Composite columns to mitigate soft storey in reinforced concrete ...

COMPOSITE COLUMNS TO MITIGATE SOFT STOREY INREINFORCED CONCRETE STRUCTURES SUBMITTED TOEARTHQUAKEPLUMIER, André 1 ; STOYCHEV, Lyubomir 2 ; DONEUX, Catherine 2ABSTRACT"Soft storey" mechanism is the most frequent failure mode of reinforced concrete(R.C.) structures - 90 % of buildings failures in Kocaeli 1999 Turkey earthquake were of thisnature. The work described hereafter proposes a constructional innovation to deal with theproblem of soft storeys. The innovation consists of encasing a steel profile in the columns ofthe lower levels of the structures. The objective is to promote safety without too muchchanging the constructional practice of reinforced concrete structures. In order to 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 innovationare presented.Key Words: seismic design, soft storey, reinforced concrete, composite, experimental1. DEFINITION OF THE PROBLEM AND RESEARCH APPROACHThe most frequent failure mode of R.C. buildings is the so called “soft storey”mechanism. It consists in a localization of buildings’ seismic deformations and rupture in thebottom storey of the building. This phenomenon is basically caused by the fact that the overallshear force applied to the building by an earthquake is higher at the base. Other factors thencontribute to create a catastrophic situation:- lower storey of buildings are often weakened by wide openings which are not present atupper levels, due to the use of the ground level for offices, shops, lobby in hotels, etc.- if the lower storey is not weakened for architectural reasons, it is however there that infillsare the most stressed, so that they fail first and create then the openings at ground level.- the sequence of concreting generally results in an interface between two different concrete atthe top section of the column, which is then a section weaker than computed.1 Professor, University of Liege, Liege, Belgium.2 Research Civil Engineer, University of Liege, Liege, Belgium

The design was made according to Eurocode 2 (EC2) [1], Eurocode 4 (EC4) [2] andEurocode 8 (EC8) [3] and sections of concrete and steel reinforcements were establishedcorresponding to 3 reference design situations of the R.C. sections (see table 1):- Static - EC2 (gravity loading only), earthquake not considered, represented by the “static”specimens (see table 1);- Low ductility - EC2 + EC8 - considering earthquake with PGA 0.2g (low seismicity), lowductility class building (q=1,5);- Medium ductility - EC2 + EC8 - considering earthquake with PGA 0.2g (low seismicity),medium ductility class building (q=3,9);The computed sections were too large to be tested in laboratory 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 soft storey mechanism would form under earthquake action, the plastic hinges inthe R.C. column at ground level would not provide much ductility; then the steel profilewould come into action, providing:- an axial resistance high enough to take gravity load + service load with values of the partialsafety factors lower than for standard design (see equation (1)).- a bending resistance similar to the M Rd of the R.C. section.However, the steel sections should bring little modification to the dynamic response ofthe structure. In particular, the stiffness should not be increased, because it would result in anincrease of the resultant shear force applied to the structure (see fig.3)S(T) dS(T d modified)S(T d initial)T modifiedT initialTFigure 3. Shape of Response Spectrum curve.Hence, the design conditions for the composite columns are as follows:- the steel section should at least be able to take alone the design axial force of the gravityloading case in pure compression:N Rd N Sd ( g G + q Q) g =1,00 and q =0,3 (1)- the steel section alone (not acting composedly) should be able to substitute the deficientconcrete section.M Rd steel M Rd concrete (2)V Rd steel V Rd concrete (3)

Figure 4. Reduced sections for the composite columns to be testedThese conditions ensure a no failure condition at ultimate limit state under realisticloading and are used for the design of the composite sections. The test sections are thoseshown on fig. 4.The idea developed in the project is to insert steel profiles in columns at ground level.It raises the problem of how to realize the details of the anchorage and how far to extend theanchorage of these steel profiles into the concrete structure of the 1st storey and of the groundlevel. Basically 2 designs were considered :- anchorage limited within the depth of the 1 st storey beams (400mm-see fig.7).- anchorage expanded into the column above ground level, up to mid height of the 1 st storeylevel columns.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 in compositesituation involving cyclic application of stress in potentially cracked concrete. That is whytests are done.In order to ensure that the axial load transfer between the concrete and the steel isefficient in 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 to use shear connectors in the loadintroduction area and in areas with change of the cross section, both to realize an easiersolution in seismic region where it is not easy to find technological solution like the weldedstuds and to minimize the cost for the fabrication of the specimens.Moreover it was decided to test specimens with and without the presence of stiffenersat the level of the bottom face of the beam (see fig 5). In this way it is possible to study if thepresence of the stiffeners plates could modify both the global behaviour of the joint and theactions transmitted from the concrete column to the inserted steel profile.3. TEST SET-UP AND TEST PROGRAMThe test set-up corresponds to a subassemblage zone of a real structure. The linkbetween the real structure and the test set-up is shown on fig.2. The test set-up (see fig. 6 and7a) is organized in such a way that infills are represented by a rigid steel stiffened plates.Figure 6. Test set-upA horizontal load is applied to the column at a distance representing the mid height ofthe storey (1750mm form the beam axis). This load is applied cyclically in positive andnegative value by a 1000 kN actuator. A vertical constant load is applied to the column beforestarting 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 in the test setup to ensure the contact (b) test instrumentationLC-2

The chosen test configuration intends to reproduce accurately the boundary conditionsof a column in a soft storey. Nonetheless it has the drawback that the determination of theaction effects in the beam and in the portion of the column between the infills cannot bedirectly deduced from external load. A specific load cell LC2 is needed to determine the axialload in the column (see fig.7). The infills in the real structure are constructed after thehardening of the concrete frame. They are in contact with the frame without special separationjoints and without structural connection to it (see also fig.2 and fig.7). The infills are notaimed to transmit significant vertical forces induced by vertical loads. If they do, it isaccidental and due to problems of excessive deformations of the beams. To ensure that theinfills do not transmit vertical loads in the test setup, a “play” ensuring no contact has to beprovided between beams and infills during vertical loading (see fig.7).Nineteen tests are planned in order to study the behavior of the reinforced concretecolumns and the columns with encased steel profile under cyclic loading and to 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 instrumentation means measurements of global data, as displacements,rotations and loads. The installation details on the specimen should not influence its behaviourin any way. The instruments used are shown on fig.7b and are as follows:-Rotation measurements by inclinometers - I-1, I-2, I-3-Displacement measurements D-1 to D-7-Load cells:LC-1: measures horizontal force applied. Maximum capacity of the cell 500 kN.LC-2: measures vertical reaction in portion of column between the infills.Maximum capacity 1000 kN.LC-3: measures vertical load applied by the actuator. Maximum capacity 1000 kN.5. VERTICAL LOADING SEQUENCEAs explained above a gap between the beam and the infills is necessary before theapplication of the vertical force, so that the sequence at the beginning of the test is as follows:1) placement of the lateral plates along the column (between column and infills), see fig.7a.2) application of the vertical force N, see fig.7a. The value of N is defined as 0.3N Rd , N Rdbeing computed referring to the R.C. section alone and the nominal properties of thematerials. The values of N are given in table 2.

Table 2. Evaluation of the maximal vertical forces N applied to 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) positioning of the horizontal bearing plates under beams (between beams and infills toensure contact), see fig. 7a.6. TEST PROCEDUREThe choice of a testing program and associated loading history 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 testing procedure [6]. The ECCSProcedure has been chosen. A reference "yield displacement" y value in the form of anabsolute value is defined a priori and kept for all specimens, in order to make directcomparison possible. For composite columns, it may be estimated that the interstorey driftangle y at yield is 0,5% = 5 mrad. The drift angle in the test is the displacement at theactuator divided by the length of the part of the specimens which is deformed during the test.The height of column which is free to deform is 1750 mm (half of a storey height), from theactuator axis to the beam axis, so that: y = y x 1750 =8,75 mm is the yield displacement atthe actuator (+ and -). Then the plastic cycles are applied as defined in the ECCS Procedure -3 cycles at intervals +/-(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 defined as follows :- M is the bending moment in 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 obtained for y = 21 mrad, followedimmediately by a sharp resistance decrease. The conventional failure (resistance reduced to50% 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 total loss of resistance corresponds to 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 obtained for y = 20 and 23 mrad,followed by a rather ductile behaviour up to y = 70 mrad, which is approximately the rotationcorresponding to conventional failure. The ductility u / y =70/21=3,33. The maximumresistance is higher then the reference R.C. specimen by a factor of 1,35 approximately. Itmay be estimated that the total loss of resistance corresponds to 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 to make direct comparison between the specimens of each phase. Table 3provides comparison data between the test specimens. The parameters in the table are definedas 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 obtained experimentally ;- M pl,th is the plastic moment of the column obtained from calculations ;- N Rd,th is the compression resistance of the column obtained from calculations ;- N Sd is the action applied during the test (see table 3);- N Rd,R.C. is the compression resistance of the column of the reinforced concrete 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 reinforced concrete 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 following detailed conclusions can be made:- The reinforced concrete 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 to not affectthe stiffness of the building.- The specimens with encased profile allow much greater rotation before they loose 50%resistance (see 50% in Table 3).- The specimens with an encased steel profile have greater resistance than the reference R.C.specimen by a factor of about 1,35. This is favorable to achieve a weak beam-strong columnmechanism.

- It can be seen from table 3 that the full composite plastic moment is developed, sinceM pl,exp =M pl,th .- The deformation comp of the composite specimen corresponding to the maximum resistanceof the R.C. specimen is almost 2.5 greater than the deformation R.C. of the reinforcedconcrete specimen. The specimens with an encased steel profile can resist many more cycles.- The specimens with encased profile have much greater capacity to dissipate energy beforethey loose 50% resistance. This can be seen comparing the area of force-displacement curveswhich is the energy dissipated, or the value of displacement u at conventional failure(resistance reduced to 50% of maximum resistance)- Additional stiffeners in the steel profile do not significantly influence the resistance. It couldbe concluded that they are not useful.- There is no difference in 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 in all bottom storey columns of R.C.buildings, can substantially and at low cost increase the ability of R.C. buildings to resist anearthquake, even in case a soft-storey mechanism is possible.Further testing should be done to gain knowledge on the following aspects:- Need for horizontal stirrups in beam-to-column connection zone. In practice, these stirrupsare difficult to put in place and for that reason sometimes omitted. The contribution of thesteel profile to shear resistance in the node may be such that these stirrups are not necessarybut this should be assessed.- Low quality concrete. The results given above indicate that short anchorage is as effective aslong anchorage of the steel profile. However, this result may be due to the good quality of theconcrete on the tests and to the presence of horizontal stirrups in the beam-to-columnconnection zone. The conclusion might change without stirrups and low quality concrete andthis should also be assessed.REFERENCES[1] CEN, (2001a). prEN 1992-1-1, Eurocode 2 –Design of concrete structures – Part 1 :General rules and rules for buildings.[2] CEN, (2001b). prEN 1994-1-1, Eurocode 4 –Design of composite steel and concretestructures – Part 1 : General rules and rules for buildings.[2] CEN, (2001c). prEN 1998-1-1, Eurocode 8 –Design of structures for earthquakeresistance – Part 1 : General rules and rules for buildings.[4] ATC24 procedure - Applied Technology Council (ATC), 1992. Guidelines for SeismicTesting of Components of Steel Structures, ATC 24, Redwood City, CA[5] ECCS (1986). Recommended Testing Procedure for Assessing the Behaviour ofStructural Steel Elements under Cyclic Loading. Technical Committee 1 – StructuralSafety and Loading.[6] Seismic resistance of composite structures - Contract 75210-SA/506 - Final Report.EU publication - Report EUR 14428 EN. 1992 - 435 pages..AcknowledgementThis work has been made possible thanks to a funding of the European Union throughthe ECSC-CECA Project 7210-PR-316 “Innovative Earthquake Resistant Design” or INERDProject.