highway bridge seismic design: summary of...

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Research Objectives

The FHWA-sponsored project titled Seismic Vulnerability of New High-way Construction (MCEER Project 112), which was completed in 1998,performed studies on the seismic design and vulnerability analysis ofhighway bridges, tunnels, and retaining structures. Extensive researchwas conducted to provide revisions and improvements to current de-sign and detailing approaches and national design specifications forhighway bridges. The program included both analytical and experi-mental studies, and addressed seismic hazard exposure and ground mo-tion input for the U.S. highway system; foundation design and soilbehavior; structural importance, analysis, and response; structural de-sign issues and details; and structural design criteria.

In the fall of 1992, MCEER commenced work on a comprehensive re-search program sponsored by the Federal Highway Administration to

evaluate the seismic vulnerability of new and existing highway construc-tion. One part of this two-contract program, Seismic Vulnerability of NewHighway Construction (Project 112), resulted in a series of special studiesrelated to seismic design of highway bridges, tunnels, and retaining struc-tures, in order to develop technical information upon which new seismicdesign approaches and details, and future specifications, could be based.The project was prompted in part because significant progress had beenmade over the last two decades in several key areas, including improvedknowledge of: (1) seismic hazard and risk throughout the United States,(2) geotechnical earthquake engineering, and (3) seismically resistantdesign. At the time Project 112 was initiated, however, there were stillmany gaps in basic knowledge, and some of the recently-developed in-formation and data required additional study before they could be ap-plied directly to highway engineering applications nationwide.Consequently, Project 112 resulted in a series of analytical and experi-mental studies related to the seismic analysis, design, and performanceof bridges, tunnels, and foundations.

Highway Bridge Seismic Design:Summary of FHWA/MCEER Project on SeismicVulnerability of New Highway Construction

by Ian M. Friedland

Federal HighwayAdministration

Project Management

George C. Lee, Director,Multidisciplinary Centerfor EarthquakeEngineering Research

Ian G. Buckle, Professor,Department of CivilEngineering, University ofNevada, Reno; formerlyDeputy Director,Multidisciplinary Centerfor EarthquakeEngineering Research

Ian M. Friedland, AssociateDirector for Development,Applied TechnologyCouncil; formerly AssistantDirector for Bridges andHighways, MultidisciplinaryCenter for EarthquakeEngineering Research

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The research conducted underProject 112 had a national focus andwas intended, in large part, to ad-dress differences in seismicity,bridge types, and typical designdetails between eastern or centralU.S. bridges, and those that had beenpreviously studied in California andthe western U.S. In particular, un-like the western U.S., design strate-gies used in the eastern and centralU.S. need to reflect the statisticalprobability that an earthquake sig-nificantly larger than the “design”earthquake can occur. In manycases, it was noted that Californiadesign practice required significantmodification before being imple-mented in the eastern and centralU.S. due to these differences in seis-micity and bridge construction type.

A range of special studies werecarried out that encompassed re-search on: seismic hazard; founda-tion properties, soil properties, andsoil response; and the response ofstructures and systems (see Figure1). These studies were conductedby a consortium of researchers, co-ordinated by MCEER. The consor-tium included a variety of academicinstitutions and consulting engi-neering firms, bringing togethermore than 20 earthquake and

bridge engineers and scientists.This consortium provided a balancebetween researchers and practicingprofessionals from the eastern, cen-tral, and western U.S.

It is anticipated that currentspecifications for the seismic de-sign of bridges will be revised, andthat new seismic design guidelineswill be prepared for other highwaysystem components, in part on thebasis of this work.

This paper summarizes some ofthe important results of the re-search conducted under the pro-gram. It draws heavily from areview of the program’s researchresults and expected impacts(Rojahn et al., 1999) and discussesissues raised in the report.

Seismic HazardExposure and GroundMotion Input

The research in the area of seis-mic hazard exposure focused on theevaluation of alternative approachesfor portraying and representing thenational seismic hazard exposure inthe U.S., quantifying and developingan understanding of the effects ofspatial variation of ground motion

• University at Buffalo

• Applied Technology Council

• Brigham Young University

• Dynamic IsolationSystems Inc.

• Earth Mechanics Inc.

• Geomatrix Consultants Inc.

• Imbsen & Associates Inc.

• Modjeski and MastersConsulting Engineers

• Princeton University

• Rensselaer PolytechnicInstitute

• University of Nevada, Reno

• University of SouthernCalifornia

It is anticipated that the results of this program will be consid-ered in future design specification development work. Specifi-cally, the AASHTO-sponsored National Cooperative HighwayResearch Program (NCHRP) initiated NCHRP Project 12-49, “De-velopment of Comprehensive Bridge Specifications and Commen-tary” in the fall of 1998. The objective of NCHRP Project 12-49,which is being conducted by a joint venture between MCEER andthe Applied Technology Council, is to develop new bridge seis-mic design specifications, commentary, and design examples,which can be incorporated into the AASHTO LRFD Bridge De-sign Specifications in the near future. Much of the basis for thespecification changes that will be recommended under NCHRPProject 12-49 are expected to be drawn from the results of thework conducted under this FHWA contract.

47Seismic Vulnerability of New Highway Construction

■ Figure 1. Special Studies Conducted by MCEER under Project 112

on the performance of highwaystructures, and the development ofinelastic design spectra for assessinginelastic deformation demands.

Representation of SeismicHazard Exposure

Research in the area of seismichazard exposure representationwas conducted in order to:

• explore a number of importantissues involved in national

representations of seismicground motions for design ofhighway facilities;

• recommend future directions fornational seismic ground motionrepresentation, especially for usein nationally applicable guide-lines and specifications such asthe AASHTO seismic design pro-visions for bridges; and

• identify areas where further de-velopment and/or research areneeded to define ground motion

• Seismic Vulnerability ofExisting Construction(Project 106)

• Seismic Vulnerability of theNational Highway System(TEA-21 Project)

• National CooperativeHighway Research Program(NCHRP) Project 12-49

SEISMIC VULNERABILITY OF NEW HIGHWAY CONSTRUCTIONFHWA Contract DTFH61-92-C-00112

TASK A: PROJECT ADMINISTRATION & HIGHWAY SEISMIC RESEARCH COUNCIL

TASK B TASK D1 TASK D8 TASK D6 TASK D4

TASK D2 TASK D5 TASK C TASK D3

TASK D9

TASK D7: STRUCTURAL RESPONSE

TASK E: IMPACT ASSESSMENT

ExistingDesignCriteriaReview

DuctilityRequire-ments

StructuralAnalysis

SpatialVariation

SoilsBehavior

andLiquefaction

StructureImportance

SeismicDetailing

HazardExposureReview

FoundationsandSoil-

StructureInteraction

NationalHazard

Exposure

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representation for the design ofguidelines and specifications.

The ground motion issues thathave emerged in recent years aspotentially important to the designof highway facilities and that wereconsidered in this work includedconsideration of the following:

• What should be the basis for thenational seismic hazard portrayalof highway facilities, and howshould this be implemented interms of design values?

• Can or should energy or dura-tion be used in a design proce-dure?

• How should site effects be char-acterized for design?

• Should vertical ground motionsbe specified for design?

• Should near-source ground mo-tions be specified for design?

The following summarize the keyelements of each issue and conclu-sions of the research.

Seismic Hazard PortrayalIn 1996, the U.S. Geological Sur-

vey (USGS) developed new seismicground shaking maps for the con-tiguous U.S. These maps depict con-tours of peak ground acceleration(PGA) and spectral accelerations(SA) at 0.2, 0.3, and 1.0 second (for5% critical damping) of ground mo-tions on rock for probabilities ofexceedance (PE) of 10%, 5%, and 2%in 50 years, corresponding to returnperiods of approximately 500, 1000,and 2500 years, respectively.

The research considered whetherthe new USGS maps should replaceor update the maps currently inAASHTO, which were developed bythe USGS in 1988. The key issueregarding whether the new USGSmaps should provide a basis for thenational seismic hazard portrayalfor highway facilities is the degreeto which they provide a scientifi-cally improved representation ofseismic ground motion. Based onan analysis of the process of devel-oping the maps, the inputs to themapping, and the resulting mapvalues, it was concluded that thesenew maps represent a major stepforward in the characterization ofnational seismic ground motion.The maps are in substantially bet-ter agreement with current scien-tific understanding of seismicsources and ground motion attenu-ation throughout the U.S. than thecurrent AASHTO maps. It wastherefore concluded that the newUSGS maps should provide the ba-sis for a new national seismic haz-ard portrayal for highway facilities.

The issue of an appropriate prob-ability level or return period for de-sign ground motions based on thenew USGS maps was also examined.Analyses were presented showingthe effect of probability level orreturn period on ground motionsand comparisons of ground mo-tions from the new USGS maps andthe current AASHTO maps. The re-search recommended that, for de-sign of highway facilities againstcollapse, consideration should be

Design and PerformanceCriteria• Review Existing Design

Criteria and Philosophies,C. Rojahn, R. Mayes,I. Buckle

• Impact Assessment andStrawman Guidelines forthe Seismic Design ofHighway Bridges,C. Rojahn, R. Mayes,I. Buckle

Seismic Hazardand Exposure• Compile and Evaluate Maps

and Other Representations,and Summarize AlternativeStrategies for Portraying theNational Hazard Exposureof the Highway System,M. Power

• Recommended Approachfor Portraying the NationalHazard Exposure,M. Power

Ductility Requirements• Establish Representative

Pier Types forComprehensive Study –Eastern & Western U.S.,J. Kulicki, R. Imbsen

• Physical and AnalyticalModeling to Derive OverallInelastic Response of BridgePiers, J. Mander

• Derive Inelastic DesignSpectra, R. Imbsen

Structure Importance• Evaluation of Structure

Importance, J. Kulicki

Technical Report: Seismic Hazard Exposure• Proceedings of the FHWA/MCEER Workshop on the National Representation of

Seismic Ground Motion for New and Existing Highway Facilities, edited by I.M.Friedland, M.S. Power and R.L. Mayes, NCEER-97-0010.


given to adopting probability lev-els for design ground motions thatare lower than the 10% probabilityof exceedance in 50 years that is cur-rently in AASHTO. This is consistentwith proposed revisions to the 1997NEHRP provisions for buildings, inwhich the new USGS maps for aprobability of exceedance of 2% in50 years (2,500 year return period)have been adopted as a collapse-prevention design basis. It was de-termined that, from a deterministicstandpoint, the 2%/50 year mapsprovided a good representation ofthe acceleration and force levelsthat had occurred in the 1800s inboth the New Madrid seismic zoneand in Charleston, South Carolina.

Consideration of Energy orDuration

At the present time, the energyor duration of ground motions isnot explicitly recognized in thedesign process for bridges or build-ings, yet many engineers are of the

opinion that the performance of astructure may be significantly af-fected by these parameters, in ad-dition to the response spectralcharacteristics of the ground mo-tion. As a result, it was concludedthat some measure of the energyof ground motions is important tothe response of a bridge, but, atpresent, there is no accepted designprocedure to account for energy. Itwas recommended that research inthis area should be continued to de-velop energy-based design methodsthat can supplement current elas-tic-response-spectrum-based designmethods. It was also concludedthat energy rather than duration isthe fundamental parameter affect-ing structural behavior.

Characterization of Site EffectsAt a Site Effects Workshop held

in 1992 at the University of South-ern California (USC), a revised quan-tification of site effects on responsespectra and revised definitions of

Foundations and Soil-Structure Interaction• Compile Data and Identify

Key Issues, I.P. Lam

• Abutment and Pile FootingStudies by CentrifugeTesting, R. Dobry

• Develop Analysis andDesign Procedures forAbutments, I.P. Lam,G. Martin

• Develop Analysis andDesign Procedures forRetaining Structures,R. Richards, K. Fishman

• Develop Analysis andDesign Procedures for PileFootings, I.P. Lam,G. Martin

• Develop Analysis andDesign Procedures forDrilled Shafts, I.P. Lam,G. Martin

• Develop Analysis andDesign Procedures forSpread Footings,G. Gazetas

• Performance andSensitivity Evaluation, andGuideline Development,P. Lam, R. Dobry,G. Martin

Soil Behavior andLiquefaction• Site Response Effects,

R. Dobry

• Identification ofLiquefaction Potential,T.L. Youd

• Development ofLiquefaction MitigationMethodologies, G. Martin

• Design Recommendationsfor Site Response andLiquefaction Mitigation,G. Martin

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site: NEHRP B-C boundary

U.S. Geological SurveyNational Seismic Hazard Mapping Project

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U.S. Geological Survey

■ Figure 2. Peak ground accelerations represented as a percentage of g with a 2% probability ofexceedance in 50 years

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site categories were proposed. Sub-sequently, these revised site factorsand site categories were adoptedinto the 1994 NEHRP provisionsand the 1997 Uniform BuildingCode (UBC). Since the develop-ment of these revised site factors,two significant earthquakes oc-curred (the 1994 Northridge and1995 Kobe earthquakes) whichprovided substantial additional datafor evaluating site effects on groundmotions, and research using thesedata has been conducted.

The site factors and site catego-ries in the current AASHTO specifi-cations are those that weresuperseded by the USC Workshoprecommendations in the NEHRPProvisions and the UBC. Under thisresearch, the question was whetherthe USC Workshop recommenda-tions should be utilized in charac-terizing ground motions forhighway facilities design andwhether they should be modifiedto reflect new data and new knowl-edge since the 1992 Workshop. Themost significant differences be-tween the USC Workshop recom-mendations and the previous sitefactors (those currently in AASHTO)are: (1) the revised site factors in-clude separate sets of factors for theshort-period and long-period partsof the response spectrum, whereasthe previous site factors were onlyfor the long-period part; (2) the re-vised site factors are dependent onrather than independent of inten-sity of ground shaking, reflectingsoil nonlinear response; and (3) therevised site factors are larger (i.e.,show a greater soil response ampli-fication) than the previous factorsat low levels of shaking, and are ap-propriate to the lower-seismicityregions in the U.S.

It was found that the post-Northridge and post-Kobe earth-quake research conducted to datewas supportive of the site factorsderived in the 1992 USC Workshop,although revisions to these factorsmight be considered as further re-search findings on site effects be-come available.

Vertical Ground MotionsAt present, the AASHTO specifi-

cations do not contain explicit re-quirements to design for verticalaccelerations. Ground motion datafrom many earthquakes in the past20 years have shown that, in thenear-source region, very high short-period vertical spectral accelera-tions can occur. For near-sourcemoderate-to-large magnitudeearthquakes, the rule-of-thumb ra-tio of 2/3 between vertical andhorizontal spectra is a poor descrip-tor of vertical ground motions. Atshort periods, the vertical-to-hori-zontal spectral ratios can substan-tially exceed unity, whereas at longperiods, a ratio of two-thirds may beconservative. It was demonstratedthat the profession’s current under-standing and ability to characterizenear-source vertical ground motionsis good, especially in the western U.S.where the near-source region is bet-ter defined (i.e., near mapped-activefaults). It was also demonstrated thathigh vertical accelerations as may beexperienced in the near-source re-gion can significantly impact bridgeresponse and design requirementsin some cases. On the basis of thesefindings, it was concluded that ver-tical ground motions should be con-sidered in bridge design in higherseismic zones for certain types ofbridge construction. However, spe-cific design criteria and procedures

Special SeismicDetailing• Capacity Detailing of

Columns, Walls, and Piersfor Ductility and Shear,J. Mander, R. Imbsen,J. Kulicki, M. Saiidi

• Capacity Detailing ofMembers to Ensure ElasticBehavior, J. Mander,R. Imbsen, J. Kulicki

• Detailing for StructuralMovements - Bridges,R. Imbsen, J. Kulicki

• Detailing for StructuralMovements - Tunnels,M. Power

• Structural Steel and Steel/Concrete Interface Details,J. Kulicki

Spatial Variation ofGround Motion• Spatial Variation of

Ground Motion,M. Shinozuka, G.Deodatis

Structural Response• Effects of Vertical

Acceleration on StructuralResponse, M. Button

Structural Analysis• Review Existing Analytical

Methods, and Identify andRecommend AnalyticalProcedures Appropriate forEach Structure Categoryand Hazard Exposure,I. Buckle, J. Mander


still need to be developed for cer-tain bridge types.

Near-source Ground MotionsThe characteristics and the ef-

fects of near-source ground mo-tions on bridge response wereexamined. As the distance to anearthquake source decreases, theintensity of ground motions in-creases, and this increase in groundmotion intensity is incorporated innew USGS maps. However, in addi-tion to their higher intensity, near-source ground motions havecertain unique characteristics thatare not found at greater distances.The most significant characteristicappears to be a large pulse of long-period ground motions when anearthquake rupture propagates to-ward a site. Furthermore, this pulseis larger in the direction perpen-dicular to the strike of the fault thanin the direction parallel to the strike.This characteristic of near-sourceground motions has been observedin many earthquakes, including mostrecently in the Northridge, Kobe,Turkey and Taiwan earthquakes.Preliminary analyses of bridge re-sponse indicate that near-sourceground motions may impose unusu-ally large displacement demands onbridge structures. It was thereforeconcluded that traditional groundmotion characterizations (i.e., re-sponse spectra) may not be ad-equate in describing near-sourceground motions, because thepulsive character of these motionsmay be more damaging than indi-cated by the response spectra ofthe motions. Recommendations

include the need for additional re-search to evaluate more fully theeffects of near-source ground mo-tions on bridge response and toincorporate these effects in codedesign procedures. Until adequateprocedures are developed, consid-eration should be given to evalu-ating bridge response usingsite-specific analyses with repre-sentative near-source accelerationtime histories.

Spatial Variation of GroundMotion

The objective of the research inthis area was to develop proceduresfor determining spectrum compat-ible time histories that adequatelyrepresent spatial variations inground motion including the ef-fects of different soil conditions.The procedures were then used toexamine the effects of spatial vari-ability on critical response quanti-ties for typical structures.

The methodology used a spectralrepresentation to simulate stochas-tic vector processes having compo-nents corresponding to differentlocations on the ground surface. Aniterative scheme was used to gen-erate time histories compatiblewith prescribed response spectra,coherency, and duration of motion.Analysis results for eight examplebridges were tabulated, showingthe relative ductility demand ratiofor column flexure due to seismicwave propagation spatial effects. Ingeneral, there was about a 10%maximum increase when using

Summary Report: Spatial Variation of Ground Motion• Effect of Spatial Variation of Ground Motion on Highway Structures, by M.

Shinozuka and G. Deodatis, Agency Final Report.

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linear analysis, and a 25% maximumincrease when using nonlinearanalysis for bridges up to 1000 feetin length. Results were also tabu-lated for relative opening and clos-ing at expansion joints for bridgeswith superstructure hinges. In gen-eral, the relative joint openingmovement was up to two timeswhen using either linear or nonlin-ear analysis for bridges up to 1000feet in length. However, the con-ducted analyses were too limited inscope on which to base specificguidance for a large variety ofbridge types and conditions at thistime, and it was recommended thatfurther studies are required beforecode language could be developed.

Inelastic Design Spectrum

The research in this area had theobjective of developing inelastic re-sponse spectrum which would al-low designers to assess the inelasticdeformation demands, ultimatelyleading to improved seismic perfor-mance for new bridge construction.The spectrum are being derived for

nationwide use, accom-modating different seis-mic environments andsite soil conditions. Theyare also being devel-oped for design applica-tions, by accounting forscattering and variabili-ties that exist in realearthquake ground mo-tions and for nonlinearstructural response.

Under the currentprogram, the researchhas not progressed tothe point where its re-sults are ready forimplementation. Whencomplete, it is likely tohave a major impact on

seismic design code requirementsfor bridges, as inelastic spectra willbe one of the key elements in a dis-placement-based or energy-baseddesign procedure. Future work inthis area should include proceduresfor determining inelastic spectra ata specific site. The current state ofresearch provides an approximatemethod that starts with an elasticspectrum rather than time history;as time histories for the eastern U.S.are currently lacking, this approachwill have an obvious appeal.

Foundation Designand Soil Behavior

Research tasks in this area inves-tigated and improved criteria forthe design and analysis of majorfoundation elements includingabutments, retaining walls, pile andspread footings, and drilled shafts.In addition, work was performed onsoil liquefaction and lateral spreadidentification and mitigation.

■ Figure 3. Research in spatial variation of groundmotion included case studies of the SR14/I-5interchange and other bridges damaged during theNorthridge, California earthquake. Further research isneeded to define the importance of these effects and todevelop simplified design procedures for a broadrange of bridge configurations.

Photographs courtesy of I.G. Buckle


Abutments and Retaining Walls

Research on bridge abutmentsand retaining walls focused on mod-eling alternatives, clarifying the pro-cess of design for service loadsversus seismic loading, and provid-ing simplified approaches for designthat incorporate key issues affect-ing seismic response. The researchalso attempted to provide a newprocedure for determining the seis-mic displacements of abutmentsand retaining walls founded onspread footings, which differ fromcurrent procedures by addressingmixed-mode behavior (i.e., includ-ing rotation due to bearing capacitymovement and sliding response).Both experimental and analyticalstudies were conducted; the experi-mental studies included sand-boxexperiments on a shaking table andcentrifuge models. Results of thisresearch included:

• Development of a simplifiedprocedure for estimating abut-ment stiffness. A key element ofthis approach is determining theportion of the wall that can berelied on to mobilize backfill re-sistance.

• Extending current AASHTO de-sign procedures to the more gen-eral case of translation androtation of walls and abutments.The results are presented in amanner that will allow the meth-ods to be easily introduced intoa future code revision. The newprocedures will be of greatestuse for free-standing gravitywalls and for active mode abut-ment and wall movements.

• Consideration of passive loadingconditions for walls and abut-ments. Current AASHTO provi-sions only address active loadingconditions. Since passive load-ing can result in forces that areup to 30 times those for activeconditions, there is a strong pos-sibility that many bridges willnot develop passive resistancewithout abutment damage.

Pile and Spread Footings, andPile Groups

Studies on pile footings, spreadfootings, and pile groups includedexperimental and analytical re-search which was intended to: (a)provide improved understanding of

Technical and Summary Reports: Foundation Design and Soil Behavior• Foundations and Soils - Compile Data and Identify Key Issues, by I.P. Lam,

Agency Final Report.• Centrifuge Modeling of Cyclic Lateral Response of Pile-Cap Systems and Seat-

Type Abutments in Dry Sands, by A. Gadre and R. Dobry, MCEER-98-0010.• Modeling of Bridge Abutments in Seismic Response Analysis of Highway Bridges,

I.P. Lam and M. Kapuskar, Agency Final Report.• Seismic Analysis and Design of Bridge Abutments Considering Sliding and Ro-

tation, by K.L. Fishman and R. Richards, Jr., NCEER-97-0009.• Modeling of Pile Footings and Drilled Shafts for Seismic Design, by I.P. Lam, M.

Kapuskar and D. Chaudhuri, MCEER-98-0018.• Development of Analysis and Design Procedures for Spread Footings, by G.

Gazetas, G. Milonakis and A. Nikolaou, Agency Final Report.• Synthesis Report on Foundation Stiffness and Sensitivity Evaluation on Bridge

Response, by I. P. Lam, G.R. Martin, and M. Kapuskar, Agency Final Report.

MCEER Highway Projecthttp://mceer.buffalo.edu/

research/HighwayPrj/default.html

MCEER Publicationshttp://mceer.buffalo.edu/

publications/default.asp

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the lateral response of pile-capfoundations; (b) evaluate the influ-ence of modeling parameters onestimated displacement and forcedemands; (c) recommend methodsfor characterizing the stiffness of pilefootings; (d) quantify the importanceof radiation damping and kinematicinteraction on response; and (e)evaluate conditions under whichuplift becomes significant and howbest to model uplift in a design pro-cedure. Results from these studiesincluded the following:

• For pile-cap systems, the re-search demonstrated that designprocedures should use simpleadditions for the contributionfrom the base, side and active/passive ends when estimatingthe lateral capacity of embeddedspread footings in dense sand,along with elastic solutions withan equivalent linear soil shearmodulus at shallow depths toestimate the secant stiffness ofthe footing. This effectively con-firms that existing procedurescan be used to obtain reasonableapproximations of pile-cap foun-dation response, as long as con-sideration is given to the levelsof deformation and embedmentfor the system.

• Axial and lateral loading re-sponse and stiffness character-istics are important parametersfor the design of single piles andpile groups, although such infor-mation is not currently ad-dressed in the AASHTOprovisions. Axial response oftencontrols rocking response of apile group. New proceduresand simplified stiffness chartsare provided for determiningthe lateral load-deflection char-acteristics of single piles andgroups.

• Nonlinear load-deflection analy-ses illustrate the sensitivity of re-sults to uncertainties in p-ystiffness, gapping, pile-head fixity,bending stiffness parameters, andembedment effects. The analyseshave demonstrated that load-de-formation response is more af-fected by input variations than bythe moment within the pile.

• For spread footings without up-lift, the research demonstratedthat (a) ignoring soil-structureinteraction reduces the funda-mental period of the system, re-sulting in higher accelerations;(b) increasing the effectivenessof embedment increases radia-tion damping and reduces thefundamental period of the sys-tem; and (c) neglecting radia-tion damping has only a minoreffect on the system. Uplift ofthe spread footing results in asofter mode of vibration for thesystem, with increasing funda-mental period as the amount ofuplift increases.

Drilled Shafts

Research on drilled shafts wasconducted in order to provide infor-mation on the influence of model-ing procedures on the response ofthe structure, evaluate the effects ofmodeling on estimated displace-ment and force demands on thefoundation, and to summarize meth-ods for characterizing the responseof drilled shaft foundations, includ-ing their limitations. Results of thiswork included the following:

• Foundation stiffness has beenshown as a key parameter andcontributor to the dynamic re-sponse of the structure, necessi-tating realistic estimates andappropriate integration into a

“Among themajor studiesconductedunder thisproject was areview, syn-thesis, andimprovement torecent devel-opments insimplifiedprocedures forevaluating theliquefactionresistance ofsoils, and thecompilationand evaluationof case studiesand proceduresfor groundliquefactionmitigation.”


detailed structural analysis. Theresponse of a soil-foundation sys-tem to load is nonlinear; how-ever, for practical purposes, anequivalent linear representationis normally used.

• Guidance is provided on the de-velopment of equivalent linearand nonlinear stiffness values,and the importance and sensi-tivity of foundation geometryand boundary conditions at theshaft head are identified. A keyconclusion is that realistic rep-resentation of pile-head fixitycan lead to a much more eco-nomical design.

• The p-y approach is recognizedas the most common method ofanalyzing the nonlinear re-sponse of the shaft to lateralload. Parameters that must beconsidered include the effects ofsoil property variation, degrada-tion effects, embedment, gap-ping, and scour effects.

Liquefaction Processes andLiquefaction MitigationMethodologies

A significant amount of researchwas conducted under the MCEER

Highway Project on liquefactionprocesses, screening for liquefactionpotential, and the development and/or improvement of liquefaction miti-gation methodologies. Much of thiswork was conducted under a com-panion FHWA contract on the seis-mic vulnerability of existingtransportation infrastructure (FHWAContract DTFH61-92-C-00106), butmuch of it is appropriate for bothnew design and existing structureevaluation. Among the major stud-ies conducted under this projectwas a review, synthesis, and im-provement to recent developmentsin simplified procedures for evalu-ating the liquefaction resistance ofsoils, and the compilation and evalu-ation of case studies and proce-dures for ground liquefactionmitigation. Results of this researchincluded:

• Identification of a consensussimplified procedure for evalu-ating liquefaction resistance.Minor modifications for the de-termination of the stress reduc-tion factor used in the calculationof the cyclic stress ratio wererecommended, which allow thestress reduction factor to be

■ Figure 4. Researchers reviewed three procedures for representing foundation stiffness in the modeling process: (a)coupled foundation stiffness matrix, (b) equivalent cantilever approach; and (c) uncoupled base-spring model.

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calculated to depths greater than30 meters.

• Identification of the latest pro-cedures for determining cyclicresistance ratios (CRR) usingcone penetration test (CPT) pro-cedures. One of the primaryadvantages of CPT is the consis-tency and repeatability of themethod. Plots for determiningthe liquefaction resistance di-rectly from CPT data, rather thanconverting to an equivalent stan-dard penetration test (SPT) N-value, are presented. Proceduresare also provided for correctingCPT data based on overburdenpressures, fines contents, and forthin layers.

• Plots for determining CRR fromshear wave velocity data havebeen prepared, and proceduresfor correcting shear wave veloc-ity data due to overburden stressand fines content are explicitlygiven.

• Methods which have been em-ployed successfully for liquefac-tion mitigation include deepdynamic compaction, deep vi-bratory densification, graveldrains, permeation grouting, re-placement grouting, soil mixing,and micro blasting. Parametersand limitations for each of theseapproaches are summarized,including typical treatmentdepths and applicable soil types.

• Flow charts for assessing grounddeformations for pre- and post-treatment conditions were de-veloped. These are accompaniedby recommendations for pre-ferred ground improvementsmethods based on differing siteconditions.

• Development of rapid screen-ing procedures for liquefactionsusceptibility of soils and foun-dations.

Structural Importance,Analysis and Response

Studies were conducted in orderto provide a definition of structuralimportance, which is necessary inthe development of design and per-formance criteria, and to evaluatemethods of analysis and structuralresponse. Structural importanceanalysis response studies also pro-vided a synthesis of current systemsand details commonly used to pro-vide acceptable seismic perfor-mance in various states and regions.Among the findings of these stud-ies were the following:

• Provisions employed by the Cali-fornia Department of Transpor-tation (Caltrans) were generallymore rigorous than those usedby the majority of states (whoprimarily used current AASHTOprovisions). However, adoption

Technical and Summary Reports: Soil Behavior and Liquefaction• Site Factors and Site Categories in Seismic Codes, by R. Dobry, R. Ramos and M.

Power, MCEER-99-0010.• Proceedings of the NCEER Workshop on Evaluation of Liquefaction Resistance

of Soils, edited by T.L. Youd and I.M. Idriss, NCEER-97-0022.• Development of Liquefaction Mitigation Methodologies/Ground Densification

Methods, by G. Martin, Agency Final Report.• Design Recommendations for Site Response and Liquefaction, by G. Martin,

Agency Final Report.

“Structuralimportance,analysis, andresponse studiesprovided asynthesis ofcurrent systemsand detailscommonly usedto provideacceptableseismicperformance invarious statesand regions.”


of Caltrans’ design provisionsnationwide would likely compli-cate designs and add to con-struction cost; this may beunjustified for many low-to-mod-erate seismic hazard states. Inaddition, if Caltrans’ experienceis to be adopted nationally, someadjustments are required in or-der to accommodate bridgetypes and details commonlyused elsewhere.

• Studies that were conducted onthe application of advancedmodeling methods for concretebridge components provided acomputer program which deter-mines moment-curvature andforce-deflection characteristicsfor reinforced concrete columns;excellent correlation was ob-tained between analytical andexperimental test results forthese components.

• A refined model to simulate thehysteretic behavior of confinedand unconfined concrete inboth cyclic compression andtension was developed. Themodel includes consideration ofthe nature of degradation withinpartial hysteresis looping andthe transition between openingand closing cracks.

• A study on energy and fatiguedemands on bridge columns re-sulted in design recommenda-tions for the assessment offatigue failure in reinforcingsteel, based on the results ofnonlinear dynamic analyses.

This methodology incorporatestraditional strength and ductilityconsiderations with the fatiguedemands. Based on parametricstudies, it was concluded that lowcycle fatigue demand is bothearthquake and hysteretic modeldependent.

• Based on an examination of ex-isting and proposed methods forquantifying bridge importance,a specific method was selectedand tested against a database ofbridge information commonlyavailable within the FHWA’s Na-tional Bridge Inventory. Onelimitation of the study is that itdeliberately avoided addressingpolitical and economic issuesrelated to bridge seismic designcriteria and highway networkconsiderations.

• Following the Northridge earth-quake, concerns were raised asto the role vertical accelerationsmay have played in causingdamage to one or more bridges.In a study conducted to investi-gate the effects of vertical ac-celeration on bridge response,preliminary results indicate thatvertical components of groundmotion could have a significanteffect on bridge response forstructures within 10 km of thefault, and even within 20 – 30km for certain conditions. Theresults of this study will be con-troversial when publicized; how-ever, a far too limited study wasconducted (only six example

Technical Reports: Structural Importance, Analysis and Response• Effect of Vertical Ground Motions on the Structural Response of Highway

Bridges, by M. Button, C. Cronin and R. Mayes, MCEER-99-0007.• Methodologies for Evaluating the Importance of Highway Bridges, by A. Tho-

mas, S. Eshenaur and J. Kulicki, MCEER-98-0002.

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bridges were analyzed) in orderto provide definitive guidance atthis time.

• In a study which investigated theapplicability of simplified analy-sis methods to various types andconfigurations of bridges, a num-ber of design and analysis limita-tions were identified. Parametersevaluated included curvature,span length ratio, pier height,skew and span connectivity.Based on the analyses, a definitionfor “regular” bridges and forwhich simplified methods areappropriate was developed. Ingeneral, regular bridges musthave three or fewer spans, varia-tion of mass distribution betweenadjacent spans varying by lessthan 50%, a maximum ratio be-tween adjacent pier stiffnesses inthe longitudinal and transversedirections not greater than 4.0,and a subtended angle in plan notgreater than 90°.

Structural DesignIssues and Details

A number of studies were con-ducted in order to improve designprocedures and structural detailingfor highway structures, but the fo-cus was primarily on bridges; onestudy also examined movementdetailing for tunnels. These studieslooked at issues of capacity detail-ing for ductility, elastic behavior, andmovements. Among the results ofthis research were the following:

• A design concept termed Dam-age Avoidance Design (DAD) wasdeveloped which attempts toavoid plastic hinging in columns,thereby avoiding loss of servicefor important bridges followinga major earthquake. The conceptevaluated details which providefor rocking of columns and piers,which rotate about their ends butare restrained from collapsethrough gravity and the optional

Photographs courtesy of J. Mander

■ Figure 5. Researchers developed a new design concept called Control and Repairability of Damage (CARD) for bridge column hinges.The CARD method controls damage while accommodating large earthquake-induced deformations.

Undamaged specimen (prior to testing) Damaged specimen (after testing)


use of central unbonded posttensioning in the column core.

• A second design concepttermed Control and Repairabil-ity of Damage (CARD) was alsodeveloped, which providedstructural and construction de-tails for reinforced concrete col-umns that provide replaceableor renewable sacrificial plastichinge zone components. In thisconcept, the hinge zones are de-liberately weakened and re-gions outside the hinge zones

are detailed to be stronger thanthe sacrificial (fuse) zone; the re-maining elements of the struc-ture then remain elastic duringstrong earthquakes.

• In a study on transverse reinforc-ing requirements for concretebridge columns and pier walls,it was found that the currentAASHTO requirements could belowered by up to 50% while stillachieving displacement ductili-ties of 4 to 7 for bridges in lowto moderate seismic zones. An

Technical and Summary Reports: Structural Design Issues and Details• Application of Simplified Methods of Analysis to the Seismic Design of Bridges,

by J.H. Kim, M.R. Button, J.B. Mander and I.G. Buckle, Agency Final Report.• Establish Representative Pier Types for Comprehensive Study: Eastern U.S., by

J. Kulicki and Z. Prucz, NCEER-96-0005.• Establish Representative Pier Types for Comprehensive Study: Western U.S., by

R.A. Imbsen, R.A. Schamber, and T.A. Osterkamp, NCEER-96-0006.• Seismic Resistance of Bridge Piers Based on Damage Avoidance Design, by J.B.

Mander and C.T. Cheng, NCEER-97-0014.• Seismic Design of Bridge Columns Based on Control and Repairability of Dam-

age, by C.T. Cheng and J.B. Mander, NCEER-97-0013.• Capacity Design and Fatigue Analysis of Confined Concrete Columns, by A. Dutta

and J.B. Mander, MCEER-98-0007.• Capacity Design of Bridge Piers and the Analysis of Overstrength, by J.B. Mander,

A. Dutta, and P. Goel, MCEER-98-0003.• Seismic Energy Based Fatigue Damage Analysis of Bridge Columns: Part I –

Evaluation of Seismic Capacity, by G.A. Chang and J.B. Mander, NCEER-94-0006.• Seismic Energy Based Fatigue Damage Analysis of Bridge Columns: Part II –

Evaluation of Seismic Demand, by G.A. Chang and J.B. Mander, NCEER-94-0013.• Ductility of Rectangular Reinforced Concrete Bridge Columns with Moderate Con-

finement, by N. Wehbe, M. Saiidi, D. Sanders and B. Douglas, NCEER-96-0003.• Capacity Detailing of Members to Ensure Elastic Behavior, by R.A. Imbsen, R.A.

Schamber, and M. Quest, Agency Final Report.• Capacity Detailing of Members to Ensure Elastic Behavior - Steel Pile-to-Cap

Connections, by P. Ritchie and J. M. Kulicki, Agency Final Report.• Structural Steel and Steel/Concrete Interface Details for Bridges, by P. Ritchie,

N. Kauhl and J. Kulicki, MCEER-98-0006.• Structural Details to Accommodate Seismic Movements of Highway Bridges and

Retaining Walls, R.A. Imbsen, R.A. Schamber, E. Thorkildsen, A. Kartoum, B.T.Martin, T.N. Rosser and J.M. Kulicki, NCEER-97-0007.

• Derivation of Inelastic Design Spectrum, by W. D. Liu, R. Imbsen, X. D. Chen andA. Neuenhofer, Agency Final Report.

• Summary and Evaluation of Procedures for the Seismic Design of Tunnels., by M. S.Power, D. Rosidi, J. Kaneshiro, S. D. Gilstrap, and S.-J. Chiou, Agency Final Report.

“Studies instructuraldesign issuesand detailslooked atcapacitydetailing forductility, elasticbehavior, andmovements.”

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important aspect of this workwas that the end anchorages fortransverse steel hoops must bemaintained for the reinforcing tobe effective; 90° bends on J-hookswere found to be inadequate.

• Research was conducted onmoment overstrength capacityin reinforced concrete bridgecolumns, and a simplifiedmethod for determining columnoverstrength was developed. Theupper-bound overstrength fac-tors developed in this task vali-date prescriptive overstrengthfactors recommended in ATC-32,but also indicate that some fac-tors in current Caltrans andAASHTO provisions may be toolow.

• A synthesis was conducted ondetails commonly used to ac-commodate expected move-ments on bridges and retainingwalls in the eastern and westernU.S. Based on the synthesis, de-sign and detailing recommenda-tions were made in order toprovided the basis for improvedbridge design standards. Thespecific elements considered inthis effort included restrainingdevices, sacrificial elements, pas-sive energy dissipation devices,and isolation bearings. A similareffort was conducted on move-ment criteria and detailing fortunnels.

• For steel superstructures, a num-ber of issues were considered,including ductility based on

cross-section configuration, ap-plicability of eccentrically-braced frames, details whichallow for easy repair of steel sec-tions following a moderate tolarge earthquake, anchor boltperformance under lateral upliftloads, and economical momentconnection details betweensteel superstructures and con-crete substructures.

Implementation ofResearch Results

In the case of highway bridges,seismic design provisions are con-tained in the two AASHTO bridgespecifications: LRFD Bridge DesignSpecifications (2nd Edition, 1998)and Division I-A, Seismic Design, ofthe Standard Specifications forHighway Bridges (16th Edition,1996). In 1997, AASHTO recog-nized that there had been many im-portant advances in the knowledgeof earthquake hazard, bridge seismicperformance, and design and detail-ing. As a result, AASHTO chargedthe Transportation Research Board’sAASHTO-sponsored National Coop-erative Highway Research Program(NCHRP) with conducting a projectto update and revise the bridge seis-mic design specifications containedin the LRFD Bridge Design Specifi-cations.

NCHRP Project 12-49, “Compre-hensive Specification for the SeismicDesign of Bridges,” was initiated in

Technical Reports: Implementation of Research Results• Impact Assessment of Selected MCEER Highway Project Research on the Seismic

Design of Highway Structures, by C. Rojahn, R. Mayes, D.G. Anderson, J.H. Clark,D’Appolonia Engineering, S. Gloyd and R.V. Nutt, MCEER-99-0009.

• Seismic Design Criteria for Bridges and Other Structures, by C. Rojahn, R. Mayes,D.G. Anderson, J. Clark, J.H. Hom, R.V. Nutt and M.J. O’Rourke, NCEER-97-0002.


August 1998. The objective ofNCHRP Project 12-49 is to developnew specifications for the seismicdesign of highway bridges, whichcan be incorporated into the LRFDBridge Design Specifications.These new specifications will benationally applicable with provi-sions for all seismic zones. The re-sults of research currently inprogress or recently completed,along with current demonstratedpractice, are the principal re-sources for this project, especiallywith respect to the research con-ducted under the FHWA-sponsoredMCEER Highway Project.

The design criteria being devel-oped under NCHRP Project 12-49will address the following: (1)strength-based and displacement-based design philosophies; (2)single- and dual-level performancecriteria; (3) acceleration hazardmaps and spectral ordinate maps;(4) spatial variation effects; (5) ef-fects of vertical acceleration; (6) siteamplification factors; (7) inelasticspectra and use of response modi-fication factors; (8) equivalent staticnonlinear analysis methods; (9)modeling of soil-structure interac-tion and structural discontinuitiesat expansion joints; (10) durationof the seismic event; and (11) de-sign and detailing requirements forboth steel and concrete super- andsubstructures.

A joint venture of the AppliedTechnology Council and MCEERwas selected by the NCHRP to con-duct Project 12-49. In the firstphase of the project, the basic phi-losophy for the new specificationshas been developed, along with rec-ommendations regarding represen-tation of seismic hazard for design,and minimum design and perfor-mance criteria for typical highwaybridges and bridge components.

The work conducted previously byMCEER, Caltrans, and others leaddirectly to the development of thisnew specification’s philosophicalframework.

The time schedule for NCHRPProject calls for the completion ofa first draft of the specification andcommentary by the end of 1999,and subsequent drafts and revisionscompleted during 2000. The targetdate for submission of a recom-mended specification to AASHTOby NCHRP is early 2001.

ConclusionAs a result of a research program

sponsored by the Federal HighwayAdministration, researchers work-ing for the Multidisciplinary Centerfor Earthquake Engineering Re-search have developed a number ofanalytical tools, methods of analy-sis, structural design details, andspecification recommendations ap-propriate for seismic design of high-way systems and structures. Theprimary focus of this work has beenon highway bridges, but some re-search on tunnels and retainingstructures was also performed. Theprogram has also resulted in recom-mendations regarding the represen-tation of seismic hazard in futuredesign codes, the performance andimprovement of soils under seismicshaking, and an improved under-standing of the behavior of struc-tural systems and componentsunder seismically-induced forcesand displacements. In addition, itis likely that additional recommen-dations regarding the use of a per-formance-based design philosophyand dual-level design and perfor-mance criteria will be made toAASHTO as a result of this work.

“MCEER hasdeveloped anumber ofanalyticaltools, methodsof analysis,structuraldesign details,andspecificationrecommendationsappropriate forseismic designof highwaysystems andstructures.”

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Rojahn, C., Mayes, R., Anderson, D.G., Clark, J.H., D’Appolonia Engineering, Goyd, S. andNutt, R.V., 1999, Impact Assessment of Selected MCEER Highway Project Researchon the Seismic Design of Highway Structures, MCEER-99-0009, April.

MCEER, 1999, Seismic Vulnerability of New Highway Construction, Final Report/Extended Technical Summary, Federal Highway Administration.

Friedland, I.M., Mayes, Ronald L., Yen, Phillip and O’Fallon, John, 2000, “Highway BridgeSeismic Design: How Current Research May Affect Future Design Practice,” TRB BridgeConference, Paper Number 5B0042.

References

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