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Another loading case that needs to be considered is down-drag on the piles due to
earthquake-induced settlements. Rollins and Strand (2006) presented measurements of
down-drag forces on a pile during reconsolidation of surrounding soils that had been
liquefied by in situ blasting. Down-drag in liquefied soils can be analyzed using neutral-
plane methods with appropriate modifications for the liquefaction case (Boulanger and
Brandenberg, 2004).
Performance-based design or evaluation of bridges for liquefaction effects requires fur-
therdiscussionofperformanceobjectivesandrelatedconsiderations.Proceduresforcon-
ducting performance-based design analyses with a formal accounting of uncertainties
havebeendevelopedforpinninganalysesatbridgeabutments(LedezmaandBray,2006)
and for global bridge systems using nonlinear dynamic finite element analyses (Kramer
et al., 2006, unpublished PEER report).
5. Summary remarks
Recommendations were presented herein for the seismic design of pile foundations for
liquefaction effects using equivalent static BNWF analysis methods. Discussions were
focusedontwolocalsubsystemsforbridges:(1)pilegroupsinlaterallyspreadingground
away from any approach embankments and (2) pile groups for approach embankment
abutmentswheretherestrainingor“pinning”effectsofthepilesandbridgesuperstructure
can be advantageous.
Forpilegroupsembeddedinlargelateralspreads,equivalentstaticBNWFanalyseswith
imposed free-field soil displacements and structural inertia forces reasonably matched
results from centrifuge model tests and results from a suite of dynamic FE analyses.
Guidelines for estimating and combining lateral spreading demands and inertia loads
were derived fromthesuiteof FEanalyses.
For pile groups in approach embankment abutments, pinning forces from the piles and
bridge superstructure can restrain the embankment against lateral spreading in the longi-
tudinaldirection.CouplingNewmarkslidingblockanalyseswithequivalentstaticBNWF
pushover analyses provided reasonable estimates of the reductions in embankment dis-
placements that were observed in centrifuge model tests of embankments with and with-
out piles. Guidelines for performing thesetypes of pinning analyses were presented.
Acknowledgments
FundingwasprovidedbyCaltransunderContractsNo.59A0162and59A0392andbythe
Pacific Earthquake Engineering Research (PEER) Center, through the Lifelines Program
andtheEarthquakeEngineeringResearchCentersProgramoftheNationalScienceFoun-
dation, under Contract No. 2312001. Recent upgrades to the centrifuge were funded by
NSFAwardNo.CMS-0086588throughtheGeorgeE.Brown,Jr.NetworkforEarthquake
Engineering Simulation (NEES). The contents of this paper do not necessarily represent
apolicyofeitheragencyorendorsementbythestateorfederalgovernment.Theauthors
