Comparison of Bridge Earthquake Resisting System Design in a Moderate Seismic Zone

Abstract

Design of bridges for seismic effects has changed significantly since it was originally included in the AASHTO Standard Specifications for Highway Bridges (the standard specifications) as Division IA in 1991. In addition to design changes, the seismic hazard in many areas of the United States has changed significantly in the AASHTO LRFD Bridge Design Specifications (the LRFD specifications) based on a change in the return period of the design earthquake and seismological research. A new option for seismic design, the AASHTO Guide Specifications for LRFD Seismic Bridge Design (the guide specifications), has been published providing an alternative, displacement-based design procedure. This paper provides a case study of three existing bridges in Alabama designed according to the standard specifications. The seismic resisting system of the existing bridges was redesigned based on the guide specifications and the LRFD specifications. The paper explores the effect of changes in design procedures and seismic hazard on detailing of bridges in a moderate seismic zone. The two specifications are compared for the worst-case hazard in Alabama; i.e., Seismic Design Category B. The results demonstrate that using the guide specifications results in larger spacing of lateral reinforcing in the plastic hinge regions of substructure components and drilled shafts resulting in more economical designs.