Current seismic design practice often relies on the use of the uniform hazard response spectrum (UHRS), which implicitly includes motions from multiple earthquake sources and envelops possible spectra, yet does not represent a single event. Seismic hazard analyses at the site of a major Mississippi River crossing near St. Louis, Missouri, showed bimodal seismic hazard dominated by small, nearby earthquakes at short periods and large, distant earthquakes in the New Madrid seismic zone at long periods. UHRS motions resulted in large seismic demands and predictions of pervasive liquefaction that were inconsistent with historical and geologic records. UHRS-compatible conditional mean spectra (CMS) were developed to bridge deterministic and probabilistic seismic hazard evaluations, and used to evaluate liquefaction, lateral spreading, and settlement potential. The computed response was consistent with the historical and geologic record. CMSs offer hazard-compatible alternatives to the UHRS and result in seismic demand consistent with historical and geologic evidence.

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