A Simplified Method of Analysis of Isolated Bridges with Yielding Substructures

Seismic isolation is an innovative design approach to minimize the effects of earthquakes on buildings and bridges. The intent is to reduce the seismic demand on a structure rather than increase its capacity, thereby avoiding structural collapse and reducing human casualties. To assist the design of an isolated bridge, the Simplified Method of analysis is recommended in the AASHTO Guide Specifications for Seismic Isolation Design, for initial design and the optimization of various design parameters. It is a displacement-based methodology assuming a single degree of freedom model of the bridge and elastic substructures. This paper describes an extension of the methodology to ductile substructures while maintaining all the attractive features of the original method. The modified method is applied to the analysis of a seismically isolated, 3-span, curved highway bridge of total length 362.5 ft. The results are compared to a nonlinear time history solution using SAP2000 software for three levels of input motion: 0.475, 1.0 and 1.4 times the 360⁰ component of the Sylmar ground motion. Displacements and substructure shear forces were found to be within 13% for the lowest level motion improving to within 8% for the highest level motion.