This paper summarizes the authors' experience with the seismic evaluations of three major highway bridges located in the Mississippi River flood plain in the Central United States. Two of the three projects involved the seismic retrofit of existing bridges, and the third involved the geotechnical seismic design for a new bridge. Site-specific ground surface response spectra were computed based on shear wave velocity measurements and one dimensional (SHAKE) analysis at all sites. The computed response spectra were compared with current and proposed AASHTO recommended spectra. The subsurface conditions at the three sites consisted of loose to medium dense recent alluvium underlain by dense alluvium. The loose recent alluvium was potentially liquefiable for the design levels of shaking. Liquefaction potential controlled foundation design for certain segments of each structure. The effects of liquefaction, liquefaction mitigation options, and site-specific spectra are discussed.

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