cyclic behaviour of a full scale rc structural wall

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  Engineering Structures 25 (2003) 835–845www.elsevier.com/locate/engstruct Cyclic behaviour of a full scale RC structural wall P. Riva  ∗ , A. Meda, E. Giuriani Civil Engineering Department, University of Brescia, Via Branze, 38, I-25123 Brescia, Italy Received 8 August 2002; received in revised form 7 January 2003; accepted 7 January 2003 Abstract The results of an experimental test on a full scale RC structural wall subjected to cyclic loading are herein presented. The testedspecimen is representative of a wall in a four storey building with one underground floor, designed for moderate seismic actions(PGA = 0.20  g ) adopting the European Seismic Code (Eurocode 8, EC8). The experimental specimen is 15.5 m long and has atransverse section of 2800 × 300 mm. The boundary conditions consist of simple supports at the foundation and ground floor levels.The wall behaviour has been studied both under service conditions, up to yielding, defined as the point to which first yield of theoutermost rebars corresponds, and ultimate conditions, up to collapse. ©  2003 Elsevier Science Ltd. All rights reserved. Keywords:  RC structural wall; Seismic design; Cyclic loading; Full scale test 1. Introduction In seismic zones, building resistance to earthquakes isoften ensured by adopting structural systems where seis-mic actions are assigned to structural walls, designed forhorizontal forces and gravity loads, while columns andbeams are designed only for gravity loads [1,2]. Thesesystems, being stiffer than earthquake resisting frames,allow a better displacement control, limiting damage ininternal partition walls and non structural elements. Onthe contrary, frame structures generally exhibit greaterductility, at the expense of large displacements and inter-action problems between structural and non-structuralelements.Extensive experimental results concerning the behav-iour of walls of different slenderness ratio subjected tovarious loading conditions are available in the literature(e.g. [3–6]). These tests are generally limited to smallscale specimens, typically from 1:2 to 1:3 scale, whileexperimental evidence on the behaviour of full-scalewalls is presently scarce. The results have shown thatthe inelastic response of slender walls, characterized byheight-over-width ratios larger or equal to 2, is con- ∗ Corresponding author. Tel.:  + 39-030-3715502; fax:  + 39-030-3715503.  E-mail address:  riva@ing.unibs.it (P. Riva). 0141-0296/03/$ - see front matter  ©  2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0141-0296(03)00020-8 trolled by flexural deformations in a plastic hinge at thebase of the wall.To achieve adequate ductility, an essential role isplayed by confining steel, placed at the edge of the sec-tion in order to control concrete crushing and longitudi-nal reinforcement buckling. Shear strength is providedby distributed vertical and horizontal reinforcement onboth wall faces. Inclined reinforcement is sometimesneeded for protection against sliding shear.The scope of the present research is to partially fill inthe gap concerning full scale tests on slender shear walls,by analysing the behaviour of a full size structural wallunder cyclic loads, with particular attention to its duc-tility, dissipated energy, damage progression, andresisting mechanisms. The tested specimen is representa-tive of a shear wall of a four storey building, with oneunderground storey and a box foundation system (Fig.1). In box foundation systems (Fig. 2), horizontal forcesare resisted by the diaphragm actions of ground andbasement level slabs, leading to a considerable reductionin the wall foundation dimensions [7]. In this case, thecritical section of the wall is located at the ground level,where bending actions are predominant, whereas theunderground part of the wall exhibits a typical shearpanel behaviour. Due to expected high shear forces inthe underground part of the wall, its thickness isincreased.The building, hence the experimental wall, was  836  P. Riva et al. / Engineering Structures 25 (2003) 835–845 Nomenclature  A gt  steel elongation at 0.99  R m  after the peak, measured over  fi ve bar diameter; F   net applied force at each jack, equal to the difference between the total jack force and the forcenecessary to obtain a bending moment equal to 0 at the critical section;PGA peak ground acceleration q  structural coef  fi cient according to EC 8 ( = 3 for class M structural walls);  R cm  mean cube concrete strength;  R e  reinforcing steel yield strength;  R m  reinforcing steel ultimate strength;  M   bending moment at the critical section; V   shear force at the critical section; d   displacement at the wall end; g  Rd  overstrength coef  fi cient according to EC8 ( = 1.15 for class M structural walls); g  c ,  g  s  partial safety factors for concrete and steel, respectively; Subscripts n nominal ultimate values;Sd acting design values;Rd resisting design values;u ultimate values at collapse;y experimental yield values;yt values corresponding to the theoretical  fi rst yield of the critical section.designed according to Eurocode 8 (EC8) [8 – 10],assuming Medium ductility class (structural coef  fi cient q = 3), a peak ground acceleration PGA = 0.20  g , typicalfor medium seismicity zones, and a Soil type B(equivalent to Soil type C in the latest EC8 version [13]). 2. Structural wall description and test set-up The structural wall was designed according to EC8[8 – 10], assuming Medium ductility class (structuralcoef  fi cient  q = 3) and a peak ground accelerationPGA = 0.20  g , typical for medium seismicity zones. Ahigher ground acceleration could not be adopted, due tolimitations in the testing loading frame available. Veri- fi cation of sectional strength was carried out accordingto Eurocode 8 and Eurocode 2 [8 – 11].Fig. 3 illustrates the wall dimensions and steelreinforcement detailing. The wall dimensions were: sec-tion 2800 × 300 mm outside of the supports,2800 × 400 mm between the supports, length outside thesupports 12.5 m, and 16 m total length. At the groundand basement levels, two ribs were inserted to simulatethe  fl oor diaphragms.As prescribed by EC8, the main  fl exural reinforcementwas concentrated in two chords at the edges of the wall,where the reinforcement was heavily con fi ned. Designshear force  V  Sd  at the critical section was determinedbased on the overstrength prescribed by EC 8 [10] as: V  Sd  e · V  ’ Sd  q    g  Rd q  ·  M  Rd  M  Sd  2  0.1  S  e ( T  C ) S  e ( T  1 )  2 · V  ’ Sd  1.50 · V  ’ Sd ,where:  q = 3 is the structural coef  fi cient,  g  Rd = 1.15 is theoverstrength factor,  M  Rd  and  M  Sd  are the resisting anddesign bending moment, respectively,  S  e (T  C )/S  e (T  1 )  isthe ratio between the elastic response spectrum ordinatesat the end of the constant acceleration branch and at thefundamental period, respectively, while  V’ Sd  is the shearforce derived from the analysis.Shear reinforcement was limited to vertical and hori-zontal bars. No inclined reinforcement was inserted, asthe theoretical strength evaluated according to the codewas in excess of the design shear force. The governingshear resisting mechanism was found to be sliding shearat the critical section, expressed as: V  Rd,s  V  dd  V  id  V  fd ,where  V  dd  is due to dowel action of the web reinforce-ment across the critical section,  V  id  is due to inclinedreinforcement ( = 0 in the experimental specimen),  V  fd  isdue to friction effects in the compression chord, whichis the predominant resisting term.  837 P. Riva et al. / Engineering Structures 25 (2003) 835  –  845 Fig. 1. Four-storey building adopted for the structural wall design.Fig. 2. Foundation box system for the wall. Table 1 shows the design bending moment and shearstrengths of the critical section computed considering thematerial safety factors ( g  c = 1.5,  g  s = 1.15) and the designstrength of concrete (C30/37) and steel (B500B) (  M  Rd , V  Rd ), and the nominal strength values determined bymeans of experimental tests on the materials (  M  n ,  V  n ).The experimental bending moment and shear force atstructural yield (  M  y ,  V  y ) and at collapse (  M  u ,  V  u ) are alsoreported in the same table. It is observed that due to thetest set-up, no axial force is present in the wall. As aconsequence, the yield and ultimate moment as well asthe shear strength of the specimen are smaller than thosein a real structural wall.The average material characteristics as determined onconcrete cube specimens (150 × 150 × 150 mm) and steelbars were:   Concrete average cube strength:  R cm = 40.7 MPa;   Reinforcing steel yield strength:  R e = 560 MPa;   Reinforcing steel ultimate strength:  R m = 640 MPa;   Reinforcing steel elongation at 0.99  R m  after thepeak:  A gt = 8.4%.Being the reaction structure available, a prestressed con-crete underground caisson which allows testing struc-tures of span up to 40 m, the wall had to be placed hori-zontally, keeping the axis of maximum inertia vertical,as shown in Fig. 4.The wall was placed on two RC supports, (a) inFig. 4, aligned with the ribs simulating the ground andbasement  fl oor diaphragms, (b) in Fig. 4, and  fi xed tothe caisson by adopting post tensioned 0.6 ”  strands andhigh strength   32 bars, (c) in Fig. 4. Strands and barspost tension was such that no decompression of the sup-port would occur during testing.Two steel frames, (d) in Fig. 4, were placed near theloading positions in order to avoid lateral instability. Thesafety of the system was improved by inserting a sup-plementary frame between the two supports, (e) in Fig.4, which would intervene whenever a lack in post-ten-sion would induce a support decompression.The loads were applied at two points by means of hydraulic jacks. The position of the jacks was de fi nedto obtain the same bending moment and shear forcearound the critical section as the one resulting from theanalysis of the four storey building (Fig. 5). Moreover,the load position was such that the same force could beapplied, greatly simplifying load control. In order toapply cyclic reverse loads, four jacks were adopted, twoacting upward, (a) in Fig. 6, and two downward, (b) inFig. 6. The jacks acting upward were placed betweenthe wall and the loading bench, while those acting down-ward were placed in two windows opened in the walland connected to the caisson with two high strength  32bars. The position of the opening was such that the jack   838  P. Riva et al. / Engineering Structures 25 (2003) 835  –  845 Fig. 3. Wall dimension and steel reinforcement.Table 1Theoretical and experimental bending moment and shear strengthvalues: design (  M  Rd ,  V  Rd,s ); nominal (  M  n ,  V  n ); structural yield  (M  y ,  V  y );collapse (  M  u,  V  u )  M  Rd = 4015 kN m  V  Rd,s = 715 kN  M  n = 5910 kN m  V  n = 1165 kN  M  y = 5300 kN m  V  y = 615 kN  M  u = 6200 kN m  V  u = 720 kNFig. 4. Test set-up: wall supports (a); ribs simulating the ground and basement  fl oor diaphragms (b); post tensioned strands and bars (c); steelframe to avoid lateral instability (d); additional frames for improving safety in the test set-up (e). would act along the wall neutral axis, thus limiting theirhorizontal displacement.The applied load was measured by means of a fullbridge resistive pressure transducer placed on the pumpmanifold. The displacements were measured using 17potentiometric transducers as shown in Fig. 6: two wiretransducers (16, 17) measured the vertical displacementof the wall; 11 linear transducers (1 – 8, 14, 15) measuredthe displacements in the upper and lower chords closeto the critical section; two linear transducers (11, 12)
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