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2 Engineering Structures 32 (21) Contents lists ville t ScienceDirect Engineering Structures journl homepge: Anlyticl study on seismic retrofitting of reinforced concrete uildings using steel rces with sher link Cengizhn Durucn, Murt Dicleli Deprtment of Engineering Sciences, METU, 6531, Turkey r t i c l e i n f o s t r c t Article history: Received 25 Novemer 29 Received in revised form 28 My 21 Accepted 29 My 21 Aville online 29 June 21 Keywords: Seismic retrofitting Reinforced concrete Building Sher link Performnce sed design This pper is focused on proposed seismic retrofitting system (PRS) configured to upgrde the performnce of seismiclly vulnerle reinforced concrete (RC) uildings. The PRS is composed of rectngulr steel housing frme with chevron rces nd yielding sher link connected etween the rces nd the frme. The retrofitting system is instlled within the ys of n RC uilding frme to enhnce the stiffness, strength nd ductility of the structure. The PRS nd conventionl retrofitting system using squt infill sher pnels (SISPs) re used in n existing school nd n office uilding. Nonliner time history nlyses of the uildings in the originl nd retrofitted conditions re conducted for three different seismic performnce levels (PLs) to ssess the efficiency of the PRS. The nlyses results reveled tht the uilding retrofitted with the PRS hs more stle lterl force deformtion ehvior with enhnced energy dissiption cpility thn tht of the one retrofitted with SISPs. For immedite occupncy PL, the mximum inter-storey drift of the uilding retrofitted with the PRS is comprle to tht of the one retrofitted with SISPs. But for life sfety nd collpse prevention PLs, the mximum interstorey drift of the uilding retrofitted with the PRS is considerly smller thn tht of the one retrofitted with SISPs. Furthermore, compred with the uilding retrofitted with SISPs, the uilding retrofitted with the PRS experiences significntly less dmge due to the more ductile ehvior of the system t the life sfety nd collpse prevention PLs. 21 Elsevier Ltd. All rights reserved. 1. Introduction In mny res round the world, reinforced concrete (RC) uildings designed using codes tht re now known to provide indequte sfety under seismic forces re potentil hzrds. In such res, the numer of RC uildings uilt prior to 198 outnumers those tht re uilt ccording to the newer codes. Therefore, these structurlly deficient uildings should e retrofitted to withstnd erthquke forces in complince with modern design codes. There re mny well known seismic retrofitting methods for RC structures. These methods cn e clssified into two groups s: (i) conventionl methods, sed on improving the strength, stiffness nd ductility of the structure, nd (ii) innovtive response modifiction methods (RMMs) imed t lleviting the effect of seismic forces on structures. methods include techniques such s dding RC infill wlls to the structurl system nd jcketing of RC columns [1,2]. The min dvntge of these methods is tht they cn e esily designed nd pplied using conventionl construction techniques. However, conventionl methods hve some technicl nd prcticl disdvntges. Strengthening the RC columns y jcketing nd/or dding RC infill wlls results Corresponding uthor. Tel.: ; fx: E-mil ddress: mdicleli@metu.edu.tr (M. Dicleli). in n increse in the weight of the structure tht produces lrger erthquke forces. Furthermore, these methods require hevy demolition nd construction work [3]. Innovtive RMMs on the other hnd include techniques such s instlling seismic isoltion devices or dmpers in the uilding. These devices re intended to modify the seismic response of the uilding so s to llevite the effect of the seismic forces. Severl retrofitting pplictions using RMMs exist in mny prts of the world [4,5]. Retrofitting techniques sed on RMMs hve significnt dvntges with respect to conventionl seismic retrofitting methods. RMMs re very effective in reducing the detrimentl effects of erthqukes on uildings. These methods usully do not require hevy demolition or construction work when used for seismic retrofitting. Nevertheless, such methods re generlly costly to implement [6]. This mkes them unsuitle for ordinry uildings. Most pplictions of RMMs re therefore found in importnt government or historicl uildings, museums or hospitls [4]. In light of the ove discussion, it is cler tht in ddition to the dvntges of ech retrofitting method, there re numerous disdvntges. Consequently, seismic retrofitting system tht comines the dvntges of oth conventionl nd modern retrofitting techniques is required. Accordingly, this reserch study is focused on proposed steel link-rce retrofitting system configured to upgrde the performnce of seismiclly vulnerle RC uildings y comining the dvntges nd eliminting most /$ see front mtter 21 Elsevier Ltd. All rights reserved. doi:1.116/j.engstruct

3 2996 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Stiffener Link Collector Bem h s P α Brces α P c d e Fig. 1. () seismic retrofitting system [8]. () Recent ppliction of the proposed retrofitting system to n office uilding. (c) Configurtion 1 of the proposed retrofitting system. (d) Configurtion 2 of the proposed retrofitting system. (e) Configurtion 3 of the proposed retrofitting system. of the disdvntges of conventionl nd modern response modifiction retrofitting techniques for RC uildings. 2. Reserch outline This reserch study is imed t studying the efficiency of the proposed retrofitting system (PRS). The efficiency of the PRS is studied using two-stories school uilding nd sixstories office uilding. These uildings re retrofitted using the PRS nd conventionl system composed of squt infill sher pnels (SISPs). The performnce of the uildings retrofitted with the PRS is then ssessed in reltion to those of the originl uildings nd the uildings retrofitted with SISPs. For this purpose, first, three possile configurtions of the PRS suitle for RC uildings re outlined. Then, 2-D nd 3-D finite element nlyses of smple two-stories RC frme with the three different PRS configurtions re conducted. From the nlyses results, the most efficient retrofitting configurtion is selected nd used throughout this study. Susequently, site specific response spectr (SSRS) re otined for retrofitting the design of the uildings used in this study nd to otin response spectrum (RS) comptile ground motions for the seismic performnce ssessment of the uildings. Next, performnce sed retrofitting design procedure tht involves RS nd nonliner sttic pushover (NLSP) nlyses is developed nd used for the design of the uildings. Following this, the seismic performnce of the uildings retrofitted with the PRS is ssessed in reltion to those of the originl uildings nd the uildings retrofitted with the SISP vi three types of nlyses. First, RS nlyses re conducted to determine the lod pttern long the height of the uildings for NLSP nlyses. NLSP nlyses re then conducted to determine the drift limits of the uildings sed on FEMA 356 [7] rottion limits nd to oserve the ehvior of the structures under monotonic lterl lod. Next, nonliner time history (NLTH) nlyses re conducted to otin the mximum drifts, memer rottions nd the deformed shpes of the uildings t the instnt of mximum interstorey drifts t three different seismic performnce levels (PLs). The mximum interstorey nd roof drifts re then compred with the drift limits of the uildings t vrious PLs to ssess the seismic performnce of the uildings retrofitted with the PRS in reltion to those of the originl uildings nd the uildings retrofitted with SISP. Dmge nlyses re lso conducted s n dditionl mesure of the seismic performnce of the uildings. 3. seismic retrofitting system Steel rces re often used for seismic retrofitting of RC uildings. However, when sujected to strong ground motions, the uckling of the rces results in loss of lterl stiffness nd strength of the structurl system [8]. Thus, seismic retrofitting of RC uildings with rces tht my potentilly uckle does not seem to e fesile retrofitting solution. Hence, this reserch study is focused on PRS tht is cple of dissipting the erthquke input energy without uckling of the rces. The PRS is composed of chevron rces nd n energy dissipting sher link connected etween the rces nd the em. Fig. 1() nd () show sketch nd photogrph from recent ppliction of the PRS y the uthors to n office uilding. Similr design or retrofitting schemes found in the literture use specil (or unconventionl) energy dissiption devices such s T-ADAS [9] (Tringulr-Added Dmping nd Stiffness) yielding under flexurl effects. In the PRS however, the sher link is designed to yield in sher efore the compression rce uckles to prevent lterl strength nd stiffness degrdtion ssocited with rce uckling. In the sher link, the sher force creted y the rces is constnt long its length. This llows for the development of lrge plstic deformtions without excessive locl strins tht normlly occur in flexurl yielding [9]. Consequently, sher yielding provides more effective energy dissiption thn tht of flexurl yielding [1] nd, hence, it is dopted for the design of the link in the PRS. The link my e uilt either using compct steel HP, Europen HE or we-stiffened W section. For the retrofitting of RC uildings, the PRS is inserted into the ys of the RC frmes to improve the stiffness, strength nd energy dissiption cpcity of the uilding s shown in Fig. 1(c) (e). The PRS cn e pplied in vrious configurtions where (i) the link nd the rces

4 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Tle 1 2-D finite element nlyses results. Configurtion Stiffness (kn/m) Bse sher (kn) Displcement (mm) Axil stress (MP) Sher stress (MP) Column Bem Column Bem 1 32, , , , re directly connected to the RC memers vi steel pltes, olts nd epoxy grouting (Fig. 1(c) - Configurtion (1), (ii) the link is connected to collector steel em ttched to the concrete em nd the rest of the memers re connected to the RC memers vi steel pltes (Fig. 1(d) - Configurtion (2) or (iii) the link nd the rces re housed in rectngulr steel frme (housing frme) where the steel frme is connected to the RC memers y olts nd epoxy grouting (Fig. 1(e) - Configurtion (3). Similr systems hve lso een proposed y other reserchers [11,12]. However, the configurtions of the proposed systems in ppliction to RC frmes hve not een studied nd verified. In most cses, the link-rce system ws directly pplied s retrofitting solution. In ddition, these reserch studies considered only hypotheticl cses with ritrry retrofitting system configurtions tht re not sed on ctul designs. Thus, the performnce ssessment of the proposed systems my not e relistic. Furthermore, comprison of the performnce of the linkrce system with those of conventionl retrofitting methods such s SISPs does not exist in the literture. This study, however, ddresses ll the issues stted ove nd provides performnce sed seismic retrofitting design methodology for the PRS. 4. Configurtion selection through finite element nlyses In this section, finite element modeling nd nlyses results of smple two-stories, one-y RC frme retrofitted using the three configurtions introduced ove re presented. From the nlyses results, the most structurlly efficient configurtion of the PRS is selected nd used throughout this study. The RC frme used in the nlyses is extrcted from one of the uildings used in this study. The frme is shown in Fig. 2(). The dimensions of the rectngulr columns on the left nd right sides of the frme re respectively.3 m.6 m nd.4 m.25 m t the first-storey level nd.25 m.4 m nd.4 m.25 m t the second-storey level. The ems t oth storey levels hve cross sections of.25 m.5 m (width depth). The retrofitting system (link, rces nd housing frme) of the smple RC frme is composed of steel HE2M, HE12M nd HE22B sections within the first storey nd HE18M, HE1M nd HE22B sections within the second storey. The finite element models of the RC frme with vrious retrofitting configurtions re uilt in two different levels of complexity. The first set of models re 2-D while the second set re 3-D solid models. The 2-D models re uilt to oserve the glol distriution of flexurl nd sher stresses within the RC memers of the frme nd to ssess the stiffness nd strength of the RC frme on the verge of yielding of the sher link for the three retrofitting configurtions. The more complicted 3-D solid models re used to oserve the stress distriutions nd concentrtions within the criticl regions of the RC frme round the link nd t the corners where the rces re connected to the frme Two dimensionl finite element modeling The 2-D finite element models of the RC frme were uilt using the nonliner finite element sed softwre ANSYS [13] s shown in Fig. 2() (d) for the three retrofitting configurtions tested. Two different em elements were used in the finite element modeling of the frme. The RC frme memers s well s the sher link nd steel frme memers of the PRS were modeled using the BEAM189 element in ANSYS [13]. The rigid joints of the RC frme were lso modeled using BEAM189 with lrge modulus of elsticity. In the cse of the steel rces of the retrofitting system, the BEAM44 element is used. BEAM189 is n element suitle for nlyzing slender to modertely stuy/thick em structures. It is sed on Timoshenko em theory. Sher deformtion effects re included. This element is well suited for liner, lrge rottion, nd/or lrge strin nonliner pplictions. BEAM44 is unixil element with tension, compression, torsion, nd ending cpilities. This element permits the end nodes to hve moment releses nd e offset from the centroidl xis of the em. Thus, it is suited for rce modeling Three dimensionl solid finite element modeling The 3-D prtil solid models of the criticl regions of the RC frme round the link nd t the corners where the rces re connected were uilt to more precisely evlute the stress concentrtions t these loctions. The models were uilt such tht the ehvior of the overll frme is simulted correctly. This required the structurl nlyses of the 2-D model of the frme under lterl lod efore the 3-D prtil models could e uilt. For the region round the link, locl T-shped section of the frme is modeled in etween the inflection points long the em to investigte the stress distriution within tht region s shown in Fig. 2() y thick solid lines. For the region round the corner where the rce is connected, 9 rotted T-shped section of the frme in etween the inflection points long the column (for the verticl memer) nd long the em (for the horizontl memer) is modeled to investigte the stress distriution within tht region s shown in Fig. 2() y thick solid lines. The solid models of the criticl prts of the frme re uilt using the progrm ANSYS [13] s shown in Fig. 2(e) nd (f) for the cses with nd without steel housing frme. An utomtic mesh genertion technique is used for the meshing of the liner elstic solid elements Anlyses results For the 2-D nlyses single lterl lod pttern is pplied on ll the frmes. The mgnitude nd distriution of the lterl lod is tken s the verge of the lods required to yield the sher link in Configurtions 1 nd 2. Consequently, the se sher is identicl for ll the three frmes considered. This enled fir comprison of the mgnitudes of the stresses in the RC memers of the frme retrofitted with three different configurtions. The results otined from the 2-D finite element nlyses re comprtively given in Tle 1 nd Fig. 2() (d). Configurtion 4 given in Tle 1 is the re RC frme without the retrofitting system. As oserved from Tle 1, the retrofitting system with the dditionl steel frme housing the link nd the rces (Configurtion 3) produces structurl system with much higher elstic stiffness compred to Configurtions 1 nd 2. Consequently, it is expected tht the lterl drift of the frme will e smller when retrofitting Configurtion 3 is used. This, in turn, my result in less

5 2998 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Moment Digrm Inflection Point F 1 V 1 F 2 α N c d e f Fig. 2. () RC frme used in the 2-D finite element nlyses s well s forces nd moment digrms used in the construction of 3-D solid models, () 2-D finite element models nd xil stresses on RC memers; Configurtion 1, (c) Configurtion 2, (d) Configurtion 3, (e) 3-D prtil finite element models nd xil stresses on RC memers; link joint of Configurtion 1, (f) link joint of Configurtion 3. dmge to the nonstructurl nd RC structurl components of the uilding during potentil erthquke. Moreover, the presence of the housing steel frme memers in Configurtion 3 incresed the se sher cpcity of the uilding y round 3% (until yielding of the link tkes plce) compred to Configurtions 1 nd 2. This mens tht the uilding retrofitted with Configurtion 3 my remin within the elstic rnge t higher seismic lods. Configurtion 3 is lso useful for providing dditionl verticl support to the RC frme to resist the grvity lods ginst ny potentil collpse sitution during or fter dmging erthquke. Tle 1 lso shows the mximum xil (minly due to ending moment nd xil lod exerted y the rces) nd sher stresses

6 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Tle 2 3-D finite element nlyses results for the criticl joints. Configurtion Axil stress (MP) Column (upper) Column (lower) Bem Upper link joint 1 N.A N.A 83 3 N.A N.A 5.5 Lower left joint within the RC memers of the retrofitted frme. For ll the cses considered the sher stresses re smll. The xil stresses re lrger for Configurtion 1 nd comprle for Configurtions 2 nd 3. The stresses re more intense round the link, t the joints nd t support loctions s oserved from Fig. 2() (d). The RC memers of the re frme (Configurtion 4) hve very lrge concrete xil nd sher stresses when it is sujected to the sme lterl lod s the other retrofitted frmes. This clerly shows the enefits of using the proposed retrofitting system even within the elstic rnge (efore the link yields). The results otined from 3-D finite element nlyses re comprtively evluted for two configurtions (1 nd 3) of the proposed seismic retrofitting system. These results re presented in Tle 2 for the upper link joint nd the lower left joint of the smple frme for the two nlyses cses considered. The distriutions of the xil stresses re shown in Fig. 2(e) nd (f) for the link joint respectively for Configurtions 1 nd 3. The results for the lower left joint re similr. The nlyses results reveled tht high xil stress concentrtions round the connections of the link nd the rces to the RC frme memers occur in the cse of Configurtion 1. This is indictive of locl dmge to the concrete memers. Such locl dmge my e mplified under cyclic loding resulting in loosening of the connections of the steel memers to the RC memers of the frme. In the cse of Configurtion 3 however, the presence of the steel housing frme results in more even distriution of forces trnsferred from the link nd rces to the RC memers of the frme. This results in much lower stresses in the RC memers s noted from the 5.5 MP nd 1 MP stresses in Tle 2. The findings from the nlyses of more refined 3-D solid models re in good greement with those from the nlyses of the 2-D models. Consequently, it is expected tht Configurtion 3 will offer etter seismic retrofitting solution for RC uildings. Accordingly, Configurtion 3 is selected for the retrofitting of the uildings used in this study. 5. Description of the uildings used in this study Two existing uildings re selected to study the structurl performnce with the PRS. The uildings hve some common properties. These properties re; nerly symmetricl floor plns, moment resisting RC frme system (i.e. no sher wlls) nd poorly detiled RC structurl memers. Furthermore, oth uildings re locted in res with high risk of seismic ctivity. Both uildings hve mjor deficiencies ccording to the current Turkish seismic design code [14]. These deficiencies re; insufficient confinement of the columns in the plstic hinging region, indequte memer sizes ccording to the code, (2 35 mm is the minimum llowed memer cross section re), lck of cpcity protected design tht leds to sher filure in some memers, the use of plin rs in the construction where the code requires deformed rs, indequte concrete compressive strength nd lck of sher wlls in the structurl system of oth uildings. The first uilding is two-storey school uilding. A photogrph of the uilding nd typicl floor pln re shown in Fig. 3() nd (). The school ws uilt in 1987 in complince with the 1975 Turkish Seismic Design Code [15]. Therefore, the seismic cpcity of the uilding is not sufficient ccording to the current 27 edition of the sme code [14]. It is noteworthy tht the Turkish seismic design code for uildings is very similr to the Interntionl Building Code [16]. Site soil type is clssified s type C per Turkish Seismic Design Code [14] or Interntionl Building Code [14]. The heights of the first nd second stories re 3.2 m. The sl thickness is.2 m in the first nd.15 m in the second storey. The first nd second-storey structurl memer sizes re identicl. The dimensions of most of the rectngulr columns re.3.5 m while the remining columns ner the wider ys hve cross sections lrger thn.3.5 m nd up to.55.5 m. All the ems supporting the lrge m sls re.3.4 m wide nd.7 m deep, while the remining ems re.3 m wide nd.5 m deep. The RC frme ys of the uilding re filled with rick msonry infill (BMI) wlls. The mterils considered in the modeling nd retrofitting design re low strength concrete clss C16 (f c = 16 MP nd E c = 18,8 MP) nd the plin reinforcing steel rs of clss St.22 (f sy = 22 MP nd E s = 2, MP) ccording to Turkish stndrds. The second uilding is six-storey RC office uilding. A photogrph of the uilding nd typicl floor pln re shown in Fig. 3(c) nd (d). The uilding ws uilt in Therefore, the seismic cpcity of the uilding is not sufficient ccording to the current Turkish seismic design code [14]. The soil chrcteristic of the site is clssified s group D. The height of the first storey (sement) is 2.9 m while tht of the second storey is 3.85 m (ove the entrnce floor). The heights of the remining stories re 2.75 m. The sl thickness is.1 m in ll the stories. The em sizes (.1 m wide nd.5 m deep) in ll the stories re generlly identicl nd the column sizes grdully decrese from.5.5 m t the lower-storey levels to.2.2 m t the upper-storey levels. Most of the RC frme ys re filled with timer window frmes nd limited numer of them re filled with thin gypsum wlls with negligile structurl resistnce. The mterils considered in the modeling nd retrofitting design re low strength concrete clss C13 (f c = 13 MP nd E c = 16,9 MP.) nd the plin reinforcing steel of clss St.22 (f sy = 22 MP nd E s = 2, MP). For the two uildings, the RC memers tht my fil in sher prior to the formtion of flexurl plstic hinges t the memer ends re determined. For the school uilding, it is found tht none of the memers fil in sher prior to the formtion of flexurl plstic hinges. However, for the office uildings, It is found tht few columns lck dequte sher cpcity to llow for flexurl plstic hinge formtion. To prevent such poor seismic performnce, these memers re retrofitted using four steel ngles ttched y olts nd epoxy grouting t column corners nd horizontl nd digonl rces connected etween the steel ngles to upgrde the sher cpcity while keeping the flexurl cpcity t the sme level s shown in Fig. 3(e) (the steel ngles used re terminted t distnce of 5 mm from the memer ends to void flexurl strengthening). Following this locl retrofitting, none of the memers will fil in sher prior to the formtion of flexurl plstic hinges t the memer ends. Moreover, in the cse of the office uilding the seismic retrofitting of the foundtion ws not required ut for the cse of the school uilding limited strengthening of the foundtions ws performed. 6. Site specific response spectr In this study, SSRS re minly required for conducting RS nlyses s prt of the itertive seismic retrofitting design procedure, for determining the lterl lod pttern long the height of the uilding for NLSP nlyses nd for otining site specific ground motions for NLTH nlyses conducted for the

7 3 C. Durucn, M. Dicleli / Engineering Structures 32 (21) d c e Sher Crcks Fig. 3. () School uilding. () Floor plns of the school uilding. (c) Office uilding. (d) Typicl floor pln of the office uilding (dimensions re in meters). (e) Column retrofitting scheme. seismic performnce ssessment of the uildings. The procedure for constructing the SSRS for the uildings under considertion is tken from the Turkish Repulic Ministry of Trnsporttion Rilwys, Hrors nd Airports Construction Generl Directorte Design Code [17]. The procedure requires the site soil type nd loction coordintes of the uildings to otin the necessry prmeters for constructing the SSRS. SRSS re otined for three different erthquke levels with different proilities of eing exceeded (5% in 5 yers, 1% in 5 yers nd 2% in 5 yers) for the performnce sed seismic retrofitting design nd nlyses of the uildings under considertion. Fig. 4() nd () show the plots of the SRSS for the three erthquke levels used in the retrofitting design nd seismic performnce ssessment of the school nd office uildings respectively. 7. Response spectr comptile ground motions Bsed on the requirements of the Interntionl Building Code [16] SSRS comptile ground motions re selected s follows. First, seven erthquke ground motions whose response spectr re comptile with the SSRS re selected from the PEER (Pcific Erthquke Engineering Reserch) strong motion dtse. Detils of the selected ground motions re given in Tle 3. Ech one of these ground motions were first scled to PGAs of the SSRS for the

8 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Tle 3 Selected erthqukes nd their properties. Erthquke Sttion/component Soil type Ap (g) Vp (cm/s) Ap/Vp (1/s) ID no. Whıttıer Nrrows Downey - Birchdle/ 18 D Imperıl Vlley Delt/ 262 D Colıng Cntu Creek School/ 27 D Lom Pri et Hollister Diff. Arry/ 255 D Imperıl Vlley , BONDS CORNER, 23 D Imperıl Vlley El Centro Arry #9/ 27 D Westmorlnd Westmorlnd Fire St/9 D Spectrl Acc. (g) % 1% 2% Spectrl Acc. (g) % 1% 2% c Spectrl Acc. (g) Period (s.) Impericl Vlley Coling-1983 Lom Priet-1989 Impericl Vlley-194 Whittier Nrrows-1987 Westmore Lnd-1981 Impericl Vlley SSRS Period (s.) d Spectrl Acc. (g) Period (s.) 1.5 SSRS Avg. Erthquke SSRS Period (s.) Fig. 4. () SSRS for school uilding. () SSRS for office uilding. (c) Design spectrum nd ccelertion spectr of the ground motions scled to the PGA of the design spectrum. (d) Comprison of the verge of the scled erthquke ground motions with the design spectrum nd 1.4 the design spectrum. three erthquke levels (5% in 5 yers, 1% in 5 yers nd 2% in 5 yers). Following this initil scling procedure, the verge vlue of the lredy scled seven SSRS comptile ground motions were rescled to otin n verge vlue lrger thn or equl to 1.4 times the SSRS within period rnge of.2t 1.5T where T is the fundmentl period of the uilding (T =.53 s for the school uilding nd T =.87 s for the office uilding). Fig. 4(c) shows the response spectr of the selected erthqukes nd the SSRS for the school uilding for the erthquke level ssocited with 1% proility of eing exceeded in 5 yers. Fig. 4(d) shows the comprison of the verge of the scled ground motions with the SSRS nd 1.4 SSRS. Response spectr for the other erthquke levels nd the office uilding re similr. Therefore, the sme erthquke ccelerogrms, ut scled using different fctors, were used in the seismic performnce ssessment of the two uildings. 8. Nonliner modelling of existing uildings NLSP nd NLTH nlyses re conducted to ssess the seismic performnce of the originl nd retrofitted uildings used in this reserch study. For this purpose, nonliner structurl models of the school uilding nd office uilding re uilt nd nlyzed using the progrm SAP2 [18]. These structurl models re cple of simulting the nonliner ehvior of the structurl components. The RC ems nd columns s well s the steel components of the PRS re modeled using 3-D em elements. The BMI wlls re modeled using digonl em elements with end moment releses while the SISPs re modeled using comintion of rigidly connected two horizontl nd one verticl em elements. The nonliner ehvior of the structurl memers re simulted y using nonliner link/hinge (link for NLTH nd hinge for NLSP nlyses) elements connected t pproprite loctions or t the ends of the em elements. Typicl structurl modeling configurtions used in SAP2 [18] re shown in Fig. 5() for RC columns nd ems, 5() for BMI wlls, 5(c) for the steel components of the PRS nd 5(d) for SISPs Modeling of the RC columns nd ems for NLTH nlyses The nonliner ehvior of the RC columns nd ems of the uildings used in this study re defined in the structurl model y using nonliner flexurl link elements t the ends of the em/column memers. Tked et l. s [19] (Fig. 5(e)) hysteresis model is the most commonly ccepted model for defining the nonliner ehvior of RC memers [2] nd it is used in the nlyses. Tked et l. s [19] model uses the monotonic moment curvture (or rottion) reltionship of the RC section s ckone curve. Therefore, n ccurte estimtion of the moment curvture reltionship is importnt to correctly simulte the nonliner cyclic ehvior of n RC memer. To otin the moment curvture reltionship of the RC memers of the uildings used in this study, computer softwre

9 32 C. Durucn, M. Dicleli / Engineering Structures 32 (21) c d e Moment Rottion f Moment (kn-m) Deformtion (mm) 2 g P 2 α 2 Fy 2 Force PP 2 P 1 Fy 1 β 1 Fy 1 PP 1 β 2 Fy 2 Fy 2 α1fy 1 P 4 P 3 Deformtion h Sher Force (kn) Top Deflection (mm) Fig. 5. Nonliner link elements nd other structurl elements used in the nonliner modeling of n exmple uilding: () RC frme, () BMI wll, (c) proposed seismic retrofitting system, (d) squt infill sher pnel, (e) Tked et l. s hysteretic model, (f) slip effect on moment-deformtion grph, (g) pivot hysteretic model, (h) sher force deformtion reltion of squt infill sher pnels. tht ws developed y Ylçın nd Stçioğlu [21] ws used. The softwre, referred to s COLA (Column Anlysis), provides sectionl moment curvture nlyses nd memer nlyses, including P effects, uckling nd nchorge slip of the reinforcement rs. Including the effect of nchorge slip in the moment curvture reltionship ecomes especilly importnt for plin rs where slippge is more likely. Therefore, the effect of the nchorge slip is included in the moment curvture reltionship of the RC memers of the uildings used in this study. Fig. 5(f) show the moment versus lterl displcement reltionship of typicl RC memer from the school uilding. In the figure, the nlyses results with nd without nchorge slip re shown. The different moment displcement reltionships clerly show the importnce of including the nchorge slip for more ccurte estimtion of RC memer ehvior Modeling of RC squt infill sher pnels for NLTH nlyses SISPs were used for the conventionl seismic retrofitting of the uildings employed in this reserch study. SISPs re structurl elements where sher ehvior is more dominnt. Pinching is common phenomenon in the hysteretic sher force deformtion reltionships of SISPs [22,23]. Pinching reduces the re under the hysteresis loop nd hence the energy dissipted y SISPs. Consequently, this ehvior should e considered in the hysteretic models of the SISPs. In this study, the hysteretic sher force deformtion ehvior of SISPs ws simulted with the pivot hysteresis model [24] (Fig. 5(g)) ville in SAP2 [2]. The model requires the sher force deformtion envelope s well s two dditionl prmeters: the stiffness degrdtion prmeter, α, nd the pinching prmeter, β, for cpturing the stiffness degrdtion nd pinching properties of RC memers. To otin the sher force deformtion envelope of the SISPs used for conventionl retrofitting, the softened truss model of Hsu nd Mo [25] is employed in the nlyses. The stress strin curve proposed y Vechio nd Collins [26] is used for the softened digonl concrete struts of the softened truss model. The nlyses re conducted following n itertive solution procedure proposed y Mnsour et l. [27] A typicl sher force deformtion envelope otined from the itertive nlysis procedure is shown in Fig. 5(h).

10 C. Durucn, M. Dicleli / Engineering Structures 32 (21) In the literture, there re limited numer of relile dt defining the stiffness degrdtion prmeter, α, nd the pinching prmeter, β. Lck of nlyticl tools for defining these prmeters forced severl reserchers to conduct mny experimentl studies to otin these prmeters [28 32]. A set of test specimens with properties similr to those of the SISPs used in this study were investigted. Then, the necessry prmeters for the sher pnels employed in this study re otined s α = 9.68, β =.41 y using the verge of the experimentl test results from these reserch studies [28 32] Modeling of rick msonry infills for NLSP nlyses c d Modeling the ehvior of BMIs is quite complex nd the presence of window or door holes mkes this tsk even more difficult. There re some modeling techniques in the literture. The common ground of these techniques is using digonl struts to model BMIs [33]. In this study, the digonl strut modeling procedure of FEMA 356 [7] is used to model the BMIs for the NLSP nlyses of the school uilding (Fig. 5()). From the NLSP nlyses it is found tht the BMI wlls fil t very smll displcements. Comprtive NLTH nlyses of smple frmes with nd without BMIs otined from the school uilding lso show comprle results. Thus, BMI wlls re not included in the NLTH nlyses of the school uilding, 8.4. Modeling of proposed steel retrofitting system for NLTH nlyses For NLTH nlyses, the hysteretic ehvior of the sher link in the PRS is modeled y using the plstic Wen element in SAP2 [18], which hs n elsto-plstic sher force displcement hysteresis. The yield force nd displcement of the link re used to define the plstic Wen element. The steel frme memers nd the rces re modeled using em elements. The steel em elements of the steel housing frme re connected to the nodes t the ends of the rigid elements defining the RC joints (Fig. 5(c)). 9. Seismic retrofitting design of the uildings A performnce sed pproch is used for the seismic retrofitting design of the uildings considered in this study. The performnce sed design pproch is sed on mtching vrious prole erthqukes with trget PLs. Three trget PLs re used for the retrofitting design of the uildings s follows: (i) Immedite Occupncy (IO) PL where no dmge is expected for minor levels of erthquke excittions, (ii) Life Sfety (LS) PL where low or repirle structurl nd non-structurl dmge is expected for moderte erthquke excittions, nd (iii) Collpse Prevention (CP) PL where irreprle or hrdly repirle structurl nd nonstructurl dmge ut no collpse is expected for mjor erthquke excittions. In FEMA 356 [7], these trget PLs re minly defined y plstic rottion limits of the RC memers. Recent reserch conducted y Acun nd Sucuoğlu reveled tht the rottion limits of Eurocode 8 nd ASCE/SEI 41(FEMA 356) re suitle for RC uilding construction in Turkey [34]. These limits re used in the retrofitting design of the uildings to define ech PL. Ground motion levels, which represent the forementioned minor, moderte nd mjor erthqukes re s follows: (i) minor erthqukes re defined s frequent erthqukes, which hve 5%/ proility of eing exceeded in 5 yers (72 yers return period), (ii) moderte erthqukes re defined s hving moderte proility of occurrence during the economicl life of structure nd hve 1% proility of eing exceeded in 5 yers (475 yers return period) nd (iii) mjor erthqukes re defined s less frequent erthqukes which hve only 2% proility of eing exceeded in 5 yers (2475 yers return period). Fig. 6. () Liner elstic se sher force vs. roof displcement reltionship. () Elsto-plstic se sher force vs. roof displcement reltionship. (c) Elstic vs. plstic se sher force -roof displcement reltionships. (d) Clcultion of required strength, Q Retrofitting design procedure The itertive procedure employed in the seismic retrofitting design of the uildings is outlined in this section. The procedure involves oth RS nd NLSP nlyses. The erthquke effects on the uildings re represented y SSRS otined for ech one of the forementioned PLs in the retrofitting design. The uildings should chieve the performnce criteri defined erlier y limiting the RC memer rottions y the rottion limits given in FEMA 356 [7] under the stted erthquke levels. In the retrofitting design procedure of the uildings, the drift limits (roof displcement limit) used in the design of the uildings for ech PL re otined from NLSP nlyses. These drift limits re determined sed on the RC memer rottion limits given in FEMA 356 [7] for ech PL. Most of the existing ordinry uildings (e.g. school, residentil nd low to mid-rise office uildings) hve fundmentl virtion periods tht fll in the intermedite period rnge. In this period rnge, the energy dissipted y n elstic system cn e ssumed to e equl to n identicl (nonliner) system tht yields t certin lterl force level. The seismic retrofitting design methodology used in this study is minly sed on this equl energy dissiption principle. In the proposed methodology, the monotonic energy dissiption cpcities of the uildings in the liner elstic rnge (sed on the roof displcement otined from RS nlyses (Fig. 6())) nd nonliner inelstic cses (from NLSP nlyses for ech PL (Fig. 6())) were clculted nd compred (Fig. 6(c)). The difference etween the res under the elstic nd inelstic se sher force vs. roof displcement curves is the required dditionl energy tht needs to e sored y the retrofitting system (e.g. PRS or SISP) (Fig. 6(d)). The step-y-step seismic retrofitting design procedure is given elow. The design procedure needs to e repeted for ech PL. The PL tht yields retrofitting design with the lrgest lterl strength governs the design. 1. In the first step, RS nlyses of the uilding in the retrofitted stge re conducted to otin the liner elstic se sher force vs. roof displcement reltionship s shown in Fig. 6(). Since the retrofitting scheme is not known t the initil stge

11 34 C. Durucn, M. Dicleli / Engineering Structures 32 (21) of the design procedure, the unretrofitted uilding is used in the nlyses. However, since seismic retrofitting results in n increse in the lterl stiffness of the uilding, the lterl stiffness of the originl uilding needs to e incresed y certin mount (e.g. initilly y 2%) in the RS nlyses y djusting the modulus of elsticity of the RC memers of the structure. 2. In the second step, the NLSP nlyses of the originl uilding re conducted to otin the se sher force vs. roof displcement reltionship. This reltionship is plotted up to the displcement level corresponding to the displcement cpcity of the uilding for the PL under considertion. The plotted curve is then idelized to hve n elsto-plstic shpe s descried in FEMA 356 [5] (Fig. 6()). The yield se sher force, V y, otined from the elsto-plstic curve is used together with the elstic stiffness of the structure in the retrofitted stge (slope of the curve in Fig. 6()) to otin new elsto-plstic se sher force vs. roof displcement reltionship (Fig. 6(c)) for susequent clcultions. 3. In the third step, first, the re, A e, under the liner elstic se sher force vs. roof displcement curve is clculted. Then the re, A p, under the elsto-plstic se sher force vs. roof displcement curve shown in Fig. 6(c) is clculted. The monotonic energy, A d, tht needs to e dissipted y the retrofitting system is then clculted s; A d = A e A p. 4. In this step, the required totl strength, Q 1, of the retrofitting system t the se of the uilding is otined with reference to Fig. 6(d) s follows. Q 11,2 = K e d p V y (V y K e d p ) 2 2K e A d (1) One of the roots from the ove eqution will give the required strength, Q 1, of the retrofitting system t the se of the uilding. 5. In this step of the design procedure, first, the seismic sher force cpcity, R Fi, of ech storey of the originl unretrofitted uilding is otined sed on FEMA 356 [7] memer rottion limits. 6. In this step of the design procedure, the retrofitting system is designed for the whole uilding. The design is sed on uniform energy dissiption throughout the height of the uilding. For this purpose, the elstic sher, V i, t ech storey level, i is otined from the RS nlyses results in Step 1. Then, the totl strength, R 1, t the se of the retrofitted uilding is otined y summing up its se sher cpcity, R F1, nd the required strength for retrofitting. Tht is; R 1 = R F1 + Q 1. To ensure uniform energy dissiption long the uilding height, the rtios of the totl strength of the retrofitted uilding t ech storey level i (R i = R Fi +Q i ) to the elstic sher, V i, t the corresponding storey level must e equl. Tht is; R 1 = R 2 = = R i = = R n (2) V 1 V z V i V n where the suscript, n, in the ove eqution denotes the numer of stories. This will ensure tht yielding is more likely to occur t ll the storey levels. The rtio of the totl strength, R 1, of the retrofitted uilding to the elstic sher V 1 t the se of the uilding is lredy known. To clculte the required retrofitting system strength t ny storey level, i, the following reltionship is used; R 1 V 1 = R Fi + Q i V i. (3) Then solving for Q i, the following eqution is otined; Q i = R 1 V 1 V i R Fi. (4) 7. The elstic stiffness of the designed uilding is reclculted nd compred with the stiffness ssumed in Step 1 of the procedure. If the difference is negligile the design is complete. Otherwise, the stiffness is updted nd Steps 1, 3, 4, nd 6 re repeted Sizing of the sher link nd rces of the proposed retrofitting system In this section, the procedure followed to determine the size of the link nd rces of the PRS is presented. Following the procedure outlined ove, the numer of link-rce systems nd the required link sher strength, Q Li, t ech storey level is lredy determined. The sher yield strength, V y, of n HP, HE or W section is given s [35]; V y =.6F y A w (5) where F y is the yield strength of steel, nd A w is the cross-section re of the we of the link. Setting V y = Q Li, the cross-section re, A wi = Q Li / (.6F y ) of the we t storey i is otined. An HP, HE or W section with the clculted we re, A wi, is then chosen. The ultimte filure mode for the sher link is inelstic we sher uckling. To ensure stle energy dissiption mechnism throughout the cyclic loding history of the link in the PRS, this mode of filure my e delyed y dding stiffeners to the we of the link. The required stiffener spcing,, for vrious sher link rottion demnds, g s, my e clculted using the following eqution proposed y AISC [35] for sher links; = C B t w.2d (6) where the constnt C B = 56, 38 nd 29 respectively for g s, =.3,.6 nd.9 rdins nd d is the sher link depth. Furthermore, the length, h s, of the link needs to e determined such tht yielding occurs in sher efore its plstic moment cpcity, M p, is reched. Ksi nd Popov [36] suggested tht, due to the strin hrdening effect, the sher link s plstic se moment, M p, nd sher, Q Li, my increse. The increse in M p nd Q Li is ccompnied y lrge flnge strins tht my result in premture filure of the welded sher link em connection. To prevent such filure, n upper ound of 1.2M p nd corresponding sher of 1.5 Q Li ws suggested y Ksi nd Popov [36]. Thus; h S < 1.2M p 1.5Q Li =.8 M p Q Li (7) In the PRS, t the verge of uckling instility of the compression rce, the xil tensile nd compressive forces in the tension nd compression rces re oth equl to the uckling lod, P. Consequently, to prevent uckling instility of the compression rce, the sum of the horizontl components of the uckling lods of the two rces must e lrger thn the yield strength of the link times n over-strength fctor, ϕ s (Fig. 1()). Thus; 2P cos α φ S Q Li. (8) In the ove eqution, α is the ngle tht the rces mke with the horizontl. Solving for P from the ove eqution, the required uckling strength of the rce is otined s; P = φ SQ Li 2cosα. (9) The rces re selected to hve minimum uckling cpcity clculted from the ove eqution. The detils of the steel sher links nd rces used for seismic retrofitting of the uildings nd the detils of the SISPs used in the conventionl seismic retrofitting of the uildings re given in Tle 4 for the x nd y directions of the uilding respectively.

12 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Tle 4 Detils of the steel nd RC retrofitting memers considered for nonliner nlyses. School uilding PRS steel sections used in x-direction SISP in x-direction f c (MP) f y (MP) Reinforcement (%) Storey # Link Brces Housing frme Thickness (m) 1 HE3M HE2M HE3M HE26M HE26M HE26M PRS steel sections used in y-direction SISP in y-direction f c (MP) f y (MP) Reinforcement (%) Storey # Link Brces Housing frme Thickness (m) 1 HE26M HE14M HE26M HE24M HE12M HE24M Office uilding PRS steel sections used in x-direction SISP in x-direction f c (MP) f y (MP) Reinforcement (%) Storey # Link Brces Housing frme Thickness (m) 2 HE2M HE12M HE22B.1 & &.25 3 HE18M HE1M HE22B.1 & &.2 4 HE18M HE1M HE22B.1 & &.2 5 HE14M HE1B HE18B.1 & &.13 6 HE14B HE1B HE18B.1 & &.13 PRS steel sections used in y-direction SISP in y-direction f c (MP) f y (MP) Reinforcement (%) Storey # Link Brces Housing frme Thickness (m) 1 HE2M HE12M HE22B HE2M HE12M HE22B HE2M HE1M HE22B HE2M HE1M HE22B HE16M HE1B HE18B HE14B HE1B HE18B Sher Force (kn) Roof Displcement (mm) Sher Force (kn) Roof Displcement (mm) Fig. 7. () The se sher force s function of the drift t the top-storey level for the school uilding in the x-direction. () The se sher force s function of the drift t the top-storey level for the office uilding in the x-direction. 1. Nonliner pushover nlyses results In this section, comprtive performnce evlution of the originl nd seismiclly retrofitted uildings is performed using the NLSP nlyses results. The NLSP nlyses results in the x direction of the school nd office uildings for the originl nd retrofitted conditions re shown respectively in Fig. 7() nd () in terms of se sher vs. roof displcement plots. The response in the y-direction is similr. From the figure, it is oserved tht the elstic lterl stiffness of the conventionlly retrofitted structure using SISPs is smller thn the structure retrofitted with the PRS. This difference is ttriuted to the lrger numer of steel sher link rce systems (PRS) required to chieve the sme yield strength level s tht of the SISPs. Thus, it is expected tht the structure retrofitted with the PRS will hve smller drift, nd hence more desirle performnce (e.g. less non-structurl dmge) under the effect of smll intensity erthqukes ssocited with the IO PL. Fig. 7() nd () lso demonstrte tht the uilding retrofitted with the PRS produces more stle lterl force displcement reltionship s compred to the uilding retrofitted with SISPs. The filure of the SISPs, which hve limited drift cpcities, cuses loss of lterl strength of the conventionlly retrofitted school uilding s oserved from the figures. This clerly shows tht the PRS hve lrger monotonic energy dissipting nd ductility cpcity compred to tht of the uilding retrofitted with SISPs nd my potentilly resist lrger erthqukes. Compred to the originl cse, the uildings retrofitted with oth PRS nd SISPs exhiited considerle increse in lterl stiffness nd strength. This is indictive of etter seismic performnce during potentil erthquke. 11. Nonliner time history nlyses results Comprtive performnce evlution of the originl nd retrofitted uildings is performed using the NLTH nlyses results. The nlyses results in terms of the mximum interstorey drift nd roof displcement re presented for ech erthquke s well s using the verge of the nlyses results from the seven erthqukes. Ech erthquke is ssigned numer (Tle 3) to fcilitte the presenttion of the results.

13 36 C. Durucn, M. Dicleli / Engineering Structures 32 (21) Story Drift (mm) Drift Limit Roof Drift (mm) 25 2 Drift Limit I.O. L.S. C.P. Performnce Level I.O. L.S. C.P. Performnce Level Story Drift (mm) Drift Limit Roof Drift (mm) Drift Limit c Story Drift (mm) Story Drift (mm) Story Drift (mm) I.O. L.S C.P I.O. L.S. C.P. Performnce Level Roof Drift (mm) Roof Drift (mm) Roof Drift (mm) I.O. L.S. C.P d Story Drift (mm) Story Drift (mm) Story Drift (mm) I.O. L.S. C.P. Performnce Level Roof Drift (mm) Roof Drift (mm) Roof Drift (mm) I.O. L.S. C.P. Fig. 8. () Averge interstorey nd roof drifts for the school uilding in the x direction. () Averge interstorey nd roof drifts for the office uilding in the x direction. (c) Mximum interstorey nd roof drifts for the school uilding in the x direction. (d) Mximum interstorey nd roof drifts for the office uilding in the x direction School uilding Fig. 8() compres the verge of the interstorey nd roof drift demnds in the x-direction from the seven erthqukes with the drift cpcities respectively for the originl uilding s well s the uilding retrofitted with the proposed (PRS) nd conventionl (SISPs) methods. The response in the y-direction is similr. As oserved from the figure, for the unretrofitted uilding, the interstorey drift demnds severely exceed the cpcities (indicted y dshed lines nd estimted sed on the rottion limits of existing columns/ems) for ll the PLs considered in the nlyses. For the uilding retrofitted with the PRS however, the interstorey drift demnds re smller thn the corresponding cpcities for ll the PLs considered in the retrofitting design. Nevertheless, this is not the cse for the uilding retrofitted with SISPs. For this cse, while the IO PL is completely stisfied, the interstorey drift demnds t the LS nd CP PLs for the first storey re lrger thn the corresponding cpcities. It is worth noting tht for the cse of the uilding retrofitted with SISPs, the interstorey drift demnds re even lrger thn those of the originl uilding t the CP PL. The min reson for this type of ehvior is the low ductility, hevy weight nd considerle pinching in the hysteresis loops of the SISPs used in conventionl retrofitting of the uilding. At the CP PL, the lrge intensity of the ground motions results in