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COMPOSITE COLUMNS TO MITIGATE SOFT STOREY INREINFORCED CONCRETE STRUCTURES SUBMITTED TOEARTHQUAKEPLUMIER, André 1 ; STOYCHEV, Lyubomir 2 ; DONEUX, Cather<strong>in</strong>e 2ABSTRACT"Soft s<strong>to</strong>rey" mechanism is the most frequent failure mode of re<strong>in</strong>forced <strong>concrete</strong>(R.C.) structures - 90 % of build<strong>in</strong>gs failures <strong>in</strong> Kocaeli 1999 Turkey earthquake were of thisnature. The work described hereafter proposes a constructional <strong>in</strong>novation <strong>to</strong> deal with theproblem of <strong>soft</strong> s<strong>to</strong>reys. The <strong>in</strong>novation consists of encas<strong>in</strong>g a steel profile <strong>in</strong> the <strong>columns</strong> ofthe lower levels of the structures. The objective is <strong>to</strong> promote safety without <strong>to</strong>o muchchang<strong>in</strong>g the constructional practice of re<strong>in</strong>forced <strong>concrete</strong> structures. In order <strong>to</strong> study thebehaviour of the composite column with encased steel profile a test program has beenrealized. Some results of the tests and assessments of the positive aspects of the <strong>in</strong>novationare presented.Key Words: seismic design, <strong>soft</strong> s<strong>to</strong>rey, re<strong>in</strong>forced <strong>concrete</strong>, composite, experimental1. DEFINITION OF THE PROBLEM AND RESEARCH APPROACHThe most frequent failure mode of R.C. build<strong>in</strong>gs is the so called “<strong>soft</strong> s<strong>to</strong>rey”mechanism. It consists <strong>in</strong> a localization of build<strong>in</strong>gs’ seismic deformations and rupture <strong>in</strong> thebot<strong>to</strong>m s<strong>to</strong>rey of the build<strong>in</strong>g. This phenomenon is basically caused by the fact that the overallshear force applied <strong>to</strong> the build<strong>in</strong>g by an earthquake is higher at the base. Other fac<strong>to</strong>rs thencontribute <strong>to</strong> create a catastrophic situation:- lower s<strong>to</strong>rey of build<strong>in</strong>gs are often weakened by wide open<strong>in</strong>gs which are not present atupper levels, due <strong>to</strong> the use of the ground level for offices, shops, lobby <strong>in</strong> hotels, etc.- if the lower s<strong>to</strong>rey is not weakened for architectural reasons, it is however there that <strong>in</strong>fillsare the most stressed, so that they fail first and create then the open<strong>in</strong>gs at ground level.- the sequence of concret<strong>in</strong>g generally results <strong>in</strong> an <strong>in</strong>terface between two different <strong>concrete</strong> atthe <strong>to</strong>p section of the column, which is then a section weaker than computed.1 Professor, University of Liege, Liege, Belgium.2 Research Civil Eng<strong>in</strong>eer, University of Liege, Liege, Belgium
The design was made accord<strong>in</strong>g <strong>to</strong> Eurocode 2 (EC2) [1], Eurocode 4 (EC4) [2] andEurocode 8 (EC8) [3] and sections of <strong>concrete</strong> and steel re<strong>in</strong>forcements were establishedcorrespond<strong>in</strong>g <strong>to</strong> 3 reference design situations of the R.C. sections (see table 1):- Static - EC2 (gravity load<strong>in</strong>g only), earthquake not considered, represented by the “static”specimens (see table 1);- Low ductility - EC2 + EC8 - consider<strong>in</strong>g earthquake with PGA 0.2g (low seismicity), lowductility class build<strong>in</strong>g (q=1,5);- Medium ductility - EC2 + EC8 - consider<strong>in</strong>g earthquake with PGA 0.2g (low seismicity),medium ductility class build<strong>in</strong>g (q=3,9);The computed sections were <strong>to</strong>o large <strong>to</strong> be tested <strong>in</strong> labora<strong>to</strong>ry and were scaled down.2. DEFINITION OF THE DESIGN CONDITIONS OF THE ENCASED STEELPROFILEIn the concept of the proposal, the steel profiles are "safety belts": if circumstances aresuch that a <strong>soft</strong> s<strong>to</strong>rey mechanism would form under earthquake action, the plastic h<strong>in</strong>ges <strong>in</strong>the R.C. column at ground level would not provide much ductility; then the steel profilewould come <strong>in</strong><strong>to</strong> action, provid<strong>in</strong>g:- an axial resistance high enough <strong>to</strong> take gravity load + service load with values of the partialsafety fac<strong>to</strong>rs lower than for standard design (see equation (1)).- a bend<strong>in</strong>g resistance similar <strong>to</strong> the M Rd of the R.C. section.However, the steel sections should br<strong>in</strong>g little modification <strong>to</strong> the dynamic response ofthe structure. In particular, the stiffness should not be <strong>in</strong>creased, because it would result <strong>in</strong> an<strong>in</strong>crease of the resultant shear force applied <strong>to</strong> the structure (see fig.3)S(T) dS(T d modified)S(T d <strong>in</strong>itial)T modifiedT <strong>in</strong>itialTFigure 3. Shape of Response Spectrum curve.Hence, the design conditions for the composite <strong>columns</strong> are as follows:- the steel section should at least be able <strong>to</strong> take alone the design axial force of the gravityload<strong>in</strong>g case <strong>in</strong> pure compression:N Rd N Sd ( g G + q Q) g =1,00 and q =0,3 (1)- the steel section alone (not act<strong>in</strong>g composedly) should be able <strong>to</strong> substitute the deficient<strong>concrete</strong> section.M Rd steel M Rd <strong>concrete</strong> (2)V Rd steel V Rd <strong>concrete</strong> (3)
Figure 4. Reduced sections for the composite <strong>columns</strong> <strong>to</strong> be testedThese conditions ensure a no failure condition at ultimate limit state under realisticload<strong>in</strong>g and are used for the design of the composite sections. The test sections are thoseshown on fig. 4.The idea developed <strong>in</strong> the project is <strong>to</strong> <strong>in</strong>sert steel profiles <strong>in</strong> <strong>columns</strong> at ground level.It raises the problem of how <strong>to</strong> realize the details of the anchorage and how far <strong>to</strong> extend theanchorage of these steel profiles <strong>in</strong><strong>to</strong> the <strong>concrete</strong> structure of the 1st s<strong>to</strong>rey and of the groundlevel. Basically 2 designs were considered :- anchorage limited with<strong>in</strong> the depth of the 1 st s<strong>to</strong>rey beams (400mm-see fig.7).- anchorage expanded <strong>in</strong><strong>to</strong> the column above ground level, up <strong>to</strong> mid height of the 1 st s<strong>to</strong>reylevel <strong>columns</strong>.Figure 5. Geometrical characteristics of the end plate and stiffenersFrom the computations, both design have a chance of success, but standard designcomputations do not master well stress concentration problems, especially <strong>in</strong> compositesituation <strong>in</strong>volv<strong>in</strong>g cyclic application of stress <strong>in</strong> potentially cracked <strong>concrete</strong>. That is whytests are done.In order <strong>to</strong> ensure that the axial load transfer between the <strong>concrete</strong> and the steel isefficient <strong>in</strong> the critical region of the composite column, an end plate is welded at the end of
the steel profile (see fig.5). It has been decided not <strong>to</strong> use shear connec<strong>to</strong>rs <strong>in</strong> the load<strong>in</strong>troduction area and <strong>in</strong> areas with change of the cross section, both <strong>to</strong> realize an easiersolution <strong>in</strong> seismic region where it is not easy <strong>to</strong> f<strong>in</strong>d technological solution like the weldedstuds and <strong>to</strong> m<strong>in</strong>imize the cost for the fabrication of the specimens.Moreover it was decided <strong>to</strong> test specimens with and without the presence of stiffenersat the level of the bot<strong>to</strong>m face of the beam (see fig 5). In this way it is possible <strong>to</strong> study if thepresence of the stiffeners plates could modify both the global behaviour of the jo<strong>in</strong>t and theactions transmitted from the <strong>concrete</strong> column <strong>to</strong> the <strong>in</strong>serted steel profile.3. TEST SET-UP AND TEST PROGRAMThe test set-up corresponds <strong>to</strong> a subassemblage zone of a real structure. The l<strong>in</strong>kbetween the real structure and the test set-up is shown on fig.2. The test set-up (see fig. 6 and7a) is organized <strong>in</strong> such a way that <strong>in</strong>fills are represented by a rigid steel stiffened plates.Figure 6. Test set-upA horizontal load is applied <strong>to</strong> the column at a distance represent<strong>in</strong>g the mid height ofthe s<strong>to</strong>rey (1750mm form the beam axis). This load is applied cyclically <strong>in</strong> positive andnegative value by a 1000 kN actua<strong>to</strong>r. A vertical constant load is applied <strong>to</strong> the column beforestart<strong>in</strong>g the cyclic application of horizontal loads (see fig.7).2 NV350AA400 100 100 4003311540400playbetween5 and 10 mmD-4LC -1875875LC-3I-3I-2I-1400D-6/7D-1D-2D-5D-3foundationLC-2170Infill representationby steel plates(a)(b)Figure 7. (a) Addition of plates <strong>in</strong> the test setup <strong>to</strong> ensure the contact (b) test <strong>in</strong>strumentationLC-2
The chosen test configuration <strong>in</strong>tends <strong>to</strong> reproduce accurately the boundary conditionsof a column <strong>in</strong> a <strong>soft</strong> s<strong>to</strong>rey. Nonetheless it has the drawback that the determ<strong>in</strong>ation of theaction effects <strong>in</strong> the beam and <strong>in</strong> the portion of the column between the <strong>in</strong>fills cannot bedirectly deduced from external load. A specific load cell LC2 is needed <strong>to</strong> determ<strong>in</strong>e the axialload <strong>in</strong> the column (see fig.7). The <strong>in</strong>fills <strong>in</strong> the real structure are constructed after theharden<strong>in</strong>g of the <strong>concrete</strong> frame. They are <strong>in</strong> contact with the frame without special separationjo<strong>in</strong>ts and without structural connection <strong>to</strong> it (see also fig.2 and fig.7). The <strong>in</strong>fills are notaimed <strong>to</strong> transmit significant vertical forces <strong>in</strong>duced by vertical loads. If they do, it isaccidental and due <strong>to</strong> problems of excessive deformations of the beams. To ensure that the<strong>in</strong>fills do not transmit vertical loads <strong>in</strong> the test setup, a “play” ensur<strong>in</strong>g no contact has <strong>to</strong> beprovided between beams and <strong>in</strong>fills dur<strong>in</strong>g vertical load<strong>in</strong>g (see fig.7).N<strong>in</strong>eteen tests are planned <strong>in</strong> order <strong>to</strong> study the behavior of the re<strong>in</strong>forced <strong>concrete</strong><strong>columns</strong> and the <strong>columns</strong> with encased steel profile under cyclic load<strong>in</strong>g and <strong>to</strong> make acomparison between them (see table 1).phase Design Strong/weakaxis sectionTable 1. Characteristics of the specimensR.C.COMPOSITEwithstiffenerslong (C1)withoutstiffenerslong (C1)withstiffenersshort (C2)withoutstiffenersshort (C2)PHASE 1 static strong axis RCL1 COL1 COL2 COL3 COL4PHASE 2 DCL strong axis RCL3 COL5 COL6 COL7 COL8PHASE 3 DCM strong axis RCL5 COL9 COL10 COL11 COL12PHASE 4static weak axis RCL2 COL13DCL weak axis RCL4 COL144. GLOBAL INSTRUMENTATION AND DATA ACQUISITIONGlobal <strong>in</strong>strumentation means measurements of global data, as displacements,rotations and loads. The <strong>in</strong>stallation details on the specimen should not <strong>in</strong>fluence its behaviour<strong>in</strong> any way. The <strong>in</strong>struments used are shown on fig.7b and are as follows:-Rotation measurements by <strong>in</strong>cl<strong>in</strong>ometers - I-1, I-2, I-3-Displacement measurements D-1 <strong>to</strong> D-7-Load cells:LC-1: measures horizontal force applied. Maximum capacity of the cell 500 kN.LC-2: measures vertical reaction <strong>in</strong> portion of column between the <strong>in</strong>fills.Maximum capacity 1000 kN.LC-3: measures vertical load applied by the actua<strong>to</strong>r. Maximum capacity 1000 kN.5. VERTICAL LOADING SEQUENCEAs expla<strong>in</strong>ed above a gap between the beam and the <strong>in</strong>fills is necessary before theapplication of the vertical force, so that the sequence at the beg<strong>in</strong>n<strong>in</strong>g of the test is as follows:1) placement of the lateral plates along the column (between column and <strong>in</strong>fills), see fig.7a.2) application of the vertical force N, see fig.7a. The value of N is def<strong>in</strong>ed as 0.3N Rd , N Rdbe<strong>in</strong>g computed referr<strong>in</strong>g <strong>to</strong> the R.C. section alone and the nom<strong>in</strong>al properties of thematerials. The values of N are given <strong>in</strong> table 2.
Table 2. Evaluation of the maximal vertical forces N applied <strong>to</strong> the specimensSpecimens A st [mm²] N Rd,conc [kN] N max applied[kN]Static 1256.64 2292 680Low ductility 3926.99 3280 980Medium ductility 1884.96 2524.5 7503) position<strong>in</strong>g of the horizontal bear<strong>in</strong>g plates under beams (between beams and <strong>in</strong>fills <strong>to</strong>ensure contact), see fig. 7a.6. TEST PROCEDUREThe choice of a test<strong>in</strong>g program and associated load<strong>in</strong>g his<strong>to</strong>ry depends on the purposeof the experiments, type of test specimens, and type of anticipated failure modes. The choiceis between ATC24 [4], ECCS [5] or ECCS modified test<strong>in</strong>g procedure [6]. The ECCSProcedure has been chosen. A reference "yield displacement" y value <strong>in</strong> the form of anabsolute value is def<strong>in</strong>ed a priori and kept for all specimens, <strong>in</strong> order <strong>to</strong> make directcomparison possible. For composite <strong>columns</strong>, it may be estimated that the <strong>in</strong>ters<strong>to</strong>rey driftangle y at yield is 0,5% = 5 mrad. The drift angle <strong>in</strong> the test is the displacement at theactua<strong>to</strong>r divided by the length of the part of the specimens which is deformed dur<strong>in</strong>g the test.The height of column which is free <strong>to</strong> deform is 1750 mm (half of a s<strong>to</strong>rey height), from theactua<strong>to</strong>r axis <strong>to</strong> the beam axis, so that: y = y x 1750 =8,75 mm is the yield displacement atthe actua<strong>to</strong>r (+ and -). Then the plastic cycles are applied as def<strong>in</strong>ed <strong>in</strong> the ECCS Procedure -3 cycles at <strong>in</strong>tervals +/-(2+2n) y with n=0,1,2,3,…, etc.7. RESULTS OF THE TESTS, CONCLUSIONS AND FURTHER DEVELOPMENTThe results presented here are those of Phase 1. Only moment-rotation (M-) diagramsare given hereafter. M and are def<strong>in</strong>ed as follows :- M is the bend<strong>in</strong>g moment <strong>in</strong> the section which should yield first – section A-A on fig.7a.M = V(measured at LC-1).1,54m (4)- is the global rotation :=D1/1,54m (5)The moment-rotation curve of the reference R.C. specimen RCL1 of Phase 1 ispresented on fig.8. The maximum resistance is obta<strong>in</strong>ed for y = 21 mrad, followedimmediately by a sharp resistance decrease. The conventional failure (resistance reduced <strong>to</strong>50% of maximum resistance) of the specimen is reached for u = 51 mrad. The ductility is u / y =51/24=2,1. It may be estimated that the <strong>to</strong>tal loss of resistance corresponds <strong>to</strong> around60 mrad.The cyclic moment-rotation curves of two composite specimens COL1 and COL2 arepresented on fig.9 and fig.10. The maximum resistance is obta<strong>in</strong>ed for y = 20 and 23 mrad,followed by a rather ductile behaviour up <strong>to</strong> y = 70 mrad, which is approximately the rotationcorrespond<strong>in</strong>g <strong>to</strong> conventional failure. The ductility u / y =70/21=3,33. The maximumresistance is higher then the reference R.C. specimen by a fac<strong>to</strong>r of 1,35 approximately. Itmay be estimated that the <strong>to</strong>tal loss of resistance corresponds <strong>to</strong> around 100 mrad. That is66% more than the reference R.C. specimen.
The envelope curves for all 5 specimens of Phase 1 have been traced on fig.11. Theygive the possibility <strong>to</strong> make direct comparison between the specimens of each phase. Table 3provides comparison data between the test specimens. The parameters <strong>in</strong> the table are def<strong>in</strong>edas follows:- y is the rotation (mrad) at yield ;- 50% is the rotation (mrad) at which the specimen has lost 50% of its resistance ;- M pl,exp is the plastic moment of the column obta<strong>in</strong>ed experimentally ;- M pl,th is the plastic moment of the column obta<strong>in</strong>ed from calculations ;- N Rd,th is the compression resistance of the column obta<strong>in</strong>ed from calculations ;- N Sd is the action applied dur<strong>in</strong>g the test (see table 3);- N Rd,R.C. is the compression resistance of the column of the re<strong>in</strong>forced <strong>concrete</strong> specimen ;- N Rd,Comp. is the compression resistance of the column with encased steel profile ;- comp is the rotation (mrad) at yield for the specimen with encased steel profile ;- R.C. is the rotation (mrad) at yield for the re<strong>in</strong>forced <strong>concrete</strong> specimen ;Table 3.Comparison between experimental and computed results.Specimen y y,50%resist. M pl,exp. M pl,th. N Rd,th. N Sd /N Rd,R.C. N Sd /N Rd,Comp. comp ./ R.C(mrad) (mrad) (kNm) (kNm) (kN)RCL1 21 51 180 175 4060 0.20 0.167 1.0COL1 20 70 240 235 4075 0.20 0.167 2.1COL2 23 81 255 235 4165 0.19 0.163 2.6COL3 22 77 249 235 4195 0.21 0.162 2.4COL4 24 83 240 235 4095 0.20 0.166 2.6Moment (kNm)300250200150100500-150 -125 -100 -75 -50 -25-500 25 50 75 100 125 150-100Rotation (mrad)-150-200-250-300Figure 11. Envelope curves for all the specimens of phase 1.RCL1COL1COL2COL3COL4The follow<strong>in</strong>g detailed conclusions can be made:- The re<strong>in</strong>forced <strong>concrete</strong> specimen and the specimens with encased steel profile have almostequal yield rotations. It could also be shown that the stiffness of those specimens are similar.Both conclusions result from the design process of the “steel fuses” which tries <strong>to</strong> not affectthe stiffness of the build<strong>in</strong>g.- The specimens with encased profile allow much greater rotation before they loose 50%resistance (see 50% <strong>in</strong> Table 3).- The specimens with an encased steel profile have greater resistance than the reference R.C.specimen by a fac<strong>to</strong>r of about 1,35. This is favorable <strong>to</strong> achieve a weak beam-strong columnmechanism.
- It can be seen from table 3 that the full composite plastic moment is developed, s<strong>in</strong>ceM pl,exp =M pl,th .- The deformation comp of the composite specimen correspond<strong>in</strong>g <strong>to</strong> the maximum resistanceof the R.C. specimen is almost 2.5 greater than the deformation R.C. of the re<strong>in</strong>forced<strong>concrete</strong> specimen. The specimens with an encased steel profile can resist many more cycles.- The specimens with encased profile have much greater capacity <strong>to</strong> dissipate energy beforethey loose 50% resistance. This can be seen compar<strong>in</strong>g the area of force-displacement curveswhich is the energy dissipated, or the value of displacement u at conventional failure(resistance reduced <strong>to</strong> 50% of maximum resistance)- Additional stiffeners <strong>in</strong> the steel profile do not significantly <strong>in</strong>fluence the resistance. It couldbe concluded that they are not useful.- There is no difference <strong>in</strong> the resistance of specimens with long and short anchorage length ofthe steel profile. At this stage it can be concluded that short anchorage is effective.As a global conclusion of the research it can be stated that the proposed constructionalmeasure, by which a steel profile would be encased <strong>in</strong> all bot<strong>to</strong>m s<strong>to</strong>rey <strong>columns</strong> of R.C.build<strong>in</strong>gs, can substantially and at low cost <strong>in</strong>crease the ability of R.C. build<strong>in</strong>gs <strong>to</strong> resist anearthquake, even <strong>in</strong> case a <strong>soft</strong>-s<strong>to</strong>rey mechanism is possible.Further test<strong>in</strong>g should be done <strong>to</strong> ga<strong>in</strong> knowledge on the follow<strong>in</strong>g aspects:- Need for horizontal stirrups <strong>in</strong> beam-<strong>to</strong>-column connection zone. In practice, these stirrupsare difficult <strong>to</strong> put <strong>in</strong> place and for that reason sometimes omitted. The contribution of thesteel profile <strong>to</strong> shear resistance <strong>in</strong> the node may be such that these stirrups are not necessarybut this should be assessed.- Low quality <strong>concrete</strong>. The results given above <strong>in</strong>dicate that short anchorage is as effective aslong anchorage of the steel profile. However, this result may be due <strong>to</strong> the good quality of the<strong>concrete</strong> on the tests and <strong>to</strong> the presence of horizontal stirrups <strong>in</strong> the beam-<strong>to</strong>-columnconnection zone. The conclusion might change without stirrups and low quality <strong>concrete</strong> andthis should also be assessed.REFERENCES[1] CEN, (2001a). prEN 1992-1-1, Eurocode 2 –Design of <strong>concrete</strong> structures – Part 1 :General rules and rules for build<strong>in</strong>gs.[2] CEN, (2001b). prEN 1994-1-1, Eurocode 4 –Design of composite steel and <strong>concrete</strong>structures – Part 1 : General rules and rules for build<strong>in</strong>gs.[2] CEN, (2001c). prEN 1998-1-1, Eurocode 8 –Design of structures for earthquakeresistance – Part 1 : General rules and rules for build<strong>in</strong>gs.[4] ATC24 procedure - Applied Technology Council (ATC), 1992. Guidel<strong>in</strong>es for SeismicTest<strong>in</strong>g of Components of Steel Structures, ATC 24, Redwood City, CA[5] ECCS (1986). Recommended Test<strong>in</strong>g Procedure for Assess<strong>in</strong>g the Behaviour ofStructural Steel Elements under Cyclic Load<strong>in</strong>g. Technical Committee 1 – StructuralSafety and Load<strong>in</strong>g.[6] Seismic resistance of composite structures - Contract 75210-SA/506 - F<strong>in</strong>al Report.EU publication - Report EUR 14428 EN. 1992 - 435 pages..AcknowledgementThis work has been made possible thanks <strong>to</strong> a fund<strong>in</strong>g of the European Union throughthe ECSC-CECA Project 7210-PR-316 “Innovative Earthquake Resistant Design” or INERDProject.