STRENGTH, DEFORMATION CAPACITY AND FAILURE MODE OF RC WALLS IN CYCLIC LOADING

Transcription

1 STRENGTH, DEFORMATION CAPACITY AND FAILURE MODE OF RC WALLS IN CYCLIC LOADING Grammatikou, S., Biskinis, D., Fardis, M.N. Strutures Laboratory, Dept. o Civil Engineering, University o Patras, 2654, Greee Abstrat A database o 621 yli tests o RC walls is utilized to evaluate past models or the yli strength and deormation apaity o the wall and to develop/alibrate new ones. From the observed damage the ailure mode is lassiied as in lexure, diagonal tension or ompression beore or ater lexural yielding, or in sliding shear. Past models being evaluated on the basis o the tests inlude models proposed by two o the authors and adopted in Euroode 8-Part 3 and/or MC21 or (a) lexural strength, (b) the yli shear strength ater lexural yielding (as aeted by the imposed dutility demand) and () the yli hord rotation apaity in lexure. Walls with height-to-length ratio less than 1.2 are onsidered separately, as their shear ailure requires dierent models: past models are ommented and a new one proposed and alibrated on the basis o 13 yli tests o squat walls. Past models or sliding shear strength are evaluated and modiied on the basis o 29 yli tests with that ailure mode: a new model is proposed or the possibility o unontrolled sliding during a load reversal at a point in time when the lexural rak at the base is open throughout the setion and only dowel ation o the still elasti vertial bars is available to resist the shear ore. There is good agreement o the predited yli strength and/or deormation apaity per ailure mode; the predition o the most likely mode in the tests o the database is also satisatory. Keywords: onrete walls, yli loading, seismi assessment, seismi design, shear strength, ultimate deormation 1. Introdution Experimental database Walls are widely used in RC buildings designed or earthquake. Their design and modeling are more omex than that o olumns, beause shear eets and shear ailures are more ommon. Besides, under yli loading they exhibit ailure modes, suh as in shear-ompression or by sliding shear at the base, whih are rare in olumns. Seismi design o walls in new building or seismi assessment o existing ones in old onstrution requires identiying the most likely ailure mode and estimating the wall yli strength and deormation apaity aordingly. This paper uses a database o over 6 yli tests o RC walls to evaluate past models and develop/alibrate new ones or the yli strength and deormation apaity o the wall. The database has been assembled at the Strutures Lab o the University o Patras and is urrently by ar the largest o its kind. It is summarized in Tables 1 and 2 (see or the data and reerenes). The data inlude 68 speimens with hollow retangular (box) setion, more representative o bridge piers than o walls, as their geometry, reinorement layout and ailure modes resemble those o walls with I-setion tested uniaxially along the web. Failure in lexure is by onrete rushing or bar rature in the lexural asti hinge at the base (Set. 3); diagonal tension or ompression ailure is due to the yli deay o shear resistane ater lexural yielding (Set. 4); or squat walls ailing in shear and or shear sliding, see Setions 5 and 6, respetively.

2 Table 1 Range and mean values o the main parameters o the speimens in the database Parameter 278 walls with 343 o non-retangular setion retangular setion (I-, U-, T-, L-, box-setion) min / max mean min / max mean setion depth, h (m).4 / / shear-span-to-depth ratio, L s /h.39 / / setion aspet ratio, h/b w 2.5 / / (MPa) 5.2 / / axial-load-ratio, N/A /.86.9 /.5.6 ratio o transverse web reinorement, w, % / / total longitudinal steel ratio, tot, %.7 / / oninement reinorement ratio, ρ s, % / / Table 2 Number o speimens per ailure mode Group Failure mode retangular walls non-retangular walls total 1 Flexural Diagonal tension - ater lexural yielding Diagonal ompression, ater lexural yielding Shear ailure o squat walls Sliding shear - ater lexural yielding No ailure in test, but yielding in lexure Moment, urvature and hord rotation at lexural yielding A prime role o the yield moment, M y, in the present ontext is as a means to identiy shear ailures beore lexural yielding rom other ailure modes taking ae ater the wall yields in lexure: i the ormer mode has a shear resistane denoted by V R, it is likely to our i V R < M y /L s, where L s is the shear span (moment-to-shear-ratio) at the end setion o the wall. The yield moment is omputed along with, and as proportional to, the yield urvature φ y o the setion, with sime expressions based on setion analysis and elasti σ-ε laws (Biskinis and Fardis, 21a); in turn φ y is important in the present ontext or the alulation o the hord rotation (: deletion o the shear span relative to the yielding end, divided by L s ) at wall yielding, θ y, as (Biskinis and Fardis, 21a, ib 212): θ L a z 3 φ d s V y bl yl y φy.13 ( yl, in the last term in MPa) (1) 8 where: a V z in the irst term (releting lexure) is the tension shit o the moment diagram, with: z = internal lever arm (: distane between the tension and ompression reinorement), a V = 1 i diagonal raking preedes lexural yielding o the base setion (i.e., i V y = M y /L s exeeds the wall shear resistane without shear reinorement, V R, ); otherwise, a V =; d bl is the diameter o the tension vertial bars and yl their yield stress, The last term is the ixed-end-rotation due to slippage o the tension vertial bars rom their anhorage zone beyond the wall end and is negleted when suh slippage is physially impossible. In the present ontext θ y is used to normalize the hord rotation demand,, into a orresponding dutility ator, θ =/ y, on whih the yli shear resistane ater lexural yielding depends (see Setions 4 to 6). Besides, it is used to ompute the lexure-ontrolled yli ultimate hord rotation, θ u, as the sum o y and an inelasti omponent, given by Eqs.(2) in Setion 3.

3 My,exp [knm] 3 2 My,exp [knm] 3 2 median:m y,exp =.99 M y,pred 1 1 median:m y,exp =1.55 M y,pred M y,pred [knm] M y,pred [knm] (a) (b) Fig.1 Experimental v predited yield moment o walls with: (a) retangular; (b) non-retangular setion. 1,6 1,6 1,4 1,4 median:θ y,exp =.995 θ y,pred 1,2 1,2 median:θ y,exp = θ y,pred 1, 1, θy,exp [%],8 θy,exp [%],8,6,6,4,4,2,,,2,4,6,8 1, 1,2 1,4 1,6 θ y,pred [%] retangular setion non retangular setion,2 retangular setion non retangular setion,,,2,4,6,8 1, 1,2 1,4 1,6 θ y,pred [%] (a) (b) Fig.2 Experimental hord rotation at yielding v predition o: (a) Eq.(1); (b) Eq.(1) with onstant term reaed by.6[1+(7/6)h/l s ]. A bilinear approximation is ommonly adopted or the monotoni moment-deletion urve o RC members up to their peak resistane, serving also as an envelope o the moment-deletion loops in yli loading. The orner point o the bilinear urve is identiied with apparent yielding o the wall. The orner moment o the bilinear approximation o the experimental moment-deletion envelopes in the present RC wall database is the "experimental yield moment", M y,exp, ompared in Fig. 1 to the value rom the sime expressions in (Biskinis and Fardis, 21a). The value o φ y to be used in Eq. (1) is the value omputed per (Biskinis and Fardis, 21a) as proportional to M y times the median bias ators o.99 and 1.55 depited in Fig. 1 or retangular and nonretangular walls, respetively. The outome o Eq.(1) is ompared in Fig. 2 to the orner hord rotation o the bilinear approximation o the experimental moment-hord rotation envelopes

4 ("experimental hord rotation at yielding", θ y,exp ) in the RC wall database. Grammatikou (213) ound that the same overall auray but better itting o the separate groups o retangular and non-retangular walls is ahieved, i.6[1+(7/6)h/l s ] with h the depth ("length") o the wall setion reaes in Eq.(1) the onstant term.13 o (Biskinis and Fardis, 21a), (ib, 212) median:θ u,exp =1.2 θ u,pred median:θ u,exp =.99 θ u,pred θu,exp [%] 4 θu,exp [%] retangular setion non retangular setion θ u,pred [%] θ u,pred [%] (a) (b) Fig.3 Experimental yli ultimate hord rotation v predition o (a) Eq.(2a) (b) Eq.(2b). Group 1 in Table 2 1 retangular setion non retangular setion 3. Flexure-ontrolled ultimate hord rotation in yli loading The ultimate ondition and deormation (in this ase hord rotation) o a RC member under yli loading is onventionally identiied with a 2% drop in the lateral ore resistane o the member ompared to the peak resistane. (Biskinis and Fardis, 21b) proposed two alternative empirial expressions adopted in (ib 212) or the asti part, θ u = θ u - y, o the lexure-ontrolled yli ultimate hord rotation, θ u, o retangular RC beams, olumns, walls and RC members with non-retangular setion (inluding a hollow retangular one): θ u.3.35 αρw yw aw, nr max(.1; ω ) L w, r ) max(.1; ω1 ) h ν.2 2 s 1ρd.25 ( MPa) min9; αst (1.44a (2a) hbw h max(.1; 2) Ls.2 d u 1 st 1.52max 1.5;min 1;.2 min 9; ( MPa) b w max(.1; 1 ) h (2b) where: or dutile (Class B, C, D) steel: a st =.143, a hbw st =.17; or brittle (Class A) steel: a st =.69, a hbw st =.73; a w,r = 1 or retangular walls, a w,,r = or other types o members; a w,nr = 1 or non-retangular setions (T-, I-, C-, hollow retangular); a w,nr = or retangular ones; b w : width o one web, even in ross-setions with two or more parallel webs. = N/(bh ), with b the ompression lange width and the axial ore N positive or ompression; 1 = ( 1 y1 + v yv )/ : mehanial reinorement ratio or the entire tension zone, inluding the tension lange (index 1) and the web longitudinal bars (index v); 2 = 2 y2 / mehanial reinorement ratio or the ompression lange; d : steel ratio o diagonal bars (i any) in eah diagonal diretion o the member; 1 3 w yw

5 yw, w = A sh /b w s h : yield stress o onining steel and its ratio parallel to the ane o bending; : oninement eetiveness ator o retangular ties, rom: s h a 1 2b o s h 1 2h o 1 with s h denoting the enterline spaing o the ties, b o, h o the dimensions o the onined ore to the tie enterline and b i the on-enter spaing along the setion perimeter o those longitudinal bars (indexed by i) engaged by a tie orner or a ross-tie hook. Fig. 3 ompares the experimental ultimate hord rotation o the speimens in Group 1 o Table 2 ailing in lexure to the outome o Eqs.(2). Grammatikou (213) ound that the median bias o 2% in Fig. 3(a) is eliminated i the oeiients multiying a w,,r and a w,nr in Eq. (2a) are redued to.42 and.23, respetively; i.6[1+(7/6)h/l s ] reaes the onstant term.13 in Eq.(1) (see end o Setion 2), the 1% median bias in Fig. 3(b) disappears while the itting o Eq. (2a) is not impaired. Witness in Fig. 3(b) the less satter and hene superiority o Eq. (2b) over (2a). b b o 2 i h o / 6 (3) VR,exp [kn] 9 6 median: V R,exp = 1.5 V R,pred VR,exp [kn] 9 6 median: V R,exp =1.1V R,pred 3 3 retangular setion non retangular setion V R,pred [kn] (a) (b) Fig.4 Experimental diagonal tension resistane ater lexural yielding v predition o (a) Eq.(4a); (b) Eq.(4b) or Group 2 o walls in Table 2 retangular setion non retangular setion V R,pred [kn] 4. Cyli shear strength o non-squat walls 4.1. Cyli resistane in diagonal tension ater lexural yielding One a member yields in lexure and a lexural asti hinge orms, its shear resistane dereases with inreasing inelasti yli disaements; so, the asti hinge may ail in diagonal tension, beore its lexure-ontrolled ultimate hord rotation is reahed. Biskinis et al. (24) proposed two alternative empirial models adopted in (CEN, 25) or the deay o diagonal tension strength, V R,ST, with inreasing inelasti yli deormation (units MN, m): V R, ST h x min 2L s Ls N;.55A 1.95 min5; μθ.16 max(.5;1 ρ ) 1.16 min5; bwd Vw tot h (4a) V R, ST h x min 2L s N ;.55 A 1.5 min 5; μ θ.16 max(.5;1 ρ tot L s ) 1.16 min5; h bwd V w (4b)

6 where: μ θ = θ -1: asti part o the dutility ator o hord rotation, with θ = / y and θ y rom Eq.(1), x: neutral axis depth at yielding per (Biskinis and Fardis, 21a) omputed alongside M y, φ y, see Setion 2, A : area o the setion, tot : total longitudinal reinorement ratio, d: eetive depth o the setion, V w = w b w z yw : ontribution to shear strength o shear links having steel w and yield stress yw As this ailure mode reers to a lexural asti hinge at the base o the wall, it an take ae only ater lexural yielding o the base setion. So, the peak shear ore in a subsequent yle annot exeed at least appreiably the shear aompanying the yield moment at the base: V y = M y /L s. The asti hinge may ail by diagonal tension, one the hord rotation demand,, inreases to the point that the orresponding asti dutility ator, μ θ =/ y -1, auses the value o shear resistane rom Eqs.(4) to drop below that o V y. Although itted to a database mainly o olumns and ew walls, Eqs.(4) do well or the more sizeable Group 2 o walls in Table 2 whih have 4.1 L s /h > 1. and ail in shear tension ater lexural yielding see Fig Cyli shear strength in diagonal ompression Ater lexural yielding RC walls experiene a redution o resistane in diagonal ompression with load yling. Biskinis et al. (24) itted to a database o suh wall ailures smaller than in Group 3 o Table 2 the ollowing empirial model adopted in (CEN, 25) or L s /h 2.5 (units MN, m): N Ls VR SC μθ ρtot bwz A h.851.6min 5; 11.8min.15; 1.25max1.75; 1 1.2min 2; (5), median:v R,exp =.995 V R,pred 2 VR,exp [kn] 15 1 VR,exp [kn] 15 1 median: V R,exp = V R,pred V R,pred [kn] retangular setion non retangular setion (a) (b) Fig.5 Experimental shear resistane v predited: (a) rom Eq.(5) or walls ailing in diagonal ompression; (Group 3 in Table 2) (b) rom Eqs.(6)-(9) or the squat walls (Group 4 in Table 2). Fig. 5(a) shows that Eq.(5) its well the more sizeable Group 3 in Table 2 o walls whih have 2.8 L s /h > 1. and whih ail in diagonal ompression ater lexural yielding. As in the ase o Eqs.(4), Eq.(5) signals shear ailure when the hord rotation demand,, inreases to the point that the orresponding asti dutility ator, μ θ =/ y -1, auses the value o shear resistane rom Eq.(5) to drop below that o V y V R,pred [kn] retangular setion non retangular setion

7 5. Cyli shear resistane o squat walls For values o L s /h o about 1. or less, the models in Set. 4 exhibit a systemati and marked bias with respet to the experimental shear resistane: Eqs.(4) underestimate the post-yield yli strength o walls, while Eq.(5) overestimates it. So, with the ew exeptions o the walls with L s /h a little over 1., whose shear ailure was well predited by Eqs. (4) or (5), those that had L s /h 1.17 and did not ail by shear sliding (see Set. 6 and Group 5 in Table 2) are grouped separately, as squat with shear ailure (Group 4 in Table 2). A dierent approah is ollowed or them, as indeed done in (CEN, 24) or L s /h < 2 and in (ASCE, 25) or L s /h < 1.5. Note, though, that on average the (CEN, 24) approah underestimates the yli shear strength o the walls in Group 4 by 9% (it is too onservative), while that o (ASCE, 25) overestimates it by 4% (it seems unsae). The shear resistane itted here to the test results o Group 4 ombines (Eq. (6)): an empirial onrete ontribution, V, ollowing the ormat in [Barda et al., (1977)] adopted in (ASCE, 25) modiied here to it the present sizeable dataset o Group 4 (see Eq.(7)), and a ontribution, V s, o the web reinorement horizontal with ratio w =A sh /b w s h and vertial with ratio v =A sv /b w s v per Eq.(8), whih extends the orresponding term in (ASCE, 25) to aount or a variable angle o the dominant rak to the vertial, r, per Eq.(9) (itted to the raking pattern at ailure o the walls o Group 4). Moreover, i the wall yields in lexure at the base beore it ails in shear as squat, its shear resistane, V R,squat, dereases with μ θ = θ -1 = / y -1 (with θ y rom Eq.(1)) aording to Eq.(6): V s V 1.7 min 6 μ V V R squat ; (6), θ s Ls N V bwd h bwh (7) 4 Ls h x Ls min1;.5ρwbw min Ls ; yw min1.5 ;1ρ v b w minl s tan θ r ; d yv h tan θ (8) r h r ( o ) = 6-15 min(1; L s /h) (9) Eqs.(6)-(9) are meant to apy only to walls with L s /h< 1.2. Unlike Eqs. (4) and (5), whih apy with μ θ = θ -1 = (/ y -1)> ater lexural yielding o the base setion, Eq.(6) apies also or μ θ =, giving a yli shear resistane o the squat wall less than V y = M y /L s. Fig. 5(b) ompares the preditions o Eqs.(6)-(9) to the experimental shear resistane o squat walls in Group 4 o Table Cyli resistane in sliding shear Under alternating positive and negative yles o bending that drive the wall base setion past yielding, that setion raks through its ull length; asti tensile strains aumulate in the vertial bars that have yielded. Beause o these strains, the rak may remain open throughout the base setion, until the ating moment beomes large enough to ause the extreme bars o the ompression lange to yield in ompression, reversing their asti tensile strains and losing the rak ("touh-down" moment, see below). One the rak loses, onrete-to-onrete rition develops at the interae over the ompression zone. At that stage horizontal sliding along the through-raked base setion is resisted aording to Eq.(1), by the sum o: the rition resistane, V, o the ompression zone, omputed here with a onrete-to-onrete rition oeiient,, equal to.8, instead o the value o.6 or smooth or.7 or rough interaes in (CEN 24); the 2nd term in the (CEN 24) expression is modiied as in Eq.(11), or non-retangular ompression zones with area A ompr and normalised neutral axis depth ; dowel resistane, V d, per Eq.(12), with A sj the area o web vertial bars ; the horizontal projetion, V i, o the resistane o any inlined bars, with total area (in both diretions) A si, aed at an angle ± to the base o the wall, see Eq.(13).

8 V V R, SLS V Vd Vi M A N ;. A min sj yl ompr z 3 (1) (11) V A min 1.6 ; 3 (12) d sj yl yl V A os φ (13) i si yl The normalised neutral axis depth,, dereases as the base setion proeeds rom yielding to the ultimate lexural ondition. Its values at these two stages, y, u, are ound by setion analysis with elasti σ-ε laws till yielding ( irst para. in Set. 2 onerning alulation o M y, φ y per (Biskinis and Fardis, 21a)); at ultimate, a bilinear σ-ε law is used or steel and a paraboli-retangular one or onrete (Biskinis and Fardis, 21b). The hord rotation at these two stages, y, θ u, may be estimated rom Eqs. (1) and (2), respetively. Linear interpolation between y, u on one hand and y, θ u on the other, or the value o =(μ θ +1) y orresponding to the value o the asti part o the hord rotation dutility ator, μ θ = θ -1=/ y -1, with θ y rom Eq.(1), gives the value o to be used in Eq.(11) alongside the orresponding area A ompr. So, dereases with inreasing θ 2 median: V R,exp =.99 V R,pred 15 VR,exp [kn] 1 5 retangular setion non retangular setion V R,s [kn] Fig.6 Experimental shear resistane o walls ailing in sliding shear (Group 5 in Table 2) v predition o Eqs.(1)-(13). This ailure mode, like those in Set. 4, is possible only ater lexural yielding at the base; beore yielding the rak is not ully open through the length o the setion and shear sliding annot our. So, like Eqs. (4) or (5), Eqs.(1) signal shear ailure when the hord rotation demand,, inreases to the point that the value o μ θ =/ y -1 auses that o V R,SLS rom Eqs. (1)-(13) to drop below the value o V y = M y /L s. The so-predited V R,SLS is ompared in Fig. 6 to the experimental sliding shear resistane o the walls in Group 5 o Table 2.

9 Besides the possibility o ull-ledged sliding at the value o =(μ θ +1) y that auses V R,SLS rom Eqs. (1)-(13) to drop below V y = M y /L s, a wall may slide prematurely, beore the peak o a post-yield yle, at a point in time when the rak is still open throughout the base setion and there is no onrete-to-onrete ontat yet. At that point the rition term, V, o Eq.(11) does not ontribute to the sliding shear resistane o Eqs.(1). So, i the sum o V d and V i rom Eqs. (12) and (13) is less than the "touh-down" shear ore, V td = M td /L s, aompanying the "touh-down" moment, M td, whih is neessary to lose a rak open throughout the wall base setion by ausing the extreme bars o the ompression edge to yield in ompression, "premature sliding" will take ae. Suh sliding may be temporary, as it may be arrested when the ating shear inreases to the "touh-down" value, V td, ativating the rition resistane, V. I the axial ompression is apable o ausing by itsel yielding o all vertial bars in the wall setion, i.e., i N > ( 1 y1 + v yv + 2 y2 )A (in dimensionless terms, i =N/A > v ), then M td = and there is no possibility o "premature sliding". I < v, a setion analysis o the through-raked, reinorement-only setion (in at with the reinorement elasti) gives readily the value o M td whih, ating together with N, will ause the extreme bars o the ompression edge to yield in ompression (Grammatikou, 213). The riterion: =N/A < v and V td = M td /L s V d +V i (14) may be used as supementary to the ondition: V y = M y /L s V R,SLS or shear sliding. This riterion involves only geometri and material harateristis o the wall and its axial ore. Shear sliding may be onsidered as very likely to our ater lexural yielding at the base, i at least one o these two riteria is met. Shear sliding is possible even in squat walls whih yield in lexure beore reahing the shear resistane V R,squat rom Eqs. (6)-(9) or μ θ =. In at, some o the walls in Group 5 have L s /h< Failure mode predition On the basis o the above, the predition o the most likely ailure mode may take ae as ollows: 1. The shear ore at yielding, V y = M y /L s, is ound rom the yield moment at the base, M y (Set. 2); 2. I L s /h 1., V R,squat is omputed rom Eqs. (6)-(9) or μ θ =; i ound less than V y, it apies; the searh ends: the wall ails in shear at a ore o V R,squat (μ θ =) beore lexural yielding; 3. For any L s /h, and unless the searh ends in 2, Eq.(14) is heked or likely "premature sliding"; 4. Unless the searh has ended in 2 or 3, the ultimate hord rotation, θ u, is alulated rom Eqs.(2); 5. The θ u -value rom 4 is translated into a value o the asti dutility ator: μ θ = u / y - 1; 6. Using the value o μ θ rom 5, the ollowing shear resistanes are omputed: a. i L s /h < 1.2, V R,squat rom Eqs. (6)-(9); b. i L s /h > 1., V R,ST rom Eqs.(4) and V R,SC rom Eq.(5);. or any L s /h, V R,SLS rom Eqs. (1)-(13); 7. I the lowest among the shear resistanes in 6 is less than V y, it apies, alongside the ailure mode; otherwise, ailure is in lexure, at a shear ore o V y and a hord rotation o θ u ; The outome o this proedure or the present dataset o wall tests is shown in Table 3. Table 3: Predited v experimental ailure mode (no. o tests) Experimental ailure mode total Predited ailure mode F ST SC squat SLS no. Flexure (F): Set. 2, Eqs.(2) Shear tension ater yielding (ST): L s /h> 1., Eqs.(4) Shear ompression ater yielding (SC): L s /h> 1., Eq.(5) Shear, as squat: L s /h < 1.2, Eqs. (6)-(9) Sliding shear ater yielding (SLS): Eqs. (1)-(13) or (12)-(14) Total no. per experimental ailure mode:

10 8. Conlusions The paper has used a database o 621 yli tests o RC walls to evaluate past models or the wall yli strength and deormation apaity to develop/alibrate new ones. From the observed damage the ailure mode has been lassiied as in lexure, diagonal tension or ompression beore or ater lexural yielding, or in sliding shear. Expressions were developed or the orresponding limit apaities: in terms o yli deormations (hord rotations, ) or lexure ailure ( u, Eqs.(2) in Set. 2); in terms o a shear ore resistane that depends on yli deormations or diagonal tension (V R,ST, Eqs.(4) in Set. 3) or ompression (V R,SC, Eq.(5) in Set. 4) ater lexural yielding, or walls with L s /h > 1.; in terms o a shear ore resistane that does not depend on yli deormations i it takes ae beore lexural yielding, or depends on them i it ours ater yielding (V R,squat, Eqs. (6)-(9) in Set. 5) or walls with L s /h < 1.2 ("squat"); in terms o a shear ore resistane that depends on yli deormations (V R,SLS, Eqs. (1)-(13) in Set. 6) or shear sliding ater lexural yielding, or walls with any L s /h (with "premature sliding" as a possibility aording to a riterion involving only geometri and material harateristis o the wall Eqs. (12)-(14)). The agreement between the predited yli strength and/or deormation apaity and the observed one per ailure mode is good, as demonstrated in Figs The predition o the most likely mode in the tests o the database is also satisatory, as shown in Table 3. Aknowledgement The researh leading to these results has reeived unding rom the European Community s 7th Framework Program [FP7/27-213] under grant agreement no (SERIES) and the Helleni General Seretariat or Researh & Tehnology under grant ERC-12 (PRESCIENT) o the Operational Program "Eduation and lielong learning", o-unded by the European Union (European Soial Fund) and national resoures. Reerenes Amerian Soiety o Civil Engineers (25) Seismi design Criteria or Strutures, Systems, and Components in Nulear Failities (ASCE/SEI 43-5). Barda, F., Hanson, J.M., Corley, W.G. (1977) Shear strength o low rise walls with boundary elements. Reinored Conrete in Seismi Zones, SP-53, ACI Speial Publiation, Biskinis D., Fardis M.N. (21a) Deormations at lexural yielding o members with ontinuous or lap-sied bars. Strutural Conrete; 11(3): Biskinis D., Fardis M.N. (21b) Flexure-ontrolled ultimate deormations o members with ontinuous or lap-sied bars. Strutural Conrete; 11(2): Biskinis D., Roupakias G., Fardis M.N. (24) Degradation o shear strength o RC members with inelasti yli disaements. ACI Strut J; 11(6): CEN (24). European Standard EN : Euroode 8: Design o strutures or earthquake resistane. Part 1: General rules, seismi ations and rules or buildings. Comite Europeen de Normalisation. Brussels. CEN (25). European Standard EN :25: Euroode 8: Design o strutures or earthquake resistane. Part 3: Assessment and retroitting o buildings. Comite Europeen de Normalisation. Brussels. ib (212) Model Code 21, Final drat. Bull. 65/66, Federation Internationale du Beton, Lausanne Grammatikou S. (213) Strength, deormation apaity and ailure mode o RC walls in seismi loading. MS Thesis, Dept. o Civil Engineering, University o Patras, Patras.