Influence of the LRFD Bridge Design Specifications on Seismic Design in Alabama

Abstract

The Alabama Department of Transportation (ALDOT) is in the process of transitioning from the AASHTO Standard Specification for Highway Bridges to the LRFD Bridge Design Specifications. One significant difference between the two specifications is the seismic design provisions. From a practical point of view, the desire is that typical details can be developed for the worst case scenarios which can be implemented for bridges throughout the state without a significant cost premium. To determine the effects of the updated seismic provisions on current practice, an initial study of existing bridges was completed. Three typical, multi-span, prestressed concrete I-girder bridges were selected for the study. In order to bracket the demands for typical bridges, the primary bridge geometry variables were span length, pier height and pier configuration. The bridges’ Earthquake Resisting Systems were re-designed for the worst conditions for the state of Alabama. This paper discusses the changes made to the three bridges in order to meet the requirements. After the re-design of the three bridges, a few conclusions were drawn. There was an increase in the connection between the substructure and substructure. Also, the amount of hoop reinforcing in the columns, drilled shafts and struts was increased. It was concluded that typical details could not be created for the worst seismic scenario in Alabama.