A Refined Model for Base-Isolated Bridges with Bi-Linear Hysteretic Bearings

Abstract

Recognizing that the inelastic displacement spectra with constant ductility ratios of an earthquake ground motion can be approximated by the equivalent elastic displacement spectra with the consideration of appropriate effective period shifts (or effective stiffness) and equivalent damping ratios, a refined equivalent linear model for the seismic analysis of base-isolated bridges with bi-linear hysteretic bearings is established in this paper using a system identification method. The parameters necessary to completely define a bi-linear hysteresis loop such as the elastic stiffness, yielding force, strain harden ratio and ductility ratio are all considered in the modeling. Eighteen California, one Washington and one Japan earthquake ground motions are used for the identification. The equivalent linear model established through the identification process is characterized as a modification of the current equivalent linear model provided by the AASHTO isolation guide specifications. Numerical comparisons indicate that the proposed model in general can predict accurately compared against the inelastic solutions and, therefore, the proposed model can be applied to the practical analysis of base-isolated bridges.