Seismic Design and Retrofit of Bridges Using Load and Resistance Factor Design: Past, Present, and Future

For almost 70 years, bridges in the United States have been designed using a working stress design specification, updated at regular intervals to reflect recent research results and field experience. Nevertheless, due to growing concern with inconsistencies in the specifications, and the fact that variability in the loads and materials were not explicitly addressed, an entirely new specification was developed in the early 1990s based on load and resistance factor design (LRFD). This new document is the Bridge Design Specifications and is the subject of this paper. In particular, the seismic design provisions are presented and the selection of load and resistance factors is discussed. Bridge performance in recent earthquakes is then reviewed and it is concluded that refinements are necessary to the LRFD methodology in order to explicitly consider response for multilevel earthquakes. Performance-based design (PBD) is introduced, and recent efforts to implement PBD for new and existing bridges are discussed. Specifically, efforts to introduce dual-level PBD for the design and retrofit of bridges, by the American Association of State Highway and Transportation Officials and the Federal Highway Administration, respectively, are summarized.