Bridges must be designed so that catastrophic collapse cannot occur from seismic forces.
Damage to a structure, even to the extent that it becomes unusable, may be acceptable, but collapse is not!
The Standard Specifications for Seismic Design of Highway Bridges of the American Association of State Highway and Transportation Officials contain standards for seismic design that are comprehensive in nature and embody several concepts that are significant departures from previous design provisions. They are based on the observed performance of bridges during past earthquakes and on recent research. The specifications include an extensive commentary that documents the basis for the standards and an example illustrating their use. LRFD specifications include seismic design as part of the Extreme Event Limit State.
Although the specifications establish design seismic-force guidelines, of equal importance is the emphasis placed on proper detailing of bridge components. For instance, one of the leading causes of collapse when bridges are subjected to earthquakes is the displacement that occurs at bridge seats. If beam seats are not properly sized, the superstructure will fall off the substructure during an earthquake. Minimum support lengths to be provided at beam ends, based on seismic performance category, is a part of the specifications. Thus, to ensure earthquake-resistant structures, both displacements and loads must be taken into account in bridge design.
Retrofitting existing structures to provide earthquake resistance is also an important consideration for critical bridges. Guidance is provided in Seismic Retrofitting Guidelines for Highway Bridges, Federal Highway Administration (FHWA) Report No. RD-83/007, and Seismic Design and Retrofit Manual for Highway Bridges, FHWA Report No. IP-87-6, Federal Highway Administration, McLean, VA 22101.