Demand Fragility Surfaces for Bridges in Liquefied and Laterally Spreading Ground

摘要

This report presents demand fragility surfaces for bridges in liquefied and laterally spreading ground that represent probability P(EDP>edp(vertical bar)IM=im), where a vector of engineering demand parameters is presented and the intensity measure is taken as free-field lateral spreading ground displacement. Equivalent global nonlinear finite element analyses were used to generate the responses of bridges due to liquefaction-induced lateral spreading, while nonlinear dynamic time-history analyses were implemented to determine the response due to seismic shaking. A classification system was developed for the bridges based on the type of superstructure, how the superstructure connects to abutments and piers, and the age of the bridges. Model input parameters were represented using random variables to capture the variability in structural and geotechnical conditions at Caltrans bridge sites. Nonlinear equivalent static analyses were conducted with inputs sampled randomly using the Monte-Carlo simulation method. Covering a wide range of different structural and geotechnical configurations, demand fragility surfaces were provided, which are a significant improvement over existing relations. Previous relations were not derived systematically and are known to be overly-conservative. The study also compares the seismic vulnerability of different classes of typical Caltrans bridges subject to seismic shaking and liquefaction-induced lateral spreading. It finds that although the fragility functions subjected to either shaking or lateral spreading show significant correlation with the structural characterizations, differences emerge for ground shaking and lateral spreading conditions. The fragility surfaces presented herein are ideally suited for implementation within the Pacific Earthquake Engineering Research Center's methodology in a transportation network analysis, or as a screening tool for identifying a manageable subset of bridges for more systematic individual retrofit evaluation.