Quake-Resistant Bridge Planned in San Francisco

By Matthew Barrows
Sacramento Bee
March 5, 2002

The new east span of San Francisco's Bay Bridge is a study in superlatives.

At $2.6 billion—more than a $1 billion a mile—it's the most expensive public works project in California's history. Building it will take 100,000 tons of steel and 67,000 workers, making it the largest ongoing bridge project in the Western Hemisphere.

Most important to motorists, an array of seismic advances—from a main tower that acts as a 600-foot (183-meter) shock absorber to hundreds of steel piles driven deep into the San Francisco Bay mud—will make it one of the safest structures in the Bay Area if "the big one" hits.

Those now crossing the bridge don't have that comfort, of course. Groundbreaking occurred last month, but the new span won't be ready for five years. The existing 66-year-old span is not built to withstand a large temblor.

About 12 years ago, the 7.1-magnitude Loma Prieta quake 60 miles (96 kilometers) south of the bridge collapsed a portion of the east span's upper deck, killing a motorist and shutting down the bridge for four weeks.

Emergency repairs strengthened the bridge some. But Bruce Bolt, a professor of seismology at the University of California–Berkeley, said that in 1989, the east span could handle only five inches (12 centimeters) of horizontal movement. The Loma Prieta quake, he said, obviously surpassed that threshold.

"Of course, that earthquake was a long way away," Bolt said. "It's nothing like what we'd expect from a large earthquake closer to the bridge. We might expect five times that."

Rolling With the Punches

The new 2.25-mile (3.6-kilometer) span will stretch from Yerba Buena Island to the Oakland shore and will consist of side-by-side roadways with five lanes each. The span from Yerba Buena into San Francisco is not part of the $2.6 billion project.

The new span will be built north of the current span, which will be torn down when the construction is completed.

The brittleness of the east span left it bruised and battered in 1989; its replacement is designed to roll with the punches.

Modern design strives to create a bridge that bends but does not break, according to Roy Imbsen, who runs a Sacramento-based engineering consulting firm. Imbsen's firm has worked on the seismic retrofits of the Golden Gate and Benicia-Martinez bridges.

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