The retrofit of the West Span
The West Span lies between Yerba Buena Island and San Francisco. It is composed of two complete suspension spans connected at a center anchorage. Retrofit work included adding massive amounts of steel and concrete to strengthen the entire West Span.
The West Span seismic retrofit project strengthened the bridge while allowing for a wider range of movement during an earthquake. This work, though not easily visible to motorists, was essential to ensuring that this vital transportation lifeline can withstand a major earthquake. Crews installed new, state-of-the art devices to isolate, absorb or diffuse the impact of an earthquake.
Much of the work came down to simple "nuts and bolts" retrofitting. The bridge's twin suspension spans were strengthened by adding new steel plates and replacing half a million original rivets with almost twice the number of high-strength bolts. In total, 17 million pounds of structural steel were added. New bracing was added under both decks, and all of the "laced" diagonal crossbeams connecting the upper and lower road decks were replaced with perforated steel. Piers were encased in heavy concrete jackets and additional anchor bolts were installed to fasten tower legs to pedestals.
Other work was simply astonishing and mind-boggling. During a dramatic lift of the entire 3-million ton span, massive rollers were installed between the roadway and the bridge supports to allow the bridge deck to roll and glide on top of the supports without major damage to the bridge.
Another state-of-the-art feature is the 96 new "viscous dampers," which will serve as "shock absorbers" during seismic activity. In the event of an earthquake, the dampers will help minimize damage to the bridge.
The span's main suspension cables were fastened by cable bands to the deck to allow uniformity of movement during an earthquake. Concrete keys cast into the bridge supports were added to keep the road deck from slipping sideways during an earthquake. And steel wind tongues will prevent sideways movement of the span.
More than 1,000 workers toiled around the clock for more than five years to accomplish this essential retrofit work. And through most of this work, the bridge remained open to traffic. To minimize impacts to motorists, the work that could not be performed during commute hours was done during the very late hours of the night and early morning. Work often required ironworkers, welders and painters to squeeze between narrow beams or to perch precipitously on scaffolding, high above traffic, in a dramatic effort to make the West Span seismically safe.