A NEW SEISMIC DESIGN APPROACH FOR BRIDGE COLUMNS IN MODERATE SEISMICITY REGIONS

Transcription

1 A NEW SEISMIC DESIGN APPROACH FOR BRIDGE COLUMNS IN MODERATE SEISMICITY REGIONS Jae-Hoon Lee 1 Jin-Ho Choi 2 and Sung-Jin Shin 3 SUMMARY Seimi performane of reinfored onrete olumn are trongly influened by amount, paing, and detail of onfining teel a well a longitudinal reinforement detail. The urrent Korean Bridge Deign Speifiation have adopted the ame onept a the AASHTO peifiation for eimi deign of bridge olumn. It peifie ontant repone modifiation fator, R, uh a 3. or 5. when plati behavior i expeted, regardle of dutility demand. Aording to the Deign Speifiation a large amount of onfinement teel i provided to guarantee the full dutility even in ae that the dutility demand are quite low. Thi ometime reult in undeirable ituation uh a diffiulty in ontrution due to teel ongetion and eonomi problem. The deign onept adopted in the urrent deign peifiation whih i appropriate in trong eimi region i not alway valid for moderate or low eimiity region uh a Korean Peninular. Thi paper introdue a newly developed onfining teel deign proedure for the bridge olumn in low to moderate earthquake region. It inlude dutility demand etimation proedure and required onfining teel etimation proedure. The equation onider natural period of bridge, olumn apet ratio, mehanial propertie of onrete and reinforement, longitudinal teel ratio, axial fore ratio, and dutility demand. Thi paper alo provide deign example of ten bridge model and diuion on eonomy, afety, and oniteny of the urrent deign method and the propoed deign method. Keyword: bridge olumn, onfining teel, moderate eimiity region, dutility demand, new deign method INTRODUCTION The urrent eimi deign riteria of the Korean Bridge Deign Speifiation [KBDS, 25] have adopted the ame eimi deign onept and requirement a the AASHTO peifiation [AASHTO, 22 and AASHTO-LRFD, 24]. Thi deign approah ha been baed on the fore-baed full dutility deign onept whih wa eentially developed for 1 Profeor, Department of Civil Engineering, Yeungnam Univerity, KOREA, jholee@yu.a.kr 2 PhD, Student, Department of Civil Engineering, Yeungnam Univerity, KOREA, jh-hoi@yumail.a.kr 3 PhD, Student, Department of Civil Engineering, Yeungnam Univerity, KOREA, ungjin@yumail.a.kr

2 trong earthquake region. It regulate the ue of ontant value of repone modifiation fator, i.e. R=3 or 5 regardle of dutility demand, and require providing a large amount of onfining teel to bridge olumn o that enough dutility an be guaranteed. The olumn dutility, whih i defined a the ability of the olumn to utain large deformation without ignifiant trength degradation at a pot-elati tage, i eential for the earthquake reiting bridge epeially in trong earthquake region. The olumn ubjeted to yli loading during earthquake event hould have enough deformation apaity without failure up to the target dutility. Therefore the deign peifiation regulate the ontant value of repone modifiation fator and the minimum amount of onfining teel for bridge olumn. Thi approah i baed on the fore-baed deign onept, whih impliitly onider the dutility of the bridge olumn. However, adopting the full dutility deign onept ometime reult in not eonomial deign and ontrution problem due to reinforement ongetion, epeially in moderate eimiity region like Korea. In moderate eimiity region, it frequently happen that the alulated elati moment i lightly greater than the flexural trength. Thi mean a large amount of onfining teel i provided aording to the urrent deign peifiation baed on the fore-baed full dutility deign onept even if the dutility demand i quite low. It ometime reult in ontrution problem uh a teel age manufaturing and onrete plaement due to reinforement ongetion a well a eonomi problem. Therefore, a deign baed on required dutility and proving required tranvere teel determined by the demand might be a reaonable approah for the moderate earthquake region. Thi paper introdue the onfining teel deign proedure developed for low to moderate eimiity. It i baed on the olumn dutility demand whih an be varied and determined by the eimi intenity, earthquake reiting trutural ytem, and flexural deign apaity of the olumn. It ue newly developed deign equation to determine the required onfining teel amount, urvature dutility demand, diplaement dutility demand, and required repone modifiation fator whih i variable and determined by elati moment demand and apaity of the olumn. Thi paper alo provide deign example of ten bridge model by the urrent deign peifiation and the propoed deign method, and diuion on the deign reult. RESEARCH ACTINITIES FOR DEVELOPING DUCTILITY DEMAND BASED DESIGN APPROACH Experimental Tet of Reinfored Conrete Column The Korean Bridge Deign Speifiation adopted the AASHTO regulation for eimi deign in 1992, ine enough reearhe were not onduted in Korea to develop eimi deign peifiation. Therefore the minimum required onfining teel ratio, either piral or perfet irular hoop, ha been defined a the greater value of Eq. (1) or Eq. (2) [KBDS, 25; AASHTO, 22; AASHTO-LRFD, 24], where f k i onrete ompreive trength, f yh i yield trength of tranvere teel, and A g /A i ratio of the gro etion area to ore onrete area. In general, it i normal that Eq. (2) govern in pratial deign for normal ize of olumn etion. A f g k ρ S =.45 1 A (1) f yh

3 f k ρ S =.12 (2) f yh A lot of experimental reearhe for bridge olumn have been onduted from the middle of 199 in Korea. Thoe ativitie have been performed primarily by everal organization uh a the Korea Earthquake Engineering Reearh Center (KEERC), the Highway and Tranportation Reearh Intitute of Korea Highway Corporation (HTRI), the Korea Intitute of Contrution Tehnology (KICT), the Korea Infratruture Safety and Tehnology Corporation (KISTEC), the Korea Bridge Deign and Engineering Reearh Center (KBRC) and everal univeritie. The tet were performed to invetigate eimi performane of reinfored onrete olumn and to verify eimi deign regulation. Majority of teted olumn had irular hape and wa onduted epeially for flexure behavior under ompreion and for hear behavior. The variable of the tet inluded apet ratio, amount and detail of longitudinal teel, axial load ratio, amount and paing of tranvere teel, detail of irular tie, and material trength of onrete and reinforement. One of the onluion of thoe tet wa that higher dutility than expeted ould be obtained, even though the olumn were lightly onfined if they had adequate detail in tranvere teel and ontinuou longitudinal teel in plati hinge region. Therefore there were extenive diuion in Korean trutural engineer oiety on the minimum requirement of onfining teel defined by Eq. (1) and Eq. (2). Baed on thi behavior and diuion, reearhe on new deign method have been onduted. Conept of New Deign Method Baed on the onluion from the experimental tet of the olumn with adequate detail in tranvere teel and ontinuou longitudinal teel in plati hinge region, dutility demand bae deign approah wa developed. Thi approah differ from the urrent traditional eimi deign riteria uh a KBDS and/or AASHTO Speifiation, and an be een to produe more ytemati and reaonable eimi deign methodology. Figure 1 how relationhip between dutility demand and onfining teel amount. In the urrently ued full dutility deign method a hown a olid line in Figure 1, a ontant large amount of onfining teel i ued regardle of the dutility demand even though the required dutility fator i lightly greater than 1.. Then the ontant repone modifiation fator, R, of 3. or 5. i applied to the elati repone. The bai onept an be deribed a indiated with the dotted line in Figure 1. It i the main idea that R fator hould be ued a variable and determined a required, whih i the ontant value by the urrent deign peifiation of KBDS and/or AASHTO. The required onfining teel amount i determined by the required dutility demand of the olumn, whih reult in a reaonable deign and to prevent any poible ontrution problem due to exeive ue of tranvere reinforement. In developing the aforementioned deign approah, it i eential to prove whih orrelation exit between required dutility and onfining teel amount among a, b and in Figure 1. And alo, appropriate deign equation hould be developed to determine the required onfining teel amount with repet to dutility demand and relevant detailing regulation hould be prepared. Development of Deign Equation Column tiffne etimation i required for alulating natural frequeny of the bridge ytem and diplaement. In general, the olumn tiffne at yield of longitudinal teel i determined by ue of nonlinear moment-urvature analyi. For deign purpoe, however, implified equation may be ueful. A imple equation for yield tiffne of reinfored olumn wa

4 developed [LEE et al, 21] whih i hown in the next hapter. Baed on the experimental tet and analytial tudie for 5,76 olumn model, orrelation equation of irular olumn between urvature dutility and diplaement dutility were developed for eimi deign purpoe a well a for aement purpoe [SON et al, 23a]. Correlation equation of irular olumn between urvature dutility demand and onfining teel amount were alo developed for eimi deign purpoe a well a for aement purpoe [SON et al, 23b]. Framework of New Deign Method and Development of Deign Proedure Baed on the previou experimental and analytial tudie, reearhe on new deign method had been onduted. A a reult of thoe ativitie, dutility demand baed eimi deign method wa propoed by reearh group on eimi bridge deign of the KBRC [LEE et al, 24]. Table 1 how omparion of the urrent deign method of the peifiation and the propoed dutility demand baed eimi deign method. Figure 2 how the proedure of the dutility demand baed eimi deign for reinfored onrete bridge olumn. The detailed proedure i hown in the following hapter. PROPOSED DESIGN METHOD: Dutility Demand Baed Deign Approah The pratial deign proedure of the dutility demand baed deign approah an be ummarized a follow. It hall be onduted after olumn ize and longitudinal teel are determined for vertial load. STEP-1: Determine M el and φ M n The trutural analyi i performed for earthquake load by either dynami or equivalent tati analyi. In general, linear elati trutural analyi i ued for deign purpoe. Therefore, elati olumn moment, M el, i alulated by ue of yield tiffne of reinfored olumn whih an be obtained by moment-urvature analyi or a implified equation uh a Eq. (3) [LEE et al, 21]. The deign flexural trength of olumn, φ M n, i determined from etion analyi by any deign ode. I eff P = ρ l +.3 I g (3) f A ' g STEP-2: Determine req' d R The required repone modifiation fator, req' d R, i determined by the ratio of elati olumn moment to the deign flexural trength of the olumn a hown in Eq. (4). req M el ' d R = (4) φ M n

5 STEP-3: Determine req' d μδ The required diplaement dutility fator, req' d μδ, i determined by the relationhip between diplaement dutility fator and repone modifiation fator a Eq. (5). The orrelation fator, λ DR, i determined by the natural period (T) of the bridge. If the natural period i le than 1.25 time ontrol period, T, Eq. (6) i applied, otherwie Eq. (7), where i the period at the end of ontant deign petral aeleration plateau. It i note that Eq. (7) i for the equal diplaement priniple, while Eq. (6) i tranition to the equal energy priniple. They were adopted in different format by propoal of deign peifiation to AASHTO-LRFD [ATC/MCEER, 21; NCHRP 12-49, 21]. req' d μ = λdr req' d R (5) Δ T 1 T < 1.25T λ DR = 1 + req' d R (6) T req' d R T 1.25 λ = 1. (7) T DR STEP-4: Determine req' d μφ The required urvature dutility fator, req' d μφ, i determined by Eq. (8), whih i the funtion of the required diplaement dutility fator and the invere of apet ratio, D / L, of the olumn. Thi equation wa derived for deign purpoe [SON et al, 23a]. h μ Δ L req' d μ = φ (8) h L STEP-5: Determine req' d ρ The required onfining teel ratio,, i determined by Eq. (9), whih wa derived for deign of irular olumn [SON et al, 23b]. Eq. (1) through (12) are ued for determination of a, b, and g in Eq. (9). Thee equation onider the variable of onrete ompreive trength ( f k ), ratio of the gro etion area to the ore onrete area ( A g / A ), urvature dutility demand, axial load ratio ( P / f k Ag ), yield trength of longitudinal ( f y ) and tranvere reinforement ( f yh ) and longitudinal reinforement ratio ( ρ l ). f A k g ρ = α β + γ f yh A (9) P u α = 3( req' d μ φ + 1) +.8req' d μφ 3. 5 fk Ag (1) = f y β (11) γ.1 ρ.1 (12) = l ( )

6 STEP-6: Deign of onfining teel (detail: ize and paing) Longitudinal teel in the potential plati hinge region of the olumn hall be retrained by tranvere teel to prevent from bukling. Therefore, the paing of the tranvere teel in the potential plati hinge region of the olumn, hall not le than Eq. (13). 4A p = (13) d ρ STEP-7: Shear trength hek (at target diplaement dutility fator) Reinfored onrete olumn with relatively mall apet ratio how flexure-hear behavior, whih i flexural behavior at initial and medium diplaement tage and hear failure at final tage. Thi type of olumn ha lower dutility than thoe with flexural failure. For prevented brittle failure, hear trength hek i needed at the req' d μδ by ue of hear model and max. plati hinging fore a hown figure 3. Lee at al. (26) propoed hear apaity model a hown in Eq. (31) through (35) in SI unit. For the hear trength of reinforement, 4 angle i adopted a hown in Eq. (34). For ultimate diplaement predition, Lee et al. model provide loer reult to the tet than the other model. COMPARISON OF DESIGN RESULTS: Propoed Method and Current Method for Flexure Seleted Bridge Model for Comparion Typial bridge model were eleted and deigned for longitudinal diretion of the bridge to ompare effiieny and afety of the urrently ued deign method and the propoed method. The main variable of the model i the elf-weight of upertruture inreaing from 5kN/m to 5kN/m with the interval of 5kN/m to make 1bridge model. The ret of the variable are idential a follow: (1) Super truture : 4 pan ontinuou beam (2) Boundary ondition : 1 hinge and 4 roller upport (3) Span length and total length : 4 x 5 m = 2 m (4) Pier ytem : T-haped ingle olumn bent (5) Column hape and etion ize : Cirular etion with diameter (D) of 24 mm (6) Column height (L) and apet ratio (L/D) : L = 8, mm, L/D = 4. (7) Conrete ompreive trength : 27 MPa (8) Yield trength of longitudinal and tranvere reinforement : 4 MPa (9) Longitudinal reinforement and teel ratio ( ρ l ) : 72-D35, ρ l =.152 (1) Deign peak ground aeleration :.154 g [KBDS, 25] (11) Applied aeleration repone petra : Korean Bridge Deign Speifiation [KBDS, 25]

7 Reult of Column Deign Table 2 ummarize the reult of olumn deign for ten bridge model by the dutility demand baed eimi deign method. It i note that axial load ratio, effetive yield tiffne, period of vibration, and elati moment demand, M el, inreae a the upertruture elf-weight inreae. The deign flexural trength of olumn alo inreae a the upertruture elf-weight inreae, even though the olumn etion of eah bridge model i idential. It i beaue the flexural trength of reinfored onrete olumn under tenion ontrol region inreae a the axial load inreae. It i note that the flexural demand inreae more rapidly than the flexural apaity. Therefore, required repone modifiation fator and the dutility demand inreae a the upertruture elf-weight inreae, and doe the required onfining teel ratio. The final deign for onfining teel with D19 or D22 reinforement i deignated in the eond lat row of Table 2. The onfining teel deigned by the urrent deign method [KBDS, 25; AASHTO, 22; AASHTO-LRFD, 24] i alo deignated in the lat row of Table 2, whih i D22 teel with 84 mm paing. It i note that Bridge Model 7 through 1 an not be deigned by the urrent deign peifiation with the hoen ize olumn ine the R-fator of 3. i permitted for ingle olumn bent [KBDS, AASHTO]. Therefore, deign modifiation for olumn etion annot be avoided in ae that the elf-weight of upertruture i greater than 33 kn/m. Effiieny of Tranvere Steel Figure 4 how the diplaement dutility demand whih an be onverted to the required R-fator for 1 bridge model. The dutility demand inreae up to 3.67 a the elf-weight of upertruture inreae from 5 kn/m to 5 kn/m. It how non-linearly inreaing a inreaing the elf-weight of upertruture, ine the demand inreae non-linearly and the apaity i not ontant. It i note that the urrently ued fore-baed deign method permit to apply 3. of the R-fator to ingle olumn bent [KBDS, AASHTO]. Therefore, the bridge model of whih upertruture elf-weight are greater than 33 kn/m an not be deigned with the eleted olumn bet ytem by the urrent deign peifiation. However, thoe bridge model an be deigned by the propoed deign method, ine 3.67 of dutility demand eem to be pratially aeptable if orreponding onfining teel doe not reult in any ontrution problem. It may be regulated that the maximum permiible R-fator hall be 4. or 5.. Figure 5 how material ot (volume) ratio of tranvere teel deigned by the propoed method to that deigned by the urrent deign method. The propoed method require 3 84% of tranvere teel determined by the urrent deign method for the bridge model that an be deigned by the urrent peifiation. For the bridge model that an not be deigned by the urrent peifiation up to 5 kn/m of upertruture elf-weight, % of tranvere teel determined by Eq.(2) are required by the propoed method. Safety and Coniteny Diplaement demand and diplaement apaity of eah bridge model were alulated and ompared in order to invetigate afety and oniteny of the deign method. The diplaement demand, Δ u, req' d, of 1 bridge model were obtained by linear elati trutural analyi with effetive yield tiffne of the olumn for deign earthquake load. Nonlinear puh-over analye by moment-urvature analyi were performed to obtain two different value of diplaement apaity of eah bridge model: Δ ap, prop, of the olumn deigned by the propoed method; and, of the olumn deigned by the urrent deign Δ ap, ode

8 peifiation. Figure 6 preent the reult. A hown in the figure, the diplaement demand, Δ u, req' d, inreae a the elf-weight of upertruture inreae from 5 kn/m to 5 kn/m. It i note that both diplaement apaitie, Δ ap, prop and Δ ap, ode, are greater than the diplaement demand, Δ u, req' d, reulting in atifatorily afe deign. However tendenie of afety margin are baially different. The diplaement apaitie,, of the olumn deigned by the Δ ap, prop propoed method alo inreae a the elf-weight of upertruture inreae, exept bridge model 1 (upertruture elf-weight i 5 kn/m) of whih longitudinal teel of olumn i in elati range under deign earthquake load. That i the ame tendeny a the diplaement demand whih alo inreae a the elf-weight of upertruture inreae. The diplaement apaitie, Δ ap, ode, of the olumn deigned by the urrent deign method, however, dereae a the elf-weight of upertruture inreae exept bridge model 1, whih i the oppoite tendeny to inreaing the diplaement demand. It i beaue that inreaing axial load reult in redued diplaement apaity even though the olumn of bridge model have the idential etion. Therefore, it an be antiipated that the urrent deign peifiation provide irrational deign reult, due to adopting full dutility deign method with ontant R-fator. And alo, rapidly dereaing afety margin a inreaing upertruture elf weight may be expeted. Figure 7 how the ratio of diplaement apaity to diplaement demand whih may repreent margin of afety. The diplaement afety fator of the olumn deigned by the urrent deign peifiation vary from 1.21 to 4.14 exept the olumn under elati deign (upertruture elf-weight i 5 kn/m). Thoe by the propoed method how range of 1.38 and 2.89, whih may be evaluated a more onitent and rational. Caltran DISPLACEMENT BASED FLEXURE-SHEAR CAPACITY MODELS CALTRANS Seimi Deign Criteria (26) define hear apaity model a Eq. (14) through (21) in SI unit. The nominal hear trength i alulated a ummation of ontribution of onrete, V, and tranvere reinforement, V. CALTRANS define hear trength of onrete, V differently by laifying inide the plati hinge region and the ret. In V determination for the plati hinge region, diplaement dutility fator μd i ued a a main variable. The volumetri ratio of tranvere teel, yield trength of tranvere teel, and axial load effet are alo onidered in V. The hear effetive area, A e in Eq. (15), i taken a.8 time gro etional area for irular etion. The hear trength of tranvere teel, V, i alulated by Eq. (21) for irular etion baed on 45 angle tru model, where A p i the area of hoop or piral and D p i the diameter of ore onrete meaured between enter to enter of hoop or piral. V = V + V (14) n e V = v A (15)

9 v = Fator (16) ' ' 1 Fator 2 f.33 f ρ f yh Fator1 = μd 12.5 (17).25 Fator 1.25 (18) P Fator 2 = (19) A g 1. Fator (2) π Ap f yhdp V = 2 (21) Ahheim and Moehle Ahheim and Moehle (1992) propoed Eq. (22) through (25) in SI unit to ompute the nominal hear trength of reinfored onrete olumn. Conidering diplaement dutility and the effet of ompreion, V i alulated by Eq. (23) and (24) for plati hinge region. The effetive hear area, A e in Eq. (23), i.8 time gro etion area (.8 A g ) for irular etion. The hear trength of reinforement, V, i alulated by Eq. (25) baed on 3 angle tru model, and.8d i ued for d in irular olumn, where D i the diameter of the etion. Prietley et al. V = V + V (22) n P ' V =.3 k + f A e 14A g (23) 4 μδ k = 1 3 (24) π Ap f yhdp V = ot3 2 (25) Prietley et al. (1996) propoed Eq. (26) through (3) in SI unit to alulate nominal hear trength of reinfored onrete olumn for deign purpoe. Conidering the effet of diplaement dutility, V i alulated by Eq. (27) and (28) for plati hinge region. The hear trength of reinforement, V, i alulated by Eq. (29) baed on 35 angle tru model for irular olumn. The effet of hear trength enhanement reulting from arh ation in axial ompreion i onidered by Eq. (3). For the olumn bent in ingle urvature, the trut form between the enter of the etion at the top where the axial load i applied and the enter of flexural ompreion at the bottom. Therefore tan α beome D / 2L, where D / 2 i the horizontal ditane between enter of the etion and enter of flexural ompreion, and L i the antilever olumn length. In the ae of irular etion,.65d i reommended for D. The oeffiient.85 in Eq. (3) i ued for eimi deign, but 1. i uggeted for eimi performane evaluation.

10 V = V + V + V (26) n V ' e p = k f A (27) μδ 2 : k =.25 ( ) 2 μδ 4 : k = μδ 2 μ = = Δ 4 : k.83 4 μδ 8 : k = ( μδ 4) 8 μδ : k =.42 (28) π Ap f yhdp V = ot35 2 (29) D Vp =.85P tanα =.85P 2L (3) Lee et al. Lee at al. (26) propoed hear apaity model a hown in Eq. (31) through (35) in SI unit, whih i baially the ame format a Prietley et al. model but modified. For the hear trength of onrete, k in Eq.(32) i modified to Eq. (33) to produe three traight line divided by the diplaement dutility of 2 and 5, while Prietley et al. model ha four traight line divided by the diplaement dutility of 2, 4, and 8. For the hear trength of reinforement, 4 angle i adopted a hown in Eq. (34). The axial load effet on hear trength i onidered a the ame format a Prietley et al. model, but 2/3 time D i reommended for D a hown in Eq. (35) for irular etion. V = V + V + V (31) V n ' e p = k f A (32) μδ 2 : k = μ = ( ) Δ 5: k.3 μδ μδ : k = (33) π Ap f yhdp V = ot 4 2 (34) D D Vp =.85P tanα =.85P =.85P 2L 3L (35)

11 EXPERIMENT Speimen and Tet Four large ize irular reinfored onrete olumn were ontruted. All the olumn have 12 mm diameter ro-etion and main variable are longitudinal teel ratio and apet ratio (pan-depth ratio). Total height of MS-HT4-N-L2 and MD-HT6-N-L2 peimen i 592 mm and loading height i 48 mm, o that the apet ratio hould be 4.. Total height of MS-HT4-N-FS peimen i 412 mm and loading height i 3 mm, o that the apet ratio hould be 2.5. Total height of MS-HT4-N-SH peimen i 331 mm and loading height i 219 mm, o that the apet ratio hould be D19 (diameter of 19 mm) and D1 bar were ued a longitudinal reinforement and tranvere reinforement, repetively, for all the olumn peimen. Detail and variable of the olumn peimen are hown in Figure 8 and Table 3. For the longitudinal reinforement, 4-D19 and 8-D19 were ued for MS-HT4-N erie olumn and MS-HT6-N-L2 olumn peimen, repetively, o that the teel ratio hould be.12 and.23, repetively. D1 irular tie were provided a tranvere reinforement. In addition to the irular tie, rotie were ued for MS-HT4 erie olumn a hown in Figure 9. Perfet hoop by ue of oupler were ued for MD-HT6-N-L2 olumn. Spae of tranvere teel in plati hinge region wa 115 mm for all the peimen, whih reulted in volumetri ratio of.23. It orrepond to 24% of the required onfining teel ratio defined by AASHTO Speifiation (22) and Korean Bridge Deign Speifiation (25). Spae of 115 mm in plati hinge i equivalent to 6 time the longitudinal bar diameter and 12 time the tranvere bar diameter. Material tet wa arried out to determine the atual mehanial propertie of onrete and teel. The onrete ompreive trength by 1 X 2 mm ylinder wa 24.8 MPa at the time of loading tet. Yield trength of the reinforement wa meaured to be 343 MPa for D19 reinforement and 373 MPa for D1 reinforement. A hown in Figure 1, quai-tati tet wa onduted under ontant axial load and inrementally inreaing lateral deformation reveral uing Hydrauli atuator with ±5 mm diplaement apaity and 35 kn fore apaity. Axial load of 1863 kn wa applied o that the axial load ratio hould be.7. Two yle of lateral load were applied at eah drift level a hown in Figure 11. Tet Reult MS-HT4-N-L2 peimen (apet ratio = 4.) howed typial flexure failure in plati hinge region, while the other three olumn howed flexure-hear failure. Figure 12 preent failure of MS-HT4-N-SH peimen (apet ratio = 1.825), whih how diagonal rak due to flexure and hear ation. Figure 13 how failure of MD-HT6-N-L2 peimen. It how flexure-hear failure even though it apet ratio i 4.. It i believed that relatively mall amount of tranvere teel ompared with longitudinal teel reulted in diagonal rak and hear related failure after plati hinge formed by flexure at the bottom of olumn. Failure mode wa eventually flexure-hear failure by frature of tranvere teel and hear rak. It hould be note that MD-HT6-N-L2 peimen ha twie the longitudinal teel ratio ompared with MS-HT4-N-L2 peimen. During the load tet, lateral load and lateral diplaement were meaured up to failure. Envelop urve of the olumn peimen were obtained from the meaured yli relationhip, whih will be ompared with the diplaement baed flexure-hear apaity model. More information about the tet an be found elewhere (Chung et al. 21).

12 Seleted Experimental Reult COMPARISION OF MODELS WITH RESULTS In order to ompare the dutility baed flexure-hear model, two et of experimental reult were eleted: (1) four large ale olumn teted by the author whih are previouly introdued in thi paper, and (2) two large ale olumn tet onduted by Kim et al. (21). Kim et al. (21) onduted quai-tati tet for two large ale irular olumn, of whih diameter wa 12 mm. FS-H-LS peimen had 495 mm of total height and 32 mm of loading height o that the apet ratio hould be The other peimen FS-L-LS had 445 mm of total height and 27 mm of loading height o that the apet ratio hould be The other variable of the two olumn peimen are idential. Thirty ix D25 (diameter of 25.4 mm) reinforing bar were ued for longitudinal teel, whih reulted in.161 of longitudinal teel ratio. Cirular hoop with 3 mm paing were ued for plati hinge region o that the volumetri lateral teel ratio hould be.169. The pae of 3 mm i equivalent to 11.8 time the longitudinal diameter and 23.6 time the tranvere teel diameter. The yield tre of D25 and D13 reinforement were 331 MPa and 326 MPa, repetively. The ompreive trength of onrete wa meaured to be 24.5 MPa at the age of tet. More information about the tet olumn an be found in their paper (Kim et al. 21). Appliation of Equation for Cro-tie Three olumn peimen (MS-HT4 erie) teted by the author have two ro-tie perpendiular to eah other. Shear fore i reited by one ro-tie parallel to the lateral loading diretion a well a the hoop reinforement. Shear trength of ro-tie i alulated by Eq. (36), where A t i area of ro-tie in a parallel diretion to the applied lateral load and D i ditane between enter of the perimeter hoop. p At f yhdp Vt = otθ (36) For MS-HT4 erie olumn, hear trength of tranvere teel, V, i alulated by ummation of hear trength produed by hoop reinforement and ro-tie. When eah model i applied, the ame value of angle uh a 45 (CALTRANS), 4 (Lee et al.), 35 (Prietley et al.) or 3 (Ahheim et al.) i ued for θ in Eq. (36). Failure Mode Predition Figure 14 how diplaement baed hear apaity model applied to the envelope urve of tet reult. The envelope urve are obtained from the meaured lateral load-diplaement relationhip under yli loading. Two envelope urve for eah olumn peimen are provided from the tet reult for puh diretion and pull diretion. The diplaement at interetion of the envelope urve and the hear apaity model preent predited diplaement apaity of the olumn under flexure-hear failure. For MS-HT4-N-SH peimen whih howed flexure-hear failure during the tet, CALTRANS model, Ahheim et al. model and Lee et al. model predit failure mode of

13 flexure-hear failure, but Prietley et al. model predit flexure failure, a hown in Figure 14(a). For MS-HT4-N-FS peimen whih alo howed flexure-hear failure during the tet, CALTRANS model and Lee et al. model predit the ame failure mode, but Ahheim et al. model and Prietley et al. model predit flexure failure, a hown in Figure 14(b). In the ae of MS-HT4-N-L2 peimen, all the model predit the ame failure mode a the tet reult of flexure failure, a hown in Figure 14(). For MD-HT6-N-L2, FS-H-S, and FS-L-S peimen whih howed flexure-hear failure during the tet, all the model predit flexure -hear failure, a hown in Figure 14(d), (e), and (f). Table 4 preent predited failure mode by eah model. CALTRANS and Lee et al. model predit the ame failure mode a tet reult for all the peimen. Auray of Diplaement Capaity Predition Table 5 how the meaured ultimate diplaement and the predited ultimate diplaement by the model for 5 peimen under flexure-hear failure. The ratio of predited ultimate diplaement to tet reult i alo preented in Table 5. It i note that Ahheim et al. model predit flexure failure for MS-HT4-N-FS peimen and Prietley et al. model predit flexure failure for MS-HT4-N-FS and MS-HT4-N-SH peimen. For five large ize olumn, the ratio of predited ultimate diplaement by CALTRANS model to tet reult are between.42 and.63, while thoe by Lee et al model are between.54 and.98. Ahheim et al. model provide the ratio between.48 and.75 for four olumn peimen. By Prietley et al. model, three peimen how the ratio between.86 and.93. CONCLUTIONS A new eimi deign approah, the dutility demand baed deign method, provide eonomial reinfored onrete bridge olumn deign ompared to the urrent deign method of the KBDS and AASHTO peifiation. It alo give afe and more onitent deign reult providing relatively narrow range of afety margin. It i believed that the propoed method i rational in onept epeially for the low to moderate earthquake region. ACKNOWLEDGEMENTS Support for thi work wa provided by the Korea Bridge Deign & Engineering Reearh Center(KBRC). Thi finanial upport i gratefully aknowledged.

14 REFERENCES AASHTO, 22, Standard Speifiation for Highway Bridge, Amerian Aoiation of State Highway and Tranportation Offiial, 17thEdition, Wahington,D.C.,USA. AASHTO, 24, LRFD Bridge Deign Speifiation, Amerian Aoiation of State Highway and Tranportation Offiial, 3rd Edition, Wahington, D.C., USA. ATC/MCEER Joint Venture, 21, Reommended LRFD Guideline for the Seimi Deign of Highway Bridge, Part I : Speifiation, ATC-49a and MCEER-2-p1,USA. CALTRANS, 26, CaltranSeimiDeignCriteria,Verion1.4, California Department of Tranportation, Saramento, USA. NCHRP 12-49, 21, Comprehenive Speifiation for the Seimi Deign of Bridge, Revied LRFD Deign Speifiation (Seimi Proviion), 3rd Draft, USA. MOCT, 25, Korean Bridge Deign Speifiation, Minitry of Contrution & Tranportation, Republi of Korea. LEE, Jae-Hoon, and BAE, Sung-Yong, 21, Yielding Effetive Stiffne of Cirular RC Bridge Column for Seimi Deign Fore, Journal of the Korea Soiety of Civil Engineer, Vol.21, No.5-A, pp LEE, Jae-Hoon, and KIM, Ik-Hyun, 24, Development of Performane Baed Deign for Conrete Bridge Column, KKBRC) Annual Report, p.49. SON, Hyeok-Soo, and LEE, Jae-Hoon, 23b, Confinement Steel Amount for Dutility Demand of RC Bridge Column under Seimi Loading, Journal of the Korea Conrete Intitute, Vol.15, No.5,pp Ahheim, M. and Moehle, J. P., 1992, Shear Strength and Deformability of RC Bridge Column Subjeted to Inelati Cyli Diplaement, Report No. UCB/EERC 92/4, Earthquake Engineering Reearh Center, Univerity of California at Berkeley. Chung, Y.S., Lee, J.H., and Kim, J.K., 21, Experimental Reearh for Seimi Performane of Exiting Reinfored Conrete Pier, ReearhReport,KoreanHighwayCorporation, Seoul, Korea. Kim, B.S., Kim, Y.J., Kwahk, I.J., Cho, C.B., and Cho, J.R., 21, Seimi Performane Evaluation of Cirular RC Bridge Pier with Shear Flexure Behavior, Journalofthe Earthquake Engineering Soiety of Korea, 5 (3) Lee, J.H., Ko, S.H., and Chung, Y.S., 26, Shear Capaity Curve Model for Cirular RC Bridge Column under Seimi load, Journal of the Earthquake Engineering Soiety of Korea, 1 (2), 1-1. Prietley, M. J. N., Seible, F., and Calvi, G. M., 1996, Seimi Deign and Retrofit of Bridge, John Wiley & Son, In., New York.

15 Table 1 Comparion of Current Deign Method and Propoed Deign Method Flexure Shear Claifiation Current deign method Propoed deign method Target Dutility Full dutility Limited and full dutility Dutility fator Impliity onidered by R Expliitly onidered R-fator Contant Variable Natural period of bridge ytem Not onidered Conidered Material trength (Conrete and tranvere teel) Variable Material trength Axial load ratio in determination of (Conrete and tranvere teel) Dutility demand onfinement teel amount Yield trength of long. Steel Longitudinal teel ratio Variable in hear trength alulation f k, f y, ο θ = 45 f k, f y, ο θ = 4, μ Δ

16 Table 2 Reult of Column Deign for Ten Bridge Model Bridge Bridge Bridge Bridge Bridge Bridge Bridge Bridge Bridge Bridge Claifiation Model Model Model Model Model Model Model Model Model Model Supertruture Self-Weight [kn/m] Axial Load Ratio ( P / f k Ag ) Effetive Yiedl Stiffne( I eff ) Period of Vibration T [e] Elati Repone Coeffiient C [g] I g.4i g.41 I g.43 I g.43 I g.44 I g.45 I g.46 I g.47 I g I g M el [kn-m] φ M n [kn-m] req ' d μ = req' d R Δ.97 (elati) req' d μ φ req' d ρ ( 1 ) Tranvere Steel Spaing [mm] Trial with D22 Final with D19 or D22 Current Deign by KBDS φ 853 6d bl 21 (D19) 3 (D16) φ 423 6d bl 21 (D19) φ 229 6d bl 17 (D19) (D22) 122 (D22) 1 (D22) 84 (D22) 85 (D22) 74 (D22) 65 (D22) 58 (D22) Not Permitted by Current Speifiation

17 Table 3 Tet Column Detail and Material Propertie Tranvere hoop tie Speimen Loading height [mm] Apet ratio Longitudinal teel ratio Plati hinge region Volumetri Spae Outide plati hinge region Volumetri Spae ratio[%] [mm] ratio[%] [mm] MS-HT4-N-L2 4, MS-HT4-N-FS 3, MS-HT4-N-SH 2, MD-HT6-N-L2 4, Table 4 Failure Mode Predition Speimen Failure mode Preited failure mode by (Tet reult) CALTRANS Ahheim et al. Prietley et al. Lee et al. MS-HT4-N-SH flexure-hear flexure-hear flexure-hear flexure flexure-hear MS-HT4-N-FS flexure-hear flexure-hear flexure flexure flexure-hear MS-HT4-N-L2 flexure flexure flexure flexure flexure MD-HT6-N-L2 flexure-hear flexure-hear flexure-hear flexure-hear flexure-hear FS-H-LS flexure-hear flexure-hear flexure-hear flexure-hear flexure-hear FS-L-LS flexure-hear flexure-hear flexure-hear flexure-hear flexure-hear Table 5 Predition Ultimate Diplaement and Auray Ratio of predited ultimate Ultimate diplaement (mm) Speimen diplaement to tet reult CAL- Ahheim Prietley Lee CAL- Ahheim Prietley Lee Tet TRANS et al. et al. et al. TRANS et al. et al. et al. MS-HT4-N-SH MS-HT4-N-FS MD-HT6-N-L FS-H-LS FS-L-LS

18 Current Deign Method baed on R-fator Propoed Deign Method baed on Dutility Fator Confinement Steel Ratio ρ,ode ρ n R = 1 A R = 3 or 5 B C point D logitudinal teel elati limited dutility full dutility Required Dutility Fig.1 Shemati relationhip between dutility and onfinement teel ratio Determine Setion type & Dimenion Deign of Longitudinal Steel Determine Elati moment Strength(Mel) by Struture Analyi Determine Deign Flexural Stength( φmn) by ACI Strength Calulation Determine (Req'd R) Repone Modifiation Fator Req'd R = Mel / φ Mn Determine Req'd Diplaement Dutility, Determine Req'd Curvature Dutility, μ Determine Req'd Confinement Steel, ρ φ μ Δ req'd μ Δ = f (req'd R, T) req'd μ = f (req'd, ar) φ μ Δ note: ar= L / D (hear pan / diameter) req'd ρ = f (fk, fyh, Ag/A, μ φ, P, fy, ρ l ) Deign of Confinement Steel (amount, paing, detail, et.) Shear Strength Chek at Target Diplaement Dutility Fator Fig.2 Deign Proedure of Dutility Demand Baed Seimi Deign

19 Req'd μ Δ or R R = μ Δ = 3 (ode) Fig.3 Shear Strength Chek at the Fore-baed Deign Dutility Demand-Baed Deign Not admitted by Current Code 1. Diameter = 2.4 m Equal Diplaement Apet Ratio = Supertruture Self-Weight [kn/m] Fig.4 Required Diplaement Dutility Fator and Required R-Fator Material Cot Ratio req' d μδ m m f m D19 D22 Not admitted by Current Code.2 Diameter = 2.4 m Anti-Bukling Apet Ratio = 4. ( = 21 mm) Supertruture Self-Weight [kn/m] Fig. 5 Ratio of Required Tranvere Steel to Confining Steel Amount of Speifiation m Lateral Diplaement [mm] elati deign deigned for Anti-Bukling ( = 21 mm) Δ ap,ode Δ ap,prop Δ u,req'd 5 Diameter = 2.4 m Apet Ratio = Supertruture Self-Weight [kn/m] Fig.6 Tendeny for lateral diplaement apaity Safety - Lateral Diplaement elati deign Δ ap,prop Δ ap,ode Δ u,req'd Diameter = 2.4 m Apet Ratio = 4. Δ u,req'd Supertruture Self-Weight [kn/m] Fig. 7 Safety ratio range for diplaement apaity

20 (a) MS-HT4-N-L2 (b) MS-HT4-N-FS () MS-HT4-N-SH (d) MD-HT6-N-L2 Figure 8. Configuration and dimenion of peimen. 25 mm oupler 16 mm 16 mm D1 D1 (a) MS-HT4 erie (b) MD-HT6-N-L2 Figure 9. Configuration of tranvere teel. Diplaement [mm] %.25 % 1. % 1.5 % 2. % be ontinued Number of Cyle Drift Level [%] Figure 1. Tet etup Figure 11. Loading pattern

21 Figure 12. Failure of MS-HT4-N-SH Figure 13. Failure of MD-HT6-N-L2 Lateral Fore [kn] Diplaement [mm] Experiment CALTRANS 24 Ahheim et al. Prietley et al. 2 Propoed model MS-HT4-N-SH Drift Ratio [%] Lateral Fore [kn] Experiment CALTRANS Ahheim et al. Prietley et al. Propoed model MS-HT4-N-FS Drift Ratio [%] (a) MS-HT4-N-SH (apet ratio = 1.825) (b) MS-HT4-N-FS (apet ratio = 2.5) Lateral Fore [kn] Diplaement [mm] Experiment CALTRANS 24 Ahheim et al. Prietley et al. 2 Propoed model MS-HT4-N-L Drift Ratio [%] Lateral Fore [kn] Diplaement [mm] Experiment CALTRANS 2 Ahheim et al. Prietley et al. Propoed model MD-HT6-N-L Drift Ratio [%] () MS-HT4-N-L2 (apet ratio = 4.) (d) MD-HT6-N-L2 (apet ratio = 4.) Lateral Fore [kn] Diplaement [mm] L/D=2.67 Kwahk et al. 22, FS-H-LS Experiment CALTRANS Ahheim et al. Prietley et al. Propoed model Drift Ratio [%] Lateral Fore [kn] Diplaement [mm] L/D=2.25 Kwahk et al. 21, FS-L-LS Experiment CALTRANS Ahheim et al. Prietley et al. Propoed model Drift Ratio [%] (e) FS-H-LS (apet ratio = 2.67) (f) FS-L-LS (apet ratio = 2.25) Figure 14. Appliation of eah model for load-diplaement of experimental reult.