Seismic Retrofit of California's Auburn–Foresthill Bridge

The Auburn–Foresthill Bridge in California was built by the Bureau of Reclamation in 1973. The 2,428-ft-long bridge is made up of three spans—639 ft, 862 ft, 639 ft—and sits 730 ft above the north fork of the American River; it would have spanned the reservoir created by the proposed, but never constructed, Auburn Dam. The superstructure is a parabolic haunched deck truss bridge that has fracture-critical, high-strength steel (100 ksi) main members. As part of the Local Agency Seismic Retrofit Program, Placer County, California, embarked on the seismic retrofit design of this bridge. This project expanded on the concepts and design methodologies that were developed during the retrofits of the California Toll Bridge Program. Strain-based design criteria were developed for all concrete and steel members of the bridge. The project-specific design criteria may serve as the “next generation” for complex steel seismic retrofit projects. The time history model incorporated material and geometric nonlinearity components and was capable of capturing the global buckling of steel members and reporting member strains. The gusset plates were evaluated to ensure that any ductile behavior was limited to the members themselves and not the connections. The project incorporated the use of large buckling restrained braces (BRBs) to improve the performance of the bridge during large seismic events. BRBs are simple, low-maintenance devices that dissipate energy and improve the seismic behavior of steel structures. This paper summarizes the innovative design concepts and their application throughout the project.