13 t World Conference on Eartquake Enneern Vancouver, B.C., Canada Auust 1-6, 24 Paper No. 2473 DEVELOPMENT OF HOOPS WITH DOUBLE IGHT ANGLE ANCHOAGE FO C COLUMNS Hros IMAI 1, Juan Jose CASTO 2 SUMMAY In ts researc a new type of oop wt double rt anle ancorae (so-called ere double ancorae type oop) s proposed. To prove te feasblty of ts type of oops an expermental proram dvded nto parts was carred out. Frst, pullout tests were carred out on bars wt double rt anle ook, and te results were compared wt bars provded wt 9 (snle ook), 135 and 18 ooks. esults sowed tat for te double ancorae type, te bars reaced te tensle strent ndcatn tat enou ancorae strent was obtaned. In te next stae experments on column elements, renforced wt double ancorae and welded closed type oops, were carred out. Te beavor of te specmens wt double ancorae oops and specmens wt welded closed type oops sowed comparable beavor n terms of ductlty and ultmate strent. INTODUCTION It s well known tat te performance of te columns aanst eartquake actons mproves consderably n terms of ductlty, wen te lateral renforcements are placed not only wt perperal oops but also wt nner oops, as sown n F. 1 (a). Studes on te collapsed buldns after te Kobe Eartquake (Japan, 1995) and recent studes on publc buldns to be strentened, sowed te fact tat n many cases te requred ancorae lent (6d, d: bar dameter) for te 135 ooks was not satsfed, ndcatn problems related wt qualty control of ts type of lateral renforcement. Te development lents, requred by te Japanese standards [1] [2], are sown n Table 1. Te lent s caned accordn to te ook type, 4d for 18 ooks, 6d for 135, and 1d for 9 ooks. As t can be understood from te table, te ven development lents for conventonal bars (39 MPa or bellow) does not depend on te concrete strent n any case. Altou te development lent of te 135 ook was recommended to be extended to 8d, by te desn udelnes based on ultmate strent concepts [3], 6d s te lent stll used n te normal practce. 1 Professor, Unversty of Tsukuba, Ibarak, Japan., Emal:ma@kz.tsukuba.ac.jp 2 Senor esearcer, Kabuk Constructon Co. Ltd., Tokyo, Japan. Emal:castro.j@kabuk.co.jp
1 61,74 4 9,98 1 1 Bent anle Table 1 Standards for ook bent for oops Type Use Bar rade Bar dameter D d Bent nner dameter 18 4d d SD295A SD295B SD345 D16 3d 135 9 6d D Hoops Strrups d 1d D SD39 D16 5d (a) 135º ook type (b) Welded closed type (c) Mult-spral type F. 1 Conventonal lateral renforcement Furtermore, n accordance wt te renforcement arranement reulatons, te poston of 135 ooks must be sfted for eac layer of lateral renforcement alon te column. Terefore, te 135 ooks at te corners make dffcult to place concrete and dsturb te proper compacton wt vbrators at te constructon ste. For tese reasons, closed welded lateral renforcements wtout ooks llustrated n F. 1(b) are ben used Japan not only for columns subjected to axal stresses but also especally n Kansa Area (Western Japan) even for low rse buldns. Altou te strat extenson beyond te bent s not necessary, n ts case, problems related to te qualty control of weldn reman as a weak pont, turnn to an ncrease of te constructon costs.
Te mult-spral oops lke s sown n F. 1(c) are also ben used. Tey ave very ood structural performance and easer qualty control, but present some dffcultes n te fabrcaton process due to te eavy wet of te bar caes and also te stron sprn actons tat causes torsonal deformatons. In ts researc [4] a new type of oop wt double rt anle ancorae (so-called ere double ancorae type oop) as sown n F. 2 s proposed. Te feature of ts lateral renforcement s tat t as a double 9 ooks placed not at te corner but also at te sde of te column. By manufacturn te double ancorae type oops, te problems presented by 135 ooks as well as te expensve welded oops can be solved. Moreover, te double ancorae metod can be adjusted to several confuratons of column man bars. One example of a more complcated but possble confuraton s sown n F. 3. Ancor lent Man Bars D dameter Dameter of of bendn 5d 4d (d: bar dameter) F. 2 Detal of double ancorae type oops F. 3 Example of complcated confuraton EXPEIMENTAL POGAM To verfy te structural applcablty of te double ancorae oops two seres of experments were carred out. Te frst seres was amed to nvestate te tensle capacty of te double rt anle ancor ook, trou pull out tests. Also ts part of te nvestaton was focused on comparn te beavor of te double rt anle ancor ook wt te 18, 135, and 9, used by te conventonal oops. Te second seres was column member testn of 14 specmens, subjected to cyclc bendn moments. Ts seres was amed to compare te beavor of te columns renforced wt te double rt anle oops wt tose renforced wt welded closed type oops.
SEIES I - PULLOUT TEST Specmens Te specmens are sown n Table 2 and F. 4. Et ooked testn bars were ancored n eac specmen at 2 mm nterval representn te ancor part of te oops. Te testn bars were D13, D1 and D16, wt specfed yeld strent of 39 MPa (Grade SD39), and te concrete strent was 24 MPa. Closed welded lateral renforcements wt expanded polystyrene nsde and plywood on bot sdes were provded n between testn bars, to mnmze te nfluence of cracks tat occurred n testn adjacent testn bars. Te concrete was cast from te top n specmens 1 to 3. However, n specmens 11 to 2, eac specmen was nverted n order to cast te concrete n te opposte drecton. Specmens No.1, 11, 14, ad 8 testn bars wt 135 ook, 18 ook and 9 ook. Te oter specmens ad also 8 testn bars wt double ancorae. Te development lents were vared as sown n Table 2 and F. 5. Eac testn bar was covered wt rubber tube up to te bennn pont of te frst bent ook n order to solate t from te concrete. M6 (6 mm screwed bar) bar was fxed on eac testn bar at te bennn pont of te frst bent ook. Spec. Bar Dameter Table 2 Outlne of Specmens (Seres I) Covern Dept Ancorae Poston Hook type Development lent (n bar dameters) 135 n=6, 8 1 18 n=4, 6 D13 3 mm Bottom 9 n=6, 8, 1, 12 2 n=6, 8 m=4, 6, 8, 1 2 x 9 3 n=1, 12 m=4, 6, 8, 1 135 n=6, 8 11 18 n=4, 6 3mm 9 n=6, 8, 1, 12 12 n=6, 8 m=4, 6, 8, 1 2 x 9 13 n=1, 12 m=4, 6, 8, 1 D13 135 n=6, 8 14 18 n=4, 6 Top mm 9 n=6, 8, 1, 12 15 n=6, 8 m=4, 6, 8, 1 16 n=1, 12 m=4, 6, 8, 1 17 n=6, 8 m=4, 6, 8, 1 D1 2 x 9 18 n=1, 12 m=4, 6, 8, 1 3mm 19 n=6, 8 m=4, 6, 8, 1 D16 2 n=1, 12 m=4, 6, 8, 1 In case of double ancor ooks: n= mddle development lent and m: end development lent
Expanded polystyrene 1mm Plywood 5mm (a) Front F. 4 Specmen (b) Sde PF 1 st bent PF Inner dameter=4d PF OF 4d F. 5 Ancorae Development Detals Loadn System and Instrumentaton Te loadn system and measurn devces are sown n F. 6. Te specmens were lad down on one of ts sdes by turnn 9 from te castn poston. Eac testn bar was subjected to a monotonc pull out load usn a center ole jack. Load, dsplacement of ancorae renforcement at te frst bent ook, and strans around bent ooks were measured. Loadn Drecton Measurn Instrument Unbonded part Specmen Ol Jack F. 6 Loadn system
Te dsplacement was measured at M6 bar from te backsde of eac specmen. Strans were recorded n 4 ponts (bennn pont, nner and outer faces of mddle pont and end pont of bent ook). Te proress of falure was observed at te backsde of eac specmen Materal Caracterstcs Te materal test results are sown n Table 3. Te strent of renforcements and concrete were er tan te specfed values. Table 3 Materal Test esults (a) Concrete (b) Steel bars (SD39) Spec. Comp. Strent E Tensle Strent Dam. Yeld Strent Yeld Strans Tensle Strent E Elon. MPa GPa MPa MPa µ MPa GPa 1-3 23.6 19.1 2.22 D13 491 458 658 193 12 11-15 19.5 18.6 1.65 D1 444 4233 76 21 16.6 16-2 2.4 21.6 1.89 D16 434 442 651 28 2. TEST ESULTS-SEIES I Fnal Falure Patterns In te case testn bars wt 9 ooks, cone type crusn of cover concrete occurred. In te case of double ancorae type tere were no cracks at te backsde of te specmens. Even f te castn drecton was caned, from te top or from te bottom, tere was no dfference n crack patterns. Maxmum Strent Te maxmum strents and types of falure are sown n Tables 4 and 5. Te values ndcated n te Tables are te dfferences between te materal test results for tensle strent taken as 1 percent and te maxmum strents. Te symbol represents te rupture n te strat unbonded part, wle symbol s for rupture n te bent ook. Symbol means falure wen te ooked porton came out of concrete wtout rupture, and symbol x denotes cone type crusn of cover concrete. Table 4 Conventonal ooks types type of ook 9 135 18 ancor lent n=4 - - - - -2.7 n=6 x -25.2-8.3 -.1 n=8 x -12.9-1. - - n=1 x 1.7 - - - - n=12 1. - - - -
Table 5 Double ancorae ooks Pos. Bottom ancorae Top ancorae Ancor Lent m=4 m=6 m=8 m=1 m=4 m=6 m=8 m=1 n=6 2.3 1.3 1.3 2.3 1.3 1.2-2..5 n=8 1.9 2.3.1 1.4 2.2-4. -.4-1.8 n=1 1.7 1.6 1.7 1. 1.3 1.5 2. 1.9 n=12 1.3.8.2 1. 1.6 -.7. 2. Te relatons between maxmum strents and total ancorae lents for D13 bars are sown n F. 7. Upper and lower broken lnes were drawn to sow te tensle stress and yeld stress, respectvely. Te total ancorae lent ncludes te bonded lent and te bent ooks. Te defnton of te total ancor lent s sown n F. 8. In case of bottom ancorae wt conventonal ooks te tensle strent was only reaced wen te total ancorae lent was more tat 18 mm (14d). For te bottom ancorae, amon te renforcements wt conventonal ooks only tose wt 18 mm reaced te yeld strent. On te oter and for te double ancorae ooks te tensle strent was reaced n almost all cases reardless of te poston and ancorae lent. 8 total ancorae lent (d) 1 15 2 25 3 tensle stress 8 total ancorae lent (d) 1 15 2 25 3 tensle stress Stress (MPa) 6 4 135º ook 18º ook 9º ook 2 x 9º ook 2 yeld stress 1 2 3 4 total ancorae lent (mm) a) bottom ancorae Stress (MPa) 6 4 2 yeld stress 1 2 3 4 total ancorae lent (mm) b) top ancorae F. 7 elaton between maxmum strent and total ancorae lent Total ancorae lent Total ancorae lent F. 8 Defnton of te total ancor lent
SEIES II COLUMN TEST Specmens Te specmens are lsted n Table 6 and te typcal specmens are sown n F. 9. For ts experment 24 MPa and 36 MPa were selected for te specfed concrete strent (Fc). Amon te tested specmens, 5 of tem were desned to ave bendn falure type, and te remann 9 specmens to ave sear falure type. Te column secton was 45 x 45 mm, and te et was 18 mm for te bendn falure type and 135 mm for te sear falure type. For te lateral renforcement, bars of 1 mm dameter and yeld strent of 39MPa were used. Welded closed type oops, double ancorae type oops, wt te ancor lent of te ook of 4 mm and 1 mm were selected for ts experment. Te test results of te materals used for te experment are lsted n Table 7. 45 45 135 @17 or 12 18 @1 4 4 12 13 12 45 12 13 12 4 4 45 Unts: mm (a) Sear Falure (b) Bendn Falure (c) Column Secton F. 9 Typcal specmen Table 6 Test Specmens Spec. Falure Mode Man Bars Lateral enforcement Lat. enf Axal Load Concrete Strent Spacn Ancorae ato ato Type p w (%) (MPa) C1W WCT D19 1 C1S BFT 4 mm SD345 mm C1L 1 mm.63 C2W WCT D19 17 C2S SFT 4 mm SD685 mm C2L 1 mm.37.15 24 C5W D22 1 WCT BFT C5S SD345 mm 4 mm.63.25 C6W 17 WCT C6S mm 4 mm.37 C7W D22 12 WCT SFT C7S SD685 mm 4 mm.53.1 36 C8W 17 WCT C8S mm 4 mm.37.25 WCT: Welded Closed Type Hoop, Secton: 45 x 45mm, Hoops: D1 (SD39) S: double ancorae bar wt ancor lent of 4 mm L: double ancorae bar wt ancor lent of 1 mm BFT: Bendn Falure Type SFT: Sear Falure Type
Table 7 Materals (a) Concrete Spec. Fc MPa σ B MPa σ t MPa E GPa C1, C2 24 28.7 2.6 24.3 C5-C8 36 36.7 3. 27.8 Fc: Specfed strent,σ B : Compressve Strent σ t : Splttn Strent, E: Youn s Modulus (b) Steel bars Sze Grade σ y MPa σ t MPa Comments D19 41 562 C1 (man bar, BFT) SD345 D22 4 527 C5 (man bar, BFT) D19 732 946 C2 (man bar, SFT) SD685 D22 722 93 C6-C8 (man bar, SFT) D1 SD39 444 656 C1-C2 (lateral renf.) 431 549 C5-C8 (lateral renf.) σ y : Yeldn Strent, σ t : Tensle Strent Loadn System Eac specmen was set under te loadn apparatus sown n F. 1. Te specmens were subjected to varyn sear forces tat were appled cyclcally to produce ant-symmetrc bendn moment dstrbuton wle ben acted upon by a constant axal load. Te loadn story was controlled n terms of lateral drft anle as sown n F. 11. Te lateral drft anle s defned as te relatve dsplacement (δ) between te lower and upper stubs dvded by et () of test porton (=δ/). For specmens wt sear falure type te appled loadn story was once at =1/8, ten twce at 1/4, 1/2, 1/1, 1/5, 1/25. For te specmens wt bendn falure type te loadn was furtermore extended once to =1/16. Specmen F. 1 Loadn system Á=δ / F.11 Loadn story
TEST ESULTS Crack Pattern Fure 12 sows te typcal crack patterns for bendn falure type and sear falure type specmens at a drft anle of =1/1 and 1/25, respectvely. For all te specmens, te ntal flexural cracks took place at te column ends wen te drft anle was =1/8. For bendn falure type specmens, as te mposed deformaton was ncreased, cracks developed wder, and crasn of concrete at te column compresson sde was observed at =1/25, follown remarkable concrete spalln off. On te oter and, sear falure type specmens, small cracks developed at te central part of te column at =1/2, ten a b daonal sear crack developed at =1/1. Bond splttn cracks were observed alon te 1st layer of man bars wen te drft anle was =1/5. Consdern te comparson between te ancor metods of te lateral renforcements, no dfferences were observed on te crack patterns amon specmens wt te same amount of renforcement (p w ). =1/1 =1/25 =1/1 =1/25 C1W C1S Welded closed type Double anc. type (a) Bendn falure type (F c =24 MPa ) =1/1 =1/25 =1/1 =1/25 C6W C6S Welded closed type Double anc. type (b) Sear falure type (F c =36 MPa) F. 12 Crack patterns Load Dsplacement Te load-dsplacement relatonsps are sown n F. 13 (a) and (b), for specmens wt bendn falure type and sear falure type, respectvely. Te flexural strent (Q mu ), n terms of sear force, was calculated usn te formulaton ven by te Buldn Center of Japan (BCJ) [5], te sear strent (Q su ) by so called Metod A of te Arctectural Insttute of Japan (AIJ) [3], and te bond strent (Q bu ) by Kaku s Equaton [6].
Te P-δ effect due to te axal load s consdered n te calculatons of te ultmate strents plotted n te fures. Consdern te dfferences between te ancorae metod and te ancor lent, no dfferences were observed amon te specmens wt te same falure pattern. Q(kN) =1/1 1/5 1/25 1/16 C1W 75 C1L (1d) Q bu 5 C1S (4d) 25 Q mu -1-5 5 1-25 δ (mm) -5-75 Q: Sear force δ: Dsplacement : Drft anle (a) Bendn falure type (Fc=24 MPa) =1/1 Q(kN) 1/5 1/25 Q su 75 Q bu C7W 5 C7S (4d) 25-5 5-25 δ (mm) Qsu: Sear strent -5 Qmu: Bendn strent -75 Qbu: Bond splttn strent (b) Sear falure type (Fc=36 MPa) F. 13 Load dsplacement curves Envelope Curves Te envelope curves of te load dsplacement ysteress loops are sown n F. 14. Comparson of te envelope curves of bendn falure type sowed no dfferences on te overall beavor between specmens wt welded closed type oops and tose wt te double ancorae type oops. Q(kN) 8 (BFT) F c =24 MPa Q(kN) 8 (S FT) F c =24 MPa 6 6 4 2 C1W C1L (1d) 1/1 1/5 1/25 1/16 4 2 Ancor Lent C2M (1d) C2S (4d) 1/1 1/5 1/25 (a) Effect of te ancor metod (b) Effect of te ancor lent Q(kN) 8 6 (SFT) F c =36 MPa Q(kN) 8 6 (S FT) F c =36 MPa 4 4 2 C6S p w =.37% C7S p w =.53% 1/1 1/5 1/25 (c) Effect of p w 2 C6S η=.1 C8S η=.25 1/1 1/5 1/25 (d) Effect of axal load rato F. 14 Envelope curves
Amon te sear falure type specmens wt double ancorae type oops, te nfluence of te ancor lent, te lateral renforcement rato p w and te axal load effect were compared. Test results sowed no nfluence of te varaton of te ancor lent, and proved tat 4d s enou to ancor te end of oops. Te test results also sowed tat maxmum strent ncreased as te lateral renforcement (p w ) and te axal load rato (η) were ncreased. However, for deformatons beyond =1/5 wen a lare axal forces are appled, remarkable strent decay was observed as sown n F. 14 (d). Stran Dstrbuton Stran development of te lateral renforcement Te poston of te stran aues for lateral renforcement s sown n F. 15. For bendn falure type, tey were placed on four layers of oops at bot sdes of te column. For te sear falure type tey were placed on four layers of oops n te mddle et of te column. Te aues (1~4) were postoned to measure strans n te loadn drecton. Because for eac of te postons (1) and (4) tere s one aue belonn to te outer oop (a) and anoter for nner oop (b), te averae stran was consdered as te stran value for tat poston. Ten te averae value for te postons 1 to 4 was calculated as te stran value for eac of te four layers were te strans were measured. Lastly, te averae stran for eac column was calculated consdern te averae stran of eac layer. Te development of strans on te lateral renforcement was plotted for eac drft anle as sown n F. 16. For bendn falure type specmens wt concrete strent Fc=36 MPa, te lateral renforcement yelded at =1/25. Te double ancorae oops presented stran development comparable to tose obtaned for te welded closed oops, presentn also adequate ancor strent. Concernn to te sear falure type, te lateral renforcement yelded at =1/1. Specmens C6 (p w =.37%) and C7 (p w =.53%) presented almost te same stran dstrbuton proress n spte of te dfferences on te lateral renforcement rato. (4b) stran aues stran aues stran aues (4a) (3) (2) (1b) (1a) Bendn Falure Type Sear Falure Type F. 15 Stran aue for lateral renforcement
µ µ F 3 c =24 MPa 3 ε C1W y C1L (1d) 2 2 C1S (4d) F c =36 MPa C5W C5S (4d) 1 1 1/8 1/2 1/5 1/16 1/4 1/1 1/25 1/8 1/2 1/5 1/16 1/4 1/1 1/25 p w =.63% η=.15 p w =.63% η=.25 (a) Bendn falure type µ µ 6 F c =36 MPa 6 C6W 4 C6S (4d) 4 F c =36 MPa C7W C7S (4d) 2 2 1/8 1/2 1/5 1/4 1/1 1/25 p w =.37% η=.1 (b) Sear falure type 1/8 1/2 1/5 1/4 1/1 1/25 p w =.53% η=.1 F. 16 Stran developments of lateral renforcement Stran dstrbuton of te ancor end Te poston of te stran aues on te ancor part of te lateral renforcement s sown n F. 17. Te stran dstrbuton of te ancor part for eac lateral drft anle s sown n F. 18. Te values plotted n te fure represent te averae values of 4 layers of oops wt 2 stran aues for eac layer. For bendn falure type specmens C1 (Fc=24 MPa) te ancor part of lateral renforcement dd not yeld, owever for specmens C5 (Fc=24 MPa) te ancor part yelded at =1/25.Te sear falure type specmens eter wt welded closed type oops or double ancorae types oops presented very smlar dstrbuton, wt ber stran values at te center of te column () and at te start of bent (), compared wt tose recorded for te ancor end (). 1 75 1 75 : ancor end : start of bent : center of column F. 17 Postons of te aues on te ancor ends
µ 1/16 µ µ µ 6 C1W 1/25 6 1/5 C1L 6 C2W 6 C2S 1/1 4 1/2 4 4 4 1/4 2 1/8 2 2 2 Fc=24 N/mm 2 Fc=24 N/mm 2 µ µ µ µ 6 C5W 6 C5S 6 C8W 6 C8S 4 2 4 4 4 2 2 2 Fc=36 N/mm 2 Fc= 36 N/mm 2 (a) Bendn falure type (b) Sear falure type F. 18 Stran dstrbuton of ancor end Stran dstrbuton at te ancor part Te stran development at te center of te column () s sown n F. 19. Te values plotted n te fure are te averae of te strans obtaned by te stran aues placed on four layers of oops. Untl =1/2 te strans were very small. After tat due to te propaaton of concrete cracks, te strans also ncreased remarkably. For sear falure type specmens te strans ncreased remarkably even after te specmens reaced te ultmate sear strent at =1/1. Specmens renforced wt te double ancorae type oops sowed adequate ancorae strent. µ 4 3 C1W C1L C1S F c =24N/mm 2 2 1 1/8 1/2 1/5 1/16 1/4 1/1 1/25 µ 1 F c =24N/mm 2 8 C2W 6 C2L C2S 4 2 1/8 1/2 1/5 1/4 1/1 1/25 µ 1 8 6 4 2 F c =36N/mm 2 C6W C6S 1/8 1/2 1/5 1/4 1/1 1/25 µ 4 3 2 1 p w =.63% η=.15 F c =36N/mm 2 C5W C5S 1/8 1/2 1/5 1/16 1/4 1/1 1/25 µ 1 8 6 4 2 p w =.37% η=.15 F c =36N/mm 2 C7W C7S 1/8 1/2 1/5 1/4 1/1 1/25 µ 1 8 6 4 2 p w =.37% η=.1 F c =36N/mm 2 C8W C8S 1/8 1/2 1/5 1/4 1/1 1/25 p w =.63% η=.15 p w =.53% η=.1 p w =.37% η=.25 (a) Bendn falure type (b) Sear falure type F. 19 Stran proress of lateral renforcement
CONCLUSIONS Te follown conclusons are deduced from te expermental results: Seres I - Pullout Test In case of te upper renforcements wt conventonal ooks, te yeldn strent was not reaced, except for te 18 ooks. In te case of te double ancorae ooks te tensle strent was reaced n all te cases reardless of te ancor lent. Te conventonal ooks are affected by te castn drecton, but te double ancorae ooks are not affected by te castn drecton n almost all cases. Seres II - Column Test Bendn falure type and sear falure type specmens eter wt welded closed type or double ancorae type of lateral renforcement sowed a comparable sesmc performance. Double ancorae type of lateral renforcement wt an ancor lent of 4 mm sowed enou ancor strent for normal strent (σ y 39 MPa) and provded enou confnement even under axal force. Wen te lateral renforcement rato p w becomes ber, under te same axal force, te sear force ressted by te truss mecansm becomes ber. Under lare axal loads te resstn component of te sear force carred by te arc mecansm becomes reater. In addton te ultmate sear capacty of te column ncreases wt te ncrement of te lateral renforcement rato and te axal load rato (N/ b D Fc). EFEENCES 1. Japanese Arctectural Standard Specfcaton (JASS5): Arctectural Insttute of Japan, 23, n Japanese. 2. ecommendaton for Detaln and Placn of Concrete enforcement: Arctectural Insttute of Japan, 24, n Japanese. 3. Desn Gudelnes for Eartquake esstant enforced Concrete Buldns Based on Ultmate Strent Concepts: Arctectural Insttute of Japan, 199, n Japanese. 4. Ima H. et al: A esearc on Double Ancorae of t Anle Hook (No.1). Annual Meetn of te Arctectural Insttute of Japan, 1999, pp. 537-549, n Japanese. 5. Buldn Center of Japan: Desn Gudelnes of enforced Concrete Structures. 6. Kaku T. et al: Te proposal of equaton for bond splttn strent of renforced concrete members ncludn strent concrete level. Concrete esearc and Tecnoloy, Vol. 3, No.1, Jan. 1992, pp.97-18, n Japanese. 7. Castro J.J., Ima H.: Structural Performance of C Columns Usn Double t Anle Ancorae Hoops Wt H Strent Bars, 13WCEE, paper No. 2468.