Update of Bridge Design Standards in Alabama for AASHTO LRFD Seismic Design Requirements

The Alabama Department of Transportation (ALDOT) has been required to update their bridge design to the Load and Resistance Factor (LRFD) Bridge Design Specifications. This transition has resulted in changes to the seismic design standards of bridges in the state. Multiple bridges, provided by ALDOT, were re-designed so that they satisfied the requirements of the LRFD Specifications. New details were used to create standard drawings for bridges in Seismic Design Category (SDC) A and B. The superstructure-to-substructure connection was also investigated for the horizontal design forces. The original connection was recommended along with an extended longitudinal seat width. A new equation for determining the minimum seat width was recommended, and this new design philosophy was incorporated into the re-design of the bridges. Nonlinear response history analyses of bridge models resulted in the acceptance of the guide specification’s design for use in Critical and Essential bridges. A comparison of the currently designed connection and an updated design was also performed, and indicated that the updated design could improve bridge behavior. Based on direct analysis of foundations, pile foundations were found to not be adequate to resist seismic loads in soft or liquefiable soils. Drilled shaft were sufficient regardless of soil type.