| During the past 30 years, from 1975 to the present, dramatic changes have occurred in American Association of State Highway and Transportation Officials (AASHTO) standard specifications for highway bridges. Before 1971, AASHTO specifications for the seismic design of bridges were based in part on the lateral force requirements for buildings developed by the Structural Engineers Association of California (SEAOC). As a result of the 1971 San Fernando, California earthquake, the California Department of Transportation (CALTRANS) issued in 1973 new seismic design criteria for bridges. This criteria formed the basis of the 1975 to 1992 AASHTO "Interim Specifications for Highway Bridges", which provided the first analyses based on real earthquake modeling. In 1981, the Applied Technology Council (ATC) issued its ATC-6 "Seismic Design Guidelines for Highway Bridges", funded by the Federal Highway Administration. This formed the basis for AASHTO's Fifteenth (1992), Sixteenth (1996) and Seventeenth (2002) Editions of the revised seismic design criteria. Unfortunately, there are many pre-1992 bridges designed per the old AASHTO standard specifications, which had no provision for seismic design. This dissertation compares the earthquake design loadings used for earthquake design criteria for highway bridges, which were assigned by AASHTO standards before 1975, between 1975 and 1992, and after 1992. For bridges built by pre-1975 or 1975--1992 standards, the weakest points of the superstructure and substructure are determined and a method is devised for the design of the retrofit of these bridges.; The earthquake design criteria and related equations included in the AASHTO Division IA standard specifications were investigated and recommendations have been made regarding how this information can be used more effectively in the retrofit of highway bridges built before 1992. |
