Updated February 2009

Overview of Golden Gate Bridge Seismic Retrofit

It was a bone rattling, concrete crushing, nerve-racking 15 seconds. At 5:04 p.m. on Tuesday evening, October 17, 1989, the 7.1 magnitude Loma Prieta earthquake caused 68 deaths, at least 3,700 injuries and an estimated dollar loss of $6 billion to $7 billion. The earthquake reminded the world that the San Francisco Bay region remains vulnerable. Although the Golden Gate Bridge suffered no observed damage from the Loma Prieta quake, since the epicenter was located some 60 miles to the south, the earthquake became a catalyst for the extensive seismic retrofit program that the historic structure is undergoing today.

Perhaps the most impressive statistic resulting from research conducted since the Loma Prieta earthquake is the conclusion by the U.S. Geological Survey (USGS) and other scientific organizations that there is a 62% probability of at least one magnitude 6.7 or greater quake capable of causing widespread damage, impacting the San Francisco Bay region within the next 30 years.

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. Spanning 1.7 miles from abutment to abutment, the Golden Gate Bridge consists of six main structures:

1. San Francisco (south) Approach Viaduct
2. San Francisco (south) Anchorage Housing and Pylons S1 and S2
3. Fort Point Arch
4. Main Suspension Bridge
5. Marin (north) Approach Viaduct
6. Marin (north) Anchorage Housing and Pylons N1 and N2

Immediately following the Loma Prieta quake, the Golden Gate Bridge, Highway and Transportation District (District), San Francisco, CA, the operator of the Golden Gate Bridge, engaged a team of consultants to conduct a vulnerability study. The conclusion of the study was that under a Richter magnitude 7.0 or greater earthquake with an epicenter near the Bridge, it would experience severe damage that could close this important transportation link for an extended period of time. If a Richter magnitude 8.0 or greater earthquake centered near the Bridge, there would be a substantial risk of impending collapse of the San Francisco and Marin Approach Viaducts and the Fort Point Arch, and extensive damage to the remaining Bridge structures, including the Main Suspension Bridge. It must be noted here, that as of April 2006, the seismic retrofit of the Golden Gate Bridge is far enough along that the Bridge no longer faces the potential for collapse and until the entire retrofit is completed, the risk of significant damage to the Main Suspension Bridge remains.

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. The site-specific, moderate earthquake was defined as one having a 10 percent chance of being exceeded in a 50-year period or having an acceleration of 0.46g. The site-specific, maximum credible earthquake was defined as one having a return period of 1,000 years or having an acceleration of 0.65g, which is equivalent to the 1906 San Francisco earthquake of a magnitude 8.3 on the Richter scale.

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. In 1996, 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

Schematic of Retrofit Measures for Golden Gate Bridge (PDF)

Phase 1 - Completed in December 2001

On June 27, 1997, the Board of Directors of the District awarded a contract for the first phase of seismic retrofit construction. It also organized a construction administration team made up of District staff and consultants.

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:

1. Strengthening the existing foundations
2. Total replacement of the four supporting steel towers and strengthening of Bent N11
3. Replacement and addition of top and bottom lateral bracing and strengthening vertical truss members and truss connections
4. The structural system has also been modified to minimize effects of ground motions on the structure by the following:
   • Connecting five, simply-supported truss spans into a continuous truss;
   • Installing seismic expansion joints at the north and south ends of the viaduct truss; and
   • Installing isolator bearings atop the new steel support towers at the Pylon N2 support and at Bent N11.

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 first work undertaken was to connect the viaduct spans to create a continuous superstructure capable of distributing lateral forces to prescribed points while the structure underwent tower replacements. Bent N11 near the Marin (north) Abutment was substantially strengthened to substitute for temporary loss of longitudinal stiffness at the removed supporting towers. Before the individual towers could be replaced, the retrofit sequence required that truss members directly above each of the towers be replaced and truss panel points be strengthened.

The contractor retrofitted the tower foundations in a two-stage operation. The first stage was constructed with the existing towers still in place, which allowed them to schedule this work outside of the project critical path.

During the first stage, cast-in-drilled-hole (CIDH) piling and pile caps were added around the perimeter of the original foundation pedestals. The new concrete to existing concrete interfaces were strengthened with post-tensioning of monostrands, clamping the new footings to the pedestals of the existing foundations. The existing grade beams between the foundation pedestals were also substantially strengthened, and additional grade beams were constructed.

After the existing tower was removed, the second stage of the foundation retrofit proceeded. First, the remaining upper portions of the existing pedestals were demolished. Then, new upper pedestals were constructed and closure pours placed to incorporate these elements into the entire foundation system. The erection of a new tower followed.

The most visually dramatic Phase 1 work was the complete removal and replacement of the four steel support towers with footprints of 50 feet by 75 feet and heights of up to 150 feet. The contractor sequentially replaced the existing towers with new ones that very closely imitate the appearance of the original towers.

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 the adjoining original tower. At the jacking points, the superstructure had to be lifted by up to 1½ inches to provide for up to ¼-inch lift at the existing bearings. This separation was sufficient for the contractor to proceed with removal of the original bearings, which was to be followed by demolition of the tower below.

The synchronous lift system used by the contractor was controlled at an electronic central control panel that is capable of raising the individual jack rams in precise increments of 0.2 inch and of shutting down the individual jacks once the superstructure was raised the prescribed height.

A total of six jacking points were used per tower; each point consisting of a cluster of four 200-ton jacks. Each jack cluster was tied to a single manifold such that all four jacks received the same hydraulic and electronic signals from the controller. This system included highly accurate (up to 0.04 inch) sensors, which were attached to the superstructure to control its position. Aside from this means of displacement monitoring, a licensed land surveyor was also deployed on a nearby hillside to monitor structure location prior to, during, and after the jacking operations so as to detect any unplanned access movement. Locking collars were placed on the jacks as a means of providing redundancy in the event of a hydraulic failure of the jacking system. Workers monitored the existing tower bearings and reported on their status via radio lift off.

The overall jacking operations typically required approximately a half hour, the majority of which was spent checking and monitoring the status of the lift, with frequent instrument readings and status verifications.

Viaduct support tower undergoes demolition. Photo property of GGBHTD	Temporary supports are in place as the viaductsupport tower is removed. Photo property of GGBHTD
Closer view of temporary supports in place with viaduct support tower removed. Photo property of GGBHTD	New support tower under construction. Photo property of GGBHTD
New support tower under construction. Photos property of GGBHTD

Phase 2 - Completed in Spring 2008

On May 11, 2001, the District Board of Directors authorized award of the Phase 2 construction contract. Phase 2 began in June 2001 and was completed in spring 2008. It was the most complex part of the project in terms of design and construction. Federal, state and regional funds totaling $189 million were authorized to complete this phase.

This phase encompasses structural retrofit of many different types of structures of the south approach: the south approach viaduct, south anchorage housing, Fort Point arch, and south pylons. Retrofit measures developed for each of these structures reflect their individual behavior under seismic ground motions and their interaction at points of interface while accommodating their already-in-place historic configuration.

Without closing the Golden Gate Bridge to traffic, the steel support towers and bottom lateral bracing of the south approach viaduct will be entirely replaced, and seismic isolation bearings and joints will be installed at the roadway level. The west wall of the south anchorage housing will be replaced and massive internal shear walls constructed. Five million pounds of external and internal steel plating will be added to south pylon walls. The historic architectural appearance of the external surfaces of the pylons will remain unchanged with the addition of a new external concrete cover on top of the new plating.

The Fort Point arch will be retrofitted with new arch bearings and energy dissipation devices, and isolation joints will be installed. Steel members throughout the entire arch will undergo extensive strengthening.

Not only were immense challenges presented in the design and engineering of this phase of retrofit construction, but the construction site itself presents very unique project limitations. The construction site is located in a very compact area bound on the west by the Pacific Ocean and on the east by very steep slopes. Severe weather including strong wind and high waves are nearly constant. Access consists of two narrow roads that must be shared with thousands of tourists visiting the Golden Gate Bridge and the Historic Fort Point Site located directly below the Fort Point arch structure of the Bridge. Construction on the arch is limited to four days per week to allow limited visitation to the Site. The small construction staging areas available near the work site further restrict the logistics of the construction operations.

Phase 3 - Phase 3A began in April 2008

The third and final phase of the Golden Gate Bridge Seismic Retrofit Construction Project has been separated into two sub phases as follows:

Phase 3A: Retrofit of the North Anchorage Housing and Pylon N1

Phase 3A will cost $125 million and will be funded using a combination of federal funds along with regional and state earmarks. A construction contract for Phase 3A was awarded in April 2008. Construction will take approximately 3.5 years to complete.

Phase 3B: Retrofit of the Main Suspension Span, Main Towers, South Tower Pier and Fender

Phase 3B will cost $260 million and will be funded using a combination of federal funds along with regional and state earmarks. Phase 3B will begin in 2010 and take approximately 3.5 years to complete.

The seismic retrofit measures for these phases consist of strengthening foundations, installation of micropiles and rock bolts, construction of reinforced concrete shearwalls, replacement of the housing roof/roadway deck with a pre-cast concrete slab-on-steel stringer deck system involving nighttime lane closures, and other structural modifications.