|Specified Sequence. Temporary towers on either side of the new permanent bridge tower (in gray) were reused at each vertical support. (Photo courtesy of Golden Gate Bridge Highway and Transportation District)|
...foundations were built for the temporary towers. "We incorporated the temporary foundation into the permanent retrofit to avoid waste," says Marvin Young, principal at Jees firm.
To install isolation bearings between the viaduct superstructure and substructure, the contractor had to jack the road deck one-quarter inch while the bridge was open to traffic. Project officials say the process has been routine. "Jacking a live bridge...is like watching grass grow," says Rich Zito, Shimmick-Obayashi project engineer. But he admits "if something were to go wrong, it could go very wrong."
|Assembly Line. Contractor works on pylon exterior plating simultaneously from five levels. (Photo by Michael Goodman for ENR)|
Strengthening the concrete pylons that support the ends of the Fort Point Arch has been far from routine. The task is necessary because movement from new energy dissipation devices installed at the base of the arch are grounded through pylons, says Young. The pylons have only minimal reinforcement and lack tensile capacity, says Kuang Yak Lim, a senior project manager with HDR Engineering Inc., Oakland, who had been part of Jees department at Sverdrup.
To accommodate those additional forces, as well as the pylons own inertial forces, engineers designed a "sandwich" system that adds steel plates, varying in size from 3 8 in. to 11 4 in. thick, on the inside and outside faces of the existing concrete pylon walls. Seams between plates are welded and high strength rods are threaded through the assembly. Grout is pumped between the plates and existing concrete.
The plating operation extends 187 ft above pylon foundations, but the contractor worked on only 20-ft-tall sections, maintaining roughly the same level of completion on each face and each pylon.
Because plating involves removing several inches of concrete cover on the exterior and demolishing interior concrete diaphragms, "all [pylons] have to stay at roughly the same stage since you dont want one more stiff than another," explains Cocotis.
The process has proved tricky because the typical exterior plate is 10 x 30 ft and weighs some 20,000 lb. "These are substantial pieces of steel," says Zito. On the lower half of the pylon exterior, the contractor used a custom-designed, self-climbing scaffold. Workers have the ability to work on the scaffolds five levels simultaneously, so the plating process is "almost more like a factory assembly line than construction," says Cocotis.
|Continuity. Lim, Jee and Young have been involved from the start. (Photo by Michael Goodman for ENR)|
For the upper half of the pylons, the contractor found a single-level window washer platform that is lighter and more easily deployable. The multi-level scaffold remains in place as a staging platform for the work higher up, says Zito.
On the interior, contractors are using a mast climber and a series of winches and cables to maneuver plates. Although those plates are narrow and tall, about 10 x 20 ft, they are still difficult to move within the approximately 26x36-ft pylon. "We couldnt just drive up with a crane," says Zito.
The contractor expects to complete plating by the end of this month. After sand blasting and epoxy coating the first 100 ft of the pylons, crews will pour a concrete cover over the exterior steel, replicating the original formwork pattern.
Work on the arch between these two pylons includes strengthening and replacing steel members and installing new energy dissipation devices. Engineers devised a custom-made but straightforward friction form of the device they say offers several advantages over off-the-shelf viscous dampers typically used on bridges.
The new device can fit into limited space, will not leak and does not need a specialty supplier for fabrication or maintenance. The damper, similar to those sometimes used in bracing systems for buildings, is made up of steel and aluminum bronze plates. A significant quake will deform "sacrificial" plates but they are easily replaced, says Lim.
|Suited Up. Painting depends on weather conditions. (Photo by Michael Goodman for ENR)|
The archs position directly above Fort Point, which is open to tourists on Fridays, Saturdays and Sundays, limited work to just four days per week. Restrictions have been especially tough on painting subcontractor Long Painting Co., Kent, Wash., whose work depends on the weather.
Atmospheric and surface conditions must be monitored constantly since paint cannot be applied when the steel temperature is above the dew point, says Scott Clark, Long project manager. "We can get all geared up, and the fog can come booming in," he says. "It is difficult to maintain an optimal flow of work." Wind also is a factor. Working inside the painting enclosure is difficult, especially during summer, when gusts are common. "The tarp is like a sail. It beats the crap out of a guy," says Clark.
On top of a limited work schedule and unpredictable weather, the painter, along with the steel erector, have had to deal with other challenges, including coordinating with each other and with engineering documents that specified the sequence of joint removal and installation. In a very selective fashion, members are disassembled, paint is removed, and the exposed steel is primed before new joints are installed, says Ronald Paz, division manager for the West Coast office of Shakopee, Minn.-based steel erector Dannys Construction Co. Inc. "There is no real rhythm," adds Clark.
To facilitate work, the erector installed steel decking suspended from wire rope below the roadway on the viaduct and on three different levels in the arch. It also installed a "stairway to heaven," a stepping steel channel system used to reach work at the base of the arch. "Easy access is the key to productivity," says Jack Geer, Dannys project sponsor.
The retrofit is scheduled for completion by March. About $12 million worth of additional repairs and painting, to be paid for with leftover contingency, will be completed about a year later.
District officials now are seeking $160 million in federal and state funds for the overall projects final phase on the north anchorage house and pylons, and the suspension span. The work will be released in two bid packages in spring 2005 and 2006. Officials also are examining options for hardening the bridge against terrorism. "We want to make certain that the seismic work will not make the bridge more vulnerable in case of an attack," says Bauer.For more information about the seismic retrofit project and the Golden Gate Bridge, see the Golden Gate Bridge Highway and Transportation District website at www.goldengatebridge.org.