EXPERIMENTAL RESEARCH ON ASEISMIC STRENGTHENING OF BLOCK MASONRY BUILDINGS USING THIN STRUCTURAL COLUMNS

Transcription

1 11th INTERNATIONAL BRICKlBLOCK MASONRY CONFERENCE TONGJI UNIVERSITY, SHANGHAI, CHINA, OCTOBER 1997 EXPERIMENTAL RESEARCH ON ASEISMIC STRENGTHENING OF BLOCK MASONRY BUILDINGS USING THIN STRUCTURAL COLUMNS Zhang Yu 1 Wang Zhengyun2 Zhang Weiping 3 1. ABSTRACT T vercme the shrtcmings f usual structural clumn due t its clumsiness, the strengthening f blck masnry buildings by use f thin structural clumns is intrduced in this paper. The tests f three masnry walls and tw single-strey masnry buildings under cyclic lading have been carried ut. Test results shw that the prpsed strengthening scheme can meet the anticipated requirement f earthquake resistance with the advantages f being easy fr practice, cnvenient t cnstruct and pleasing t the eye. It can als be used as a reference fr aseismic strengthening f masnry buildings. 2. INTRODUCTION A number f middle-size PF A blck masnry buildings, whse earthquake resistant measures had been left ut f cnsideratin, had been built in Shanghai during 1950's - 70's. Nw Shanghai has been cnsidered as seven-degree aseismic regin, s the study n methds t strengthen the existing blck masnry building has been placed n the agenda. T vercme the shrtcmings f a usual structural clumn due t its big dimensin and induced clumsiness, instead fusual structural KEYWORDS: Aseismic Strengthening, Blck Masnry Building, Structural Clumn I Prfessr, Department f Structural Engineering, Tngji University, 1239 Siping Rad, Shanghai , China 2 Engineer, Design Institute f Qingda, Shandng , China ) Graduate Student, Department fstructural Engineering, Tngji University, 1239 Siping Rad, Shanghai , China 626

2 ciumn. 240 x 70mm and 400 x 70mm thin structural clumns are intrduced. The aseismic behavir f masnry walls befre and after strengthening with the abve tw different methds has been tested under cyclic reversed lading, and the test results betre and after strengthening are cmpared. In additin, cmpared with the unstrengthened building, ne ne-third scale single-strey blck masnry building has als been tested under cyclic lading. Experimental results and aseismic cajculating shw that the prpsed aseismic strengthening scheme can meet the anticipated requirements f earthquake resistance. 3. OUTLINE OF TEST 3.1 Test Specimens Three half scale silicate blck masnry walls and tw ne-third scale single-strey blck masnry building have been tested. One wall and ne building are kept unstrengthened as base specimen. The strength grades f masnry and mrtar mix are MU7.5 and MU5, respectively. Accrding t statistical data f20 types fmiddle-size blck masnry buildings cnstructed during 50's - 70's, the height t width rati f the wall is taken as Tw f the walls are strengthened by use f thin RC structural clumns, dimensins f the structural clumns are shwn in Table 1 and Fig.l. The structural clumns are reinfrced with 3<1> I O and made frm C20 cncrete. Cnsidering that the mrtar strength used in third stry r abve is usually lwer than that in the bttm tw stry, the vertical cmpressive stress applied n specimens is taken as that f the third stry, that is, cr=0.325mpa. The Blck masnry building is strengthened with thin structural clumns reinfrced with 3<1> 10, and 70 x 180mm girth reinfrced with <l)6@ 150mm, The thickness f the flr slab is 60mm. Details and dimensins fthe specimen are given in Fig.2. N. DQ-l DQ-2 DQ-3 FQ-l FQ-2 Table 1 Parameters f Specimens Specimen Dimensins Strengthening Methd Type H x W x T(mm) Single Wall 1220 x 3620 x Single Wall 1220 x 3620 x 240 Thin Structural Clumn Single Wall 1220 x 3620 x 240 Thin Structural Clumn Building 1220 x 3620 x Building 1220 x 3620 x 2240 Thin Structural Clumn+Glued Plate Size f Structura! Clumn (mm) x x x 70 Tensin Rd L ~ ~~~ _N* J 4- b J (a) DQ walls Fig.l. (b)dq-2 structural clumn (c)dq-3 structural clumn Specimens fblck masnry walls 627

3 Girth "1.,. t N... N ~~[."" ~t===~=========tjj ~ ' " l:l 2-2 Fig.2. Single-strey blck masnry building mdel ~4i' ~ 3.2 Lading Methd and Instrument Arrangement Vertical cmpressive stress is unifrrnly applied n the tp f the waus by fur simultaneus il jack, lateral lad is applied by the pseud-dynamic test machine n the axis f distributin beam t simulate the earthquake frce. At first the lad is cntrued by frce, after limit lad it is cntrued by displacement. Electrical resistance strain gauges n the lwer part f lngitudinal reinfrcement in structural clumns are used t measure the steel strain. Tw displacement meters are arranged n the axis f the tp distributin beam and the bttm beam, respectively, and three displacement meters are als fixed n the upper, middle and lwer part f the structural clumn t investigate lateral displacement. The lading methd f the blck masnry buildings is similar t that f the waus, but tw pseud-dynamic test machine are used t lad simultaneusly n the tw respnding strengthened waus, the cmpressive stress applied n each wau is als Q.325Mpa. 3.3 Test ResuIts (1) Failure Pattern When the single wau was laded l1p t po percent f limit lad, the vertical mrtar jint between the tw blcks in the middle f the wau began t crack. With further lading, cracking develped frm vertical jint t hrizntal jint in a step-like pattern. Due t the stiffness reductin f the wall after cracking, displacement and lad were n lnger in linear relatinship. Cracking develped rather quickly in the unstrengthened DQ-l, and cracking develped slwly in the strengthened DQ-2 - DQ-3. When the lateral frce was up t 90 percent f the limit lad, fracture in the blck masnry culd be fund at the bttm f the flange f DQ-l, but the clumns and blck masnry didn't crack, and the steel strain in structural clumns was very smau «450flE). With the increasing f lateral lad, steel strain increased abruptly in the clumns fdq-2, DQ-3. Cracking in the flange develped slwly due t cnfining fthe structural clumns, the blck masnry at the bttm fthe flange fdq-l wau was cmpressed int fracture with 10mm vertical cracking at 8mm lateral displacement. Shearing cracking ccurred at the bttm f the structural clumns f DQ-2 and DQ-3 waus when strain f the tensin rd reached the maximum withut yielding. Cracking and failure pattern fthe walls are shwn in Fig.3. Hrizntal cracking ccurred after th~ unstrengthened building is laded up t

4 percent f limit lad. With the increasing f lad, cracking develped in a step-like pattem. Hrizntal cracking between the tp twe blck masnry frmed int cntinuus cracking at 90 percent f limit lad. Shearing failure happened at the interface f lngitudinal and lateral walls When hrizntal displacement reached 8mm. The building strengthened with thin structural clumns cracked at 70 percent f limit lad, relatively later than the unstrengthened ne. With the further increasing f the lad, cracking develped mainly in the hrizntal directin, Only when the limit lad is apprached which is 10% greater than that f the wall strengthened with usual structural clurnns, minr vertical cracking ccurred. F1 8!l~ rr-rf , / Structural Clumn Structural Clumn (a) Walls strengthened with thin structural clurnns r.:===~=;::::~t:=~==~structural Clumn Fig.3 (b) Building strengthened with ~hin structural clumns Cracking and failure pattem f the strengthened walls and building P(kN) (a) DQ-l Fig. 4 (b) DQ-2 (c) FQ-2 Lad-displacement hysteresis curve (2) Hysteresis Curve Behavir The lad-displacement hysteresis curves f the strengthened walls and building are shwn in Fig.4. The hysteresis curve lks like a straight line and residual defrmatin hardly exist befre specimen cracks. After cracking, hysteresis lp behaves like 629

5 shuttle, and the hysteresis lps f the strengthened walls and building have bigger areas than thse fthe unstrengthened nes. Afier ultimate lad, ske1etn curves fthe hysteresis curves f the strengthened walls and building descends mre smthly than thse f unstrengthened nes, that is t say, the strengthened walls and building have greater ductility. 4. ANAL YSIS OF THE STRENGTHENING EFFECT Cracking strength, ultimate shearing strength, defrmatin capacity and energy depassitatin capacity fr the strengthened and unstrengthened walls and buildings are all presented in Table 2. Cnsidering utilizabie ductility afier limit lad, the limit displacement is defined as the displacement n the descending part f the laddisplacement skeletn curve when lad is reduced t 0.85Pu. Based n this, efficient ductility cefficient is btained. Energy dissipatin capacity is valued as the area f hysteresis curve crrespnding t limit lad. Table 2 Experimental results Shear Strength Defrmatin Capacity EDC N. Pc(kN) Pu(kN) D-y(mm) D-u(mm) D-/D-y (kn. mm) DQ-l DQ DQ FQ-l FQ 'EDC' IS shrtened frm 'Energy DIssslpatJn Capaclty'. Cnc1usins can be reached frm Table 2 as fllws: (I)Cmpared with that f the unstrengthened walls, cracking lad and ultimate shear strength f the wall strengthened with 70 x 240mm thin structural clumns increased 15% and 10%, respectively; Defrmatin capacity and energy depassitatin capacity increased 52% and 55%, respective1y. (2)Cmpared with that f the unstrengthened walls, cracking lad and ultimate shear strength f the wall strengthened with 70 x 400mm thin structural clumns increased 27% and 18%, respective1y, Defrmatin capacity and energy depassitatin capacity increased 63% and 86%, respectively. (3)Cmpared with the wall strengthened with 70 x 240mm thin structural clumns, cracking lad and ultimate shear strength f the wall strengthened with 70 x 400mm thin structural clumns increased 10% and 7.5%, respectively, Defrmatin capacity and energy dissipatin capacity increased 7% and 20%, respectively. Strengthened with 70 x 240mm thin structural clumn, girth and flr slab, the building can imprve its cracking lad, ultimate shear strength, defrmatin capacity and energy dissipatin capacity by 15.4%, 10%,4% and 53%, respectively.cmpared with the wall strengthened with pure 70 x 240mm thin structural clumn, its cracking lad, ultimate shear strength, defrmatin capacity and energy dissipatin capacity rise 18.5%, 12%, 18% and 10.8%, respectively. Cnc1usin can be reached that the 630

6 unifrmity f the building is reinfrced due t the cnfinement f the girth and the fir slab. 5. SHEARING STRENGTH OF ASEISMIC STRENGTHENED BLOCK MASONRY BUILDING The shearing strength f the strengthened wall is mainly made up f tw parts: the shearing strength f wall itself and elastic restring frce f structural clumns. Based n experimental study, masnry cracks in the middle f the wall where the maximum shearing frce ccur. The frmer inc1udes bnding shearing strength f hrizntal crack and frictin shearing strength f blck masnry alng the surface f mrtar, as shwn in Equatin (1).,= f vm + J.l. (J" (1) where: f vm - cr - Shearing strength f masnry alng hrizntal crack, Vertical cmpressive stress,!l - Frictin cefficient f blck masnry alng the surface f mrtar,!l=0.65. The latter f Equatin (1 ) inc1udes shearing strength f cncrete and dwel frce f lngitudinal steels. Shearing strength f reinfrced stee1 can be taken as '-, = f y /.J3, therefre the ultimate shear ~trength fthe structural clumn can be btained: (2) where: fc - fy - Ac - As - Axial cmpressive strength f cncrete, Tensile strength f reinfrcement, Sectin area f single structural clumn, Sectin area f reinfrcement in single structural clumn The ultimate shear strength f the wall strengthened with thin structural clumns can be calculated as fllws: (3) where: As - Area f the masnry sectin, SI,S2 - Respective reductin cefficient fmasnry and structural clumns due t their interactin, SI=0.89, S2=0.57. The difference between the calculated limit shearing strength by Equatin (3) and the measured nes is n mre than 5%. 6. ELASTIC-PLASTIC SEISMIC ANAL YSIS OF THE STRENGTHENED BLOCK MASONRY BUILDING Taking ne five-strey blck masnry building (See Fig.5) as example, this paper carried ut the elastic-plastic seismic respnse analysis f the building befre and after being strengthened. Flr slab is assumed t be abslute1y rigid. Structural gemetrical nnlinearities are neglected. Flr-t-fir shearing defrmatin is assumed t dminate in the calculating mde!. The infiuence f the interactin f structural 631

7 clumns and girth is cnsidered. Dynamic equatin is slved by the Wilsn-9 Methd.Damping matrix is the Rayleign damping matrix. Flr-t-flr restring frce mdel is taken as a cmbined mdel in which hysteresis curve f the restring frce mdel is simplified as 'Half-degradatin tri-line' befre flr-t-flr lirnit lateral strength f the building, and in which the influence f the lading stiffness reductin due t the descending part is cnsidered after limit lateral strength. t 3300 I I ~ ij i\., ' t '" Ir) '" Ir) '"... Ir) 00 a- Fig.S Plane figure fthe five-strey masnry building Fig.6 - Skeletn curve f interflr displacement Accrding t flr-t-flr ske1etn curve (as shwn in Fig.6), structural failure cri teria is described as fllws: When Ók~ó<Óy, the building is subjected t such minr damage that it need nly repainting and nt strengthening. When Óy~<óu, the building is subjected t mderate damage, its walls are seriusly destryed, it can nt serve withut strengthening. When Óu~Ó<Ód, the building is subjected t serius damage, with lcal blck masnry cmpressed int fracture. It C&Il serve after being strengthened with cnsiderable cst. When Ó~Ód, the building cllapses s that it is unable t be repaired t use. The' quantities Ó, t1k, ó y, 4u, Ód are interflr displacement and respective displacement at cracking, yielding, limit lad and limit displacement. 632

8 The Shanghai Categry III man-made earthquake wave was used as input. When the maximum acceleratin f the earthface mvement reached 280gal, the building respnded as fllws : in the directin f lateral w~ll f the unstrengthened building, the tp tw stry cracked and belnged t minr damage; the secnd and third stry belnged t mderate damage because its flr-t-flr shearing frce surpassed yielding lad, the shear frce f the bttm flr surpassed limit lad and the building was seriusly destryed, the maximum interflr displacement reached 12.7mm. After strengthening, the tp three stries belnged t minr damage, the bttm tw stries belnged t mderate damage, the maximum interflr displacement reached nly 7.3mm. In the directin fthe lngitudinal wall, the unstrengthened building cllapsed with 15. 9mm maximum interflr displacement, the strengthened ne was als seriusly destryed, but did nt cllapse, with 12.3mm maximum interflr displacement. The ca1culatin f elastic-plastic seismic respnse indicated that the displacement respnse f the building strengthened with thin structural clumns was bviusly smaller than that befre strengthening, and it culd resist the actin f a seven-degree earthquake. As far as the building is cncemed, the lngitudinal wall behaved weaker in aseismic capacity than the lateral wall. 7. CONCLUSION (I)The shear capacity f the blck masnry wall can be imprved after being strengthened by use f thin structural clumns. Althugh the shear capacity usually increases n mre than 20%, the defrmatin capacity and energy dissipatin capacity will be bviusly enhanced. The defrmatin capacity at 0.85P u can be increased by 50% r greater, hence the aseismic behavir f the blck masnry building is much imprved. (2)The thin structural clumn with mderately bigger width has better effect. Test results shwed that the wall with 70 x 400mm thin structural clumns imprved its shear strength and defrmatin capacity by 11.5% and 7% than tt>.at with 70 x 240mm thin structural clumns. (3)The ca1culated results f the prpsed methd are well in agreement with the test nes. (4 )Analytic results shw that the strengthening methd by means f the interactin f thin structural clumns, tensin rd and girth can meet the seven-degree aseismic requirement. 8. REFERENCES (I)Zhu Blng et.ai:, Study n the Imprvement Of.Aseismic Capacity fthe Masnry Building by Adding the Reinfrced Cncrete Clumns, Jumal f Tngji University, N.l, 1983 (2)Liu Xihui et.al., Study n Strengthening Aseismic Behavir f the Masnry Building by Use f Structural Clumns, Building Structure(China), N.3, 1985 (3)Niu Zeqing et.al.,aseismic Behavir f the Masnry Wall Strengthened with RC Clumns, Research Institute f Architecture in China 633'