A simple procedure for evaluating seismic isolation in bridges

This paper presents a simple method for evaluating the performance of seismic isolation for most bridge structures. Through application of the basic theory of seismic isolation and the graphic representation of the modified acceleration and displacement spectra of the AASHTO Guide Specification for Seismic Isolation Design (1999), the user can determine with reasonable accuracy the force and displacement levels achievable with any isolation system. The method employed can be used to independently verify the force and displacement levels reported by isolation bearing suppliers. Detailed analyses will generally result in overall performance similar to that obtained using the method presented here. The method presented is not intended as a replacement for detailed analysis, and the analysis requirements of the AASHTO Guide Specifications for Seismic Isolation Design (1999 and 2000) must be satisfied. This paper does not describe methods for configuring isolation bearing units. This task should be done by one or more isolation bearing suppliers contacted during the design phase of a project. Such an approach will better insure that the isolation system performance and bearing geometry shown on the plans can be achieved. In addition, the isolation bearing suppliers will insure that the most current and economical bearing designs are used.