EFFECT OF WALL REINFORCEMENTS, APPLIED LATERAL FORCES AND VERTICAL AXIAL LOADS ON SEISMIC BEHAVIOR OF CONFINED CONCRETE MASONRY WALLS

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1 EFFECT OF WALL REINFORCEMENTS, APPLIED LATERAL FORCES AND VERTICAL AXIAL LOADS ON SEISMIC BEHAVIOR OF CONFINED CONCRETE MASONRY WALLS 984 Koji YOSHIMURA 1, Kenji KIKUCHI 2, Masauki KUROKI 3, Lizhen LIU 4 And Lin MA 5 SUMMARY In order to investigate the effect of wall reinforceents, applied lateral forces and vertical axial loads on seisic behavior of confined concrete asonr walls, eighteen different wall speciens with and without wall reinforceent are tested under constant copression or tension vertical axial load, and alternatel repeated lateral forces. Test results indicated that if the wall reinforcing bars are provided in both horizontal and vertical directions, then this wall sste can develop higher lateral strength and better ductilit. Also, ost of the speciens which have lower height of the applied lateral forces failed in shear failure ode. On the contrar, all the speciens having higher height of the applied lateral forces failed in flexural failure ode. Ultiate lateral shear strength increases in proportion to the applied vertical axial load. In addition, ost of the ultiate lateral strengths which failed in flexural failure odes can be predicted well b the existing theor on reinforced asonr walls. While in soe of the speciens which failed in shear failure odes can not be predicted well b the existing theor on reinforced asonr walls. One of the ain reasons to this cause is that the effect of the applied vertical axial loads on ultiate lateral shear strength is considerabl larger than the prediction based on the existing theor. INTRODUCTION Confined concrete asonr walls, which are defined as the concrete asonr walls confined b reinforced concrete (R/C) coluns and beas, are widel accepted as the structural bearing walls in Latin Aerican countries as well as the People's Republic of China. The confined asonr walls shown in Figures 1(a) and 1(b) are coposed of hollow concrete block or cla brick asonr units which are confined b cast-in-place R/C sall coluns and beas (and/or floor slabs) along the perieter of each asonr wall. These structural wall sstes are ver popular and have been frequentl designed and constructed in those earthquake countries, although the seisic behavior of these wall sstes have not been cleared sufficientl. A series of experiental studies have been conducted b authors in order to investigate the seisic behavior of confined asonr walls and to develop ore seisic reinforcing ethod for R/C confining coluns and asonr wall panels. In the previous experiental studies conducted b authors, the lateral loads carried b the confined asonr wall using the hollow concrete block asonr units were considerabl higher than the loads given b superposing the pure R/C oent resisting frae and the asonr wall. Although four longitudinal Re-bars andrectangular hoops are ordinar provided in the narrow cross-sectional area of the confining colun as shown in Figure 1(b), confined concrete asonr walls with uch ore sipler reinforcing details using the onl one longitudinal Rebar with large diaeter and circular spiral hoops for confining colun sections could develop alost the siilar seisic behavior to the walls adopting the ordinar reinforcing details for confining coluns [Yoshiura et.al., 1995] Departent of Architectural Engineering, Oita Universit, Oita, Japan Eail: kojisr@cc.oita-u.ac.jp Departent of Architectural Engineering, Oita Universit, Oita, Japan Eail: kikuchi@cc.oita-u.ac.jp Departent of Architectural Engineering, Oita Universit, Oita, Japan Eail: kuroki@cc.oita-u.ac.jp Departent of Architectural Engineering, Oita Universit, Oita, Japan Eail: litin@arch.oita-u.ac.jp Departent of Civil Engineering, Universit of Toronto, Toronto, Canada, Eail: alin@ecf.utoronto.ca

2 (a) An exaple using hollow concrete block (b) An exaple using solid cla brick units in units in Mexico, 1993 P.R. China, 1999 Main objective of the present stud shown here is to investigate the effect of wall reinforceents, height of applied lateral forces (or height of inflection point in flexural deforation of the walls) and vertical axial loads on seisic behavior of confined concrete asonr walls. Each of the experiental results presented here was alread presented in the following literatures [Yoshiura et.al., 1996, Liu et.al., 1997, Yoshiura et.al., 1998, Yoshiura et.al., 1999]. Herein, test results of eighteen speciens are rearranged and discussed in detail. TEST SPECIMENS Eighteen confined concrete asonr wall speciens with five different tpes of wall reinforceents are selected as being listed in Table 1 and Figure 2. All the speciens are approxiatel one-half scale odels of one-baone-stor asonr walls using hollow concrete block asonr units with Grade C in the Japanese Industrial Standard (JIS). Wall length (l) of the specien is 1,8, and wall height (h) is 1,512 as shown in Figure 2, where wall height to wall length ratio (h/l) is approxiatel.84. Thickness of all the asonr walls are 1c, and all the asonr walls are confined b the cast-in-place R/C confining coluns with 1c x 1c cross-sections along their extree edges of the wall. 1 Grouting ortar 1 R/C confining ertical Re-bar colun A-A' Section C C' ' Strain gages R/C collar bea Spiral hoops (2-675) A' A Joint ortar Horizontal Re-bar (14) R/C confining colun ertical Re-bar (14) otto of wall Wall height: h1512 Top of wall 15 Grouting ortar Spiral hoop (675) Longitudinal Re-bar (1-19) Spiral hoops (2-675) R/C foundation bea 21 C 16 Wall length: l ' Section C-C' Section Unit in Figure 2: Test specien having wall Re-bars in both horizontal and vertical directions 2 984

3 The speciens are classified into two test series, L-series and H-series, depending on the adopted test setup as shown in Figures 3(a) and 3(b). Each of the specien is designated b a four sbol code, such as L1-H4V4- HC. The first letter L or H represents that the height of repeated lateral forces is low or high as shown in Figures 3(a) or 3(b), respectivel. Based on the calculation results on the height of inflection point in the first stor asonr bearing walls in case when two different five-stor asonr buildings are subjected to design earthquake forces [Liu et.al., 1997], the heights of repeated lateral forces adopted for the test were deterined as.67 ties the wall height (h in Figures 2 and 3) for L-series speciens and 1.11 ties the wall height for H- series speciens, respectivel. The second nueral 1 after the letter L or H represents that the onl one longitudinal Re-bar with bar-size of D19 (or #6) is provided in each of the R/C confining colun-section, which is transversel reinforced b circular spiral hoops of D6 (#2) as shown in Figure 2. The third and fourth sbols such as H4 or V4 indicate that the horizontal or vertical Re-bars are respectivel provided in the asonr wall, where spacings of these wall reinforceents are 4 c and 4 c, respectivel. In case of the HV speciens, there are no wall Re-bars in both horizontal and vertical directions. The final letter HC or LC represents that the constant vertical axial load applied to the speciens are respectivel High Copression or Low Copression. while in case of T, constant vertical Tension axial load is applied to the wall speciens. Based on the theoretical stud for investigating the vertical axial loads induced in the first stor asonr bearing Table 1: List of test speciens Speciens Height of applied Wall reinforcing bar Axial stress lateral forces Horizontal bar Vertical bar (MPa) L1-HV-HC None None L1-H4V4-HC 1.8 L1-HV-LC None None L-series L1-H4V4-LC h L1-H4V-LC None ( h : Wall height) L1-HV4-LC None L1-H2V-LC None L1-HV-T None None L1-H4V4-T H1-HV-HC None None H1-H4V4-HC 1.8 H1-HV-LC None None H-series H1-H4V4-LC h H1-H4V-LC None ( h : Wall height) H1-HV4-LC None H1-H2V-LC None H1-HV-T None None H1-H4V4-T * Spacing of bars in Table 2: Mechanical properties of concrete, pris, joint ortar and reinforcing bars Copressive strength of Copressive Copressive Speciens concrete (MPa) strength of strength of joint Yield strength of Re-bar (MPa) Colun Bea pris (MPa) ortar (MPa) D6 (#2)* D1 (#3) D19 (#6) L1-HV-HC L1-H4V4-HC L1-HV-LC L1-H4V4-LC L1-H4V-LC L1-HV4-LC L1-H2V-LC L1-HV-T L1-H4V4-T uneasured uneasured H1-HV-HC H1-H4V4-HC H1-HV-LC H1-H4V4-LC H1-H4V-LC H1-HV4-LC H1-H2V-LC H1-HV-T H1-H4V4-T *.2% offset ield strength 3 984

4 walls of the five stor buildings [Yoshiura et.al., 1998], the values of vertical axial loads are deterined as 1.8MPa for HC speciens,.84mpa for LC speciens and -.11MPa for T speciens, respectivel. Mechanical properties of aterials used for all the speciens are shown in Table 2. TEST SETUPS Test setups adopted for L-series and H-series tests are respectivel shown in Figures 3(a) and 3(b). Corresponding constant vertical axial load was applied b a hdraulic jack (V), and alternatel repeated lateral forces were applied b another double-acting hdraulic jack (H) as shown in Figure 3. As entioned before, the height of the repeated lateral forces applied to the L-series speciens is.67 ties the wall height (h), while in the H-series speciens 1.11 ties the wall height is adopted. Based on those height of the applied lateral forces and the wall height to wall length ratio of the speciens, the shear span to depth ratio (M/Qd) of the L-series and H-series tests becoe to be.58 and.96, respectivel. One auxiliar jack (A) installed between Loading frae and Reaction frae is for counterbalancing and setting the test speciens. Iportant displaceents and strains in reinforcing bars and wall surfaces were easured b displaceent transducers and strain gages, and all the inforation easured were processed siultaneousl b a personal coputer. Reaction wall Auxiliar jack (A) Loading bea Reaction frae Hdraulic jack (V) : Displaceent transducer Reaction wall Reaction frae Auxiliar jack (A) Loading bea Positive Hdraulic jack (V) Positive Negative Double acting hdraulic jack (H) Test bed.67x h Specien h1512 (Wall height) (Unit in ) Negative Double acting hdraulic jack (H) Test bed 1.11x h Specien h1512 (Wall height) (a) Test setup for L-series speciens (b) Test setup for H-series speciens Figure 3: Test setups TEST RESULTS Soe of the tpical hsteresis loops between applied lateral force (Q) versus stor drift (R) relations are shown in Figures 4(a) through 4(e). In Figure 4, the stor drift (R) is defined as the interstor displaceent between top and botto of the wall divided b the wall height of the specien (h), and ean shear stresses (τ ), which are defined as the applied lateral force divided b the gross horizontal sectional-area of the wall, are also shown in Figures 4(a) through 4(e). Observed ultiate lateral strengths and failure odes of all the eighteen speciens are listed in Table 3. Envelope curves obtained fro the Q-R hsteresis loops of all the eighteen speciens are shown in Figures 5 and 6, where lateral forces (Q) are given b the siple average calculated fro the positive and negative-loading curves. A light dashed line parallel to the horizontal axis in each figure represents the allowable lateral shear force provided b the current AIJ (Architectural Institute of Japan) Design Standard for Hollow Concrete-block Masonr Structures [Architectural Institute of Japan, 1994]. The test results obtained can be suarized as; 1. Ultiate lateral strengths of the test speciens are at least ore than 1.8 ties the AIJ allowable design strength as shown in Figures 5 and In Figures 5 and 6, ost of the L-series speciens failed in shear failure ode at around the stor drifts of.2x1-2 rad to.3x1-2 rad before developing their flexural oent capacities. On the contrar, all the H- series speciens failed in flexural failure ode in the region of the stor drifts of.2x1-2 rad to.3x1-2 rad, 4 984

5 Lateral force: Q (kn) Lateral force: Q (kn) Theoretical [SL] 2. 3 lateral strength [ S ] [ S ] =539 (kn) [ S ] [F S] Positive Negative (a) L1- H4V4-HC [F S] (d) H1- H4V4-HC [SL] [F S] Figure 4: Q-R Hsteresis loops of tpical speciens (b) L1- HV-LC [F S] (e) H1- H4V-LC Stor drift: R(x1-2 rad) Stor drift: R(x1-2 rad) Shear stress Fτ (MPa) [ S ] [Rearks] (c) L1- H4V4-LC Shear stress Fτ (MPa) : Predicted ultiate flexural strength : Predicted ultiate shear strength : Initial flexural crack in colun : Initial shear crack in asonr wall : Initial tension ield in vertical Re-bar in asonr wall : Initial tension ield in horizontal Re-bar in asonr wall : Initial tension ield in longitudinal Re-bar in confining colun [ F ]: Flexural failure, [ S ]: Shear failure [SL]: Sliding failure Table 3: Ultiate lateral strengths and failure odes Speciens Observed experiental results Positive loading Negative loading Ultiate Ultiate Failure Failure strength strength ode* ode* +Q ax (kn) -Q ax (kn) Average ultiate strength Q ax (kn) Prediction b existing theor Ultiate strength Shear ode Q su (kn) Flexure ode Q u (kn) Predicted failure ode* Q ax /Q u or (Q ax /Q su ) L1-HV-HC 343 S 356 S S (1.64) L1-H4V4-HC 395 S 411 S S (3) L1-HV-LC 239 SL 246 SL S - L1-H4V4-LC 3 S 285 S S (1.27) L1-H4V-LC 24 S 242 SL S - L1-HV4-LC 221 S 242 S S (1.28) L1-H2V-LC 29 S 24 S S (1.14) L1-HV-T 15 S 16 F S S - L1-H4V4-T 21 F SL 212 F SL S.89 H1-HV-HC 264 F S 268 F S S.99 H1-H4V4-HC 37 F S 314 F S S.95 H1-HV-LC 186 F S 185 F S F 1.8 H1-H4V4-LC 24 F S 218 F S F 1. H1-H4V-LC 196 F S 186 F S F 1.1 H1-HV4-LC 222 F S 228 F S S.98 H1-H2V-LC 195 F S 183 F S F 1.9 H1-HV-T 13 F S 99 F S F 1.9 H1-H4V4-T 147 F SL 144 F SL F.98 * S: Shear failure, F: Flexural failure, SL: Sliding failure and finall failed in shear failure ode. Ultiate lateral strengths carried b the L-series speciens are generall higher than the H-series speciens. It is noted that ductilit (or deforabilit) developed b the L-series speciens are considerabl saller than the H-series speciens. 3. In Figure 6, the ultiate lateral strengths carried b the speciens becae graduall higher with the increase of the vertical axial load applied to the specien. On the contrar, ductilit developed b the speciens becae graduall saller with the increase of the vertical axial load. 4. Aong the L-series test speciens, the L1-VH-LC and L1-V4H-LC speciens without an vertical wall Re-bars did not failed in flexural or shear failure ode but failed in sliding failure ode. The sliding failure occurred between botto of the asonr wall panel and top surface of the R/C foundation bea at about the stor drift of.15x1-2 rad

6 Lateral force: Q (kn) L1-HV-LC L1-HV4-LC L1-H4V4-LC L1-H2V-LC L1-H4V-LC AIJ Allowable design strength (.3MPa).5 1. Stor drift: R (x1-2 rad) (b) H-series speciens Figure 5: Q-R (a) Envelope L-series speciens curves of the speciens with different wall Re-bars Shear stress: τ (MPa) Lateral force: Q (kn) Flexural failure Shear failure Sliding failure H1-HV4-LC H1-H4V4-LC H1-H4V-LC H1-H2V-LC H1-HV-LC AIJ Allowable design strength (.3MPa).5 1. Stor drift: R (x1-2 rad) Shear stress: τ (MPa) Lateral force: Q (kn) L1-H4V4-HC L1-HV-HC L1-H4V4-LC L1-HV-LC L1-HV-T L1-H4V4-T AIJ Allowable design strength (.3 MPa).5 1. Stor drift: R (x1-2 rad) Shear stress: τ (MPa) Lateral force: Q (kn) (a) L-series speciens 5. In case of the asonr wall speciens with both vertical and horizontal wall Re-bars, their ultiate shear Figure 6: Q-R Envelope curves of the speciens subjected to the different vertical axial loads AIJ Allowable design strength (.3 MPa).5 1. Stor drift: R (x1-2 rad) strengths becoe higher than the speciens without an wall reinforceents or with wall reinforceents in onl one direction as shown in Figure 5(a). Flexural failure Shear failure Sliding failure H1-H4V4-HC H1-HV-HC H1-H4V4-LC H1-HV-LC H1-H4V4-T (b) H-series speciens H1-HV-T Shear stress: τ (MPa) DISCUSSIONS Evaluation of Ultiate Lateral Strengths b Existing Equations Dotted lines in Q-R hsteresis loops shown in Figures 4(b) through 4(e) represent the ultiate lateral strengths deterined b the ultiate flexural oent capacit at the botto of each wall, which is calculated b using Equation (1). Also, dashed lines in Figures 4(a) through 4(e) are the ultiate lateral strengths in shear failure ode of the asonr wall with flexural reinforceent in its wall-edges (or R/C confining coluns), which is calculated b using Equation (2). Equations (1) and (2) are for the reinforced hollow concrete-block bearing walls without an R/C confining coluns [Architectural Institute of Japan, 199]. M u 6 ( a l +.5a l +.5N l ) 1 = t w w w w w (1) Q su.76 3 = ku k p ( ) f' ph h f' t j (2) h' d +.7 where, M u : ultiate flexural oent capacit ( kn ), Q su : ultiate lateral shear strength (kn) 6 984

7 a t : cross-sectional area of longitudinal Re-bar in confining colun ( 2 ) : ield strength of longitudinal Re-bar in confining colun (MPa) l w : distance between longitudinal Re-bars in confining coluns () a w : cross-sectional area of vertical Re-bars in asonr wall ( 2 ) w : ield strength of vertical Re-bar in asonr wall (MPa), N : applied vertical axial load (N) k u : reduction factor for partiall grouted concrete asonr =.64 k : 1.16, ( ) (%) p p.3 t p = a t d, h' : height of asonr wall () t t d : distance between extree copression fiber and tension bar in confining colun () f' : copressive strength of pris (MPa) p h : a h ( t x), ah : cross area of horizontal Re-bars in asonr wall ( 2 ) x : spacing of horizontal Re-bars in asonr wall () : ield strength of horizontal Re-bar in asonr wall (MPa) : vertical axial stress in asonr wall (MPa), t : thickness of asonr wall (), j : ( 7 8)d () h Observed axiu lateral strengths divided b the theoretical ultiate strength in the corresponding failure odes (Q ax /Q u or Q ax /Q su ) are listed in Table 3. It can be understood that the observed flexural strengths can be evaluated within the error of 1 percents b Equation (1). On the contrar, observed axiu lateral strengths of the speciens failed in shear failure ode are generall higher than those of the evaluations especiall in the speciens subjected to higher copression axial load as shown in Q ax /Q su in Table 3. Effect of Applied Vertical Axial Loads on Ultiate Shear Strength In Figure 7, ultiate shear stresses ( τ u ) of the two tpes of speciens with and without wall reinforceents, which failed in shear failure ode, are plotted against the vertical axial stresses ( ) applied to the speciens. In this figure, it appears that τ u increases in proportion to. The regression lines for each tpe of the speciens are given b solid lines in Figure 7, where increasing factors of which are.74 for the specien having both horizontal and vertical wall reinforceents and.68 for the specien without an wall reinforceents. Effect of the vertical axial stresses on the ultiate shear stress ( τ u ) can be expressed b Equation (3) b adopting the siple average of these increasing factors. τ u =.71 (3) This increasing factor of.71 is considerabl larger than.2 which appears in Equation (2). This is one of the ain reasons wh observed axiu lateral strengths of the speciens, which failed in shear failure ode, are generall uch higher than those of the evaluations given b Equation (2). Effect of Wall Reinforceents on Ultiate Shear Strength The effects of applied vertical axial loads on ultiate shear stresses are subtracted fro the observed ultiate shear stresses of all the speciens failed in shear b using Equation (3), and reaining shear stresses ( τ u ) ) are plotted against their horizontal reinforceent indexes ( p f' h h ( = ) in Figure 8. Dashed and dotted line represents the relationship between τ u ( =) and ph h f' provided b Equation (2), where siple average of the τ u ( =) of the speciens without an wall reinforceents is substituted into the first ter in the right side of Equation (2). Effect of ph h f' on τ u of the speciens with wall reinforceents in both horizontal and vertical directions can be evaluated well b Equation (2), but corresponding effect of the speciens with onl horizontal reinforceents sees to be evaluated uch higher than being shown in Figure 8. Effect of Height of Lateral Forces on Ultiate Shear Strength Effect of the height of repeated lateral forces on ultiate lateral shear strength could not be cleared because all the H-series speciens failed in flexural failure ode as shown in Table 3. Further experiental studies are 7 984

8 τ u = Q u /(te j) (MPa) L1-H4V4-HC (Positive, Negative) L1-H4V4-LC (Positive, Negative) L1-HV-HC (Positive, Negative) L1-HV-T (Positive) (MPa) τ u( =) = τ u - τ u (MPa) 2 1 FL1-HV-HC i Positive, Negative j œfl1-h4v4-hc i V j F L1-H4V4-LC i V j.5 FL1-HV4-LC i V j FL1-H2V-LC i V j FL1-H4V-LC i Positive j FL1-HV-T i V j p f' (MPa) h h Figure 7: τ u versus Figure 8: τ u ( =) versus ph h f' necessar to investigate the effect of height of the repeated lateral forces b using the additional speciens, where wall height to wall length ratio (h/l) of the speciens are lower than the present H-series speciens. CONCLUSIONS 1. Ultiate lateral strengths of the confined concrete asonr walls failed in flexure can be predicted well b the existing theor on reinforced asonr walls. 2. Shear strengths of the confined concrete asonr walls increases in proportion to the applied vertical axial load. The increasing factor of which is considerabl higher than that appeared in the existing equation on reinforced asonr walls without an confining colun. 3. Ultiate lateral shear strengths of the confined concrete asonr walls can be iproved better if the wall reinforcing bars are provided in both horizontal and vertical directions. The increasing factor of those ultiate lateral shear strength can not be predicted well b the existing equation on reinforced asonr walls. 4. Effect of the height of applied repeated lateral forces on ultiate lateral shear strengths can not be cleared in the present paper because all the H-series speciens, which have higher height of applied lateral forces, failed in flexural failure ode. Further experiental stud is necessar to clarif the effect of the height of repeated lateral forces on the ultiate shear strength of the confined concrete asonr walls. REFERENCES Architectural Institute of Japan (199), Ultiate Strength and Deforation Capacit of Buildings in Seisic Design, in Japanese. Architectural Institute of Japan (1994), AIJ Standards for Structural Design of Masonr Building Structures (1989 Edition), in English, (Spanish version is also available in CENAPRED (National Center for Disaster Prevention) in Mexico D.F., MEXICO.). Architectural Institute of Japan (1997), 1997 edition of the AIJ Standards for Structural Design of Masonr Structures, in Japanese. Liu, L., Kajiwara, K., Yoshiura, K., Kikuchi, K. and Sanchez, P.T. (1997), Effect of Applied Lateral Forces and Wall Reinforceent on Seisic Behavior of Confined Concrete Masonr Walls, Proc. of the 11th International Brick/Block Masonr Conference, Shanghai, P.R. of China, Vol.1. pp Yoshiura, K. and Kikuchi, K. (1995), Experiental Stud on Seisic Behavior of Masonr Walls Confined b R/C Fraes, Proc. of the Pacific Conference on Earthquake Engineering Vol. 3, Melbourne, Australia, pp Yoshiura, K., Kikuchi, K., Okaoto, T. and Sanchez, P.T. (1996), Effect of Vertical and Horizontal Wall Reinforceent on Seisic Behavior of Confined Concrete Masonr Walls, Proc. of the 11th World Conference on Earthquake Engineering, Acapulco, Mexico, Paper No.191. Yoshiura, K., Kikuchi, K., Kuroki, M., Liu, L., Kajiwara, K. and Ushijia, M. (1998), Effect of the Vertical Axial Loads and Wall Reinforceents on Seisic Behaviour of Confined Concrete Masonr Walls, Proc. of the 23rd Conference on Our World in Concrete & Structures, Singapore, pp Yoshiura, K., Kikuchi, K., Kuroki, M., Liu, L., Ma, L. (1999), Effect of Vertical Axial Loads and Repeated Lateral Forces on Seisic Behavior of Confined Concrete Masonr Walls, Proc. of the 8th Canadian Conference on Earthquake Engineering, Vancouver, pp