LATERAL RESISTANCE OF CONFINED BRICK WALL UNDER CYCLIC QUASI-STATIC LATERAL LOADING

Transcription

1 LATERAL RESISTANCE OF CONFINED BRICK WALL UNDER CYCLIC QUASI-STATIC LATERAL LOADING A. Bourza 1, T. Ikeoto 2 and M. Miyajia 3 1 PhD Cand., Kanazawa University, Graduate School of Natural Science and Technology, Kanazawa University, Japan E-ail: bourza@gail.co 2 Assistant Prof.,Dr. Eng., Kanazawa University, Graduate School of Natural Science and Technology, Kanazawa University, Japan E-ail: tikeoto@t.kanazawa-u.ac.jp 3 Professor, Dr. Eng., Kanazawa University, Graduate School of Natural Science and Technology, Kanazawa University, Japan E-ail: iyajia@t.kanazawa-u.ac.jp ABSTRACT: Usually in asonry building systes, the piers between openings are the ost vulnerable in case of earthquake. The failure of such walls is due in the ajority of cases to shear. Accordingly, this study is elaborated to copare three different analytical approaches regarding the prediction of lateral shear resistance of confined asonry walls. The principle is to calculate the total lateral resistance of the wall by considering the participation of both structural ebers, i.e. the brick panel and the RC tie-coluns. In this study, the tie-coluns participation is evaluated fro the dowel action of confined coluns reinforceent and the brick panel participation is evaluated using the theory of elasticity with different assuptions. A full scale experiental study on confined clay brick wall subjected to a constant vertical and quasi-static cyclic lateral loads is perfored to estiate the efficiency degree of each ethod. Depending on the ethod, the coparison of calculated values to test results has shown that the real lateral resistance is overestiated by 17.54%, 42% and 54.1%. On the light of these results, the appropriate approach to predict the shear capacity of the asonry structural eber needs to be selected carefully to be consistent to a certain extent with the results of the experiental testing. KEYWORDS: confined asonry, brick panel, tie-coluns, cyclic load, dowel action, shear capacity. 1. INTRODUCTION The developent of practical ethods for evaluating the shear strength of bearing confined asonry walls is a difficult task since experiental and analytical data of its echanical behavior are less nuerous and their interpretations are less precise. Indeed, the diversity of the available aterial and the iperfection of its production techniques ake it coplex to characterize and noralize this type of construction aterial. Most of the research works and the existing regulations applied to the construction industry, siplify the behavior of the asonry with the ai to provide practical and easier criteria for the analysis of the structural behavior. In general, these regulations recoend the use of a linear odel by considering the asonry such as a hoogeneous aterial 1). Thus, despite of the high costs, the difficulties of carrying out experiental work in the laboratory and the coplexity of the asonry characteristics; experients reain incontestably iperative for identifying the echanical paraeters of this structural syste.

2 As known, flexural failure is favored in seisic resistant design because it is accopanied by large plastic deforation and energy absorption and dissipation capacities 2). Shear failure, on the other hand, is ore brittle, with liited ductility, fro where; confining the plain asonry can reedy in a certain extent this weakness. In this respect, Toaževič and Kleenc 3) proposed a ethod to predict the shear capacity of confined asonry walls, where the seisic behavior of brick panel is odeled by considering the effect of interaction forces between bondbea, tie-coluns and asonry panel. According to Sucuoglu et al. 2) the ultiate shear strength of the asonry wall is reached when an inclined cracking initiates at the iddle of the wall due to the principal stress coponents at their critical values. Another expression proposed by Turnsek and Cacovic 4) predicts shear failure of asonry wall according to tension failure criterion, which occurs when the principal tensile stress in the wall attain the tensile strength of the asonry. The tie-coluns participation in any case is odeled by the dowel action of coluns reinforceent as developed by Priestley and Bridgean 5). In this study the three approaches entioned here will be investigated to deterine the appropriate ethod conduct to the best assessent of the shear capacity of such structural syste. 2. TEST SPECIMEN, MATERIAL CARACTERISTICS AND LOADING SYSTEM The test specien is the full scale of a coon window pier with h/l ratio equal to 1.5 surrounded by RC ties, noted JCM. The panel of the wall is coposite of Japanese solid clay brick units with 210x100x60 of noinal diensions. The bricks are laid with plain ceent ortar, with a joint thickness of 10. The surrounding frae which serves to confine the brick panel is ade by reinforced concrete. Mechanical characteristics of used aterials, details of diension, arrangeent of reinforceent and eber sections are suarized in Table 1 and Fig.1. The wall is erected on an RC rigid bea foundation. A siilar bea is cast in-place on the top of the wall after constructing the brick panel and the RC tie coluns. This bea perits a sufficient anchorage of the vertical reinforcing bars in both coluns and provides also an adequate transfer of the applied lateral load to the wall. The RC beas are bolted to the reaction frae (fixed-ended). The copressive strength of asonry is obtained by testing 03 stack bonded priss of five bricks each and jointed between the on the building face by 10 of ortar according to the specifications of LUMB1 6). The asonry tensile strength is obtained by testing 03 asonry square panel speciens (35x35x10 c) under diagonal copression load as specified by ASTM 519 (ASTM C1391). The value of the tensile strength is evaluated as the principal tensile stress in the center of the panel 1), 7) which is equal to P d /A where P d and A are the axiu diagonal copressive load and the panel section, respectively. Modulus of elasticity E,Ec and shear odulus G,G c of asonry and concrete are deterined at 1/3 of copression strength, their values are reported in Table 1. The loading syste used for perforing the experients of the confined asonry wall JCM is shown in the Fig. 2. The vertical and horizontal actuators exert forces self-controlled by coputer. Two Transducers of LVDT type are used to easure the lateral displaceent. During the test process, the vertical pressure generated by the vertical actuator, the steel loading bea and the top RC bea is set equal to 4 Kg/c 2. The cyclic loading test applied to the specien JCM was considered as a quasistatic test.

3 Cyclic horizontal displaceents of increasing aplitude have been iposed to the wall and expressed in drift angles, the first value is of 1/3200 and the latest one corresponds to the rupture of the wall. Two loading cycles have been perfored at each aplitude. The hysteresis loops, showing the relationship between the easured lateral load and the average horizontal displaceents for the tested wall are shown in Fig. 3. Table1 Material properties Property Experiental value (MPa) Brick copression strength f b 30 Mortar copression strength f or 27 Concrete copression strength f c 20 Yielding stress of vertical reinforceents f y 415 Masonry copression strength f 9 Masonry tensile strength f t 1.1 Modulus of elasticity of asonry E 8241 Shear odulus of asonry G 1723 Modulus of elasticity of concrete E c Shear odulus of concrete G c 7633 V 100 h= D4 D8 100 l= [All diensions are in ()] Figure 1 Test specien (JCM) 3. TEST RESULTS AND CRACK PATTERN During the third cycle of loading which corresponds to a drift angle of 1/1600, and at lateral load V= kn and horizontal displaceent δ= -0.49, a sall bending cracks appeared horizontally at the basis of the left RC tie. At about the sae loading level during the fourth cycle (V= kn, δ= 0.60 ), other sall cracks appeared at the interface of the brick panel and the left tie-colun, as well as a diagonally oriented cracks were observed on the strut of the wall as entioned in the Fig.4. Syetric cracks were created quickly by a negative loading within the

4 sae level of drift angle. By definition this step arks the crack liit (elastic liit) of the wall. The followed cracks propagated and spread siilarly to the crack pattern which initiated at the elastic liit. The axiu lateral load was recorded in the ninth cycle which corresponds to the story drift angle of 1/500. The average values of the axiu shear load and its corresponding horizontal deflection obtained at loading in positive and negative direction were kn and 3.26, respectively. The ductile behavior of the wall was confired by the large horizontal displaceents and the decreasing of the lateral load in the last loading cycles as shown in the Fig. 3. The openings of the diagonal cracks becae iportant and the cracks passed through the tie-coluns to for the corner-to-corner diagonal shear cracks. 4. ANALYTICAL METODS Few attepts have been ade to analytically predict the seisic behavior of confined asonry walls 8). In this respect, this study has been elaborated to copare three different approaches which lead to predict the axiu shear capacity of confined asonry walls expressed as the su of the brick panel shear resistance and the shear supported by the R.C tie-coluns Brick Panel Contribution By considering the asonry wall as an elastic, hoogeneous and isotropic structural eleent, the basic equation for the evaluation of the shear resistance of plain asonry walls can be derived by taking into account the assuptions of the eleentary theory of elasticity. Under the cobination of a vertical load V and a lateral load, the principal stresses of copression σ c and tension σ t are developed in the iddle section of the wall. Figure 2 Loading syste

5 Negative loading direction Positive loading direction 90 V (kn) δ () Colun Side view Figure 3 Lateral load-displaceent hysteresis loops of JCM JCM Figure 4 Final observed cracks In this study, three failure hypotheses for splitting are considered; failure at a critical in-plane tensile strength as proposed by Turnsek and Cacovic 4), failure at a critical tensile strength by considering the interaction effect of confineent eleents which is developed by Toaževič and Kleenc 3) and failure by a critical biaxial cobination of noral principal stresses according to Sucuoglu and McNiven 2). The three approaches are conveyed by the following Eqns. (1), (2) and (3): = A f t b σ f V t + 1 After Turnsek Cacovic (1) Where: : the lateral resistance force of asonry brick panel. b: the shear stress distribution factor depends of the height h and the width l of the brick panel, b= 1 for h/l 1, b= h/l for 1< h/l <1.5 and b= 1.5 for h/l 1.5. σ V = the average copression stress on the brick panel due to vertical load V. A = the horizontal cross-section area of the brick panel only (tie-coluns are not included). f t = the asonry tensile strength, obtained by diagonal copression test as specified by ASTM C1391 as defined previously. f = t A 2 σ V Ci 1 bci f t After Toaževič and Kleenc (2)

6 Where: l C i = 2αb the interaction coefficient (α = 5/4 is a paraeter of shape and distribution of interaction forces, h h and l are the height and the width of the brick panel, respectively). ( 1 + β ) 2 2 ( β f ) + β f σ ( 1 β ) β σ = V V After Sucuoğlu et al. (3) b A Where: β = f t/f f = the asonry copressive strength obtained by testing stack bonded priss of five bricks according to the specifications of LUMB1(1994). Pd ft = f obtained fro diagonal copression test After Yokel and Fattal (1976). ( f A 1.683Pd ) (P d and A are the axiu diagonal copressive load and the specien cross section, respectively) Tie Coluns Effect In the confined asonry syste the RC confining eleents prevent the collapse of the asonry and cause additional copression stress in the vertical and horizontal directions; hence a certain aount of additional shear can be transitted by dowel action of the vertical bars. This echanis occurs ainly at wall corners. The dowel strength of vertical reinforceents crossing the crack can be estiated by assuing the reaction of concrete on these bars as described in the Fig. 5. In confined asonry syste, the additive shear force which can be caused by stirrups is usually neglected because of the large span between two successive stirrups. owever, the aount of shear generated by the tie-coluns is evaluated by the Eqn. (4). 1 Crack 1 R c f c l M ax f c R c l 0.577l V ax = 2/3 R c 0.577l Figure 5 Dowel action echanis of vertical reinforceents

7 dowel = n Rc = n λ d f y f c (4) 3 3 Where: R c = the reaction of the concrete on the ain bar. n = the total nuber of vertical bars in the RC tie-coluns. λ = in case of elastic behavior and in case of a full plastic cross section of the ain bar. d = the ain bar diaeter. f y = the yield stress of vertical bar. f c = the copression strength of concrete. As a result, the total shear resistance of a confined asonry wall w is then: w 2 = + dowel = + 2 n λ d f y f c (5) 3 Table 2 Analytical and experiental lateral shear resistance of the wall (kn) dowel (kn) w = + dowel (kn) w Exp. (kn) ( w - w Exp. )/ w Exp. % After Turnsek After Sucuoglu After Toaževič CONCLUSION By applying Eqns. (1), (2), (3) and (4), one observes that the contribution of the asonry brick panel to the total shear capacity represents an average value of 81%, while the reaining 19% is attributed to the tie-coluns which are in agreeent with Uek (1971). Results obtained fro the three proposed approaches and test results suarized in Table 2 reveal however that the calculated value of the lateral shear resistance overestiates the real shear capacity by 17.54%, 42% and 54.1%, depending on the ethod. This can be explained in a certain way by the fact that while assessing the shear capacity of the asonry structural ebers, any factors influence their resistance; such as the heterogeneity caused by the ortar joints the variability of the copressive strength of the brick units, the intensity of the gravity load etc. On the light of these results, the appropriate approach to predict the shear capacity of the asonry structural eber needs to be selected carefully to be consistent to a certain extent with the results of the experiental testing. owever, for ore precision on the general applicability of any approach, ore experiental data are required.

8 REFERENCES 1) Yokel, F. Y., and Fattal, S, G. (1976): Failure hypothesis for asonry shear walls, J. Struct. Div., ASCE, 102(3), pp ) Sucuoglu,. and McNiven,. D. (1991) : Seisic shear capacity of reinforced asonry piers, J. Struct. Engrg.Vol. 117, No 7, pp ) Toasevic, M. and Kleenc, I. (1997): Seisic behaviour of confined asonry walls, Earthquake Engrg. Struct. Dynaics,Vol. 26, pp ) Turnsek, V. and Cacovic, F. (1971): Soe experiental results on the strength of brick asonry walls, Proc.2 nd int. brick asonry conf., Stoke-on-Trent, pp ) Priestley, M.J.N. and Bridgean, D.O. (1974): Seisic resistance of brick asonry walls, Bull. of the New Zealand Nat. Soc. for Earthquake Engrg. 7 (4) (New Zealand National Society for Earthquake Engineering, Wellington), pp ) Rile. Technical recoendations for the testing and use of constructions aterials (1994), LUMB1, Copressive strength of sall walls and priss. 7) Chiostrini, S., Galano, L. and Vignoli, A.(2000): On the deterination of strength of ancient asonry walls via experiental test, Proc. of 12th World Conf. Earthquake Engrg., CD-ROM. 8) Toaževič, M. (1999): Earthquake-resistant Design of Masonry Buildings, (Innovation in Structures and Construction, Vol. (I), Iperial college press. 9) Uek, A. (1971): Coparaison between unreinforced, confined and horizontally reinforced asonry walls, Civil Engrg. J.,Vol. 20 (Society of Civil Engineers, Ljubljana), pp