Within the past decade, seismic isolation systems have gained rapid popularity in the earthquake resistant design of bridge structures. This popularity has come in response to the inadequacy of earlier seismic design and retrofit methodologies. The mechanical shortcomings and subsequent failure mechanisms associated with the early elastic seismic design philosophy are first presented, followed by a study of the applicability of seismic isolation systems in bridge structures. This investigation includes a discussion of the susceptibility of simple span bridges to seismic failure, an overview of the behavior of base isolated structures, a detailed description of isolation system components, an explanation of the mechanics of elastometric isolation bearings, and a presentation of the linear theory of seismic isolation. The qualitative investigation of seismic isolation systems is supplemented by a quantitative nonlinear time-history analysis that illustrates the response of a simple span bridge to seismic excitation. The analysis examines the effects of the varying stiffness of a lead-rubber elastometric bearing upon structural response when subjected to the 1940 El Centro earthquake. The results confirm the expected behavior of isolated structures and emphasize the need for site-specific studies in the design of effective seismic isolation systems for bridge structures.
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