Seismic Retrofit of the North Approach Viaduct of the Golden Gate Bridge

The north viaduct seismic retrofit construction project is the first of several projects to upgrade the Golden Gate Bridge crossing. The upgrade design strategy for this portion of the crossing includes changing the five-span simply-supported truss bridge to a partially continuous bridge, replacing the substructure towers and foundations, constructing new expansion joints at the ends of the bridge, and installing new isolation bearings. Construction is being carried out under traffic and will include erecting temporary supports during the building of the new substructure. The construction techniques anticipated for use in transforming a bridge designed with yesterday’s engineering to one that reflects today’s technology are described. The north approach viaduct of the Golden Gate Bridge extends from the north end of the north anchorage housing to the north abutment. In 1982, the viaduct was retrofitted to the California Department of Transportation’s seismic retrofit guidelines. In 1996, a subsequent retrofit design was completed to upgrade the viaduct so it could remain open after a major earthquake. Using the results of a nonlinear earthquake analysis of the structure, seismic deficiencies were identified in the superstructure, bearings, towers, foundations, and abutments. The overall strategy used on the north viaduct includes changing the simple spans of the steel truss superstructure to a system that is continuous in the lateral direction by making the bottom chord continuous. This strategy ties the overall system together and distributes the seismic forces to all the towers. In addition, the overall system is tuned by replacing steel bearings with isolation bearings.