The catastrophic collapse of the cypress street viaduct during the loma prieta earthquake of october 17, 1989 and the loss of spans in both the struve slough bridge and the san francisco-oakland bay bridge has emphasized, very forcibly, the importance of rigorous seismic design procedures for highway bridges. More than eight bridges were damaged in this moderate eqrthquake; some were old, some were new, some were on soft soil and some on rock. Damage patterns confirm the seismic vulnerability of older structures on poor ground, but equally disturbing is the damage sustained by some new structures designed to so-called "modern" codes. This paper first reviews common bridge failures during earthquakes. It then summarizes the present situation regarding seismic codes for bridges in the united states, andfinally it speculates on future developments. It is concluded that existing codes should be reexamined in the light of recent events and in the process, the following issues should be addressed: the design loads (acceleration coefficients, seismic hazard maps, and soil types); the response modification factors (redundancy and ductility);the seismic performance categories (importance issues); methods of analysis (curved bridges, single vs multimode methods, inelastic methods); seatwidths and design methods for reinforced concrete joints.This paper appears in transportation research record no. 1290, Third bridge engineering conference, march 10-13, 1991, denver, colorado, volume 2.
Abstract