Phase 1 (1997-2001)
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:
- Strengthening the existing foundations
- Total replacement of the four supporting steel towers and strengthening of Bent N11
- Replacement and addition of top and bottom lateral bracing and strengthening vertical truss members and truss connections
- 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.
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One of the viaduct support towers undergoes demolition.
© GGBHTD
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Temporary supports are in place as
the viaduct support tower is removed.
© GGBHTD
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Closer view of temporary supports in place with viaduct support tower removed.
© GGBHTD
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New support tower under construction.
© GGBHTD
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New support tower under construction.
© GGBHTD
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