Golden Gate Bridge

Golden Gate Bridge

Introduction

The Golden Gate Bridge represents a vital transportation link to the San Francisco Bay Area, serving more than 40 million vehicles a year. The Bridge is recognized by the American Society of Civil Engineers as one of seven civil engineering wonders of the United States. The Bridge is a national treasure known and admired around the world. After determining that retrofitting the Bridge would be more cost-effective than replacing it, in 1992, the District hired engineering consultants to develop seismic retrofit design criteria. As part of this task, the site-specific design ground motions associated with different magnitudes of earthquakes and expected performance levels were defined as the basis for the Bridge retrofit design.

Analysis

Because of financial constraints, the District proceeded with phasing the construction of the seismic retrofit in a manner that reflected the degrees of structural vulnerabilities. The three construction phases were established as follows:

Phase 1 would retrofit the Marin (north) Approach Viaduct

Phase 2 would retrofit the San Francisco (south) Approach Viaduct, San Francisco (south) Anchorage Housing, Fort Point Arch, and Pylons S1 and S2

Phase 3 would Main Suspension Bridge and Marin (north) Anchorage Housing

Phase 1

The scope of retrofit within the viaduct truss was significantly reduced through the installation of lead-core-rubber type isolator bearings. These bearings enable displacements of the truss relative to its supports, thereby significantly reducing the transfer of seismic forces onto the truss. The maximum credible earthquake is predicted to create up to 12-inch displacements of the truss. To prevent the truss from crushing against the Marin (north) Abutment and Pylon N2, seismic expansion joints were constructed at these locations by removing a section of the orthotropic steel deck of the viaduct at Pylon N2 and removing and reconstructing the Marin Abutment backwall. These joints enable truss displacements of up to 15 inches, thereby preventing damage that could jeopardize the integrity of the structure.

A primary challenge of Phase 1 was to construct the retrofit measures under continuous traffic. The construction inspection team closely monitored the structure throughout the complex process of installing temporary bracing, constructing and loading temporary supports for replacement of the towers, removing and replacing members, and strengthening members and connections.

The seismic retrofit measures applied to the Bridge structures consist of various methods of structural upgrades and include both the strengthening of structural components and the modification of structural response of the structures so they can better respond to strong motions without damage. The cost of Phase 1 totaled $71 million, which was funded using Golden Gate Bridge tolls. The major strengthening measures implemented on the Marin (north) Approach Viaduct included the following:

Jacking of the superstructure continuously under traffic was an interesting aspect of the tower removal and replacement operation. Once erection of the temporary supports was completed on the sides of the original tower, a series of synchronized jacks lifted the superstructure from the six original tower bearings by loading the six temporary support bearings. The temporary supports and jack were located 25 feet away from ...