As part of an ongoing seismic retrofitting program undertaken by the Illinois Department of Transportation (IDOT), PB Booker Associates, Inc. was selected to perform the final seismic retrofit design of dual structures on Interstate 57 over Illinois Route 3. Located in southern Illinois near the New Madrid seismic zone, these bridges each consisted of thirty spans of steel welded plate girders. The evaluation procedure, in general, was based on the May 1995 Federal Highway Administration publication entitled "Seismic Retrofitting Manual for Highway Bridges". According to this manual, bridge elements are evaluated quantitatively to determine their ability to resist the seismic forces generated by a specific seismic event. This procedure provided for the identification of deficient bridge elements and the development of an overall retrofitting strategy for the structures. These retrofitting concepts were incorporated into revised global and local models and analyzed to determine their impact on the overall behavior of the structures. This paper summarizes the development of the selected retrofitting concepts and discusses the structural enhancements incorporated into the existing bridges. The most effective retrofitting strategy for these bridges, based on the analysis, was one that combined a seismic protection system, incorporating elements of isolation and damping, along with measures for increasing the ductility and strength of critical substructure elements. Overall, the seismic performance of the subject bridges was greatly enhanced with the implementation of state-of-the-art technology.

¹PB Booker Associates, Inc., 1831 Chestnut Street, St. Louis, Missouri 63103-2225, Telephone: (314) 421-1476, Fax: (314) 421-1741, E-Mail: Grundler@pbworld.com

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