Report - PEER - University of California, Berkeley

2. PERFORMANCE-BASED ASPECTS OF PART 1 OF EUROCODE 8 FORTHE DESIGN OF NEW BUILDINGS2.1 Performance Objectives and Their implications for DesignEC8-Part 1 specifies a two-level seismic design with explicit performance objectives:1. Protection of life under a rare seismic action, by preventing collapse of thestructure or parts of it and ensuring structural integrity and residual load capacity.2. Limited property loss in a frequent earthquake, via limitation of structural andnon-structural damage.Performance level 1 is achieved by proportioning and detailing structural elements fora combination of strength and ductility that provides a safety factor between 1.5 and 2against substantial loss of lateral load resistance. The damage limitation performancelevel is pursued by limiting the overall deformations (lateral displacements) of thebuilding to levels acceptable for the integrity of all its parts (including non-structuralones) and through non-engineered measures for the integrity of (masonry) infills.The three Ductility Classes (DCs) were essentially reduced to two: DC Medium(M) and High (H). The third class (DC L or Low), amounting to design essentially forstrength (with q=1.5 due to overstrength) without engineered ductility, is limited tolow seismicity (design PGA not more than 0.1g). For the other two DCs a third — butnot explicitly stated — performance objective is prevention of global collapse underan extremely strong earthquake, like the “Maximum Considered Earthquake” (MCE)of US codes. It is recognized, though, that repair after that earthquake may beunfeasible or economically prohibitive and that the damaged structure may collapse ina strong aftershock. This performance objective is pursued by control of the inelasticresponse mechanism through systematic and across-the-board application of capacitydesign.The Eurocodes have adopted a policy of letting National Authorities control thesafety and cost-effectiveness provided by structures in their territory, by choosing thevalues of certain key parameters (termed Nationally Determined Parameters or NDPs)that control safety and economy. Within this policy, the hazard levels correspondingto the two performance levels are left for the countries to determine. Eurocode 8recommends though the following, for structures of ordinary importance:i. A seismic action for (local) collapse prevention — termed “design” seismicaction — with 10% exceedance probability in 50 yrs (return period: 475 yrs).ii. A 10% in 10 yrs “serviceability” earthquake for damage limitation (mean returnperiod: 95 yrs).Enhanced performance of essential or large occupancy facilities is achieved not byupgrading the performance level for given earthquake level, as in US codes, but bymodifying the hazard level for which the performance level is pursued. For essentialor large occupancy structures the seismic action at both performance levels should beincreased so that its exceedance probability in 50 or 10 years, respectively, is less than10%. At the collapse prevention level the recommended value of the NDP-importance3