For several days, the roadway will remain suspended in midair while construction workers 100 feet above the ground insert an 8-foot-long bearing, a device used to absorb energy and make bridges safer during earthquakes.
It will have taken three years to prepare for that day because failure would be catastrophic, and thus completely unthinkable. "We've taken all the precautions," says Nick Panayotou, who will head the endeavor for Caltrans, the state agency that oversees California's roads and bridges. "We don't want to screw up. And I don't want to be run out of this town."
Jacking up the bridge with a live load of traffic is a remarkable engineering feat, but it is only one aspect of Caltrans' grand Bay Bridge seismic retrofit, the most ambitious ever performed on a suspension bridge.
The project is so massive that it will take more than 200 construction workers, laboring nearly around the clock, five years to complete the job. Construction began in the summer of 1998, and by the time it is finished in 2003, workers will have removed nearly half a million rivets and replaced them with 1.1 million high-strength bolts. About 19 million pounds of steel will have been added to the bridge, along with 96 shock absorbers the size of redwood trees, and four giant bearings, which will allow the bridge to skate a few feet back and forth in an earthquake. All told, the retrofit will cost about $200 million -- nearly three times as much as it did to build the bridge in the 1930s.
Yet despite its enormous scale, the retrofitting goes virtually unnoticed by the public. Most of the attention has focused on constructing the new and flashy east span, scheduled to be rebuilt by 2006.
Caltrans says it is proud of its invisibility in the west span retrofit -- if people don't know it's happening, then it isn't disrupting traffic, the agency says. And it is traffic on the busy bridge -- even more than the size or complexity of the project -- that has posed the greatest challenge. Caltrans, for example, is restricted to closing the bridge at night 30 times in the next couple of years, when it will be lifting 25-foot steel plates onto the bridge towers. For everything else, engineers must design the work around a constant stream of traffic, and they can only close a certain number of lanes during specific hours.
"It's difficult to do this work without causing an impact on the 280,000 vehicles crossing the bridge," says Peter Siegenthaler, the supervising bridge engineer. "Yes, we take up lanes, but we're working off-peak hours. And it's not the safest working environment. The challenge of this work is underappreciated."
Caltrans Structural Representative Thom Fresquez likes to put it another way: "It's like trying to rebuild your engine while driving on the freeway," he says.
On Oct. 17, 1989, the infamous Loma Prieta earthquake tore through the Bay Area in a 15-second fury, shaking the land for a hundred miles around, killing about 60 people, and causing about $7 billion in damage. The force of the 7.1-magnitude quake ripped open a 250-ton section of the Bay Bridge's eastern deck, which collapsed on the lower roadway, killing one person.
It was the worst damage the bridge had seen in 50 years -- though, all things considered, the span held up remarkably well, given that engineers designing the bridge in the late 1920s had assumed that wind forces -- not earthquakes -- would pose the greatest threat.
The Bay Bridge was in fact a magnificent engineering feat for its time. Bridge towers had to be sunk into bedrock at depths never reached before, and the span itself required 6 percent of the nation's total steel output in 1933, plus 1.3 million barrels of cement, and 18,500 tons of cable wire, enough to circle the earth 2.6 times.
The $77 million bridge opened on Nov. 7, 1936. Over the years, it has become the nation's most traveled bridge, and carries twice the number of cars as the Golden Gate. The 1989 failure of such a vital structure urged then-Gov. George Deukmejian to immediately call for retrofits of all nine toll bridges in the state, a mandate that is still being accomplished today.
Though it is impossible to bring a 60-year-old bridge up to modern design standards, the retrofit will make the bridge strong enough to withstand an earthquake 10 times the size of Loma Prieta. Caltrans considers the Bay Bridge a crucial "lifeline," and it needs to survive the next big shaker so emergency vehicles can cross the bay immediately afterward.
In anticipation of the Big One, the retrofit of the Bay Bridge has two goals: to make the bridge stronger and to allow it more freedom of motion in an earthquake.
The towers will be strengthened with concrete and metal, and the road decks bolstered with more steel. At the same time, giant shock absorbers and bearings will allow the bridge to diffuse the quake's destructive forces.
Each "friction pendulum bearing" used on the bridge is essentially an upside-down iron mushroom resting in a shallow dish topped by a "slider plate." The plate contains a groove that fits over the stem of the mushroom so the bridge can slide about three feet east and west (in the direction of traffic) on the support columns. A bearing will be placed between the roadway and each of four steel columns holding up the bridge at the San Francisco end near the Embarcadero.