Response Modification Factors for Seismically Isolated Bridges
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The 1997 AASHTO "Guide Specifications for Seismic Isolation Design" specify response modification factors for the substructures of isolated bridges that are lower than those specified for the substructures of non-isolated bridges. This report presents the rationale behind these specifications and presents research results that lead to the establishment of appropriate response modification factors for isolated bridges. The research concentrated on the dynamic analysis of simple models of seismic-isolated and non-isolated bridges for a range of isolated system and substructure behaviors, and for seismic excitation characterized by AASHTO ground motion spectra for a range of soil conditions and acceleration coefficients. The study investigated the displacement ductility demands in the substructure of these bridges and established the appropriate value of the ductility-based portion of the response modification factors. This was achieved by comparing the displacement ductility ratio for the substructures of isolated and non-isolated bridges. The study concludes that response modification factors should be lower in the substructures of isolated bridges than in the substructures of non-isolated bridges because: (a) elastic or nearly elastic substructure behavior is required for proper behavior of the isolation system, and (b) isolated bridges exhibit more sensitivity in the substructure inelastic response due to variability in the seismic input.