Behavior and Design of Moment-Reducing Details for Bridge Column-Foundation Connections

Transcription

1 16 TRANSPORTATON RESEARCH RECORD 1319 Behavior and Design of Moment-Reduing Details for Bridge Column-Foundation Connetions DAVD. MLEAN, KuANG Y. LM, AND EDWARD H. HENLEY, JR. Bridge foundations in seismi regions are usually designed to withstand the plasti hinge moments that develop at the bases of the olumns. Various hinge details have been proposed to redue or even eliminate the plasti moments transferred to the foundations, and thereby, redue the sizes and osts of the foundations. However, no ode speifiations for these moment-reduing hinge details urrently exist. The behavior of olumn speimens inorporating different moment-reduing hinge details was investigated. Tests were performed on reinfored-onrete olumn speimens subjeted to inreasing levels of yled inelasti displaements under onstant axial load. The effets on hinge performane of several parameters were investigated, inluding providing vertial disontinuity in the hinge detail, level of axial load, low-yle fatigue harateristis, olumn aspet ratio, and different amounts of longitudinal and transverse reinforement. Using the test results, hinge details an be inorporated into olumns to signifiantly redue the moment apaity at the bases of the olumns. The moments are not negligible, as is sometimes assumed for design with the moment-reduing hinge details. Providing vertial disontinuity in the moment-reduing hinge details results in redued distress in the longitudinal reinforement and improved performane of the hinge. Preliminary design reommendations are proposed for the omprehensive design of momentreduing hinge details at the bases of bridge olumns. Bridge foundations in seismi regions are designed to withstand the plasti hinge moments that develop at the bases of bridge olumns. n olumns that are oversized for arhitetural or other reasons, this approah results in exessively large foundations. Various hinge details for the bases of bridge olumns have been proposed to redue the plasti moments transferred to the foundations, and hene, redue foundation sizes and osts. The basi onept inherent in the modified hinge details is to provide a redued moment apaity in the plasti hinging region at the bases of the olumns. This is aomplished by plaing a layer of easily ompressed material at the base of the olumn that provides partial disontinuity between the olumn and the foundation. The disontinuity results in a smaller effetive ross setion at the olumn base and, thus, in a redued hinge apaity in the olumn. To a great extent, the modifiations that have been suggested have been based on engineering judgment, and the behavior and safety of the moment-reduing details have not been fully established. D.. MLean, Department of Civil and Environmental Engineering, Washington State University, Pullman, Wash K. Y. Lim, Sverdrup Corporation, 134 Treat Boulevard, No. 1, Walnut Creek, Calif E. H. Henley, Jr., Bridge and Strutures Branh, Washington State Department of Transportation, Transportation Building KF-1, Olympia, Wash The objetives of this study were to evaluate urrent design praties for inorporating moment-reduing hinge details at the bases of oversized bridge olumns, to experimentally investigate the seismi performane of olumns inorporating suh details, to identify any symptomati problems assoiated with the suggested details, and to develop design reommendations for the detailing of the hinge region of oversized olumns to redue the moment transfer between the olumns and foundations. Details of the experimental program and preliminary findings were presented by Lim et al. (1). Herein, fin;:il onlnsions from the study ;:ire presented and preliminary reommendations for the design of moment-reduing hinge details at the bases of bridge olumns are proposed. CURRENT PRACTCE Codified guidelines for the design of moment-reduing hinge details do not urrently exist. As a result, there is onsiderable variation in the speifiations, and even the use, of these details. One approah to the design of the moment-reduing hinge details is to determine the size of the hinge required from the pure axial ompressive apaity of the setion, and to design for shear aross the setion by providing the amount of longitudinal steel required as determined from shear frition theory. A horizontal joint onsisting of V4- to 1 /2-in.-thik expansion joint material is provided at the throat region around the hinge perimeter to reate partial disontinuity between the olumn and the footing. To further redue the moment developed at the hinge setion, the longitudinal bars are sometimes lustered at the enter of the hinge, and the hinge is treated as a pin with no moment apaity. Both irular and retangular arrangements of the reinforement in the hinges have been used. Normally, only nominal transverse steel is provided. Oasionally, no transverse steel is used. An example design for a olumn inorporating a hinge of this type is shown in Figure 1 (left). Several questions about the behavior of this hinge detail under seismi loading an be raised. The hinge is designed for the axial load apaity of the setion, and researh (2) has shown that reinfored-onrete olumns tested under axial loads lose to the maximum axial load allowed by Amerian Conrete nstitute (AC) (3) exhibited signifiantly redued dutility. Also, even though the hinge is assumed to be a pin onnetion, substantial moment atually develops at the hinge setion even if the longitudinal bars are lustered. This proess results in an inrease in the shear and axial load in the olumn

2 MLean et al. 17 'o -,.., ' 'D "-.J.'.' EXPANSON 2 JONT FLLER '-BEARNG AREA T ;... ' "' l ' 24' o" f-6'-o" j--6' -o"--t---6 ' -o"--j l::r :r1u:::n111 :i1 2-2 ',! '... 1 _, N l ' ' - 4'-" +- 3'-6" - -., ' _ ' L W3.5 SPRAL (l) 3",,...--f EPANSON JONT FLLER f' SHEAR KEY 6" CC s PACED TY p -..., CONTR UCTON JON T WTH ROUG HEN ED SURF ACE..._ -j1'-5"- 4'-a" J 2'-. -,- 3'-6".l_ ;-., r-..i'- 'r-- ;-. 2.f' zf 1 ;,...,..,,.., f--6'-o"--+-s -o s -o / f' PRE MOLDED JONT FLLER v 2"EXPA NSON TRYENE i,!:.olys..., FO PE DESTAL OTNG FGURE 1 Moment-reduing hinge details for the bases of bridge olumns. over that assumed for design. Prying ation, aused by ontat of the olumn edges with the top of the footing, develops under inelasti loading if insuffiient horizontal joint thikness is provided. This prying ation leads to higher moments and inreased degradation in the hinge. Beause of the sharp hanges in setion properties at the hinge, plasti deformations in the hinge are onentrated at the loation of the horizontal disontinuity, resulting in inreased distress in the hinge. Finally, the assumed design fores for the footing are unonservative as the atual moment transferred by the hinge to the footing is not onsidered. Other designs have been proposed to spread the zone of plasti ation over a greater vertial length by providing both horizontal and vertial disontinuity in the moment-reduing hinge detail. nreased disontinuity joint thiknesses are also speified to prevent ontat of the outer olumn with the footing. An example design for a hinge inorporating both horizontal and vertial disontinuity is shown in Figure 1 (right). EXPERMENTAL TESTNG PROGRAM Test Speimens and Parameters Experimental tests were onduted on reinfored-onrete olumn speimens inorporating several different momentreduing hinge details. The test speimens onsisted of a single olumn member onneted at the base to a retangular footing. The speimens were subjeted to inreasing levels of yled inelasti displaements under a onstant axial load. The speimens were arranged in groups of three: one speimen inorporating a hinge detail with horizontal disontinuity only (CA series), one speimen inorporating a hinge detail with both horizontal and vertial disontinuity (WA series), and one referene or ontrol speimen onsisting of a olumn with the same dimensions and reinforement as the hinge onnetion of the speimens inorporating the moment-reduing hinge details (CON series). These three types of speimens are shown in Figure 2. Tests were performed on two different sizes of speimens: small-sale speimens (approximately 1: 2) and moderatesale speimens (approximately 1:6). More than fifty 1:2- sale speimens were tested. The small-sale study provided a ost-effiient parametri study and also guided the seletion of variables for the larger-sale tests. Fourteen 1:6-sale speimens were tested. The larger 1:6-sale tests resulted in a more realisti representation of the hinging behavior in atual bridge olumns, and size effets were redued when ompared to the small-sale tests. Dimensions and reinforement for a typial 1:6-sale olumn speimen are shown in Figure 3. Parameters investigated in the experimental testing program inluded olumn aspet ratio, magnitude of axial load, amount of both longitudinal and transverse reinforement,

3 18 TRANSPORTATON RESEARCH RECORD 1319 [QJ (a) CA detail (b ) WA de tail () CON detail FGURE 2 Hinge details studied. r r "--i T " l 28" 18"!>..._l_-, H (VARES) 8 NQ 6 LONGTUDNAL BAR NO. 3 TES AT 6" O.C. AROilTECTURAL COLUMN 8 N. 4 LONGTUDNAL BARS 9GAGE SPRAL PTCH 718" FGURE 3 Typial 1:6-sale speimen dimensions and reinforement. DSCONTNUTY JONT DETALS VARY) vertial disontinuity length, thikness of horizontal disontinuity, olumn shape, hinge ross-setional shape (irular and square), and low-yle fatigue harateristis. A summary of the details of the speimens of the 1 :6-sale testing program is presented in Table 1. Additional details of the testing program are provided by Lim et al. (1,4,5). Test Setup and Proedures The test setup and proedures for the 1 :2- and 1 :6-sale speimens were similar. Figure 4 shows the test setup for the 1:6-sale speimens. The footing of the test olumn was anhored to a laboratory strong floor. Axial load was first applied to the top of the olumn using a 55-kip atuator operated in fore ontrol. Axial loads were maintained at a onstant level during a test. Lateral fore was then applied slightly below the top of the olumn using a 22-kip atuator operated in displaement ontrol. An analog signal of a presribed ramp funtion was generated using a personal omputer and sent to the servoontroller of the 22-kip atuator. Strain gages were used to monitor the strains in the longitudinal and transverse reinforement within the hinging region, and linear variable displaement transformers (L VDTs) were mounted to the sides of the olumns to measure rotations at the olumn base. All data were reorded intermittently on the same personal omputer used to generate ontrol signals for the horizontal atuator. The determination of the yield displaement, Y' and the loading sequene were similar to the proedures used by Priestley and Park (6-8). However, on the basis of preliminary tests, it was found that the ultimate moment apaities

4 TABLE 1 SUMMARY OF THE 1:6-SCALE TESTNG PROGRAM Speimen No. Variable Studied Aspet Ratio Axi a l Load Yield Displaement Measured Yield Moment Measured Peak Moment Measured Peak Moment Maxi nun Applied Shear Load CH/) (P/f'Ag> C in.) Cin.-kips) Cin. - kips) AC Predited Moment (kips) CA1 hinge detail lla CON1 * CA2 aspet ratio lla CON2 ** CA3 axial load lla CON3 " *** CA4 low-yle fatigue lla lla5 joint height CA6 repeatability lla * rular ontrol oll.llrl w th the same he ght as Un ts CA1 and lla1 ** rular ontrol olu111'1 w th the same he ght as Un ts CA2 and lla2 *** rular ontrol olunn w th the same he ght as Un ts CA3 and lla3

5 2..., 1' 28"x 14"x 1 1/2" END PLATE 1 TRANSPORTATON RESEARCH RECORD Unit CA "'C L.. Cl> , Displaement (in.) 1 Unit WA2 STRONG FLOOR FGURE 4 Test setup for the 1:6-sale speimens. and stiffnesses, and hene the yield displaements, varied in olumns with different details. However, to better ompare the hinging behavior of olumns with different hinge details, parallel sets of olumns were subjeted to the same displaement history. The typial loading sequenes used for the tests was two yles at displaement dutility fators (i.e., multiple values of ily) ofµ = 1, 2, 4, 6, 8, 1 and 12 unless premature failure of the speimen aused a halt to the testing. TEST RESULTS AND DSCUSSON A summary uf lhe lesl resulls fur all 1 :6-srnle lesl spet:imens is presented in Table 1. Column performane was evaluated with respet to the moment apaity and displaement dutility attained, the overall hysteresis behavior, and degradation and energy dissipation harateristis. Rather than disuss the results of eah speimen individually, results of groups of speimens are presented to failitate orrelation of the influene of various parameters with olumn performane and to obtain behavioral trends. General Behavior Hysteresis Behavior Figure 5 shows typial load-displaement hysteresis urves for 1:6-sale olumns inorporating details CA and WA and a omparable ontrol olumn (Units CA2, WA2, and CON2). These olumns were subjeted to an axial load level of.24fag. The aspet ratio for the olumns inorporating the modified details was 1.25 measured with respet to the outer olumn, whih orresponded to an aspet ratio of 3.75 for the ontrol olumn. Longitudinal and volumetri reinforing ratios in the hinges were 5.7 and 1.46 perent, respetively. The lateral loads presented in these plots are the true loads on UJ 1 a.... "'C L.. Cl> ,... UJ a.... "'C L.. Cl> t Displaement (in.) , 14 Unit CON , Displaement (in.) FGURE 5 Load-displaement hysteresis urves for Units CA2, W A2, and CON2. the speimens, inluding P-ll effets and seondary effets from the axial load. The hysteresis urves for all three speimens are very stable even at displaement levels of µ = 12. No evidene of any sudden drop in load-arrying apability was observed, and the plasti hinges ontinued to absorb energy throughout the tests. The theoretial ultimate lateral load, alulated from AC methods and using measured material strengths with a material redution fator of one, is also shown in eah of these figures. Figure 5 indiates that substantial enhanement of the measured flexural strength above the AC-predited values was obtained for these olumns. This strength enhanement is aused by inreases in onrete strength and dutility

6 MLean et al. 21 beause of the onfinement provided by the spiral, and by the inreased strength of the steel in the strain-hardening region. Results from the 1:6-sale tests indiated average enhanement values of 1.17, 1.35, and 1.52 for ontrol olumns, olumns with detail WA, and olumns with detail CA, respetively. The greater strength enhanement in the olumns with the modified hinge details is aused by the additional onfinement provided by the outer olumn surrounding the hinge detail. Figure 5 also indiates that hysteresis urves for the olumn inorporating detail CA are somewhat narrower than those for the olumn with detail WA and for the ontrol olumn. This result indiates a redued energy dissipation apaity in the hinge with the CA detail. Shear Degradation Figure 6 shows the plots of the shear strength envelope urves for Units CAl, WAl, and CONl. The shear strength envelope urve is obtained by plotting the maximum shear fore attained at eah peak displaement level with respet to that displaement. The olumns with the moment-reduing details exhibited less strength degradation than did the ontrol olumn, possibly beause of the additional onfinement provided around the hinge region by the outer olumn. Figure 6 also indiates that the olumn with detail CA exhibited the greatest stiffness and that the ontrol olumn exhibited the least stiffness. Two reasons an be ited for the differene in stiffnesses observed in these speimens. First, the elasti stiffness of the ontrol olumn is less than that of the outer olumns with the moment-reduing details. Seond, the moment-reduing details produe pinhing of the rebars rossing the olumn-tofoundation onnetion, thereby induing larger strain values in the rebars of the moment-reduing details. Strain profiles of the longitudinal bars measured at the base of these olumns are shown in Figure 7, illustrating this pinhing effet. The largest strains were measured in the olumn with detail CA; the strains in the olumn with detail WA and in the ontrol olumn were onsiderably lower. Examining the distributions in the figure of the strains over the vertial height of the olumns also indiates that the plasti hinging ation in the olumn with detail CA was largely onentrated at the throat region of the hinge. n ontrast, the plasti ation i " CD 9 E 7 " u 5 E 3 LL u - 1 "' -3 Ci E4 1.6E4 2.E4 2.4E4 Longitudinal Strain at µ=6 (µe) FGURE 7 Longitudinal bar strain profiles. was distributed over a greater vertial length of the hinge region in the olumn with the WA detail and in the ontrol olumn. Energy Dissipation The energy dissipated by a olumn during a partiular load yle is represented by the area enlosed by the loaddisplaement hysteresis urve. The energy dissipated by a perfetly elastoplasti system during a omplete displaement yle, as shown in Figure 8, is the area of the parallelogram BCDE. For a partiular displaement dutility fator, µ, the ideal plasti energy dissipated, EP, an be omputed as where VP is the maximum shear fore attained at that displaement level (7). n order to evaluate quantitatively the energy dissipation apability of the various hinge details, the measured energy dissipation was divided by the EP value of the olumn for the same displaement dutility fator. This ratio is referred to as the relative energy dissipation index. Values of the energy dissipation effetiveness of Units CA2, WA2, and CON2 are shown in Figure 9. The low values of v (1) 7. 6.,... Q_ "' 5. :.>: 1'.l :----_-_-_-_-_-_-_ CA1 1st,,: CA 1, 2nd // ', ;; -- WA1, 1st ' - WA1,2nd 1 CON1 1st CON1:2nd yle Displaement Dutility Fator, µ FGURE 6 Shear strength envelope urves for Units CA, WA, and CON. Ep "4 ( µ.-1) V b.y FGURE 8 Atual and idealized perfetly elastoplasti hysteresis urves.

7 22 TRANSPORTATON RESEARCH RECORD w.4... w - a - O.JO 2 J B Displaement Dutility Fator, µ FGURE 9 Relative energy dissipation index urves for Units CA2, WA2, and CON2. EEP atµ = 2 and 4 for the ontrol olumn, Unit CON2, are aused by the inexatness in defining the atual yield displaement in the different olumns, with the result that response of the ontrol olumn is still largely elasti at these displaement levels. The figure indiates that the ontrol olumn exhibited the greatest energy dissipation effetiveness and olumn with detail CA exhibited the least effetiveness. The redued effetiveness in the olumn with moment-reduing details may be aused by the onfining of the plasti ation at the base of the olumn, partiularly with the CA detail. Effet of Various Parameters on Column Performane Apet Rutia n order to evaluate the effets of olumn aspet ratio on the behavior of the hinge details, 1 :6-sale test results for modified rnlumns with asptl'.l ialios of 2.5 (Units WAl and CAl) and 1.25 (Units WA2 and CA2) and omparable ontrol olumns with aspet ratios of 7.5 (Unit CONl) and 3.75 (Unit CON2) are ompared. The hysteresis urves for Units WA2, CA2, and CON2 and Units WAl, CAl, and CONl are shown in Figures 5 and 1, respetively. The hysteresis urves for the two sets of speimens are similar, indiating that flexure dominated the behavior of the olumns even with low aspet ratios. The shear strength envelope urves for the two sets of speimens are shown in Figure 11. To aount for the different lateral load levels assoiated with olumns of different heights, the shear fore, V, is plotted normalized with respet to the yield shear fore, VY. The figure indiates that greater strength degradation ourred in the olumns with the higher aspet ratio. Level of Axial Load n order to examine the effet of level of axial load on hinge performane, 1:6-sale Units WA2, CA2, and CON2 and Units WA3, CA3, and CON3 were tested with axial load levels of.24/; A g and.35/: A g, respetively. The shear strength envelope urves for these speimens are shown in Figure 12. From the figure, higher axial load resulted in greater a -2 Cl> -J..., a,,..._, Ul a.,.. :i: - a - a -2 Cl> -J..., a,,..._, : (/).J a. J,.. :i: r a _, - a -2 J..., u -.l _; a J _, i Un it WA1 Unit CON1-2 Q Displaement (in.) -2 Displaement (in.) -2 Displaement (in.) FGURE O Load-displaement hysteresis urves for Units CA, WA, and CONl. degradation in the ontrol olumns. However, axial load seemed to have little effet in the olumns with the modified hinge details, partiularly in the olumn with the CA detail. The reason that these olumns are relatively unaffeted by axial load level may be the onfining effet provided around the hinge region by the outer olumn. Horizontal Disontinuity Joint Thikness When insuffiient disontinuity joint thikness was provided in the moment-reduing hinge details, large prying fores developed from ontat of the edges of the outer olumn with the top of the footing. This prying ation resulted in greatly

8 MLean et al. 23 <!: 2 > J.OO.,.-,W,,-,A-1,-:C,-A-1,..,. CO.,-N_l_: "'"' 45:-" -o- lu-m-ns-, -,--fly--o-3" 2 8 WA2, CA2. CON2 : 22 5" olumns, <'>yo l 5" 2 6. ; 1 /_/ CON ' CA Ul 1.2,' \..CAl Displaement Dutilily Fator,µ FGURE 11 Shear strength envelope urves for Units CA, WA, and CON and Units CA2, WA2, and CON2. > "' '- > BO WA2, CA2, CON2: axial loodz:q,24f Ag WA3, CAJ, CON3: axial lood=.3 5 ( Ag ll Displaement Dutility Fator,µ, FGURE 12 Shear strength envelope urves for Units CA2, WA2, and CON2 and Units CAJ, W AJ, and CONJ. inreased strains in the longitudinal bars and larger moments than that in olumns with no prying ation. Energy dissipation effetiveness was also redued beause of the prying fores. Detail WA Vertial Joint Height The test results indiated that providing vertial disontinuity in the moment-reduing hinge detail resulted in a greater plasti hinge length and lower longitudinal bar strains when ompared to that for the hinge detail providing only horizontal disontinuity. As the length of vertial disontinuity was inreased from one to two hinge diameters, the behavior of the olumn approahed that of the unmodified ontrol olumns. Longitudinal Reinforing Ratio n order to evaluate the influene of the longitudinal reinforing ratio on hinge performane, small-sale speimens with hinge reinforing ratios of 4, 6, and 8 perent were tested. n general, the behavior of the speimens with the different longitudinal steel ontents was similar. However, less degradation and greater energy dissipation effetiveness was observed in the olumns with the larger reinforing ratios. Transverse Reinforing Ratio Small-sale speimens with spiral reinforing ratios in the hinge of,.94, and' 3.2 perent were tested. Greater degradation and less energy dissipation apability were observed in the speimens with no transverse reinforement, partiularly in the speimen inorporating both horizontal and vertial disontinuity in the hinge detail. However, no sudden failure ourred in the speimens beause of the onfinement provided around the hinge region by the outer olumn. There was little differene in behavior between the speimens with.94 and 3.2 perent transverse reinforement. Cirular Versus Square Hinge Cross Setion The performane of olumns inorporating square momentreduing hinge details with tie reinforement was ompared to olumns inorporating irular hinge details with spiral reinforement in the small-sale study. Test results indiated that olumns with square hinges experiened signifiantly more rapid strength degradation than that for olumns with irular hinges. Effets of Low-Cyle Fatigue n the small-sale study, frature of the longitudinal bars was observed in olumns inorporating detail CA when subjeted to repeated loadings at large displaement levels. This result was taken as evidene of greater distress in the longitudinal reinforement in detail CA than in detail WA. To further examine the low-yle fatigue harateristis of the momentreduing hinge details, tests were onduted on Units W A4 and CA4 in the 1 :6-sale study. Both units were yled to a displaement level of µ = 1 and then subjeted to multiple yles at this displaement level. The hysteresis urves for these speimens are shown in Figure 13. For both speimens, little degradation ourred after the ompletion of the seond yle at µ = 1. The hinges ontinued to exhibit stable plasti behavior even after being yled up to 16 times at that displaement level. APPLCATONS AND MPLEMENTATON Design Reommendations On the basis of the results of this investigation and a survey of the literature, the following preliminary reommendations are proposed for the design of moment-reduing hinge details at the onnetion between the olumn and footing. There are two appliations for the proposed hinge detail. The first appliation is to redue the moment apaity in a olumn that has been oversized for arhitetural or other reasons. For this ase, the olumn foundation onnetion is designed to arry the required fores resulting from the bridge analysis. The seond appliation is to reate as near as possible a pinned onnetion. For this ase, the reommended proedures result in a hinge onnetion with the smallest possible moment apaity that is apable of arrying the required fores. 1. From equilibrium requirements for the olumn, determine the design shear fore, V,,. For multiple olumn bents, Vu is determined on the basis of the flexural overstrength,

9 24,... Q. "' "O...J,.. Unit CA4 12 D e -2.,....J -e,...., Q. "O...J _,,. Unit WA J _,. -2 Displaement (in.) -1 Displaement (in.) FGURE 13 Load-displaement hysteresis urves for Units CA4 and WA4. MP, of the plasti hinging region at the top of the olumn. The overstrength moment is alulated as where MP = plasti moment of setion, M" = AC nominal moment (of setion), and <l>o = overstrength fator, speified by AASHTO as Determine the required hinge area on the basis of the greatest area from the following: a. Shear frition theory: Ag V)(.2<j>f/) Ag V,,/(8<!>) where A 8 = gross area of hinge setion (in 2 ); v" = design shear fore (lb); J: = onrete ompressive strength (psi); and <!> = strength redution fator, taken as.85. (2) (3) (4) TRANSPORTATON RESEARCH RECORD 1319 b. Maximum allowable diagonal shear: A = V,,![1 (f/) 112 <1>] (5) where A is the ore area of setion (A 8 over area) (in 2 ) and <Pis.85.. Axial stress limit of.7// (to ensure dutility): A 8 "" P,,1(.?<Pf') (6) where P,, is the fatored design axial load (lb) and t> is. 75 for irular, spirally reinfored setions. 3. Determine the longitudinal steel required on the basis of the greatest of the following: a. Shear frition theory: where Avr = area of reinforement required for shear fration (in. 2 ); µ = oeffiient of frition (1.); f,. = yield strength of longitudinal steel (psi); and <P =.85. b. Minimum longitudinal reinforement permitted by AASHTO: A,= O.OlAg (8) where A, is the area of longitudinal reinforement (in 2 ).. For the ase of a single olumn or an oversized olumn in whih a redued moment apaity is desired, the longitudinal steel area required on the basis of the design loads resulting from the bridge analysis. 4. Determine the plasti moment apaity of the olumn base hinge: where Mpbh = plasti moment apaity of the base hinge, M,, = AC nominal moment (of the hinge); and j> = overstrength fator ( = 1.6 to aount for the inreased moment strength enhanement of the moment-reduing hinge detail). 5. As appliable, revise the alulated shear fore and axial load (developed from framing ation) to reflet the atual moment apaity of the hinge at the base of the olumn. 6. Repeat Steps 2 through 5 until the design loads onverge within 1 perent. 7. Determine the spiral reinforing ratio required on the basis of the greater of the following: a. Confinement requirements: Ps.45 [(A.A) - 1] le' 11;. [ P,,( j>f/ A 8 ) ] (1) (7) (9)

10 MLean et al. and (11) The design for the moment-reduing hinge detail is presented in the step-by-step proedure of the reommendations outlined previously. 25 where [ P.J(<l>f;A 8 )] is greater than or equal to 1. and <l> is.75. where b. Diagonal shear requirements (V" = <J>V + <J>Vs): S = 4 A,p/(psd), A sp = ross-setional area of spiral bar, de = outside diameter of spiral, s = spaing of spiral, and <l> =.85. (12) 8. Detailing of the moment-reduing hinge: a. Provide a '12-in.-thik vertial disontinuity joint with a height equal to the hinge diameter. b. Provide a length of 1.25 times the rebar development length for the longitudinal bars above the top of the vertial disontinuity joint for anhorage into the olumn, and ensure proper anhorage into the footing.. Provide a horizontal disontinuity joint thikness of at least 2 in. (in some ases, greater thiknesses may be needed to prevent the ontat of the outer olumn edge with the footing). d. Provide lear spaing of the spiral reinforement not greater than 6 times the hinge longitudinal bar diameter and not more than 3 in. e. Provide a 1 /2-in. shear key at the olumn-foundation onnetion. 9. Base the design of the footing on the maximum axial load and atual plasti moment at the base of the olumn. Design Example This example illustrates the appliation of the proposed design reommendations. The olumn setion and design fores used in the example were obtained from the example problem presented in Appendix A of the 1983 AASHTO Guide Speifiations for Seismi Design of Highway Bridges (9). The olumn has a lear height of 22 ft and an overall diameter of 4 ft. The fatored design axial load and moment for this olumn are Pu = 1,141 kips and Mu = 3,84 kip-ft. The speified onrete ompressive strength, J', is 4, psi, and a yield strength of fv = 6, psi is speified for both the longitudinal and transverse reinforements. The slenderness ratio for the olumn seleted for the example is slightly greater than that for whih slenderness effets may be negleted, and thus slenderness effets should be onsidered. However, for simpliity, slenderness is not onsidered in this example. Using the appropriate strength redution fators and the AC olumn hart, the olumn requires 43 No. 1 bars for longitudinal reinforement. This yields a longitudinal reinforing ratio of p, =.3, whih is within the limits speified by AASHTO (9). Step 1. The olumn shear fore is obtained from the overstrength plasti moment apaity of the top of the olumn (initially, the olumn is onsidered pinned at the base): = 1.3 x 5,46/22 = 319 kips (13) where L., is the height of the olumn. The nominal moment apaity, Mn = 5,46 kip-ft, is obtained for the olumn using the AC design hart for a longitudinal reinforing ratio of 3 perent and a lear over of 2 in., with the strength redution fator taken as unity. Step 2. The required irular hinge area is determined on the basis of the following: a. Shear frition theory: A 8 V.J(.2<j>f/) = 319/(.2 x.85 x 4) = 469 in. 2 Ag Vj8<j> = 319 x 1,/(.85 x 8) = 469 in. 2 b. Maximum allowable diagonal shear: A V., [1(f')l12<J>J x 1 x (4 ) 1 n - m. (14) (15) (16) On the basis of the required ore area, a ore diameter of 28 in. is required. With a 2-in. over, the ore and gross areas required are A = 615 in. 2 and Ag = 84 in. 2, respetively.. Axial stress limit of. 7f': A 8 Pj(.7f'<l>) = 1,141/(.75 x.7 x 4) = 543 in. 2 (17) Therefore, a gross hinge area of A 8 = 84 in. 2 should be provided. Step 3. On the basis of shear frition theory, the longitudinal steel required is A,,r V,,( <j>µ.f,.) = 319/(.85 x 1 x 6) = 6.25 in. 2 (18) Beause this value is less than 1 perent of the gross hinge area, a total of 8 No. 9 bars will be used to provide a longitudinal reinforement ratio of 1 perent. Step 4. The plasti moment apaity of the base hinge is Mpbh = <l>omnbli = 1.6 X 991 = 1,586 kip-ft (19) Step 5. Using the preliminary design of the base hinge, the olumn shear fore is revised to reflet the atual moment apaity at the base of the olumn. The revised olumn shear

11 26 fore V resulting from the plasti moments developed at both the top and bottom of the olumn is given by (1.3 x 5, x 991)/22 = 392 kips (2) where Mp,h and Mpbh are the plasti moments at the top and bottom of the olumn, respetively. Step 6. Beause of the plasti moment at the base hinge, the olumn shear fore is inreased by 23 perent. The design of the base hinge is revised by repeating Steps 2 through 5 until the shear fore onverges within 1 perent. A final base hinge with a gross diameter of 35 in. and 1 No. 9 longitudinal bars is obtained. The plasti moment apaity of the hinge, <f>omnbh is 2,35 kip-ft. Step 7. The transverse reinforement required hased on onfinement is the greater of p,.45[(a 8 /A) - l]f'lf, [.5 + l.25p)(<f>f/ag)] 962 ) 4 ( 1,141 ) =.4 5 ( x.75 x 4 x 962 and =.82 (21) p,.12 (f'lfy)[.5 + l.25p,,l(<!>f'ag)] =.12 x (4/6) (o x '.75 x 4 x 962 ) =.8 (22) 148' ld N, TENSON ' 48 HE +-- 2''- / V ' GHT OF VERTCAL 2 DSCONTNUTY JONT BASE HNGE DA 35" z" f SHEAR KEY i,..--1-#9 # 5 SPRAL ta) / 2f CC "" _l_ TOP OF FOOTNG FGURE 14 Cross-setion of the moment-reduing hinge detail for the design example. TRANSPORTATON RESEARCH RECORD 1319 The volumetri ratio required based on shear onsiderations is p, 2/fy [(V)<f>A) - 2 (f') 112 J = 2 x ( x (4,)' 12 ) 6.85 x 755 1, =.17 (23) Therefore, a transverse reinforement ratio of 1.7 perent is provided using No. 5 spiral at a pith of 2.5 in. Step 8. A ross-setional view showing the details of the moment-reduing hinge is shown in Figure 14. For the olumn seleted for the example, the plasti moment apaities of the olumn and the hinge, inluding the overstrength fators, are 7,28 and 2,35 kip-ft, respetively. Thus, a 7 perent redution of moment transferred to the foundation is obtained by inorporating the moment-reduing hinge detail when ompared to a foundation onnetion onsisting of the onstant ross setion and reinforement provided in the olumn. CONCLUSONS AND RECOMMENDATONS Conlusions On the basis of the results of this investigation, the following onlusions are made: 1. Columns with the moment-reduing hinge details of this study exhibited stable hinging behavior similar to that of a onventional olumn with the same dimensions and reinforement as that of the hinge. 2. Substantial enhanement of the measured flexural strength over that predited by urrent design approahes was observed for all olumns. Average enhanement values of 1.17, 1.35, and 1.52 were obtained for onventional olumns, olumns inorporating moment-reduing hinge details providing both horizontal and vertial disontinuity, and olumns inorporating moment-reduing hinge details providing only horizontal disontinuity, respetively. 3. Greater distress in the longitudinal bars and redued energy dissipation effetiveness was observed in the hinge detail with only horizontal disontinuity when ompared to the other hinge details of this study. 4. Flexure ontrolled the behavior of all of the olumns, inluding those with an aspet ratio of However, greater strength degradation ourred in the olumns with higher aspet ratios. 5. Higher axial load levels had only a minor effet on the performane of olumns with the moment-reduing hinge details. This lak of effet was attributed to the onfinement around the hinge provided by the outer olumn. 6. Columns tested with small horizontal disontinuity joint thiknesses experiened prying ation beause of ontat of the olumn edges with the footing. This prying ation resulted in inreases in the hinge moments and reinforement strains. 7. The onrete of the outer olumn provided signifiant lateral onfinement around the hinge region in the olumns

12 MLean et al. with the moment-reduing hinge details. However, adequate onfining reinforement was still required in order to obtain stable plasti hinging behavior and satisfatory energy dissipation in the olumn. 8. Columns with irular spirally reinfored hinge details exhibited better performane than did olumns with square hinge details with tie reinforement. Reommendations The following preliminary reommendations are made on the basis of the results of this study and a survey of the literature. 1. Both vertial and horizontal disontinuity should be provided in the moment-reduing hinge detail. The thikness of the disontinuity joint should be seleted to aommodate the antiipated rotation requirements of the olumn base. 2. The olumn and the moment-reduing hinge detail should be designed on the basis of the atual moment apaity of the hinge detail. Rational proedures based on known priniples of performane should be used in the design. 3. Cirular hinge setions should be used in the momentreduing hinge, and spiral reinforement should be provided over the full length of the hinge detail. 4. The hinge setion at the base of the olumn should be designed for a value lower than the maximum allowable axial load apaity to ensure dutility when subjeted to seismi loadings. 5. Conservative evaluations were used for several parameters not investigated in this study, inluding anhorage requirements of the reinforing bars, very high axial load levels, shear strength, and the effet of lustering the longitudinal bars. Further researh is needed to preisely define the influene of these parameters on the behavior of the momentreduing hinge details. The urrent information should also be supplemented by testing multiple olumn bents inorporating moment-reduing hinge details at the bases of the olumns. ACKNOWLEDGMENTS The researh presented in this paper was funded by the Washington State Transportation Center (TRAC). The authors aknowledge the valuable assistane of Umesh Vasishth of Exelteh Engineering. REFERENCES 1. K. Y. Lim, D.. MLean, and E. H. Henley. Moment-Reduing Hinge Details for the Bases of Bridge Columns. n Transportation Researh Reord 1275, TRB, National Researh Counil, Washington, D.C., 199, pp K. Sakai and S. A. Sheikh. What Do We Know about Confinement in Reinforement Conrete Columns? (A Critial Review of Previous Work and Code Provisions). AC Strutural Journal, Vol. 86, No. 2, Amerian Conrete nstitute, Detroit, Mih., Marh April 1989, pp AC Committee 318. Building Code Requirements for Reinfored Conrete. AC Amerian Conrete nstitute, Detroit, Mih., D.. MLean, K. Y. Lim, and E. H. Henley. Moment-Reduing Hinge Details for the Bases of Bridge Columns. Washington State Department of Transportation, Olympia, July K. Y. Lim and D.. MLean. Sale Model Studies of Moment Reduing Hinge Details in Bridge Columns. AC! Strutural Journal, Vol. 88, No. 4, Amerian Conrete nstitute, Detroit, Mih., July-Aug. 1991, pp M. J. N. Priestley and R. Park. Strength and Dutility of Bridge Substru1ures. RRU Bulletin 71. National Roads Board, Wellington, New Zealand, 1984, 12 pp. 7. B. G. Ang, M. J. N. Priestley, and T. Paulay. Seismi Shear Strength of Cirular Bridge Piers. Researh Report Department of Civil Engineering, University of Canterbury, New Zealand, M. J. N. Priestley and R. Park. Strength and Dutility of Conrete Bridge Columns Under Seismi Loading. AC Strutural Journal, Vol. 84, No. 1, Amerian Conrete nstitute, Detroit, Mih., Jan. Feb. 1987, pp Guide Speifiations for Seismi Design of Highway Bridges. AASHTO, Washington, D.C., Publiation of this paper sponsored by Commiltee on General Strutures. 27