Optimum Isolation Design for Highway Bridges Using Fragility Function Method

Abstract eng:
Seismic isolation can be used to improve the seismic response and reduce damages of bridges. This paper aims to use the performance-based evaluation approach to investigate the performance of highway bridges with base isolation. Fragility functions are derived for typical base-isolated highway bridges using a simplified two dimensional numerical model representing the transverse response of bridges. The nonlinear models for piers and isolation devices are incorporated and various combinations of isolation parameters, e.g. elastic stiffness, characteristic strength and post-yielding stiffness representing common types of isolation devices are evaluated. Fragility curves are derived based on nonlinear time history analyses using a suite of 250 recorded earthquake motions in California. Damage criteria for both pier and isolation device are established to relate the response quantities to damage index of the bridge. The derived fragility functions define the probability of exceeding a predefined performance state at varying levels of earthquake intensity. By evaluating the earthquake intensity required to achieve specified damage states of base-isolated bridges, the optimum parameters of isolation devices are identified as function of structural properties. The study shows that the mechanical properties of isolation devices play an important role to the bridge response. The fragility function method provides an effective way to achieve the optimum design to minimize the damaging potential of bridges while incorporating the uncertainties in ground motions and variability of structural properties.

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