Seismic Fragility Analysis of Highway Bridges

Abstract

Past earthquakes, such as the 1971 San Fernando earthquake, the 1994 Northridge earthquake,
\nthe 1995 Great Hanshin earthquake in Japan, and the 1999 Chi-Chi earthquake in Taiwan, have
\ndemonstrated that bridges are vulnerable to earthquakes. The seismic vulnerability of highway
\nbridges is usually expressed in the form of fragility curves, which display the conditional
\nprobability that the structural demand (structural response) caused by various levels of ground
\nshaking exceeds the structural capacity defined by a damage state. Fragility curves of structures
\ncan be generated empirically and analytically. Empirical fragility curves are usually developed
\nbased on the damage reports from past earthquakes, while analytical fragility curves are
\ndeveloped from seismic response analysis of structures and the resulting fragility curves are
\nverified with actual earthquake data, if available. Since earthquake damage data are very scarce
\nin the central and eastern United States, the analytical method is the only feasible approach to
\ndevelop fragility curves for structures in this region.
\nThis report presents an analytical method for the development of fragility curves of highway
\nbridges. In this method, uncertainties in the parameters used in modeling ground motion, site
\nconditions, and bridges are identified and quantified to establish a set of earthquake-site-bridge
\nsamples. A nonlinear time history response analysis is performed for each earthquake-sitebridge
\nsample to establish the probabilistic characteristics of structural demand as a function of a
\nground shaking parameter, for example, spectral acceleration or peak ground acceleration.
\nFurthermore, bridge damage states are defined and the probabilistic characteristics of structural
\ncapacity corresponding to each damage state are established. Then, the conditional probabilities
\nthat structural demand exceeds structural capacity are computed and the results are displayed as
\nfragility curves. The advantage of this approach is that the assessment of uncertainties in the
\nmodeling parameters can be easily verified and refined. To illustrate the proposed method, the
\nmethod is applied to a continuous concrete bridge commonly found in the highway systems
\naffected by the New Madrid seismic zone.