Three-dimensional nonlinear seismic performance evaluation of

Engineering Structures 30 (2008) 1869–1878www.elsevier.com/locate/engstruct

Three-dimensional nonlinear seismic performance evaluation of retrofitmeasures for typical steel girder bridges

Jamie E. Padgetta,∗, Reginald DesRochesb,1

a Department of Civil and Environmental Engineering, Rice University, 6100 Main Street, MS-318, Houston, TX 77005, USAb School of Civil and Environmental Engineering, Georgia Institute of Technology, 790 Atlantic Dr., Atlanta, GA 30332-0355, USA

Received 3 August 2006; received in revised form 11 December 2007; accepted 12 December 2007Available online 22 January 2008

Abstract

Steel girder bridges are common bridges in North America which have exhibited considerable vulnerabilities in past earthquake events. Thispaper conducts nonlinear time history analysis using detailed three-dimensional models of typical multi-span simply supported and multi-span continuous steel girder bridges to evaluate the effectiveness of various retrofit strategies. Restrainer cables, steel jackets, shear keys,and elastomeric isolation bearings are assessed for their influence on the variability and peak longitudinal and transverse response of criticalcomponents in the bridges. The results indicate that different retrofit measures may be more effective for each class of bridges. For example, therestrainer cables are effective for the multi-span simply supported (MSSS) bridge, yet not the multi-span continuous (MSC) bridge; shear keysimprove the transverse bearing response in the MSC bridge but are not effective in the MSSS bridge which exhibits less transverse vulnerability;and elastomeric bearings improve the response of the vulnerable columns in both the MSSS and MSC bridges yet lead to increased abutmentdemands in the MSC bridge. The study reveals that while a retrofit may have a positive influence on the targeted component, other criticalcomponents may be unaffected or negatively impacted. This lends support to vulnerability assessments that consider the impact of retrofit onsystem vulnerability reflecting the contribution of multiple components.c© 2007 Elsevier Ltd. All rights reserved.

Keywords: Retrofit; Steel bridges; Seismic response; Steel jackets; Restrainer cables; Isolation; Shear keys

1. Introduction

Steel bridges are often potentially vulnerable highwaybridges found across various seismic regions of North America.Tens of thousands of steel bridges exist in the current NorthAmerican bridge inventory, most of which have not beendesigned considering earthquake loading [1]. More than one-third of the highway bridges in the Central and SoutheasternUnited States (CSUS) are slab-on-steel girder bridges andwere constructed during the 1950s through the 1980s, priorto the initiation of seismic design practices in the region [2].Concrete slab-on-steel girder bridges are also the most commonsteel bridges found in Canada, which like the CSUS areoften supported on non-seismically designed concrete piers and

∗ Corresponding author. Tel.: +1 713 348 2325.E-mail addresses: [email protected] (J.E. Padgett),

[email protected] (R. DesRoches).1 Tel.: +1 404 385 0826.

0141-0296/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2007.12.011

abutments [3]. Multi-span simply supported (MSSS) and multi-span continuous (MSC) slab-on-girder bridges account for amajority of the steel bridges in North American.

While these bridge types are less common on the WestCoast, there have been examples of damage to steel bridgesin past earthquake events in this part of the US (Fig. 1).The state of California has hundreds of steel girder bridges,several of which experienced damage during the 1994Northridge earthquake [5]. Typical damage included poundingdamage, damage to bearings and fracture of anchor bolts,and damage to piers and abutments [6]. A number ofsteel bridges were damaged in the 1995 Hyogoken-Nanbu(Kobe, Japan) earthquake. These bridges have details andcharacteristics that are similar to the steel girder bridges foundin North America and particularly the Central and EasternUS [7]. Damage to non-seismically detailed reinforced concretesubstructures included flexural failure of columns due toinadequate confinement and shear failures due to inadequate

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1870 J.E. Padgett, R. DesRoches / Engineering Structures 30 (2008) 1869–1878

Fig. 1. Damage to steel girder bridges in past earthquake events: (a) pounding damage in Northridge, (b) column damage and (c) shifted superstructure in Kobe [4],and (d) bearing damage in Nisqually (courtesy of WDOT).

shear reinforcement [3]. There was a significant amount ofobserved damage to steel bearings, as well as excessive deckdisplacements as a consequence of bearing failure and theloss of connection between the superstructure and substructure.Approximately 8 steel girder bridges were damaged in the 2001Nisqually Earthquake in the state of Washington. These bridgeswere often constructed prior to 1975 and suffered bearingdamage, spalling of concrete columns, and cracking at theabutments [8].

A number of studies have analytically evaluated the seismicresponse of typical MSSS and MSC steel girder bridges inorder to better understand the seismic behavior and impact ofmodeling fidelity on the performance of these bridges [1,2,9,10]. Using a linear elastic analysis, Dicleli and Bruneau [1]found that bearing stiffness significantly affects the responseof MSSS steel girder bridges, and indicated that if poundingwere considered in the longitudinal direction, there could bea large potential for shearing of bearings and span unseating.They stated that for MSC bridges, damage to steel bearingsis probable, but may serve as an effective way of isolatingthe superstructure and preventing further column damage [9].Nielson and DesRoches [2] performed nonlinear time historyanalyses with three-dimensional models and concluded thatat a hazard level of 2% in 50 years, the MSSS steel girderbridge could be expected to have fractured or toppled steel fixedand rocker bearings, as well as excessive demands placed onthe columns leading to cracking, spalling, and potential lap-splice failure. They concluded that considerable damage to theabutments was not expected.

In a study evaluating the effect of steel bearing stiffnesson the seismic response of MSSS steel girder bridges,Ala Saadeghvaziri and Rashidi [10] concluded that even inmoderate earthquakes the bearing capacity would often beexceeded. As a result they recommended bearing replacement,increased seat lengths, or restrainer cables as potentialseismic retrofits. In general, there have been numerous studiesevaluating the efficacy of retrofits for other types of bridges,such as multi-frame concrete bridges, or the impact ofretrofit on improving a particular component response. Theseapproaches include strengthening, such as the use of steelrestrainer cables [11,12]; capacity improvement, for examplesteel jacketing of columns [13,14]; force limitation or response

modification, such as implementing an isolation strategy [15,16]; and other bridge retrofit approaches [17,18]. However,there have been relatively few studies that have explored theimpact of various retrofit measures on the seismic response ofmulti-span simply supported or continuous steel girder bridges,which have been revealed to have potentially significant seismicvulnerabilities.

Saiidi et al. [19] evaluated different restrainer designmethods for MSSS bridges using two- and five-span steel girderbridges. They concluded that while the use of restrainers tendedto decrease the critical relative displacements, the reductionwas not necessarily proportional to the number of restrainersused, and that the bearing strength has a significant impacton the response. Maleki [20] considered the effects of sideretainers (or keeper plates) on single span steel girder bridgeswith non-seismic elastomeric bearings, and concluded that thegap between the bearings and keeper should be considered inorder to avoid underestimating the forces transferred to thesubstructure. Ductile end-diaphragm systems were investigatedas retrofits which help us to provide energy dissipation and limitthe forces transferred to vulnerable substructure elements inwork by Zahrai and Bruneau [21]. They found that the retrofitsshowed promise, particularly for bridges having long spans andfew steel girders.

Even fewer studies have provided comparisons of the impactof different retrofit measures on the seismic performanceof typical steel girder bridges. One study by DesRocheset al. [22] examined the response of steel girder bridgesretrofit with elastomeric bearings, lead-rubber bearings, andrestrainer cables. Detailed two-dimensional analytical modelswere evaluated using nonlinear time history analysis to assessthe impact of the retrofits in the longitudinal direction.However, past studies have indicated that classes of steelgirder bridges may potentially be vulnerable in the transversedirection [9].

To the authors’ knowledge, comparative studies have notbeen performed using detailed three-dimensional nonlinearmodels to assess the longitudinal and transverse seismicresponse of MSSS and MSC steel girder bridges with anarray of retrofit measures—ranging from strengthening, toresponse modification, to increasing capacity. These types ofcomparative studies are essential to enhance our understanding

J.E. Padgett, R. DesRoches / Engineering Structures 30 (2008) 1869–1878 1871

of the seismic response of steel girder bridges with differentretrofit strategies, and assist in identifying the relative impactof various retrofits on the response of critical components.Such insight supports decision making regarding retrofit ofthese classes of vulnerable bridges common in North America.To address this need, the study presented herein assessesand compares the impact of four commonly used retrofitmeasures, which target previously identified vulnerabilities,on the longitudinal and transverse seismic response of MSSSand MSC steel girder bridges. The three-dimensional modelsare subjected to nonlinear time history analysis consideringthe effects of pounding and the nonlinear behavior of criticalcomponents, such as the steel bearings and columns. Thisfacilitates a comparison of the impact of steel jackets, shearkeys, elastomeric isolation bearings, and restrainer cables onthe seismic performance of each class of steel bridges.

2. Three-dimensional analytical modeling and retrofitmeasures for typical steel girder bridges

2.1. MSSS and MSC bridge characteristics and model

The multi-span simply supported and multi-span continuoussteel girder bridges examined in this study have characteristi-cally non-seismic detailing, including multi-column bents hav-ing approximately a 1% longitudinal reinforcement ratio withwidely spaced transverse ties providing limited confinement,and high-type steel fixed and expansion (rocker) bearings. Thedetails and geometry for the bridges are based on past studieswhich have examined bridge plans from over 150 bridges andpresented typical representative configurations for MSSS andMSC steel bridges found in the CSUS [23,24]. Fig. 2 shows aphoto of a typical MSC steel girder bridge in Illinois, similarto the bridges analyzed in this study. The bridges consideredare zero-skew, three-span concrete slab-on-girder bridges with15 m wide decks consisting of eight steel plate girders. Eachbridge is supported on three column bents, having 915-mm-diameter columns that are 4.6 m tall, and seat-type pile-bentabutments. The primary difference in the representative bridgesfor the MSSS and MSC bridges are the lengths and weights oftypical spans, presence and size of gaps, and bearing configu-ration. The continuous bridge has expansion bearings at eachabutment and fixed bearings between the continuous deck andpier, while the simply supported bridge has alternating fixedand expansion bearings supporting each deck. The span lengths,weights, and gaps for each bridge are listed in Table 1. Furtherdetails can be found in work by Nielson and DesRoches [2,24].

While modeling details for the as-built bridge are presentedelsewhere [2,24], the general modeling scheme for both the as-built and the retrofitted bridge developed in the finite elementanalysis package OpenSees [25] is presented in Fig. 3. Thedeck is modeled as elastic beam–column elements, and thecolumns and bent caps are modeled using fiber elements.The steel fixed and expansion bearings are modeled usingnonlinear translational springs, based on previous large-scaletesting of similar bearings [26]. The pile foundation stiffnessis considered using translational and rotational linear springs,

Fig. 2. Example multi-span continuous (MSC) slab-on-steel girder bridgefound in Illinois, with location of rocker and fixed steel bearings identified.

Table 1Properties of MSSS and MSC bridge spans

Length (m) Weight (kN/m) Gap (mm)

MSSS BridgeEnd Span 12.2 39.0 38.1Mid Span 24.4 52.0 25.4

MSC BridgeEnd Span 30.3 68.3 76.2Mid Span 30.3 68.3 N/A

and the abutments are represented in the active (tension),passive (compression), and transverse directions by nonlinearinelastic springs [27]. Pounding between decks and the deckand abutment is accounted for using a trilinear contact element.

2.2. Retrofit measures

The retrofit measures evaluated for the classes of typicalMSSS and MSC steel girder bridges include steel jacketing ofcolumns, transverse shear keys, elastomeric isolation bearings,and steel restrainer cables. Examples of these retrofits installedin steel girder bridges are shown in Fig. 4.

2.2.1. Steel jacketsSteel jacketing has been used as a retrofit measure to enhance

the flexural ductility, shear strength, or performance of lapsplices in reinforced concrete bridge columns. The steel jacketencasement provides enhanced confinement, which ultimatelyimproves the ductility capacity through increased compressivestrength and ultimate strain in the concrete [13]. For theMSSS and MSC concrete bridge columns, a grouted gap of19.1 mm is assumed, and a minimum jacket thickness of10 mm is sufficient to provide for lap-splice confinement anda significant increase in ductility capacity (ultimate curvatureductility demand of over µφ = 30). The steel jacketing iscaptured in the analytical bridge model by altering the fibermodel for the columns to reflect the enhanced compressivestrength and ultimate strain in the concrete fibers, estimatedfollowing Chai’s procedure [13]. Though it is not a desiredeffect, past testing has found that full height steel jacketsalso lead to an increase in column stiffness in the range of20%–40% [18]. The elastic modulus of the section is increasedin the analysis such that the steel jacketed column stiffness


Fig. 3. Three-dimensional nonlinear analytical model of MSSS bridge focusing on modeling of retrofitted components: elastomeric bearings, restrainer cables, steeljackets, and shear keys, as well as select as-built components.

Fig. 4. Steel girder bridges retrofitted in CSUS with (a) steel jackets, (b) elastomeric bearings, (c) shear keys, and (d) restrainer cables.

is 30% larger than the as-built concrete columns. In general,while the analytical model is slightly affected by the use of steeljacketing, the primary impact of the retrofit is to increase thecolumn ductility capacity.

2.2.2. Shear keysShear keys serve to restrain the deck motion when a bridge

is excited in the transverse direction and facilitate shear forcetransfer to the substructure. These devices are often concreteblocks provided at each bearing location, as the example inFig. 4c illustrates. The assumed shear key design for this studyfollows a shear friction approach and limits the shear forcestransferred to the substructure to half of the shear strength of theconcrete columns, thus preventing excessive column demands.Shear keys are provided at every bearing (girder) location, asillustrated in Fig. 3 for one section of the bridge, and are

provided on at each abutment and bent beam. The shear keysare represented by a coulomb friction model, as shown in Fig. 3.

2.2.3. Elastomeric isolation bearingsElastomeric bearings are a form of isolation bearings that

have been used in bridge and building construction for over35 years [28]. The general concept of isolation is to shift thenatural period of the structure out of the region of dominantearthquake energy, to increase the damping, and to limit theforces transferred from the superstructure to the substructure. Itis often adopted as a retrofit scheme because isolation systemstend to reduce the need for costly retrofit of deficient pierand foundation elements. Koh and Kelly [29] have identifiedelastomeric bearings (or laminated-rubber bearings) as thesimplest method of isolation, making them prime candidatesfor retrofit of typical MSSS and MSC steel girder bridges.


The elastomeric isolation bearings are designed in this studytargeting a fundamental period increase of 2–3 times the initialperiod for the MSSS and MSC steel bridges. The designresults in a fundamental period increase (of the longitudinallydominant mode) from 0.29 to 0.72 s for the MSSS bridge, andfrom 0.44 to 1.06 s for the MSC bridge using 76.2 mm and82.6 mm total height of elastomer, respectively. The analyticalmodel of the bearings is shown in Fig. 3, following Kelly’s [16]work which showed that elastomeric bearings can be modeledusing a bilinear element. Keeper plates are provided in thetransverse direction, which have an approximate yield force of140 kN and a gap of 12.7 mm between the keeper and bearing.

2.2.4. Restrainer cablesRestrainer cables are devices which serve to limit the deck

displacement and relative hinge openings in order to preventunseating of bridge spans, or as fail-safe mechanisms to supportbridge decks in case of unseating. They are often employed inbridges with insufficient seat widths, which has been found tobe common in many MSSS and MSC steel girder bridges [10].The restrainers are placed at the hinge locations at the deck-abutment and deck-bent cap interfaces in the MSSS bridge,and only at the deck-abutment location in the MSC bridge.The restrainer cables are designed in such a way that they cansupport half of the weight of their adjacent deck as a simplisticrestrainer design procedure, which is often employed in regionsof low-to-moderate seismicity where steel girder bridges arecommon. This results in a total cable area of 2.0 cm2 at thedeck-abutment and deck-bent cap location of the end spans and5.2 cm2 at the deck-bent cap location of the center span of theMSSS bridge. For the MSC bridge a total area of 8.5 cm2 isprovided at the end of the MSC bridge at the deck-abutmentlocation. The restrainers are modeled as bilinear componentsthat act only in tension after exhausting the initial 9.5 mm slackin the 228.6-cm-long cable, which is represented by a gap in theanalytical model. This bilinear stress–strain model for the steelrestrainer cables is shown in Fig. 3.

3. Seismic response of retrofitted MSSS & MSC steel girderbridges

The seismic response of the retrofitted multi-span simplysupported and multi-span continuous steel girder bridges isassessed through nonlinear time history analysis using a suiteof synthetic ground motions that were developed by Wenand Wu [30]. They simulated ground motions for three citiesin mid-America for use in seismic performance assessmentsof structures, such that the median of the response spectramatch the uniform hazard response spectra corresponding toa probability of exceedance of 2% and 10% in 50 years. Thesuite of 10 ground motions representative of a 2% in 50 yearevent in Memphis, TN are used to assess the seismic responseof the retrofitted MSSS and MSC bridges. Fig. 5 shows theacceleration response spectra for each ground motion in thesuite along with a plot of the mean and mean plus or minusone standard deviation response spectra.

Fig. 5. Response spectra for the 2% in 50 year Memphis ground motion suiteused in the analysis. Ground motion #5 is used to illustrate typical componentresponses.

3.1. Typical seismic responses of bridge components

In order to illustrate the impact of the retrofit measures onthe performance of various components in the bridge, typicalnonlinear responses are shown for the as-built and retrofittedMSC and MSSS steel girder bridges subjected to groundmotion #5, which has a peak ground acceleration of 0.47g. Notethat the acceleration response spectrum for this ground motionis highlighted in Fig. 5. The ground motion is applied separatelyalong the principle orthogonal axes of the bridges to facilitatea comparison of how the retrofits affect both longitudinal andtransverse motion.

3.1.1. ColumnsAs indicated in past studies, the ductility demands placed

on the columns in both the as-built MSSS and MSC steelgirder bridges is significant, and could be expected to result inconsiderable damage to the columns in the form of cracking,spalling, and lap-splice failure. Fig. 6 shows a plot of themoment-curvature response of the left column in the MSCbridge when loaded longitudinally. It is seen that throughthe use of elastomeric isolation bearings, the peak curvatureductility demands reduce from µ = 2.7 to µ = 0.6, and thatwith restrainer cables demands reduce to µ = 1.8. Hence theelastomeric bearings effectively isolate the superstructure fromthe substructure and reduce column demands, and in limitingthe deck displacements the restrainers also improve the columnresponse. Similar results are seen in the MSSS bridge, howeverthe initial demands are of the order of µ = 1.3.

3.1.2. Expansion bearingsThe expansion bearing deformations are a direct result

of the induced motion of the bridge decks, and the relativedisplacement between the bridge decks and abutments in theMSC bridge, as well as the decks and bent caps in the MSSSbridge. The longitudinal response of the steel rocker bearingsin the MSSS steel girder bridge are shown in Fig. 7a. Therestrainer cables reduce the peak deformations from 59.5 to46.8 mm for the case examined. It is noted that the elastomericisolation bearing at this location has a 36.2 mm deformation,though this is an altogether different component. The transverse


Fig. 6. Curvature ductility demands placed on the columns in the as-built MSC bridge compared to the bridge retrofit with elastomeric bearings and restrainer cables(longitudinal response and loading).

(a) MSSS longitudinal response under longitudinal loading.

(b) MSC transverse response under transverse loading.

Fig. 7. Effect of retrofit on the bearing deformations at (a) expansion bearing 1 in the longitudinal direction of the MSSS bridge, and (b) expansion bearing 2 in thetransverse direction of the MSC bridge.

deformations are relatively small (4.9 mm) and tend to beunaffected by the various retrofits, other than replacement withelastomeric bearings. In the continuous bridge, the deformationof the expansion bearings located at the abutments are in excessof 100 mm and reduced to 80 mm by using the restrainercables, though yielding of the cables occurs at this level. Inthe transverse direction, these bearings exhibit highly nonlinearbehavior, with deformations of 47.7 mm reduced to 13.5 mmthrough the use of shear keys which engaged to limit out-of-plane motion of the deck and bearings (Fig. 7b).

3.1.3. Fixed bearingsThe fixed bearings also exhibit nonlinear behavior in the

transverse direction for the MSC steel girder bridge, withdeformations of 16.6 mm. As shown in Fig. 8, the deformationsare reduced to 5.3 mm with shear keys. In the longitudinaldirection, the fixed bearings on the MSC bridge deform very

little, as the bridge deck, fixed bearings, and columns tendto respond like a single degree of freedom system. However,for the MSSS bridge, the longitudinal deformations are of theorder of 20 mm and transverse are 10 mm, yet are primarilyunaffected by the retrofits.

3.1.4. AbutmentsThe deformation of the abutment in active action (tension),

passive action (compression), and in the transverse directionare relatively low for both the as-built MSSS and the MSCsteel bridges. However, the use of elastomeric bearings in theMSC bridge increases the passive deformations from 17.4 to29.0 mm due to pounding, while the restrainers reduce them to1.0 mm. Fig. 9 illustrates that the initial transverse deformationsare 6.0 mm, yet they increase to 10.0 mm with shear keys and34.0 mm with elastomeric bearings having the keeper platedetail. This is a result of the physical mechanisms enabling


Fig. 8. Transverse deformation of fixed bearings in the MSC bridge loaded transversely reduced by shear keys.

Fig. 9. Abutment response in the transverse direction when loaded in the transverse direction, increased due to shear keys and elastomeric bearings in the MSCsteel girder bridge.

force transfer to the abutments in the transverse direction whilethey are limiting the displacement and deformation of otherbridge components.

3.2. Comparison of retrofit impact for MSSS and MSC steelgirder bridges

Composite results of the peak component responses (bear-ing deformations, abutments deformations, column ductility de-mands) using the suite of 10 ground motions provide furtherinsight on the effect of retrofit on the response statistics foreach of the bridge types. Figs. 10 and 11 provide box plots ofthe peak component responses for the MSSS and MSC bridgesin the as-built condition, and retrofit with restrainer cables,steel jackets, elastomeric bearings, and shear keys. A box plotpresents a visual summary of several key statistical quantitiesof the responses and allows for comparison of how retrofittingthe bridges affect the distribution of the peak component re-sponses [31]. The boundary of the box plots represents the 25thand 75th percentiles of the peak component responses, whilethe whiskers plotted above and below the box represent the 5thand 95th percentile with outliers indicated. A black line is plot-ted in the box at the 50th percentile, or median value, and thedashed white line indicates the mean of the peak responses forthe suite of time history analyses. Each bar in the figure repre-sents the statistical quantities for a particular bridge in its as-built or retrofitted condition.

3.2.1. Multi-span simply supported steel girder bridgeIt is observed for the MSSS steel girder bridge in Fig. 10

that the mean peak expansion bearing deformations in thelongitudinal direction are reduced by nearly 28% with the

restrainer cable retrofit, yet the other retrofits tend to have littleeffect (with the exception of the elastomeric bearings whichactually replace the vulnerable steel bearings). The relativelysmall transverse expansion bearing deformations (4.1 mm),longitudinal (22.4 mm) and transverse (10.8 mm) fixed bearingdeformations are not impacted by retrofit. However, thesesmall levels of deformations may still be sufficient to producemoderate damage. For example, past tests of similar bearingsrevealed that longitudinal deformations of over 20 mm inthe fixed bearings may be associated with cracking in theconcrete pier, prying of the bearings, and severe deformationsin the anchor bolts [26]. Replacement of these bearings withelastomeric bearings may be a viable retrofit option, wheredamage to the vulnerable steel bearings is altogether avoided.While this option leads to larger variability in the bearingresponse, as illustrated in Fig. 10, the level of deformation inthe elastomeric isolation bearings is well under strain levelsof 100% and within acceptable deformation ranges for theseflexible bearings [32].

The demands placed on the columns under transverseloading are not anticipated to compromise the columns, but maylead to potential damage in the longitudinal direction. Althoughthe restrainer cables reduce the mean peak demands by roughly17% to 1.0, the upper quartile have exceeded the yield andnear the level of expected cracking. These levels of damagemay be considered acceptable, however, and the restrainers arefound to be effective in both reducing the expansion bearingdeformations and column demands for the MSSS steel bridge.The elastomeric bearings significantly reduce the columndemands when loaded in each of the orthogonal directions,and effectively isolate the superstructure. Finally, although the


Fig. 10. Box plot of component responses for retrofitted MSSS steel girder bridge. (AB = as-built; RC = restrainer cables; SJ = steel jackets; EB =

elastomeric isolation bearings; SK = shear keys).

response of the bridge is not significantly affected by the steeljackets, the observed demands placed on the columns evenat the 95th percentile are well below the enhanced ductilitycapacity of the columns.

As found in past studies, the level of abutment deformationin the longitudinal and transverse directions of the as-builtMSSS bridge are not likely to result in abutment damage [2].While most of the retrofits do not impact the abutmentdeformations, Fig. 10 illustrates that the elastomeric bearingsincrease the passive and transverse action of the abutment dueto pounding, as well as increase the variability in response.

3.2.2. Multi-span continuous steel girder bridgeThe mean peak transverse deformations of the steel fixed and

expansion bearings in the MSC bridge (Fig. 11) reach levelsof potential damage, at 13.1 mm and 41.7 mm respectively.At these levels, the shear keys become effective in restrictingtransverse motion and reducing the bearing deformations.Toppling of the rocker bearings in the longitudinal direction isa particular concern for this bridge type, because past studieshave indicated potential instability at a level of 94 mm, whichis exceeded on average [2,10]. While the restrainers tend toreduce the deformation and variability in response, the meanpeak deformations are only decreased to 82.8 mm because therestrainers often yield at such levels. This indicates that seatextenders may be a viable option to provide additional support

length at the abutments in the case that the deck falls off ofthe supporting bearings. Alternatively, the use of elastomericbearings is a feasible approach, since there is less potential forbearing damage.

As seen in Fig. 11, the demands placed on the columnsof the MSC steel girder bridge are excessive, particularly inthe longitudinal direction. The elastomeric bearings effectivelydecrease the ductility demands to well below µ = 0.8,even at the 95th percentile, and reduce the variability incolumn demands. Additionally, the peak demands placed onthe columns retrofit with steel jackets are within an acceptablelevel, ranging from roughly µ = 1.6 to µ = 2.6 at the5th and 95th percentiles, which are well below the ultimatecapacity estimated at over µ = 30. In passive action, theuse of elastomeric bearings increases the mean peak abutmentdeformations by nearly 64% from 16.0 to 26.3 mm—againa result of pounding between the continuous deck and theabutments. The variability in response is also increased, withsome responses exceeding 50 mm. The passive deformationsare actually decreased through the use of the restrainer cableretrofit. The elastomeric bearings also increase the transversedeformations significantly to a mean of 20.4 mm, and the shearkeys nearly double the deformations to 9.0 mm. Both increasescan be attributed to the load transfer from the deck to theabutment through contact with either the shear key or bearingkeeper plate.


Fig. 11. Box plot of component responses for retrofitted MSC steel girder bridge. (AB = as-built; RC = restrainer cables; SJ = steel jackets; EB =

elastomeric isolation bearings; SK = shear keys) (*Note: The longitudinal Elastomeric Bearing (EB) deformations at the location of FB 1 are not shown forscaling purposes but are the same as those plotted at EX 1.).

4. Conclusions

High-fidelity three-dimensional nonlinear analytical modelsof typical multi-span simply supported and multi-spancontinuous steel girder bridges are analyzed with variousretrofit measures to provide a comparative seismic performanceevaluation. The use of restrainer cables, steel jackets,elastomeric isolation bearings, and shear keys, which targetthe identified deficiencies of these non-seismically designedbridges, are assessed for their impact on typical criticalcomponent responses. A synthetic ground motion suiterepresentative of the 2% in 50 years hazard level for Memphis,TN is used to evaluate the response under different retrofitmeasures and support the discernment of a number ofsignificant conclusions.

The analyses reveal that shear keys are more effective for theMSC steel girder bridge which exhibits bearing vulnerability inthe transverse direction. However, there is still a need to addressthe potential for bearing damage in the longitudinal direction aswell as other components in the MSC bridge. Since the typicalMSSS steel girder bridge exhibits predominant response andvulnerability in the longitudinal direction, the shear keys are oflittle impact.

While the initial column demands in the longitudinal andtransverse directions are considerably higher in the typical

MSC bridge, which responds similar to a single degree offreedom system with larger inertial loading, there is risk ofcolumn damage in both bridge types. The steel jackets provideample capacity improvement in the columns and significantlyreduce the likelihood for column damage in both the MSSS andMSC steel girder bridges, yet do not impact the response ofother vulnerable components, such as the bearings.

The elastomeric bearings are highly effective in reducingpotential component damage in both bridges, and offer a viableretrofit option. They considerably reduce the column demandsby isolating the superstructures, and diminish the likelihood forbearing damage by replacing the vulnerable steel bearings withflexible isolation bearings. It is noted, however, that their useincreases the demands placed on the abutments in the transversedirection and in passive action due to pounding, which requirescareful consideration, particularly for the MSC bridge.

A less invasive and less costly retrofit measure using steelrestrainer cables may be an acceptable alternative for the MSSSbridge, but is not as effective for the MSC bridge. In theMSSS steel bridge, the restrainer cables cut the mean peakexpansion bearing deformations below the levels of expectedbearing toppling, and reduce the column demands to levelsnearing yield or potential cracking, which may be tolerable.It is noted that the deformation demands placed on the steelfixed bearings are not improved with the restrainers because


of their magnitude relative to the restrainer slack. Althoughthe deformations are relatively small and not indicative ofunseating (10.8 mm longitudinal, 22.4 mm transverse) thereis still potential for bearing damage. The restrainer cables areless effective for the MSC bridge which has higher bearingdeformations and inertial loads which tend to yield the cables.

This study provides insight on the impact of retrofit on theseismic performance of the two classes of steel girder bridges.It is exemplified that different retrofits may be more appropriatefor either the MSSS or MSC steel girder bridge. Owing tothe nature of the system response, the demands placed on thecomponents in a typical MSC steel girder bridge in both thetransverse and longitudinal direction pose a larger threat to thepost-earthquake bridge performance, and present a challengein identifying the most effective retrofit measure. In general, itis illustrated for both bridge types that while the performanceof some components is improved through retrofit, others maybe negatively impacted or not affected. This lends supportto system-wide vulnerability assessments which consider thecontribution of multiple components when assessing retrofitimpact. The results of the analysis presented support decisionmaking for future evaluations of the seismic risk to thesebridges and identification of potential retrofit measures. It offersan enhanced understanding of the seismic response of theseclasses of retrofitted bridges and indicates key considerationsfor retrofit impact on critical components in the bridges.

Acknowledgement

This study has been supported by the EarthquakeEngineering Research Centers program of the National ScienceFoundation under Award Number EEC-9701785 (Mid-AmericaEarthquake Center).

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Three-dimensional nonlinear seismic performance evaluation of