The Seismic Design of Bridges — Geotechnical and Foundation Design Issues

Current seismic design guidelines for bridges, as reflected by the recently published NCHRP12-49 Recommended LRFD Guidelines, are placing increasing emphasis on earthquake induced displacements as a measure of seismic demand and performance. Such an approach places increasing importance on improving our understanding of earthquake time histories and nonlinear deformation and ductility behavior of bridge components. Whereas structural engineers have made considerable advances in developing and applying displacement based design concepts over the past 10 years, geotechnical engineers have only recently been addressing issues related to displacement based foundation design, including nonlinear deformation and capacity modeling under earthquake loading. Foundation design issues associated with nonlinear deformation and capacity modeling of piled foundations, shallow footings, and abutments, are reviewed in the context, of displacement based design. As competent foundation soils such as clays and dense sands are ductile and not susceptible to strength degradation, mobilization of foundation capacity leads to performance criteria more related to the effect of ductility demands on the bridge structure and to earthquake induced foundation settlement. Examples associated with earthquake time history loading on piled and shallow footing foundations are used to illustrate nonlinear modeling approaches and the difficulties in making reliable settlement or permanent deformation estimates. The case of displacement demands on pile foundations arising from liquefaction induced lateral spreads, is also discussed, and illustrated with design examples.