Task 45, FY 1991, "Revisions to the AASHTO Guide Specification for Seismic Design of Highway Bridges" (Dr. Ian G. Buckle). This task was requested by the AASHTO Highway Subcommittee on Bridges and Structures. The objective of the study was to provide recommended revisions to the seismic Guide Specification. The 1983 AASHTO Guide Specification for Seismic Design was based on work completed in 1981 by the Applied Technology Council under contract to the Federal Highway Administration. Since it was issued, relatively few revisions have been made to the Guide Specification. However, a substantial amount of new knowledge has been gained concerning the seismicity of the Eastern and Central United States since 1981. In addition, a significant amount of damage was sustained by bridges in California during the 1989 Loma Prieta earthquake. Finally, NCHRP Project 12-33, Development of a Comprehensive Bridge Specification and Commentary, which developed a new probability-based bridge design specification for consideration by AASHTO, requires state-of-the-art seismic design information. Therefore, this task was initiated to provide the basis for reviewing and revising the provisions in the Guide Specification as necessary. The contractor had several updates of a comprehensive list of changes that were made to the specification. However, some of these changes were considered dependent on research performed for Caltrans by the Applied Technology Council (ATC), and the project suffered some delays because of delays in the ATC work. A decision was made to bring this project to conclusion without waiting for additional ATC progress. The final recommended revisions from the project were adopted by AASHTO in 1994. The project is complete; the recommended specifications have been distributed to the program sponsors.