Influence of the Guide Specifications for LRFD Seismic Bridge Design in Alabama

摘要

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. After doing the initial calculations with Site Class D as the soil condition, it was found that the Seismic Design Catergory B was the worst case for Alabama. The AASHTO Guide Specifications for LRFD Seismic Bridge Design was used for analysis and design of the three bridges. This paper discusses the changes made to two of the bridges in order to meet the requirements. The reinforcing detailing requirements in the pier for the hinge location were increased, the pier cap and drilled shaft were designed in accordance with capacity-based design, and the connection between the pier cap and girder was re-designed.