining the concrete. 10-14Components: The basic structural members thatconstitute the building, such as beams, columns, slabs,braces, piers, coupling beams, and connections.Components, such as columns and beams, are combinedto form elements (e.g., a frame). 2-47 3-17Component, flexible: A component, including itsattachments, having a fundamental period greater than0.06 seconds. 11-28Component, rigid: A component, including itsattachments, having a fundamental period less than orequal to 0.06 seconds. 11-28Composite masonry wall: Multiwythe masonry wallacting with composite action. 7-23Composite panel: A structural panel comprising thinwood strands or wafers bonded together with exterioradhesive. 8-31Concentric braced frame (CBF): A braced frame inwhich the members are subjected primarily to axialforces. 5-40Concrete masonry: Masonry constructed with solid orhollow units made of concrete. Hollow concrete unitsmay be ungrouted, or grouted. 7-2 31Condition of service: The environment to which thestructure will be subjected. Moisture conditions are themost significant issue; however, temperature can have asignificant effect on some assemblies. 8-31Connection: A link between components or elementsthat transmits actions from one component or element toanother component or element. Categorized by type ofaction (moment, shear, or axial), connection links arefrequently nonductile. 5-40 8-31Contents: Movable items within the buildingintroduced by the owner or occupants. 11-28Continuity plates: Column stiffeners at top and bottomof the panel zone. 5-40Control node: The node in the mathematical model of abuilding used to characterize mass and earthquakedisplacement. 3-17Corrective measure: Any modification of a componentor element, or the structure as a whole, intended toreduce building vulnerability. 2-47Coupling beam: Flexural member that ties or couplesadjacent shear walls acting in the same plane. Acoupling beam is designed to yield and dissipateinelastic energy, and, when properly detailed andproportioned, has a significant effect on the overallstiffness of the coupled wall. 10-14Cripple studs: Short studs between header and top plateat opening in wall framing or studs between base sill andsill of opening. 8-31Cripple wall: Short wall between foundation and firstfloor framing. 8-31Critical action: That component action that reaches itselastic limit at the lowest level of lateral deflection, orloading, for the structure. 2-47Crosstie: A beam or girder that spans across the widthof the diaphragm, accumulates the wall loads, andtransfers them, over the full depth of the diaphragms,into the next bay and onto the nearest shear wall orframe. 10 -14DDecay: Decomposition of wood caused by action ofwood-destroying fungi. The term “dry rot” is usedinterchangeably with decay. 8-31A-2 Seismic Rehabilitation Guidelines FEMA 273

Appendix A: GlossaryDecking: Solid sawn lumber or glued laminateddecking, nominally two to four inches thick and fourinches and wider. Decking may be tongue-and-groove orconnected at longitudinal joints with nails or metalclips. 8-31Deep foundation: Piles or piers. 4-19Deformation: Relative displacement or rotation of theends of a component or element. 3-17Deformation-sensitive nonstructural component: Anonstructural component sensitive to deformationimposed on it by the drift or deformation of the structure,including deflection or deformation of diaphragms. 11-29Demand: The amount of force or deformation imposedon an element or component. 2-47Design displacement: The design earthquakedisplacement of an isolation or energy dissipationsystem, or elements thereof, excluding additionaldisplacement due to actual and accidental torsion. 9-25Design resistance: Resistance (force or moment asappropriate) provided by member or connection; theproduct of adjusted resistance, the resistance factor,confidence factor, and time effect factor. 8-31Diagonal bracing: Inclined structural memberscarrying primarily axial load, employed to enable astructural frame to act as a truss to resist horizontalloads. 5-40Diaphragm: A horizontal (or nearly horizontal)structural element used to distribute inertial lateralforces to vertical elements of the lateral-force-resistingsystem. 2-47 8-31Diaphragm chord: A diaphragm component providedto resist tension or compression at the edges of thediaphragm. 8-31 10-15Diaphragm collector: A diaphragm componentprovided to transfer lateral force from the diaphragm tovertical elements of the lateral-force-resisting system orto other portions of the diaphragm. 2-47Diaphragm ratio: See aspect ratio. 8-31Differential compaction: An earthquake-inducedprocess in which loose or soft soils become morecompact and settle in a nonuniform manner across asite. 4-19Dimensioned lumber: Lumber from nominal twothrough four inches thick and nominal two or moreinches wide. 8-31Displacement: The total movement, typicallyhorizontal, of a component or element or node. 3-17Displacement restraint system: Collection ofstructural components and elements that limit lateraldisplacement of seismically-isolated buildings duringthe BSE-2. 9-25Displacement-dependent energy dissipation devices:Devices having mechanical properties such that theforce in the device is related to the relative displacementin the device. 9-25Dowel bearing strength: The maximum compressionstrength of wood or wood-based products whensubjected to bearing by a steel dowel or bolt of specificdiameter. 8-31Dowel type fasteners: Includes bolts, lag screws, woodscrews, nails, and spikes. 8-31Drag strut: A component parallel to the applied loadthat collects and transfers diaphragm shear forces to thevertical lateral-force-resisting components or elements,or distributes forces within a diaphragm. Also calledcollector, diaphragm strut, or tie. 8-31Dressed size: The dimensions of lumber after surfacingwith a planing machine. Usually 1/2 to 3/4 inch less thannominal size. 8-31Dry service: Structures wherein the maximumequilibrium moisture content does not exceed 19%. 8-31Dual system: A structural system included in buildingswith the following features: 5-41• An essentially complete space frame providessupport for gravity loads. 5-41• Resistance to lateral load is provided by concrete orsteel shear walls, steel eccentrically braced frames(EBF), or concentrically braced frames (CBF) alongwith moment-resisting frames (Special MomentFrames, or Ordinary Moment Frames) that arecapable of resisting at least 25% of the lateralloads. 5-41• Each system is also designed to resist the total lateralload in proportion to its relative rigidity. 5-41FEMA 273 Seismic Rehabilitation Guidelines A-3

Appendix A: GlossaryEEccentric braced frame (EBF): A diagonal bracedframe in which at least one end of each diagonal bracingmember connects to a beam a short distance from abeam-to-column connection or another brace end. 5-41Edge distance: The distance from the edge of themember to the center of the nearest fastener. When amember is loaded perpendicular to the grain, the loadededge shall be defined as the edge in the direction towardwhich the fastener is acting. 8-31Effective damping: The value of equivalent viscousdamping corresponding to the energy dissipated by thebuilding, or element thereof, during a cycle ofresponse. 9-25Effective stiffness: The value of the lateral force in thebuilding, or an element thereof, divided by thecorresponding lateral displacement. 9-25Element: An assembly of structural components that acttogether in resisting lateral forces, such as momentresistingframes, braced frames, shear walls, anddiaphragms. 2-47 3-17Energy dissipation device (EDD): Non-gravity-loadsupportingelement designed to dissipate energy in astable manner during repeated cycles of earthquakedemand. 9-25Energy dissipation system (EDS): Complete collectionof all energy dissipation devices, their supportingframing, and connections. 9-25FFault: Plane or zone along which earth materials onopposite sides have moved differentially in response totectonic forces. 4-19Flexible connections: Connections betweencomponents that permit rotational and/or translationalmovement without degradation of performance.Examples include universal joints, bellows expansionjoints, and flexible metal hose. 11 -2 9Flexible diaphragm: A diaphragm that meetsrequirements of Section 3.2.4. 3-17Footing: A structural component transferring the weightof a building to the foundation soils and resisting lateralloads. 4-19Foundation soils: Soils supporting the foundationsystem and resisting vertical and lateral loads. 4-19Foundation springs: Method of modeling toincorporate load-deformation characteristics offoundation soils. 4-19Foundation system: Structural components (footings,piles). 4-19Framing type: Type of seismic resisting system. 3-1Fundamental period: The first mode period of thebuilding in the direction under consideration. 3-1 7GGauge or row spacing: The center-to-center distancebetween fastener rows or gauge lines. 8-31Glulam beam: Shortened term for glued-laminatedbeam. 8-31Grade: The classification of lumber in regard tostrength and utility, in accordance with the grading rulesof an approved agency. 8-31Grading rules: Systematic and standardized criteria forrating the quality of wood products. 8-31Gypsum wallboard or drywall: An interior wallsurface sheathing material sometimes considered forresisting lateral forces. 8-31HHazard level: Earthquake shaking demands of specifiedseverity, determined on either a probabilistic ordeterministic basis. 2-4 7Head joint: Vertical mortar joint placed betweenmasonry units in the same wythe. 7-23Hold-down: Hardware used to anchor the vertical chordforces to the foundation or framing of the structure inorder to resist overturning of the wall. 8-32Hollow masonry unit: A masonry unit whose net crosssectionalarea in every plane parallel to the bearingsurface is less than 75% of the gross cross-sectional areain the same plane. 7-237A-4 Seismic Rehabilitation Guidelines FEMA 273

Appendix A: GlossaryIInfill: A panel of masonry placed within a steel orconcrete frame. Panels separated from the surroundingframe by a gap are termed "isolated infills". Panels thatare in tight contact with a frame around its full perimeterare termed “shear infills.” 7-23In-plane wall: See shear wall. 7-23Inter-story drift: The relative horizontal displacementof two adjacent floors in a building. Inter-story drift canalso be expressed as a percentage of the story heightseparating the two adjacent floors. 10-15Isolation interface: The boundary between the upperportion of the structure (superstructure), which isisolated, and the lower portion of the structure, whichmoves rigidly with the ground. 9-25Isolation system: The collection of structural elementsthat includes all individual isolator units, all structuralelements that transfer force between elements of theisolation system, and all connections to other structuralelements. The isolation system also includes the windrestraintsystem. 9-25Isolator unit: A horizontally flexible and vertically stiffstructural element of the isolation system that permitslarge lateral deformations under seismic load. Anisolator unit may be used either as part of or in additionto the weight-supporting system of the building. 9-25JJoint: Area where two or more ends, surfaces, or edgesare attached. Categorized by type of fastener or weldused and method of force transfer. 5-41KKing stud: Full height stud or studs adjacent to openingsthat provide out-of-plane stability to cripple studs atopenings. 8-32LLandslide: A down-slope mass movement of earthresulting from any cause. 4-1 9Lateral support member: Member designed to inhibitlateral buckling or lateral-torsional buckling of acomponent. 5-41Lateral-force-resisting system: Those elements of thestructure that provide its basic lateral strength andstiffness, and without which the structure would belaterally unstable. 2-47Light framing: Repetitive framing with smalluniformly spaced members. 8-32Linear procedure: Analysis based on a straight-line(elastic) force-versus-displacement relationship. A-1Link: In an EBF, the segment of a beam that extendsfrom column to brace, located between the end of adiagonal brace and a column, or between the ends of twodiagonal braces of the EBF. The length of the link isdefined as the clear distance between the diagonal braceand the column face or between the ends of two diagonalbraces. 5-41Link intermediate web stiffeners: Vertical webstiffeners placed within the link. 5-41Link rotation angle: The angle of plastic rotationbetween the link and the beam outside of the link derivedusing the specified base shear, V. 5-41Liquefaction: An earthquake-induced process in whichsaturated, loose, granular soils lose a substantial amountof shear strength as a result of increase in pore-waterpressure during earthquake shaking. 4-19Load duration: The period of continuous application ofa given load, or the cumulative period of intermittentapplications of load. (See time effect factor.) 8-32Load path: A path that seismic forces pass through tothe foundation of the structure and, ultimately, to thesoil. Typically, the load travels from the diaphragmthrough connections to the vertical lateral-forceresistingelements, and then proceeds to the foundationby way of additional connections. 10-15Load sharing: The load redistribution mechanismamong parallel components constrained to deflecttogether. 8-32Load/slip constant: The ratio of the applied load to aconnection and the resulting lateral deformation of theconnection in the direction of the applied load. 8-32FEMA 273 Seismic Rehabilitation Guidelines A-5

Appendix A: GlossaryLRFD (Load and Resistance Factor Design): Amethod of proportioning structural components(members, connectors, connecting elements, andassemblages) using load and resistance factors such thatno applicable limit state is exceeded when the structureis subjected to all design load and resistance factorcombinations using load and resistance factors such thatno applicable limit state is exceeded when the structureis subjected to all design load combinations. 5-41Lumber: The product of the sawmill and planing mill,usually not further manufactured other than by sawing,resawing, passing lengthwise through a standard planingmachine, crosscutting to length, and matching. 8-32Lumber size: Lumber is typically referred to by sizeclassifications. Additionally, lumber is specified bymanufacturing classification. Rough lumber and dressedlumber are two of the routinely used manufacturingclassifications. 8-32MMasonry: The assemblage of masonry units, mortar andpossibly grout and/or reinforcement. Types of masonryare classified herein with respect to the type of themasonry units such as clay-unit masonry, concretemasonry, or hollow-clay tile masonry. 7-2 3Mat-formed panel: A structural panel designationrepresenting panels manufactured in a mat-formedprocess, such as oriented strand board andwaferboard. 8-32Maximum Considered Earthquake (MCE): Anextreme earthquake hazard level used in the formation ofRehabilitation Objectives. (See BSE-2.) 2-47Maximum displacement: The maximum earthquakedisplacement of an isolation or energy dissipationsystem, or elements thereof, excluding additionaldisplacement due to actual or accidental torsion. 9-25Mean return period: The average period of time, inyears, between the expected occurrences of anearthquake of specified severity. 2-47Model Building Type: Fifteen common building typesused to categorize expected deficiencies, reasonablerehabilitation methods, and estimated costs. See Table10-2 for descriptions of Model Building Types. 10-15Moisture content: The weight of the water in woodexpressed as a percentage of the weight of the ovendriedwood. 8-32Moment frame: A building frame system in whichseismic shear forces are resisted by shear and flexure inmembers and joints of the frame. 5-41NNarrow wood shear wall: Wood shear walls with anaspect ratio (height to width) greater than two to one.These walls are relatively flexible and thus tend to beincompatible with other building components, therebytaking less shear than would be anticipated whencompared to wider walls. 10-15Nominal size: The approximate rough-sawncommercial size by which lumber products are knownand sold in the market. Actual rough-sawn sizes varyfrom the nominal. Reference to standards or grade rulesis required to determine nominal to actual finished sizerelationships, which have changed over time. 8-32Nominal strength: The capacity of a structure orcomponent to resist the effects of loads, as determinedby computations using specified material strengths anddimensions and formulas derived from acceptedprinciples of structural mechanics, or by field tests orlaboratory tests of scaled models, allowing for modelingeffects, and differences between laboratory and fieldconditions. 5-41Nonbearing wall: A wall that supports gravity loadsless than as defined for a bearing wall. 7-23Noncompact member: A steel section in compressionwhose width-to-thickness ratio does not meet thelimiting values for compactness, as shown in Table B5.1of AISC (1986). 10-15Noncomposite masonry wall: Multiwythe masonrywall acting without composite action. 7-23Nonlinear procedure: Analysis based on and includingboth elastic and post-yield force-versus-displacementrelationships. A-1Nonstructural component: An architectural,mechanical, plumbing, or electrical component, or itemof interior equipment and furnishing, permanentlyinstalled in the building, as listed in Table 11-1. 11-29A-6 Seismic Rehabilitation Guidelines FEMA 273

Appendix A: GlossaryNonstructural Performance Level: A limiting damagestate for nonstructural building components used todefine Rehabilitation Objectives. 2-47OOrdinary Moment Frame (OMF): A moment framesystem that meets the requirements for OrdinaryMoment Frames as defined in seismic provisions fornew construction in AISC (1994a), Chapter 5. 5-41Oriented strandboard: A structural panel comprisingthin elongated wood strands with surface layers arrangedin the long panel direction and core layers arranged inthe cross panel direction. 8-32Out-of-plane wall: A wall that resists lateral forcesapplied normal to its plane. 7-23Overturning: When the moment produced at the baseof vertical lateral-force-resisting elements is larger thanthe resistance provided by the foundation’s upliftresistance and building weight. 10-15PPanel: A sheet-type wood product. 8-32Panel rigidity or stiffness: The in-plane shear rigidityof a panel, the product of panel thickness and modulusof rigidity. 8-32Panel shear: Shear stress acting through the panelthickness. 8-32Panel zone: Area of a column at the beam-to-columnconnection delineated by beam and column flanges.5-41 10 -1 5Parametric analysis: Repetitive analyses performed inwhich one or more independent parameters are variedfor the ultimate purpose of optimizing a dependent(earthquake response) parameter. A-1Parapet: Portions of a wall extending above the roofdiaphragm. Parapets can be considered as flanges to roofdiaphragms if adequate connections exist or areprovided. 7-23Partially grouted masonry wall: A masonry wallcontaining grout in some of the cells. 7-24Particleboard: A panel manufactured from small piecesof wood, hemp, and flax, bonded with synthetic ororganic binders, and pressed into flat sheets. 8-32P-∆ effect: Secondary effect of column axial loads andlateral deflection on the shears and moments in variouscomponents of a structure. 5-41Perforated wall or infill panel: A wall or panel notmeeting the requirements for a solid wall or infillpanel. 7-24Pier: Similar to pile; usually constructed of concrete andcast in place. 4-19Pile: A deep structural component transferring theweight of a building to the foundation soils and resistingvertical and lateral loads; constructed of concrete, steel,or wood; usually driven into soft or loose soils. 4-19 8-32Pitch or spacing: The longitudinal center-to-centerdistance between any two consecutive holes or fastenersin a row. 8-32Plan irregularity: Horizontal irregularity in the layoutof vertical lateral-force-resisting elements, thusproducing a differential between the center of mass andcenter of rigidity, that typically results in significanttorsional demands on the structure. 10-15Planar shear: The shear that occurs in a plane parallelto the surface of a panel, which has the ability to causethe panel to fail along the plies in a plywood panel or ina random layer in a nonveneer or composite panel. 8-32Platform framing: Construction method in which studwalls are constructed one floor at a time, with a floor orroof joist bearing on top of the wall framing at eachlevel. 8-32Ply: A single sheet of veneer, or several strips laid withadjoining edges that form one veneer lamina in a gluedplywood panel. 8-32Plywood: A structural panel comprising plies of woodveneer arranged in cross-aligned layers. The plies arebonded with an adhesive that cures upon application ofheat and pressure. 8-32Pole: A round timber of any size or length, usually usedwith the larger end in the ground. 8-33Pole structure: A structure framed with generally roundcontinuous poles that provide the primary vertical frameand lateral-load-resisting system. 8-33FEMA 273 Seismic Rehabilitation Guidelines A-7

Appendix A: GlossaryPounding: Two adjacent buildings coming in contactduring earthquake excitation because they are too closetogether and/or exhibit different dynamic deflectioncharacteristics. 10-15Prescriptive ultimate bearing capacity: Assumptionof ultimate bearing capacity based on propertiesprescribed in Section 4.4.1.2. 4-1 9Preservative: A chemical that, when suitably applied towood, makes the wood resistant to attack by fungi,insects, marine borers, or weather conditions. 8-33Pressure-preservative treated wood: Wood productspressure-treated by an approved process andpreservative. 8-33Presumptive ultimate bearing capacity: Assumptionof ultimate bearing capacity based on allowable loadsfrom original design. 4-1 9Primary (strong) panel axis: The direction thatcoincides with the length of the panel. 8-33Primary component: Those components that arerequired as part of the building’s lateral-force-resistingsystem (as contrasted to secondary components). 3-17Primary element: An element that is essential to theability of the structure to resist earthquake-induceddeformations. 2-47Punched metal plate: A light steel plate fasteninghaving punched teeth of various shapes andconfigurations that are pressed into wood members toeffect transfer shear. Used with structural lumberassemblies. 8-33RRedundancy: Quality of having alternative paths in thestructure by which the lateral forces are resisted,allowing the structure to remain stable following thefailure of any single element. 10-15Re-entrant corner: Plan irregularity in a diaphragm,such as an extending wing, plan inset, or E-, T-, X-, orL-shaped configuration, where large tensile andcompressive forces can develop. 10-15Rehabilitation Method: A procedural methodology forthe reduction of building earthquake vulnerability. 2-47Rehabilitation Objective: A statement of the desiredlimits of damage or loss for a given seismic demand,which is usually selected by the owner, engineer, and/orrelevant public agencies. (See Chapter 2.) 2-47 10-15Rehabilitation strategy: A technical approach fordeveloping rehabilitation measures for a building toreduce its earthquake vulnerability. 2-47Reinforced masonry (RM) wall: A masonry wall thatis reinforced in both the vertical and horizontaldirections. The sum of the areas of horizontal andvertical reinforcement must be at least 0.002 times thegross cross-sectional area of the wall, and the minimumarea of reinforcement in each direction must be not lessthan 0.0007 times the gross cross-sectional area of thewall. Reinforced walls are assumed to resist loadsthrough resistance of the masonry in compression andthe reinforcing steel in tension or compression.Reinforced masonry is partially grouted or fullygrouted. 7-24Repointing: A method of repairing a cracked ordeteriorating mortar joint in masonry. The damaged ordeteriorated mortar is removed and the joint is refilledwith new mortar. 10-15Required member resistance: Load effect (force,moment, stress, action as appropriate) acting on anelement or connection, determined by structural analysisfrom the factored loads and the critical loadcombinations. 8-33Required strength: Load effect (force, moment, stress,as appropriate) acting on a component or connectiondetermined by structural analysis from the factored loads(using most appropriate critical load combinations). 5-4 1Resistance: The capacity of a structure, component, orconnection to resist the effects of loads. It is determinedby computations using specified material strengths,dimensions, and formulas derived from acceptedprinciples of structural mechanics, or by field orlaboratory tests of scaled models, allowing for modelingeffects and differences between laboratory and fieldconditions. 8-33Resistance factor: A reduction factor applied tomember resistance that accounts for unavoidabledeviations of the actual strength from the nominal value,and the manner and consequences of failure. 5-41 8-33A-8 Seismic Rehabilitation Guidelines FEMA 273

Appendix A: GlossaryRetaining wall: A free-standing wall that has soil onone side. 4-19Rigid diaphragm: A diaphragm that meetsrequirements of Section 3.2.4 3-17Rough lumber: Lumber as it comes from the saw priorto any dressing operation. 8-33Row of fasteners: Two or more fasteners aligned withthe direction of load. 8-33Running bond: A pattern of masonry where the headjoints are staggered between adjacent courses by morethan a third of the length of a masonry unit. Also refersto the placement of masonry units such that head jointsin successive courses are horizontally offset at least onequarterthe unit length. 7-24SSeasoned lumber: Lumber that has been dried.Seasoning takes place by open-air drying within thelimits of moisture contents attainable by this method, orby controlled air drying (i.e., kiln drying). 8-33Secondary component: Those components that are notrequired for lateral force resistance (contrasted toPrimary Components). They may or may not actuallyresist some lateral forces. 2-47Secondary component: Those components that are notrequired for lateral force resistance (contrasted toprimary components). They may or may not actuallyresist some lateral forces. 3-17Secondary element: An element that does not affect theability of the structure to resist earthquake-induceddeformations. 2-47Seismic demand: Seismic hazard level commonlyexpressed in the form of a ground shaking responsespectrum. It may also include an estimate of permanentground deformation. 2-47Shallow foundation: Isolated or continuous spreadfootings or mats. 4-19Shear wall: A wall that resists lateral forces appliedparallel with its plane. Also known as an in-planewall. 7-24Sheathing: Lumber or panel products that are attachedto parallel framing members, typically forming wall,floor, ceiling, or roof surfaces. 8-33Short captive column: Columns with height-to-depthratios less than 75% of the nominal height-to-depthratios of the typical columns at that level. Thesecolumns, which may not be designed as part of theprimary lateral-load-resisting system, tend to attractshear forces because of their high stiffness relative toadjacent elements. 10-15Shrinkage: Reduction in the dimensions of wood due toa decrease of moisture content. 8-33Simplified Rehabilitation Method: An approach,applicable to some types of buildings and RehabilitationObjectives, in which analyses of the entire building’sresponse to earthquake hazards are not required. 2-47 10-15Slip-critical joint: A bolted joint in which slipresistance of the connection is required. 5-41Solid masonry unit: A masonry unit whose net crosssectionalarea in every plane parallel to the bearingsurface is 75% or more of the gross cross-sectional areain the same plane. 7-24Solid wall or solid infill panel: A wall or infill panelwith openings not exceeding 5% of the wall surface area.The maximum length or height of an opening in a solidwall must not exceed 10% of the wall width or storyheight. Openings in a solid wall or infill panel must belocated within the middle 50% of a wall length and storyheight, and must not be contiguous with adjacentopenings. 7-24Special Moment Frame (SMF): A moment framesystem that meets the special requirements for frames asdefined in seismic provisions for new construction. 5-41SPT N-Values: Using a standard penetration test(ASTM Test D1586), the number of blows of a 140-pound hammer falling 30 inches required to drive astandard 2-inch-diameter sampler a distance of 12inches. 4-19Stack bond: In contrast to running bond, usually aplacement of units such that the head joints in successivecourses are aligned vertically. 7-24 7-24Stiff diaphragm: A diaphragm that meets requirementsof Section 3.2.4. 3-17FEMA 273 Seismic Rehabilitation Guidelines A-9

Appendix A: GlossaryStorage racks: Industrial pallet racks, movable shelfracks, and stacker racks made of cold-formed or hotrolledstructural members. Does not include other typesof racks such as drive-in and drive-through racks,cantilever wall-hung racks, portable racks, or racksmade of materials other than steel. 11-29Strength: The maximum axial force, shear force, ormoment that can be resisted by a component. 2-47Stress resultant: The net axial force, shear, or bendingmoment imposed on a cross section of a structuralcomponent. 2-48Strong back system: A secondary system, such as aframe, commonly used to provide out-of-plane supportfor an unreinforced or under-reinforced masonrywall. 10-16Strong column-weak beam: The capacity of thecolumn at any moment frame joint must be greater thanthose of the beams, in order to ensure inelastic action inthe beams, thereby localizing damage and controllingdrift. 10-16Structural Performance Level: A limiting structuraldamage state, used in the definition of RehabilitationObjectives. 2-48Structural Performance Range: A range of structuraldamage states, used in the definition of RehabilitationObjectives. 2-48Structural system: An assemblage of load-carryingcomponents that are joined together to provide regularinteraction or interdependence. 5-41Structural-use panel: A wood-based panel productbonded with an exterior adhesive, generally 4' x 8' orlarger in size. Included under this designation areplywood, oriented strand board, waferboard, andcomposite panels. These panel products meet therequirements of PS 1-83 or PS 2-92 and are intended forstructural use in residential, commercial, and industrialapplications. 8-33Stud: Wood member used as vertical framing memberin interior or exterior walls of a building, usually 2" x 4"or 2" x 6" sizes, and precision end-trimmed. 8-33Subassembly: A portion of an assembly. 2-48Subdiaphragm: A portion of a larger diaphragm used todistribute loads between members. 8-33Systematic Rehabilitation Method: An approach torehabilitation in which complete analysis of thebuilding’s response to earthquake shaking isperformed. 2-48 10-16TTarget displacement: An estimate of the likelybuilding roof displacement in the design earthquake. 3-17Tie: See drag strut. 8-33Tie-down: Hardware used to anchor the vertical chordforces to the foundation or framing of the structure inorder to resist overturning of the wall. 8-33Tie-down system: The collection of structuralconnections, components, and elements that providerestraint against uplift of the structure above theisolation system. 9-25Timbers: Lumber of nominal five or more inches insmaller cross-section dimension. 8-33Time effect factor: A factor applied to adjustedresistance to account for effects of duration of load. (Seeload duration.) 8-33Total design displacement: The BSE-1 displacementof an isolation or energy dissipation system, or elementsthereof, including additional displacement due to actualand accidental torsion. 9-25Total maximum displacement: The maximumearthquake displacement of an isolation or energydissipation system, or elements thereof, includingadditional displacement due to actual and accidentaltorsion. 9-25Transverse wall: A wall that is oriented transverse tothe in-plane shear walls, and resists lateral forces appliednormal to its plane. Also known as an out-of-planewall. 7-24UUltimate bearing capacity: Maximum possiblefoundation load or stress (strength); increase indeformation or strain results in no increase in load orstress. 4-19A-10 Seismic Rehabilitation Guidelines FEMA 273

Appendix A: GlossaryUnreinforced masonry (URM) wall: A masonry wallcontaining less than the minimum amounts ofreinforcement as defined for masonry (RM) walls. Anunreinforced wall is assumed to resist gravity and lateralloads solely through resistance of the masonrymaterials. 7-24VV-braced frame: A concentric braced frame (CBF) inwhich a pair of diagonal braces located either above orbelow a beam is connected to a single point within theclear beam span. Where the diagonal braces are belowthe beam, the system also is referred to as an “invertedV-brace frame,” or “chevron bracing.” 5-42Velocity-dependent energy dissipation devices(EDDs): Devices having mechanical characteristicssuch that the force in the device is dependent on therelative velocity in the device. 9-25Vertical irregularity: A discontinuity of strength,stiffness, geometry, or mass in one story with respect toadjacent stories. 10-16WWaferboard: A nonveneered structural panelmanufactured from two- to three-inch flakes or wafersbonded together with a phenolic resin and pressed intosheet panels. 8-33Wind-restraint system: The collection of structuralelements that provides restraint of the seismic-isolatedstructure for wind loads. The wind-restraint system maybe either an integral part of isolator units or a separatedevice. 9-25Wythe: A continuous vertical section of a wall, onemasonry unit in thickness. 7-24XX-braced frame: A concentric braced frame (CBF) inwhich a pair of diagonal braces crosses near the midlengthof the braces. 5-42YY-braced frame: An eccentric braced frame (EBF) inwhich the stem of the Y is the link of the EBFsystem. 5-42FEMA 273 Seismic Rehabilitation Guidelines A-11

B.Seismic Rehabilitation Guidelines Project ParticipantsProject Oversight CommitteeChairmanEugene ZellerDirector of Planning and BuildingDepartment of Planning and BuildingLong Beach, CaliforniaASCE MembersPaul SeaburgOffice of the Associate DeanCollege of Engineering and TechnologyOmaha, NebraskaAshvin ShahDirector of EngineeringAmerican Society of Civil EngineersWashington, D.C.ATC MembersThomas G. AtkinsonAtkinson, Johnson and SpurrierSan Diego, CaliforniaChristopher RojahnExecutive DirectorApplied Technology CouncilRedwood City, CaliforniaBSSC MembersGerald H. JonesConsultantKansas City, MissouriJames R. SmithExecutive DirectorBuilding Seismic Safety CouncilWashington, D.C.ASCE Project ParticipantsProject Steering CommitteeVitelmo V. BerteroEarthquake Engineering Research CenterUniversity of CaliforniaRichmond, CaliforniaWilliam J. HallUniversity of Illinois at Urbana-ChampaignUrbana, IllinoisClarkson W. PinkhamS. B. Barnes AssociatesLos Angeles, CaliforniaUsers WorkshopsTom McLane, ManagerAmerican Socity of Civil EngineersWashington, D.C.Debbie Smith, CoordinatorAmerican Socity of Civil EngineersWashington, D.C.Paul SeaburgUniversity of OmahaOmaha, NebreskaRoland L. SharpeConsulting Structural EngineerLos Altos, CaliforniaJon S. TrawInternational Council of Building OfficialsWhittier, CaliforniaResearch Synthesis SubcontractorJames O. JirsaUniversity of TexasAustin, TexasSpecial Issues SubcontractorMelvyn GreenMelvyn Green and AssociatesTorrance, CaliforniaFEMA 273 Seismic Rehabilitation Guidelines B-1

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsATC Project ParticipantsSenior Technical CommitteePrincipal Investigator (PI)Christopher RojahnApplied Technology CouncilRedwood City, CaliforniaCo-PI and Project DirectorDaniel ShapiroSOH & AssociatesSan Francisco, CaliforniaCo-Project DirectorLawrence D. ReaveleyUniversity of UtahSalt Lake City, UtahSenior Technical AdvisorWilliam T. HolmesRutherford & ChekeneSan Francisco, CaliforniaTechnical AdvisorJack MoehleEERC, UC BerkeleyRichmond, CaliforniaATC Board Representative (Ex-Officio)Tom AtkinsonAtkinson, Johnson, & SpurrierSan Diego, CaliforniaGeneral Requirements TeamRonald O. Hamburger, Team LeaderEQE InternationalSan Francisco, CaliforniaSigmund A. FreemanWiss, Janney, Elstner AssociatesEmeryville, CaliforniaProf. Peter Gergely (deceased)Cornell UniversityIthaca, New YorkRichard A. ParmeleeConsulting EngineerSt. George, UtahAllan R. PorushDames & MooreLos Angeles, CaliforniaModeling and Analysis TeamMike Mehrain, Team LeaderDames & MooreLos Angeles, CaliforniaRonald P. GallagherR. P. Gallagher AssociatesSan Francisco, CaliforniaHelmut KrawinklerStanford UniversityStanford, CaliforniaGuy J. P. NordensonOve Arup & PartnersNew York, New YorkMaurice S. PowerGeomatrix Consultants, Inc.San Francisco, CaliforniaAndrew S. WhittakerEERC, UC BerkeleyRichmond, CaliforniaB-2 Seismic Rehabilitation Guidelines FEMA 273

Geotechnical/Foundations TeamAppendix B: Seismic Rehabilitation GuidelinesProject ParticipantsJeffrey R. Keaton, Team LeaderAGRA Earth & EnvironmentalPhoenix, ArizonaCraig D. ComartinConsulting EngineerStockton, CaliforniaPaul W. GrantShannon & Wilson, Inc.Seattle, WashingtonGeoffrey R. MartinUniversity of Southern CaliforniaLos Angeles, CaliforniaMaurice S. PowerGeomatrix Consultants, Inc.San Francisco, CaliforniaConcrete TeamJack P. Moehle, Co-Team LeaderEERC, UC BerkeleyRichmond, CaliforniaLawrence D. Reaveley, Co-Team LeaderUniversity of UtahSalt Lake City, UtahJames E. CarpenterBruce C. Olsen Consulting EngineersSeattle, WashingtonJacob GrossmanRosenwasser/Grossman Cons Engrs.New York, New YorkPaul A. MurrayStanley D. Lindsey & AssociatesNashville, TennesseeJoseph P. NicolettiURS/John A. Blume and AssociatesSan Francisco, CaliforniaKent B. SoelbergRutherford & ChekeneBoise, IdahoJames K. WightUniversity of MichiganAnn Arbor, MichiganMasonry TeamDaniel P. Abrams, Team LeaderUniversity of IllinoisUrbana, IllinoisSamy A. AdhamAgbabian AssociatesPasadena, CaliforniaGregory R. KingsleyKLFA of ColoradoGolden, ColoradoOnder KustuOak EngineeringBelmont, CaliforniaJohn C. TheissEQE - TheissSt. Louis, MissouriFEMA 273 Seismic Rehabilitation Guidelines B-3

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsSteel TeamDouglas A. Foutch, Team LeaderUniversity of IllinoisUrbana, IllinoisNavin R. AminSkidmore, Owings & MerrillSan Francisco, CaliforniaJames O. MalleyDegenkolb EngineersSan Francisco, CaliforniaCharles W. RoederUniversity of WashingtonSeattle, WashingtonThomas Z. ScarangelloThornton-TomasettiNew York, New YorkWood TeamJohn M. Coil, Team LeaderCoil & WelshTustin, CaliforniaJeffery T. MillerReaveley Engineers & Assoc., Inc.Salt Lake City, UtahRobin ShepherdForensic Expert Advisers, Inc.Santa Ana, CaliforniaWilliam B. VaughnVaughn EngineeringLafayette, CaliforniaNew Technologies TeamCharles A. Kircher, Team LeaderCharles Kircher & AssociatesMountain View, CaliforniaMichael C. ConstantinouState University of New York at BuffaloBuffalo, New YorkAndrew S. WhittakerEERC, UC BerkeleyRichmond, CaliforniaNonstructural TeamChristopher Arnold, Team LeaderBuilding Systems DevelopmentPalo Alto, CaliforniaRichard L. HessHess Engineering, Inc.Los Alamitos, CaliforniaFrank E. McClureConsulting Structural EngineerOrinda, CaliforniaTodd W. PerbixRSP/EQESeattle, WashingtonB-4 Seismic Rehabilitation Guidelines FEMA 273

Simplified Rehabilitation TeamAppendix B: Seismic Rehabilitation GuidelinesProject ParticipantsChris D. Poland, Team LeaderDegenkolb EngineersSan Francisco, CaliforniaLeo E. ArgirisOve Arup & PartnersNew York, New YorkThomas F. HeauslerHeausler Structural EngineersKansas City, MissouriEvan ReisDegenkolb EngineersSan Francisco, CaliforniaTony TschanzSkilling Ward MagnussonBarkshire, Inc.Seattle, WashingtonQualification of In-Place Materials ConsultantsCharles J. Hookham (Lead)Black & VeatchAnn Arbor, MichiganDr. Richard Atkinson (deceased)Atkinson-Noland & AssociatesBoulder, ColoradoRoss EsfandiariRoss Engineering ServicesWalnut Creek, CaliforniaLanguage & Format ConsultantJames R. HarrisJ. R. Harris & CompanyDenver, ColoradoReport Preparation ConsultantsRoger E. Scholl (deceased), LeadCounterQuake CorporationRedwood City, CaliforniaRobert K. ReithermanThe Reitherman CompanyHalf Moon Bay, CaliforniaDocument ProductionLasselle-Ramsay, Inc.Mountain View, CaliforniaATC StaffChristopher RojohnExecutive DirectorA. Gerald BradyDeputy Executive DirectorPatty ChristoffersonManager, Administration & Public RelationsPeter N. MorkComputer SpecialistFEMA 273 Seismic Rehabilitation Guidelines B-5

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsBSSC Project Participants1997 Board of DirectionChairmanEugene ZellerDirector of Planning and BuildingDepartment of Planning and BuildingLong Beach, CaliforniaSecretaryMark B. HoganVice President of EngineeringNational Concrete Masonry AssociationHerndon, VirginiaVice ChairmanWilliam W. Stewart, FAIAStewart•Schaberg/ArchitectsClayton, Missouri(representing the American Institute of Architects)MembersJames E. Beavers, Ex officioJames E. Beavers ConsultantsOak Ridge, TennesseeEugene ColeCole, Yee, Schubert and AssociatesCarmichael, California(representing the Structural Engineers Association ofCalifornia)S. K. GhoshPortland Cement AssociationSkokie, IllinoisNestor IwankiwVice President, Technology and ResearchAmerican Institute of Steel ConstructionChicago, IllinoisGerald H. JonesKansas City, Missouri(representing the National Institute of Building Sciences)Joseph NicolettiSenior ConsultantURS/John A. Blume & AssociatesSan Francisco, California(representing the Earthquake Engineering Research Institute)John R. “Jack” ProsekProject ManagerTurner Construction CompanySan Francisco, California(representing Associated General Contractors of America)W. Lee Shoemaker, Ph.D.Director of Research and EngineeringMetal Building Manufacturers AssociationCleveland, OhioJohn C. TheissVice PresidentEQE - TheissSt. Louis, Missouri(representing the American Society of Civil Engineers)Charles H. ThorntonChairman/PrincipalThornton-Tomasetti EngineersNew York, New York(representing the Applied Technology Council)David P. TyreeRegional ManagerAmerican Forest and Paper AssociationColorado Springs, ColoradoB-6 Seismic Rehabilitation Guidelines FEMA 273

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsMembers (continued)David WismerDirector of Planning and Code DevelopmentDepartment of Licenses and InspectionsPhiladelphia, Pennsylvania(representing Building Officials and Code AdministratorsInternational)Richard WrightDirectorBuilding and Fire Research LaboratoryNational Institute of Standards and TechnologyGaithersburg, MarylandBSSC StaffJames R. SmithExecutive DirectorThomas R. HollenbachDeputy Executive DirectorLarry AndersonDirector, Special ProjectsClaret M. HeiderTechnical Writer-EditorMary MarshallAdministrative AssistantSocietal Issues SubcontractorRobert A. OlsonRobert Olson Associates, Inc.Sacramento, CaliforniaBSSC Project CommitteeChairmanWarner HoweConsulting Structural EngineerGermantown, TennesseeConsultantRobert A. OlsonRobert Olson Associates, Inc.Sacramento, CaliforniaMembersGerald H. JonesKansas City, MissouriHarry W. MartinAmerican Iron and Steel InstituteAuburn, CaliforniaAllan R. PorushStructural EngineerDames and MooreLos Angeles, CaliforniaF. Robert PreecePreece/Goudie and AssociatesSan Francisco, CaliforniaWilliam W. Stewart, FAIAStewart Schaberg/ArchitectsClayton, MissouriFEMA 273 Seismic Rehabilitation Guidelines B-7

Seismic Rehabilitation Advisory PanelChairmanGerald H. JonesKansas City, MissouriMembersAppendix B: Seismic Rehabilitation GuidelinesProject ParticipantsDavid E. AllenStructures DivisionInstitute of Research in ConstructionNational Research Council of CanadaOttawa, Ontario, CanadaJohn BattlesSouthern Building Code Congress, InternationalBirmingham, AlabamaDavid C. BreiholzChairman, Existing Buildings CommitteeStructural Engineers Association of CaliforniaLomita, CaliforniaMichael CaldwellAmerican Institute of Timber ConstructionEnglewood, ColoradoGregory L. F. ChiuInstitute for Business and Home SafetyBoston, MassachusettsTerry DooleyMorley Construction CompanySanta Monica, CaliforniaSusan M. DowtyInternational Conference of Building OfficialsWhittier, CaliforniaSteven J. EderEQE Engineering ConsultantsSan Francisco, CaliforniaS. K. GhoshPortland Cement AssociationSkokie, IllinoisBarry J. GoodnoProfessorSchool of Civil EngineeringGeorgia Institute of TechnologyAtlanta, GeorgiaCharles C. GutberletU.S. Army Corps of EngineersWashington, D.C.Warner HoweConsulting Structural EngineerGermantown, TennesseeHoward KunreutherWharton SchoolUniversity of PennsylvaniaPhiladelphia, PennsylvaniaHarry W. MartinAmerican Iron and Steel InstituteAuburn, CaliforniaRobert McCluerBuilding Officials and Code Administrators, InternationalCountry Club Hills, IllinoisMargaret Pepin-DonatNational Park Service RetiredEdmonds, WashingtonWilliam PetakProfessor, Institute of Safety and Systems ManagementUniversity of Southern CaliforniaLos Angeles, CaliforniaHoward SimpsonSimpson, Gumpertz and HegerArlington, MassachusettsWilliam W. Stewart, FAIAStewart Schaberg/ArchitectsClayton, MissouriJames E. ThomasDuke Power CompanyCharlotte, North CarolinaL. Thomas TobinTobin and AssociatesMill Valley, CaliforniaB-8 Seismic Rehabilitation Guidelines FEMA 273

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsRepresentatives of BSSC Member Organizations and their Alternates(as of September 1997)AFL-CIO Building and Construction Trades DepartmentRepresentativeSandra TillettAFL-CIO Building and Construction TradesWashington, D.C.AlternatePete StaffordCenter to Protect Workers' RightsWashington, D.C.AISC Marketing, Inc.RepresentativeRobert PyleAISC Marketing, Inc.Buena Park, CaliforniaAmerican Concrete InstituteRepresentativeArthur J. MullkoffAmerican Concrete InstituteFarmington Hills, MichiganAlternateWard R. MalischAmerican Concrete InstituteFarmington Hills, MichiganAmerican Consulting Engineers CouncilRepresentativeRoy G. JohnstonBrandow and Johnston AssociatesLos Angeles, CaliforniaAlternateEdward BajerAmerican Consulting Engineers CouncilWashington, D.C.American Forest and Paper AssociationRepresentativeDavid P. Tyree, P.E.American Forest and Paper AssociationColorado Springs, ColoradoAlternateBradford K. Douglas, P.E.American Forest and Paper AssociationWashington, D.C.American Institute of ArchitectsRepresentativeWilliam W. Stewart, FAIAStewart-Schaberg/ArchitectsClayton, MissouriAlternateGabor LorantGabor Lorant Architect Inc.Phoenix, ArizonaAmerican Institute of Steel ConstructionRepresentativeNestor IwankiwAmerican Institute of Steel ConstructionChicago, IllinoisFEMA 273 Seismic Rehabilitation Guidelines B-9

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsAmerican Insurance Services Group, Inc.RepresentativeJohn A. MineoAmerican Insurance Services Group, Inc.New York, New YorkAlternatePhillip OlmsteadITT Hartford Insurance GroupHartford, ConnecticutAmerican Iron and Steel InstituteRepresentativeHarry W. MartinAmerican Iron and Steel InstituteAuburn, CaliforniaAmerican Plywood AssociationRepresentativeKenneth R. AndreasonAmerican Plywood AssociationTacoma, WashingtonAmerican Society of Civil EngineersRepresentativeJohn C. TheissEQE - TheissSt. Louis, MissouriAmerican Society of Civil Engineers - Kansas City ChapterRepresentativeHarold SpragueBlack & VeatchOverland, MissouriAlternateWilliam A. BakerAmerican Plywood AssociationTacoma, WashingtonAlternateAshvin ShahScarsdale, New YorkAlternateBrad VaughanBlack & VeatchOverland, MissouriAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.RepresentativeWilliam StaehlinState of CaliforniaSacramento, CaliforniaAlternateBruce D. Hunn, Ph.D.ASHREAtlanta, GeorgiaAmerican Society of Mechanical EngineersRepresentativeEvangelos MichalopoulosHartford Steam Boiler Inspection and Insurance CompanyHartford, ConnecticutAmerican Welding SocietyRepresentativeHardy C. Campbell IIIAmerican Welding SocietyMiami, FloridaAlternateRonald W. HauptPressure Piping Engineering AssociatesFoster City, CaliforniaAlternateCharles R. FassingerAmerican Welding SocietyMiami, FloridaB-10 Seismic Rehabilitation Guidelines FEMA 273

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsApplied Technology CouncilRepresentativeChristopher RojahnApplied Technology CouncilRedwood City, CaliforniaAssociated General Contractors of AmericaRepresentativeJack ProsekTurner / VANIR Construction ManagementOakland, CaliforniaAssociation of Engineering GeologistsRepresentativeEllis KrinitzskyArmy Corps of EngineersVicksburg, MississippiAssociation of Major City Building OfficialsRepresentativeArthur J. Johnson, Jr.City of Los AngelesDepartment of Building and SafetyLos Angeles, CaliforniaAlternateKarl DeppeCity of Los AngelesDepartment of Building and SafetyLos Angeles, CaliforniaBuilding Officials and Code Administrators InternationalRepresentativeDavid WismerDepartment of Licenses and InspectionsPhiladelphia, PennsylvaniaAlternateCharles N. ThorntonThornton-TomasettiNew York, New YorkAlternateChristopher MonekAssociated General Contractors of AmericaWashington, D.C.AlternatePatrick J. BaroshPatrick J. Barosh & AssociatesConcord, MassachusettsBrick Institute of AmericaRepresentativeJ. Gregg BorcheltBrick Institute of AmericaReston, VirginiaAlternateMark NunnBrick Institute of AmericaReston, VirginiaAlternatePaul K. HeilstedtBOCA, InternationalCountry Club Hills, IllinoisBuilding Owners and Managers Association InternationalRepresentativeMichael JawerBOMA, InternationalWashington, D.C.California Geotechnical Engineers AssociationRepresentativeAlan KroppAlan Kropp & AssociatesBerkeley, CaliforniaAlternateJohn A. BakerAnderson Geotechnical ConsultantsRoseville, CaliforniaFEMA 273 Seismic Rehabilitation Guidelines B-11

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsCalifornia Seismic Safety CommissionRepresentativeFred TurnerSeismic Safety CommissionSacramento, CaliforniaCanadian National Committee on Earthquake EngineeringRepresentativeR. H. DevallRead Jones Christoffersen Ltd.Vancouver, British Columbia, CanadaConcrete Masonry Association of California and NevadaRepresentativeStuart R. BeaversConcrete Masonry Association of California and NevadaCitrus Heights, CaliforniaConcrete Reinforcing Steel InstituteRepresentativeDavid P. GustafsonConcrete Reinforcing Steel InstituteSchaumburg, IllinoisDivision of the State ArchitectRepresentativeVilas MujumdarDivision of the State ArchitectSacramento, CaliforniaEarthquake Engineering Research InstituteRepresentativeJoseph NicolettiURS ConsultantsSan Francisco, CaliforniaHawaii State Earthquake Advisory BoardRepresentativePaul Okubo, Ph. D.Hawaii State Earthquake Advisory BoardDepartment of DefenseHonolulu, HawaiiAlternateD. A. LutesNational Research Council of CanadaDivision of Research BuildingOttawa, Ontario, CanadaAlternateDaniel ShapiroSOH & Associates, Structural EngineersSan Francisco, CaliforniaAlternateH. James NevinConcrete Reinforcing Steel InstituteGlendora, CaliforniaAlternateAlan WilliamsDivision of the State ArchitectSacramento, CaliforniaAlternateF. Robert PreecePreece/Goudie & AssociatesSan Francisco, CaliforniaAlternateGary ChockMartin, Bravo & Chock, Inc.Honolulu, HawaiiB-12 Seismic Rehabilitation Guidelines FEMA 273

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsInsulating Concrete Form AssociationRepresentativeDick WhitakerInsulating Concrete Form AssociationGlenview, IllinoisInstitute for Business and Home SafetyRepresentativeGregory L.F. ChiuInstitute for Business and Home SafetyBoston, MassachusettsInteragency Committee on Seismic Safety in ConstructionRepresentativeRichard WrightNational Institute of Standards & TechnologyGaithersburg, MarylandInternational Conference of Building OfficialsRepresentativeRick OkawaInternational Conference of Building OfficialsWhittier, CaliforniaInternational Masonry InstituteRepresentativeRichard FilloramoInternational Masonry InstituteGlastonbury, ConnecticutMasonry Institute of AmericaRepresentativeJohn ChryslerMasonry Institute of AmericaLos Angeles, CaliforniaMetal Building Manufacturers AssociationRepresentativeW. Lee Shoemaker, Ph.D., P.E.Metal Building Manufacturers AssociationCleveland, OhioAlternateDan MistickPortland Cement AssociationSkokie, IllinoisAlternateKaren GahaganInstitute for Business and Home SafetyBoston, MassachusettsAlternateH. S. LewNational Institute of Standards and TechnologyGaithersburg, MarylandAlternateSusan M. DowtyInternational Conference of Building OfficialsLaguna Niguel, CaliforniaAlternateDiane ThroopInternational Masonry InstittuteGreat Lakes OfficeAnn Arbor, MichiganAlternateJames E. AmrheinMasonry Institute of AmericaLos Angeles, CaliforniaAlternateJoe N. NunneryAMCA Building DivisionMemphis, TennesseeNational Association of Home BuildersRepresentativeEd SuttonNational Association of Home BuildersWashington, D.C.FEMA 273 Seismic Rehabilitation Guidelines B-13

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsNational Concrete Masonry AssociationRepresentativeMark B. Hogan, P.E.National Concrete Masonry AssociationHerndon, VirginiaAlternatePhillip J. SamblanetNational Concrete Masonry AssociationHerndon, VirginiaNational Conference of States on Building Codes and StandardsRepresentativeRichard T. Conrad, AIAState Historical Building Safety BoardSacramento, CaliforniaAlternateRobert C. WibleNational Conference of States on Building Codes andStandardsHerndon, VirginiaNational Council of Structural Engineers AssociationsRepresentativeHoward SimpsonSimpson, Gumpertz & HegerArlington, MassachusettsAlternateW. Gene CorleyConstruction Technology LaboratoriesSkokie, IllinoisNational Elevator Industry, Inc.RepresentativeGeorge A. KappenhagenSchindler Elevator CorporationMorristown, New JerseyNational Fire Sprinkler Association, Inc.RepresentativeRussell P. FlemingNational Fire Sprinkler AssociationPatterson, New YorkAlternateKenneth E. IsmanNational Fire Sprinkler AssociationPatterson, New YorkNational Institute of Building SciencesRepresentativeGerald H. JonesKansas City, MissouriNational Ready Mixed Concrete AssociationRepresentativeAnne M. Ellis, P.E.National Ready Mixed Concrete AssociationSilver Spring, MarylandPortland Cement AssociationRepresentativeS. K. GhoshPortland Cement AssociationSkokie, IllinoisAlternateJon I. Mullarky, P.E.National Ready Mixed Concrete AssociationSilver Spring, MarylandAlternateJoseph J. MessersmithPortland Cement AssociationRockville, VirginiaB-14 Seismic Rehabilitation Guidelines FEMA 273

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsPrecast/Prestressed Concrete InstituteRepresentativePhillip J. IversonPrecast/Prestressed Concrete InstituteChicago, IllinoisRack Manufacturers InstituteRepresentativeVictor AzziRack Manufacturers InstituteRye, New HampshireSouthern Building Code Congress InternationalRepresentativeJohn BattlesSouthern Building Code Congress InternationalBirmingham, AlabamaSteel Deck InstituteRepresentativeBernard E. CromiSteel Deck InstituteFox River Grove, IllinoisAlternateDavid A. SheppardD.A. Sheppard Consulting Structural Engineer, Inc.Sonora, CaliforniaAlternateJohn NofsingerRack Manufacturers InstituteCharlotte, North CarolinaAlternateT. Eric Stafford, E.I.T.Southern Building Code Congress InternationalBirmingham, AlabamaAlternateRichard B. HeaglerNicholas J. Bouras Inc.Summit, New JerseyStructural Engineers Association of ArizonaRepresentativeRobert StanleyStructural Engineers Association of ArizonaScottsdale, ArizonaStructural Engineers Association of CaliforniaRepresentativeEugene ColeCole, Yee, Schubert and AssociatesCarmichael, CaliforniaStructural Engineers Association of Central CaliforniaRepresentativeRobert N. ChittendenDivision of State ArchitectGranite Bay, CaliforniaStructural Engineers Association of ColoradoRepresentativeJames R. HarrisJ.R. Harris and CompanyDenver, ColoradoAlternateThomas WosserDegenkolb EngineersSan Francisco, CaliforniaAlternateTom H. HaleImbsen & AssociatesSacramento, CaliforniaAlternateRobert B. HunnesJVA, IncorporatedBoulder, ColoradoFEMA 273 Seismic Rehabilitation Guidelines B-15

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsStructural Engineers Association of IllinoisRepresentativeW. Gene CorleyConstruction Technology LaboratoriesSkokie, IllinoisStructural Engineers Association of Northern CaliforniaRepresentativeRonald F. Middlebrook, S.E.Middlebrook + LouisSan Francisco, CaliforniaStructural Engineers Association of OregonRepresentativeJoseph C. GehlenKramer Gehlen Associates, Inc.Vancouver, WashingtonStructural Engineers Association of Southern CaliforniaRepresentativeSaif HussainSaif Hussain & AssociatesWoodland Hills, CaliforniaStructural Engineers Association of San DiegoRepresentativeAli SadreESGIL CorporationSan Diego, CaliforniaStructural Engineers Association of UtahRepresentativeLawrence D. ReaveleyUniversity of UtahSalt Lake City, UtahStructural Engineers Association of WashingtonRepresentativeJames CarpenterBruce Olsen Consulting EngineerSeattle, WashingtonAlternateEdwin G. ZacherH. J. Brunnier AssociatesSan Francisco, CaliforniaAlternateGrant L. DavisKPFF Consulting EngineersPortland, OregonAlternateSaiful IslamNabih Youssef and AssociatesLos Angeles, CaliforniaAlternateCarl SchulzeSan Diego, CaliforniaAlternateNewland MalmquistStructural Engineers Association of UtahWest Valley City, UtahAlternateBruce C. OlsenBruce Olsen Consulting EngineerSeattle, WashingtonThe Masonry SocietyRepresentativeJohn KariotisKariotis and AssociatesSierra Madre, CaliforniaB-16 Seismic Rehabilitation Guidelines FEMA 273

Appendix B: Seismic Rehabilitation GuidelinesProject ParticipantsWestern States Clay Products AssociationRepresentativeJeff I. Elder, P.E.Western States Clay Products AssociationWest Jordan, UtahWestern States Council of Structural Engineers AssociationRepresentativeGreg SheaMoffatt, Nichol & Bonney, Inc.Portland, OregonWire Reinforcement Institute, Inc.RepresentativeRoy H. Reiterman, P.E.Wire Reinforcement Institute, Inc.Findlay, OhioAlternateWilliam T. RaffertyStructural Design NorthSpokane, WashingtonAlternateRobert C. RichardsonConsultantSun Lakes, ArizonaFEMA Project ParticipantsProject OfficerUgo MorelliFederal Emergency Management AgencyWashington, D.C.Project Technical AdvisorDiana ToddConsulting EngineerSilver Spring, MarylandFEMA 273 Seismic Rehabilitation Guidelines B-17

IndexAacceleration time histories 2-24acceleration-sensitive nonstructural components 11-7acceptance criteriaalternative force deformation curve and 2-46backbone curve for experimental data 2-45definition 2-32description 3-15for acceleration-sensitive and deformation-sensitivecomponents 11-7for alternative construction materials andmethods 2-44for braced frames and steel shear walls 5-26for braced horizontal diaphragms 8-29for cast-in-place concrete diaphragms 6-54for concrete braced frames 6-52for concrete frames with masonry infills 6-36for concrete members 6-44for concrete members controlled by flexure 6-46for concrete members controlled by shearwalls 6-48for concrete moment frames 6-18for concrete slab-column moment frames 6-30for double diagonally sheathed wooddiaphragms 8-27for double straight sheathed wood diaphragms 8-25for fully restrained steel moment frames 5-14, 5-16for members 6-43for nonlinear procedures 5-21for partially restrained steel moment frames 5-20,5-21for precast concrete diaphragms 6-54for precast concrete shear walls and wallsegments 6-50for reinforced concrete beam-column joints 6-25for reinforced concrete beams 6-23for reinforced concrete infilled frames 6-36for single straight sheathed wood diaphragms 8-25for steel braced frames 5-28for steel piles 5-39for structural panel sheathed wooddiaphragms 8-28for two-way slabs and slab-columnconnections 6-29for wood floors 8-26for wood structural panel overlays on existing woodstructural wood panel diaphragms 8-29for wood structural panel overlays on straight ordiagonally sheathed diaphragms 8-28See also deformation acceptance criteria; strengthacceptance criteriaactioncomponentsprimary and secondary 3-4definition 3-16determining with Linear Static Procedure (LSP) 3-7actions and deformationsdetermining with Linear Dynamic Procedures 3-10determining with Nonlinear DynamicProcedure 3-14determining with Nonlinear Static Procedure(NSP) 3-12active control systems 9-24adhered veneer exterior wall elements 11-13adjacent buildings 2-27corrective measures for deficiencies in 10-3hazards from 2-27alternative construction materials and methods 2-43backbone curve for experimental data 2-45data reduction and reporting 2-44design parameters and acceptance criteria for 2-44experimental setup 2-43analysis and design requirementsbuilding separation 2-40continuity 2-39diaphragms 2-39directional effects 2-37general 2-36nonstructural components 2-40overturning 2-37P-∆ effects 2-37structures sharing common elements 2-40torsion 2-37walls 2-40See also analysis procedures; stiffness for analysisAnalysis Proceduresgeneral description and limitations 2-28Linear Dynamic Procedures (LDP) 3-9Linear Static Procedure 3-6Nonlinear Dynamic Procedure (NDP) 3-14Nonlinear Static Procedure 3-10See also analysis and design requirements; stiffnessfor analysisAnalytical Procedurefor rehabilitation 11-9anchorage to masonry walls 7-22anchored veneer exterior wall elements 11-13archaic diaphragms 5-37strength and deformation acceptance criteria 5-38FEMA 273 Seismic Rehabilitation Guidelines Index-1

architectural components 11-13canopies and marquees 11-19ceilings 11-17chimneys and stacks 11-19exterior wall elements 11-13interior veneers 11-17Nonstructural Performance Levels and damageto 2-15parapets and appendages 11-18partitions 11-16stairs and stair enclosures 11-19architectural, mechanical, and electrical components andsystems 11-1acceleration-sensitive components 11-7architectural components 11-13definitions for 11-28deformation-sensitive components 11-8furnishings and interior equipment 11-25mechanical, electrical, and plumbingcomponents 11-20nonstructural componentsassessment of 11-5procedures for rehabilitation 11-9references for 11-29rehabilitation concepts 11-13Rehabilitation Objectives 11-5structural-nonstructural interaction 11-7as-built information 2-25for building configurations 2-25for existing components 2-25of adjacent buildings 2-27site characterization 2-27Bbackbone curve 2-45base isolation. See seismic isolationbase shear. See pseudo lateral loadBasic Safety Objective (BSO)definition of 2-5beam-column jointsmodeling parameters and acceptance criteria forin reinforced concrete 6-21beamsmodeling parameters and acceptance criteria forin reinforced concrete 6-19bookcases 11-26braced frames 6-51braced horizontal steel diaphragms 5-36braced horizontal wood diaphragms 8-23, 8-24, 8-29braced masonry walls 7-8BSO (Basic Safety Objective)definition of 2-5Building Performance Levels 1-1, 2-10Collapse Prevention Level 2-10Damage Control 2-11Immediate Occupancy Level 2-10Life Safety Level 2-10Operational Level 2-10quantitative specifications of building behavior 1-3recommendations for combining Structural andNonstructural Performance Levels 2-17See also performance levelsbuilding pounding 2-27building separation 2-40for seismic isolation systems 9-10buildingsclassifying 3-3codes and standards for 1-1evaluating characteristics of 1-8historical useof concrete in 6-1of masonry in 7-1of steel and cast iron in 5-1of wood and light metal framing in 8-1identifying as-built information 2-25Model Building Types for SimplifiedRehabilitation 10-20simplified corrective measures for deficienciesin 10-10Ccanopies and marquees 11-19cast iron 5-1, 5-5cast-in-place concrete connections 6-16cast-in-place concrete diaphragms 6-53cast-in-place pile foundations 6-55ceilings 11-17chimneys and stacks 11-19chord and collector elementsstrength and deformation acceptance criteria 5-38chord rotationdefinition of 5-11, 6-42in concrete shear walls 6-42chordsand steel diaphragms 5-38and wood diaphragms 8-22clip angle connections 5-22Collapse Prevention Levelas Building Performance Level 2-10Collapse Prevention Performance Level 1-1as Structural Performance Level 2-8site foundation conditions 4-1column base plates 5-15columnsacceptance criteriafor reinforced concrete beams 6-24Index-2 Seismic Rehabilitation Guidelines FEMA 273

acceptance criteria forin fully restrained steel moment frames 5-13,5-14, 5-16, 5-26modeling parameters and acceptance criteria forin reinforced concrete 6-20required strength adjacent to infill panels 7-19stiffness ofin concentric braced steel frames 5-25in partially restrained steel momentframes 5-18compressive strengthof masonry in-place materials and components 7-2of masonry piers and walls 7-9, 7-13of steel columns and braces 5-13of structural concrete 6-4of wood 8-5computer access floors 11-26concrete 6-1braced frames 6-51cast-in-place diaphragms 6-53connections 6-11, 6-16default material properties for 6-7design strength and deformabilities of 6-13diaphragmscast-in-place 6-53precast 6-54flanged construction for 6-13foundations 6-55frames with masonry shear wallsModel Building Typestypical deficiencies 10-26general analysis and design assumptions 6-11historical use of 6-1jacketing 6-22joint strength 6-22knowledge (κ) factor for 6-10material properties and condition assessmentfor 6-2Model Building Typesdescription of 10-21typical deficiencies 10-25model buildingssimplified corrective measures 10-3moment frames 6-16beam-column moment frames 6-16post-tensioned beam-column momentframes 6-26moment frames with infills 6-33concrete infills 6-37masonry infills 6-34precast concrete diaphragms 6-54precast concrete frames 6-31precast concrete shear walls 6-48precast framesModel Building Typestypical deficiencies 10-28precast/tilt-up wallsmodel buildingsdescription 10-22properties of in-place materials 6-2shear and torsion for 6-14shear wallscast-in-place 6-39corrective measures for deficiencies in 10-5Model Building Typestypical deficiencies 10-26precast 6-48strength development for reinforcing 6-15strength of 6-4testing for 6-5, 6-9condition assessmentfor concrete 6-8quantifying results for 6-9scope and procedures for 6-8for masonry 7-4nondestructive and supplemental tests 7-5visual examination of 7-4for steel 5-4, 5-8quantifying results for 5-8scope and procedures for 5-8for wood and light metal framing 8-6quantifying results for 8-7scope and procedures for 8-6continuity 2-39control nodes 3-11conveyors 11-28DDamage Control Performance Rangeas Structural Performance Range 2-8egress and 11-6dampingcalculating effectivefor energy dissipation devices 9-23coefficients for modifying design responsespectra 2-23effectivefor isolation systems 9-13energy dissipation devices and 9-14DCRs (demand-capacity ratios) 2-29default material propertiesfor concrete 6-7for masonry 7-2for steel 5-5for wood and light metal framing 8-5FEMA 273 Seismic Rehabilitation Guidelines Index-3

deficienciesfor Simplified Rehabilitation Method 10-3, 10-23deformationdetermining with Linear Static Procedure (LSP) 3-7deformation acceptance criteriadefinition of 2-32description 3-15deformation-controlled actionsdefinition 2-32linear procedures 3-15mathematical modeling of 3-3nonlinear procedures 3-16deformation-sensitive nonstructural components 11-8demand-capacity ratios (DCRs) 2-29design and construction reviewfor seismic isolation systems 9-10design reviewfor passive energy dissipation 9-21foundation loads 4-2diagonal lumber sheathing shear walls 8-9, 8-15diagonal sheathing with straight sheathing or flooringabove diaphragms 8-26diaphragmsanalysis and design requirements 2-39chords 2-39collectors 2-39concrete 6-53effective stiffness 3-12floor 3-14force-deflection curve coordinates for nonlinearanalysis of 8-16Linear Dynamic Procedure (LDP) 3-8, 3-10Nonlinear Dynamic Procedure (NDP) 3-15precast concrete 6-54simplified corrective measures for deficienciesin 10-8steel 5-32ties 2-39wood 8-22differential compaction 4-4, 4-6directional effects 2-37drift ratios and relative displacements 11-10drilled shafts 4-8, 4-16ductwork 11-23dynamic vibration absorption control systems 9-24Eearthquake ground shaking hazard 2-19eccentric braced frames (EBF) 5-29rehabilitation measures for 5-31stiffness for analysis 5-29strength and deformation acceptance criteria 5-28,5-30economic factors of rehabilitation 1-12, 2-38egress 11-6eigenvalue (dynamic) analysis 3-6elastic modulus 5-41concrete components 6-12masonry walls 7-2wood diaphragms 8-22wood walls 8-8elastomeric isolators 9-3electrical componentselectrical and communications distributioncomponents 11-24electrical and communications equipment 11-24Nonstructural Performance Levels and damageto 2-16elementsprimary and secondary 3-3elevators 11-27energy dissipation devicesdetermining force-displacement characteristicsof 9-23displacement-dependent devicesLinear Dynamic Procedure for 9-18Linear Static Procedure for 9-17modeling 9-15general information about 9-21linear procedures 9-17modeling of 9-15Nonlinear Static Procedure for 9-19other types 9-16prototype tests for 9-22system adequacy and 9-24energy dissipation systems. See passive energy dissipationsystemsEnhanced Rehabilitation Objectives 2-6equivalent base shear. See pseudo lateral loadexpected strengthcriteria for use of 2-34definition 3-16exterior wall elementsadhered veneer 11-13anchored veneer 11-13glass block units 11-14glazing systems 11-15prefabricated panels 11-14Ffault ruptureas seismic hazard 2-24, 4-2mitigation of effect of 4-5fiberboard shear wallsfiberboard sheathing shear walls 8-10, 8-22stucco on 8-10, 8-19fire suppression piping 11-22fixed base assumptions 4-16Index-4 Seismic Rehabilitation Guidelines FEMA 273

flange plate connectionsfor fully restrained steel moment frames 5-15for partially restrained moment frames 5-23illustrated 5-24flanged walls 7-13flexible base assumptions 4-16floodingas seismic hazard 2-24, 4-5mitigation of 4-6fluid piping 11-22fluid viscoelastic damping devices 9-16fluid viscous damping devices 9-16footingsmasonry 7-22rigid 4-15shallow bearings 4-14spread 4-18wood 8-30force-deformation curve, alternative 2-46force-controlled actionsacceptance criteria for linear analysis 3-16acceptance criteria for linear procedures 3-15acceptance criteria for nonlinear procedures 3-16mathematical modeling of 3-2foundation acceptability criteria 4-16foundation loads 4-2design 4-2foundation shape correction factors 4-12foundation soil information 4-1, 4-17foundation strength and stiffness 4-6foundation acceptability criteria 4-16foundation ultimate bearing pressures 4-7load-deformation characteristics forfoundations 4-8foundation ultimate bearing pressure 4-7foundationsconcrete 6-55deep 6-55definitions for 4-19foundation soil information 4-1load-deformation characteristics for 4-8masonry 7-22mathematical modeling 3-3mitigating liquefaction hazards 4-5references for 4-21retaining walls 4-17shallow 6-55, 6-56shallow bearing 4-8, 4-15simplified corrective measures for deficienciesin 10-10soil foundation rehabilitation 4-18steel pile 5-39stiffness of 4-6symbols for 4-19ultimate bearing pressures for 4-7wood 8-29See also geotechnical site hazards; pile foundationsframes with infills 6-33concrete infills 6-37masonry infills 6-34full penetration welded connections 5-15fully restrained steel moment frames 5-9acceptance criteriafor linear procedures 5-14for nonlinear procedures 5-16full penetration welded connections for 5-15rehabilitation measures for 5-17stiffness for analysis 5-9strength and deformation acceptance criteria 5-12furnishings and interior equipment 11-25bookcases 11-26computer access floors 11-26computer and communication racks 11-27conveyors 11-28elevators 11-27hazardous materials storage 11-27storage racks 11-25Ggeneral component behavior curvesillustrated 2-32general requirements. See rehabilitation requirementsgeotechnical site hazardscorrective measures for foundations 10-10definitions for 4-19guidelines for 4-1mitigation of site-seismic hazards 4-5references for 4-21retaining walls 4-17site characterization 2-27, 4-1symbols for 4-19See also seismic hazards; seismic site hazardsglass block units 11-14glazing systems 11-15global structural stiffening and strengthening 2-35, 2-36gravity loads and load combinations 3-5ground motion characterization 3-14ground shaking hazards 2-18ground water conditionsdifferential compaction 4-4liquefaction and 4-2grout injections 7-7gypsum plaster shear wallsgypsum plaster on gypsum lath 8-10, 8-20gypsum plaster on wood lath 8-10, 8-20gypsum sheathing 8-10, 8-21FEMA 273 Seismic Rehabilitation Guidelines Index-5

Hhazardous materials storage 11-27hazards. See geotechnical site hazards; seismic hazards;seismic site hazardsHazards Reduced Nonstructural Performance Level 2-9high-pressure piping 11-22historic buildingscharacteristics of as-built conditions 1-10effects of rehabilitation on 1-13general considerations for 1-14historic preservation 1-13historical perspectivenonstructural components 11-5historical useof concrete 6-1of masonry 7-1of steel and cast iron 5-1of wood and light metal framing 8-1horizontal lumber sheathing with cut-in braces or diagonalblocking shear walls 8-10, 8-21IImmediate Occupancy Levelas Building Performance Level 2-10Immediate Occupancy Performance Level 1-1as Nonstructural Performance Level 2-9Immediate Occupancy Performance Levelsas Structural Performance Level 2-8impact echo 7-5infill masonry shear wallmodel buildingsdescription 10-21infill panelsm factors for masonryinfill panels 7-20simplified force deflection relations masonryinfill panels simplified force deflectionrelations 7-20infill shear strength 7-18infilled openings 7-6infills. See concrete; masonryin-place materials and components 7-3, 7-4concretecomponent properties of 6-4default properties of 6-7material properties of 6-2minimum number of tests for 6-5test methods to quantify 6-5masonry 7-2compressive strength 7-2flexural tensile strength 7-3location and minimum number of tests 7-4masonry elastic modulus in compression 7-2steelcomponent properties of 5-2default properties of 5-4, 5-5material properties of 5-2minimum number of tests for 5-3test methods to quantify 5-2wood and light metal framing 8-3component properties 8-3default properties 8-5material properties 8-3minimum number of tests 8-4test methods to quantify properties 8-4in-plane discontinuitiesillustrated 2-30in-plane masonry infillsdeformation acceptance criteria 7-19stiffness 7-18strength acceptance criteria 7-18inspectionfor seismic isolation systems 9-10inspectionsby regulatory agency 2-42for construction quality assurancerequirements 2-42for passive energy dissipation devices 9-21of concrete 6-8of masonry 7-4interior veneers 11-17introduction to 9-1inundation. See floodingirregularities and discontinuities 2-30, 2-35isolation systems. See seismic isolationisolatorselastomeric 9-3modeling of 9-3sliding 9-3Kknee-braced frames 8-12knowledge (κ) factorfor concrete 6-10for masonry 7-5for steel 5-8for wood and light metal framing 8-8Llandslidesas seismic hazard 2-24, 4-4mitigation of 4-6lateral patternsload 3-11Life Safety Levelas Building Performance Level 2-10Index-6 Seismic Rehabilitation Guidelines FEMA 273

Life Safety Performance Level 1-1as Nonstructural Performance Level 2-9as Structural Performance Level 2-8site foundation conditions 4-1light fixtures 11-25light gage metal frame shear walls 8-12, 8-22limitationsNonlinear Dynamic Procedure (NDP) 2-31Nonlinear Static Procedure (NSP) 2-31of Simplified Rehabilitation Method 10-18Limited Rehabilitation Objectives 2-6Limited Safety Performance Range 2-8linear analysis procedures. See Linear DynamicProcedure; Linear Static ProcedureLinear Dynamic Procedure (LDP)basis of 3-9description of 3-9determination of actions and deformations 3-10diaphragms 3-4, 3-10modeling and analysis considerations for 3-9torsion 3-2linear proceduresgeneral description and applicability 2-29m factor 3-16Linear Static Procedure (LSP)basis of 3-6description of 3-6diaphragms 3-4, 3-8horizontal distribution of seismic forces 3-8modeling and analysis considerations for 3-6torsion 3-2vertical distribution of seismic forces 3-8liquefactionas seismic hazard 4-2mitigation of 4-5susceptibility to 4-3load capacity for pile foundations 4-16load path discontinuitiescorrective measures for deficiencies in 10-3loadsdetermining load combinations 3-5local risk mitigation programsactive or mandated programs 1-15choosing active programs 1-16historic buildings 1-13initial considerations for 1-12passive seismic rehabilitation standards 1-15potential costs of 1-13selecting of buildings for 1-15timetables and effectiveness for 1-13triggers for seismic rehabilitation 1-15lower bound strengthcriteria for use of 2-34definition 3-16LSP. See Linear Static ProcedureMm factordefinition 3-16manufacturing quality controlfor energy dissipation devices 9-21mapped response spectrum acceleration parameters. Seeresponse spectrum acceleration parametersmasonrycondition assessment 7-4elastic modulus in compression 7-2engineering properties of masonry infills 7-14engineering properties of masonry walls 7-5flexural tensile strength 7-3foundation elements 7-22historical use of 7-1infillsengineering properties of 7-14enhanced infill panels 7-17existing 7-17in concrete frames 6-34, 6-51in-plane 7-18new 7-17out-of-planestiffness for 7-21types of 7-17knowledge (κ) factor for 7-5material properties and condition assessment 7-2model buildingscorrective measures 10-3properties of in-place materials 7-2shear modulus 7-4shear strength 7-3shear wallscorrective measures for deficiencies in 10-6simplified corrective measures for deficienciesin 10-12strength and modulus of reinforcing steel 7-4testing 7-4wallsanchorage to 7-22enhanced 7-6existing 7-6new 7-6RM in-plane walls and piers 7-11RM out-of-plane walls 7-14URM in-plane walls and piers 7-8URM out-of-plane walls 7-10FEMA 273 Seismic Rehabilitation Guidelines Index-7

masonry, reinforcedModel Building Typestypical deficiencies 10-29masonry, unreinforcedModel Building Typestypical deficiencies 10-29mass reduction 2-36material properties and condition assessmentfor concrete 6-2connections 6-11knowledge (κ) factor 6-10properties of in-place materials andcomponents 6-2rehabilitation issues 6-10for masonry 7-2, 7-4knowledge factor for 7-5properties of in-place materials 7-2for steel 5-1condition assessment 5-4knowledge (κ) factor 5-8properties of in-place materials andcomponents 5-2for wood and light metal framing 8-2condition assessment for 8-6knowledge (κ) factor for 8-8rehabilitation issues for 8-8materials properties and condition assessmentfor concrete 6-8mathematical modeling. See modelingmechanical pulse velocity 7-5mechanical systemsmechanical equipment 11-20Nonstructural Performance Levels and damageto 2-16mechanical, electrical, and plumbing components 11-20metal deck diaphragmsbare metal deck diaphragms 5-32with nonstructural concrete topping 5-35stiffness for analysis 5-35strength and deformation acceptancecriteria 5-36with structural concrete toppingstiffness for analysis 5-34strength and deformation acceptancecriteria 5-34mitigationguidelines for initial risk 1-10of differential compaction 4-6of faulting 4-5of flooding 4-6of landslides 4-6of liquefaction 4-5See also local risk mitigation programsModel Building Typesdescription 10-20typical deficiencies 10-23modelingfor Linear Dynamic Procedure (LDP) 3-9for Linear Static Procedure (LSP) 3-6for Nonlinear Dynamic Procedure (NDP) 3-14for Nonlinear Static Procedure (NSP) 3-11of energy dissipation devices 9-15displacement-dependent devices 9-15other types of devices 9-16velocity-dependent devices 9-15of isolation system and superstructure 9-3of soil-structure interaction 3-4, 4-8Systematic Rehabilitation Method 1-11moment framescorrective measures for deficiencies in 10-4slab-column moment frames 6-27types of 6-16NNDP. See Nonlinear Dynamic Procedurenondestructive examination (NDE) methodsfor concrete 6-9nonlinear analysis procedures. See Nonlinear DynamicProcedure; Nonlinear Static ProcedureNonlinear Dynamic Procedure (NDP)basis of 3-14diaphragms 3-4, 3-15general description and applicability 2-31limitations on 2-31modeling and analysis considerations for 3-14torsion 3-3, 3-14Nonlinear Static Procedure (NSP)basis of 3-10control nodes and 3-11description of 3-10determining of actions and deformations for 3-12diaphragms 3-4general description and applicability 2-31lateral load patterns 3-11limitations on 2-31modeling and analysis considerations for 3-11period determination 3-11target displacement 3-12torsion 3-3nonstructural componentsanalysis and design requirements 2-40assessment of 11-5coefficients for 11-11for seismic isolation systems 9-8historical perspective on 11-5Performance Levels for 11-5regional seismicity 11-6Index-8 Seismic Rehabilitation Guidelines FEMA 273

Rehabilitation Objectives 11-5rehabilitation procedures for 11-9Nonstructural Performance Levels 1-2, 1-11, 2-8, 11-5damage to architectural components 2-15damage to building contents 2-17damage to mechanical, electrical, and plumbingsystems 2-16NSP. See Nonlinear Static ProcedureOoperating temperaturefor passive energy dissipation devices 9-21Operational Levelas Building Performance Level 2-10Operational Performance Level 1-1as Nonstructural Performance Level 2-9out-of-plane wall forces 2-40overdriven nailscorrective measures for deficiencies in 10-11overturning factors 2-37for seismic isolation systems 9-10overturning issuesalternative methods 2-37overturning moment 4-15Ppanel zones 5-18parapets and appendages 11-18Partial Rehabilitation 2-6partially restrained moment framesacceptance criteria for 5-21partially restrained steel moment frames 5-18linear procedures 5-20rehabilitation measures for 5-24stiffness for analysis 5-18strength and deformation acceptance criteria 5-19particleboard sheathing shear walls 8-22partitions 11-16passive energy dissipation systems 9-14criteria selection for 9-15design and construction review 9-21detailed system requirements for 9-21general requirements for 9-14linear procedures 9-17modeling of energy dissipation devices 9-15nonlinear procedures 9-19required tests of energy dissipation devices 9-22passive pressure 4-13passive programs for mitigationselecting standards for 1-15triggers for 1-15P-∆ effectsanalysis and design requirements for 2-37Systematic Rehabilitation Method and 3-4Performance Levels 2-7for nonstructural components 11-5Nonstructural Performance Levels 2-8Structural Performance Levels and Ranges 2-7See also Building Performance Levelsperformance objectives for seismic isolation 9-2Performance RangesDamage Control 2-8, 11-6for nonstructural components 11-6Limited Safety 2-8period determination 3-11piersmasonrycompressive strength of 7-9, 7-13expected flexural strength of wallsmasonryexpected flexural strength of 7-12lateral strength of 7-8lower bound shear strength of 7-12piers and piles 4-18pile capslateral load pathcorrective measures for deficiencies in 10-4pile foundations 4-8concrete 6-55soil load-deformation characteristics for 4-15steel 5-39stiffness parameters for 4-15vertical load capacity for 4-16wood 8-29pipingfire suppression 11-22fluid 11-22high-pressure 11-22plan irregularities 2-35corrective measures for deficiencies in 10-3plansfor quality assurance 2-41verifying for Systematic RehabilitationMethod 1-11plaster on metal lath shear walls 8-10, 8-21plastic hinge rotationin concrete shear walls 6-43plate steel shear walls 5-31plumbing systems and componentsductwork 11-23fire suppression piping 11-22fluid piping 11-22high-pressure piping 11-22Nonstructural Performance Levels and damageto 2-16storage vessels and water heaters 11-21pole structures 8-30FEMA 273 Seismic Rehabilitation Guidelines Index-9

political considerations of rehabilitation 1-12, 2-38post-installed concrete connections 6-16post-tensioned concrete beam-column momentframes 6-16, 6-26post-tensioned reinforcementconcrete 6-26post-tensioning anchorscorrective measures for deficiencies in 10-11precast/tilt-up concrete wallsModel Building Typestypical deficiencies 10-27model buildingsdescription 10-22prefabricated panels 11-14Prescriptive Procedurefor rehabilitation 11-9prestressing steelslaboratory testing and 6-7testing for 6-7pseudo lateral load 3-7Qquality assurance 2-41construction requirements for 2-42plans for 2-41quality controlfor seismic isolation systems 9-10quantifying test resultsfor concrete 6-5for masonry 7-4for steel 5-2for wood and light metal framing 8-4Rrackscomputer and communication 11-27storage 11-25radiography 7-5Reduced Rehabilitation 2-6regional seismicityand nonstructural components 11-6Rehabilitation 6-33rehabilitation measuresfor concentric braced steel frames 5-29for eccentric braced frames 5-31for masonry foundation elements 7-23for steel pile foundations 5-39rehabilitation methods. See Simplified RehabilitationMethod; Systematic Rehabilitation MethodRehabilitation Objective 1-2Rehabilitation Objectives 2-4rehabilitation process flowchart 1-9rehabilitation requirementsrehabilitation procedures 11-9rehabilitation process flowchart 1-9seismic hazard 2-18, 2-19symbols used 2-48See also analysis and design requirements;rehabilitation strategies; systemrequirementsrehabilitation strategies 2-35and seismic isolation and energy dissipationsystems 9-1for cast-in-place concrete diaphragms 6-54for concrete 6-10for concrete beam-column moment frames 6-22for concrete braced frames 6-53for concrete foundations 6-56for concrete infills in concrete frames 6-38for concrete slab-column moment frames 6-31for deep foundations 6-57for emulated beam-column moment frames 6-32for existing irregularities and discontinuities 2-35for fully restrained steel moment frames 5-17for masonry infills in concrete frames 6-37for partially restrained steel moment frames 5-24for post-tensioned concrete beam-column momentframes 6-27for precast concrete beam-column momentframes 6-32for precast concrete diaphragms 6-55for reinforced concrete shear walls and wallelements 6-45for shallow foundations 6-56for steel plate shear walls 5-32global structural stiffening and strengthening 2-35,2-36local modification of components in 2-35mass reduction 2-36new technologies in 1-4precast concrete beam-column momentframes 6-33precast concrete frames unable to resist lateralloads 6-33seismic isolation and energy dissipationsystems 2-36social, economic, and political considerationsof 1-12, 2-38reinforced concrete braced frames 6-51reinforced concrete columns supporting shearwalls 6-40reinforced concrete coupling beams 6-40reinforced concrete shear walls and wall elements 6-40design strengths 6-42general modeling considerations 6-40rehabilitation measures 6-45Index-10 Seismic Rehabilitation Guidelines FEMA 273

stiffness for analysis 6-41reinforced masonry bearing wallsModel Building Typestypical deficiencies 10-28Model Building Types for 10-22reinforcement 6-15repointing 7-7reporting and compliance proceduresfor construction quality assurancerequirements 2-42regulatory agency permitting and inspections 2-42response spectrum acceleration parametersadjusting for variations of viscous damping 2-23adjusting mapped 2-20general response spectrum for 2-23mapped BSE-1 2-19mapped BSE-2 2-19values as a function of site class and mapped shortperiod spectral response acceleration 2-21rigid footingsconcentration of stress at edge of 4-15elastic solutions for spring constants 4-11riveted clip angle steel connections 5-21RM in-plane walls and piers 7-11deformation acceptance criteria for 7-13m factors for 7-15stiffness 7-11strength acceptance criteria for 7-12RM out-of-plane walls 7-14rod-braced frames 8-12Ssafety regulations 1-12seismic hazards 1-5, 2-18, 2-19determining 2-19differential compaction 4-4, 4-6faulting 2-24, 4-2flooding 2-24, 4-5general response spectrum 2-22liquefaction as 4-2mapped response spectrum accelerationparameters 2-19other than ground shaking 2-24response spectrum accelerationfor variations of viscous damping 2-23seismicity zones 2-24site-specific response spectra 2-23See also geotechnical site hazards; seismic sitehazardsseismic isolation 9-1adequacy of system 9-12and superstructure modeling 9-3as rehabilitation strategy 9-1background for 9-2definitions for 9-25design 9-4design and construction review 9-9design properties of 9-13detailed system requirements for 9-9determination of force-deflection characteristicsfor 9-12general criteria for design 9-4isolation system testing and design properties 9-11linear procedures 9-6nonlinear procedures 9-7performance objectives for 9-2prototype tests for 9-11rehabilitation strategies 2-36seismic isolation systems 9-2seismic isolators 9-3system design and construction review 9-10seismic isolation systemsmechanical properties and modeling of 9-2seismic site hazardsacceleration time histories 2-24differential compaction 4-4fault ruptures 4-2flooding 2-24, 4-5ground shaking for Basic Safety Objective 2-18landsliding 2-24, 4-4liquefaction 4-2mapped response spectra and 2-19mitigation of 4-5response spectra and 2-19site-specific response spectra 2-23seismicity zones 2-24shallow bearing footings 4-14shallow bearing foundations 4-8capacity parameters for 4-15shallow foundations 6-55, 6-56shared structural elementsanalysis and design requirements for 2-40collecting data for 2-27shear wallsconcretecorrective measures for deficiencies in 10-5shear wave velocity 4-9shotcrete applications 7-7Simplified Rehabilitation Method 2-28amendments to FEMA 178 10-12applying guidelines for 1-3, 1-10, 10-1comparison of Guidelines and FEMA 178requirements 10-17comparisons of standards for shear walls 10-17corrective measures for deficiencies in 10-3cross reference between Guidelines and FEMA 178deficiency numbers 10-30deficiencies for 10-3FEMA 273 Seismic Rehabilitation Guidelines Index-11

description of Model Building Types 10-20limitations of use of 10-18procedural steps of 10-2scope of 10-1typical deficiencies in 10-23single straight sheathed diaphragms 8-22, 8-24site classesdefined 2-21site soil characterization 2-27, 4-1site soil foundation conditions 4-1site-specific ground shaking hazards. See site-seismichazardsslab-column concrete moment frames 6-27slab-column connections 6-29sliding isolators 9-3social considerations of rehabilitation 1-12, 2-38soilembedment correction factors 4-12foundation acceptability summary 4-17foundation rehabilitation 4-18foundation soil information 4-1load-deformation behavior for 4-8material improvements 4-18mitigating liquefaction hazards 4-5presumptive ultimate foundation pressures 4-7spring constants 4-11susceptibility to liquefaction 4-3soil-structure interaction (SSI) 3-4solid viscoelastic devices 9-15spread footings and mats 4-18SSI (soil-structure interaction) 3-4stairs and stair enclosures 11-19static lateral forcesNonlinear Static Procedure (NSP) 3-12steelbraced frames 5-25concentric braced frames 5-25corrective measures for deficiencies in 10-7eccentric braced frames 5-29linear procedures 5-26Model Building Typestypical deficiencies 10-24Model Building Types for 10-20, 10-21condition assessment 5-4connectionscolumn base plates 5-15composite partially restrained 5-23end plate 5-15, 5-23flange plate connections 5-15, 5-23full penetration welded 5-15of fully restrained moment frames 5-9riveted clip angle 5-21riveted or bolted T-stub 5-22stiffness of partially restrained steel momentframes 5-18default material properties for 5-4, 5-5diaphragmsarchaic 5-37bare metal deck 5-32chord and collector elements 5-38metal deck with structural concretetopping 5-34steel truss 5-36frames with concrete shear wallsModel Building Typestypical deficiencies 10-24frames with infill masonry shear wallsModel Building Typestypical deficiencies 10-25historical use of 5-1knowledge (κ) factor 5-8material properties and condition assessment 5-1Model Building Typesdescriptions of 10-20typical deficiencies 10-22model buildingssimplified corrective measures 10-3moment frames 5-9fully restrained 5-9joint modeling 5-10Model Building Typestypical deficiencies 10-23partially restrained 5-18with infills 5-32pile foundations 5-39properties of in-place materials 5-2tensile and yield strengths 5-6testing of 5-2steel connectionsof fully restrained moment frames 5-15steel diaphragms 5-32steel plate shear walls 5-31steel truss diaphragmsstrength and deformation acceptance criteria 5-37stiffnessanalysis and design assumptions for concrete 6-11and RM in-plane walls and piers 7-11calculating effective 3-12diaphragms 3-4for in-plane masonry infills 7-18for out-of-plane masonry infills 7-21for seismic isolation systems 9-13for URM in-plane walls and piers 7-8lateral foundation-to-soil 4-13of foundations 4-6parameters for pile foundations 4-15RM out-of-plane walls 7-14Index-12 Seismic Rehabilitation Guidelines FEMA 273

steel pile foundations and 5-39vertical modeling for shallow bearing footings 4-14See also foundation strength and stiffness; stiffnessfor analysisstiffness for analysisarchaic diaphragms 5-38bare metal deck diaphragms 5-33chord and collector elements 5-38concrete beam-column moment frames 6-17concrete braced frames 6-52concrete infills in concrete frames 6-38concrete slab-column moment frames 6-28diagonal sheathing with straight sheathing or flooringabove wood diaphragms 8-26double diagonally sheathed wood diaphragms 8-26double straight sheathed diaphragms 8-25eccentric braced frames 5-29for braced horizontal diaphragms 8-29for cast-in-place concrete diaphragms 6-53for concentric braced frames 5-25for diagonal lumber sheathing shear walls 8-15for fiberboard or particle board shear walls 8-22for fully restrained steel moment frames 5-9for gypsum plaster shear walls 8-19, 8-21for gypsum sheathing 8-20for horizontal lumber sheathing with cut-in braces ordiagonal blocking shear walls 8-21for partially restrained steel moment frames 5-18for plaster on metal lath shear walls 8-21for reinforced concrete beam-column momentframes 6-17for steel plate shear walls 5-31for structural panel or plywood panel sheathing shearwalls 8-19for stucco on studs, sheathing, or fiberboard shearwalls 8-19for wood structural panel overlays on straight ordiagonally sheathed diaphragms 8-28masonry infills in concrete frames 6-34metal decks with nonstructural concretetopping 5-35metal decks with structural concrete topping 5-34of single layer horizontal lumber sheathing or sidingshear walls 8-12post-tensioned concrete beam-column momentframes 6-27precast concrete beam-column momentframes 6-32precast concrete frames unable to resist lateralloads 6-33precast concrete shear walls 6-49reinforced concrete shear walls and wallelements 6-41single diagonally sheathed diaphragms 8-26single straight sheathed diaphragms 8-24steel pile foundations 5-39steel truss diaphragms 5-37wood structural panel overlays on existing woodstructural panel diaphragms 8-28wood structural panel sheathed diaphragms 8-27See also analysis and design requirements; analysisproceduresstorage racks 11-25storage vessels and water heaters 11-21story driftconcrete shear wall 6-42strength acceptance criteriadefinitions 2-32description 3-15descriptions 2-32Structural Performance Levels 1-2and Ranges 2-7comparing damage for horizontal elements 2-14comparing damage for vertical elements 2-11Structural Performance Ranges 1-2structural-nonstructural interaction 11-7supplemental damping devices. See passive energydissipation systemssystem requirementsfor passive energy dissipation systems 9-21, 9-24for seismic isolation systems 9-9See also rehabilitation requirementsSystematic Rehabilitation Method 2-28applying guidelines for 1-11classifying buildings by configuration 3-3definitions for 3-16diaphragms and 3-4gravity loads and load combinations 3-5mathematical modeling for 3-2multidirectional excitation effects 3-5P-∆ effects and 3-4preliminary design for 1-11soil-structure interaction 3-4Structural Performance Levels 1-11Ttarget displacementdescription of 3-10horizontal torsion 3-2Nonlinear Static Procedure (NSP) and 3-12tenants 1-12tensile propertiesof concrete reinforcement bars 6-2tensile strengthsof steel in-place materials and components 5-3testingfor concrete materials and components 6-5FEMA 273 Seismic Rehabilitation Guidelines Index-13

for construction quality assurancerequirements 2-42for masonry materials and components 7-4for seismic isolation devices 9-11for steel materials and components 5-2for wood and light metal framing 8-4nondestructivefor concrete 6-9for masonry 7-5prototypes for energy dissipation devices 9-22required for energy dissipation devices 9-22Time-History Analysis 3-14torsionaccidental 3-2actual 3-2analysis and design requirements 2-37in Nonlinear Dynamic Procedure (NDP) 3-14mathematical modeling of 3-2triggers for local risk mitigation programs 1-15Uultrasonic pulse velocity 7-5URM bearing walls 10-22, 10-29URM in-plane piersmasonryforce-deflection relation for 7-11URM in-plane wallsmasonryforce-deflection relation for 7-11URM in-plane walls and piers 7-8deformation acceptance criteria 7-9m factors for 7-10stiffness 7-8strength acceptance criteria 7-8URM out-of-plane walls 7-10Vvelocity-dependent damping devices 9-15veneeradhered 11-13anchored 11-13vertical irregularitiescorrective measures for deficiencies in 10-3See also irregularities and discontinuitiesvertical load stabilityfor seismic isolation systems 9-10viscoelastic damping devices 9-16viscous damping devices 2-23, 9-16visual inspections. See inspectionsWwallsanalysis and design requirements for 2-40calculating out-of-plane wall forces 2-40masonryanchorage to 7-22compressive strength of 7-9, 7-13lateral strength of 7-8lower bound shear strength of 7-12retaining 4-17RM in-plane walls and piers 7-11RM out-of-plane 7-14URM bearing 10-22Model Building Typestypical deficiencies 10-29URM in-plane walls and piers 7-8URM out-of-plane walls 7-10wood and light metal 6-16wood and light metal framingcondition assessment 8-6connectionscorrective measures for deficiencies in 10-9for braced horizontal diaphragms 8-29for gypsum plaster shear walls 8-20for single layer horizontal lumber sheathing orsiding shear walls 8-15for single straight sheathed diaphragms 8-25for stucco on studs, sheathing, or fiberboardshear walls 8-20for wood and light metal framing 8-4force-deflection curve coordinates for nonlinearanalysis of 8-16numerical acceptance factors for linearprocedures 8-13rehabilitation of wood and light metal shearwalls 8-11default material properties 8-5diaphragmsbraced horizontal diaphragms 8-23, 8-24, 8-29diagonal sheathing with straight sheathing orflooring above 8-26double diagonally sheathed 8-23, 8-26double straight sheathed diaphragms 8-22,8-25effects of chords and openings in 8-29enhanced for rehabilitation 8-23new 8-24single diagonally sheathed diaphragms 8-26single straight sheathed diaphragms 8-22, 8-24structural panel overlays on existing woodstructural diaphragms 8-24structural panel overlays on existing woodstructural panel diaphragms 8-28Index-14 Seismic Rehabilitation Guidelines FEMA 273

structural panel overlays on straight ordiagonally sheathed diaphragms 8-23,8-28structural panel sheathed diaphragms 8-27types of 8-22evolution of 8-1footings 8-30force-deflection curve coordinates for nonlinearprocedures 8-16foundations 8-29general information about 8-2historical use of 8-1material properties and condition assessment 8-2Model Building Typesdescriptions of 10-20typical deficiencies 10-23model buildingssimplified corrective measures 10-3piling 8-29properties of in-place materials 8-3scope of 8-1shear walls 8-8corrective measures for deficiencies in 10-6diagonal lumber sheathing 8-9, 8-15fiberboard or particleboard sheathing 8-10,8-22gypsum plaster on gypsum lath 8-10, 8-20gypsum plaster on wood lath 8-10, 8-20gypsum sheathing shear walls 8-10, 8-21horizontal lumber sheathing with cut-in braces ordiagonal blocking 8-10, 8-21knee-braced and miscellaneous timberframes 8-12light gage metal frame shear walls 8-12, 8-22plaster on metal lath shear walls 8-10, 8-21single layer horizontal lumber sheathing orsiding 8-9, 8-12structural panel or plywood panelsheathing 8-10, 8-19stucco on studs, sheathing, or fiberboard 8-10,8-20types of 8-9vertical wood siding 8-9, 8-18wood siding over diagonal sheathing 8-9, 8-18wood siding over horizontal sheathing 8-9,8-18siding 8-9, 8-18strength 8-5structural panel overlayson existing wood structural paneldiaphragms 8-28on exiting wood structural diaphragms 8-24on straight or diagonally sheatheddiaphragms 8-23, 8-28Ystructural panel sheathed diaphragms 8-27test methods 8-4yield strength of component 2-33FEMA 273 Seismic Rehabilitation Guidelines Index-15

US CUSTOMARY TO SI UNIT CONVERSION TABLESTo Convert To Multiply ByLENGTHAREAFORCEFORCE LENGTH(BENDING MOMENT,TORQUE)FORCE/LENGTHFORCE/AREA(MODULUS,PRESSURE, STRESS)inches (in.)feet (ft)square inches (in. 2 )square feet (ft 2 )pounds (lb)kips (k)inch-pounds (in.-lb)foot-pounds (ft-lb)inch-kips (in.-k)foot-kips (ft-k)pounds/inch (lb/in.)pounds/foot (lb/ft)kips/inch (k/in.)kips/foot (k/ft)pounds/inch 2 (lb/in. 2 )pounds/foot 2 (lb/ft 2 )kips/inch 2 (k/in. 2 )kips/foot 2 (k/ft 2 )millimeters (mm) 25.4meters (m) 0.0254millimeters (mm) 304.8meters (m) 0.3048square millimeters (mm 2 ) 645.16square meters (m 2 ) 0.00064516square millimeters (mm 2 ) 92903square meters (m 2 ) 0.092903newtons (N) 4.4482kilonewtons (kN) 0.004482newtons (N) 4448.2kilonewtons (kN) 4.4482newton-millimeters (N-mm) 112.98newton-meters (N-m) 0.11298newton-millimeters (N-mm) 1355.8newton-meters (N-m) 1.3558kilonewton-millimeters (kN-mm) 112.98kilonewton-meters (kN-m) 0.11298kilonewton-millimeters (kN-mm) 1355.8kilonewton-meters (kN-m) 1.3558newtons/millimeter (N-mm) 0.17513newtons/meter (N-m) 175.13newtons/millimeter (N-mm) 0.014594newtons/meter (N-m) 14.594kilonewtons/millimeter (kN-mm) 0.17513kilonewtons/meter (kN-m) 175.13kilonewtons/millimeter (kN-mm) 0.014594kilonewtons/meter (kN-m) 14.594pascals (Pa) 6894.8kilopascals (kPa) 6.8948pascals (Pa) 47.88kilopascals (kPa) 0.04788pascals (Pa) 6894800kilopascals (kPa) 6894.8pascals (Pa) 47880kilopascals (kPa) 47.88

Note: 1.0 Pa = 1.0 N/m 2US CUSTOMARY TO SI UNIT CONVERSION TABLESTo Convert To Multiply By