Silverado Contractors/California Engineering Contractors joint venture – San Francisco Bay Bridge demolition

The completion of the new eastern span of the San Francisco-Oakland Bay Bridge in September 2013 was a major accomplishment for state transportation agency Caltrans and a host of contractors including Oakland's own Silverado Contractors. The project – which cost $6.3 billion and took 11 years to build – finally put to rest concerns about the bridge's ability to withstand an earthquake akin to the 7.1-magnitude Loma Prieta earthquake in 1989, which collapsed a portion of the span.

Although traffic is now moving on the bridge, another large project is currently underway: the removal of the now-closed 2,400-foot-long and 70-foot-wide cantilever portion of the east span, which rises 400 feet above San Francisco Bay at its highest point. A joint venture of Silverado Contractors and California Engineering Contractors is working to demolish the cantilever and related truss structures piece by piece. 

In addition, the team is demolishing a 1,725-foot-long and 70-foot-wide detour structure built to accommodate traffic during construction. The structure, a double-decked truss system, is built over and adjacent to an active U.S. Coast Guard station, says Rich Riggs, project manager and one of four founding partners of Silverado Contractors. 

Other improvements include a new east-bound on-ramp to the bridge and the completion of a bike path that begins in Oakland and ends at Yerba Buena Island/Treasure Island. Both the cantilever and the detour structure must be removed before the ramp and path can be completed. The $86 million demolition and related projects started shortly after the new span opened and will be completed in 2016, Riggs says.

The San Francisco-Oakland Bay Bridge was initially completed in 1936. Work on the new portion began in 2002 after years of planning following the 1989 collapse, which shut down the bridge for more than a month for emergency repairs. The new east span includes a number of seismic enhancements including the placement of 160 rebar and concrete-filled steel piles driven up to 300 feet below the water's surface, and 196 feet into bedrock, to anchor it into stable soil. Twenty 60-foot-long hinge pipe beams – which resemble massive metal dowels – were also placed on sections of the bridge. These beams are designed to absorb the energy of an earthquake by deforming in their middle section, minimizing damage to the bridge's main structure, Caltrans notes.

Piece by Piece

The joint-venture team is taking great care to remove the cantilever structure. The first phase of demolition, now complete, included the removal of 1,400 feet of the upper deck of the cantilever, the tallest portion of the bridge. An estimated 2,125 tons of concrete, 374 tons of rebar and 1,300 tons of steel deck supports are being removed.

Later this year, the team will begin to slice the cantilever section in two and begin dismantling its steel pieces. Portions of the upper deck were dismantled first, with pieces being lowered to the bottom deck for removal. The lower deck will then be deconstructed. “Our plan is to remove the structure in the reverse order that it was built,” Riggs says. “We'll do this by installing temporary supports under the truss sections of the structures and dismantle it piece by piece from the existing deck.”

J. Coleman, principal of the engineering firm Foothills Bridge Co., led a team of engineers in the design of the demolition plan. Planning processes included 3-D modeling, Riggs adds. 

Crews will first dismantle the cantilever section to the west of the split, and then work east. Following completion of the removal of the cantilever structure, Caltrans will award separate contracts for the removal of the 504-foot-long truss spans that descend from the cantilever, followed by the 288-foot incline truss spans. A third and final contract will be awarded to remove the foundations down to the mud line.

A total of 58,209 tons of steel and 245,470 tons of concrete will ultimately be removed, and the majority of the materials will be recycled or reused. 

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