Development of Site-Specific Acceleration Response Spectra for a Liquefiable Bridge Site-Paper

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The study developed site-specific acceleration response spectra (ARS) for a proposed bridge spanning a canyon in southern California, where the side supports are founded on nonliquefiable terrace and bedrock deposits and the middle supports overlie potentially liquefiable alluvium.<br /><br />Researchers evaluated three earthquake hazard levels using seven selected ground-motion records. They compared one-dimensional equivalent-linear analyses with nonlinear effective-stress analyses in DEEPSOIL, using soil properties informed by field and laboratory data. The nonlinear analyses modeled soil strength and liquefaction explicitly. At the highest hazard level, liquefaction occurred in shallow sandy layers in all seven records, with some records also liquefying deeper layers.<br /><br />Results showed that equivalent-linear analysis generally produced higher short- and mid-period spectral accelerations, making it potentially conservative and costly for design. Nonlinear effective-stress results were generally lower at short periods, but could exceed equivalent-linear results at longer periods under lower-intensity shaking. For the strongest shaking, liquefaction usually reduced surface spectral accelerations, though long-period effects remained important for bridge design.<br /><br />Because the bridge crosses both liquefiable and nonliquefiable ground, the authors recommend a single design spectrum that envelopes the applicable Caltrans or site-specific nonliquefied-soil spectrum and the nonlinear effective-stress results. This approach accounts for short-period motions affecting supports on firm ground and potential long-period amplification associated with deep, soft deposits, while avoiding excessive amplification from equivalent-linear methods.<br /><br />The authors emphasize that one-dimensional models assume liquefiable deposits extend indefinitely, which may overstate ground-motion damping where liquefiable soils are localized. Accordingly, deposit extent should be investigated carefully, and nonlinear results should be assessed alongside equivalent-linear analyses, code requirements, and engineering judgment.

Keywords

site-specific acceleration response spectra

bridge design

liquefiable alluvium

equivalent-linear analysis

nonlinear effective-stress analysis

DEEPSOIL

liquefaction

ground-motion records

spectral accelerations

southern California