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Risks considered in deciding opening of Bay Bridge

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POSTED May 26, 2013 6:25 p.m.

SAN FRANCISCO (AP) — Caltrans and the Metropolitan Transportation Commission officials must consider the risks in deciding whether to delay the scheduled Labor Day weekend opening of the eastern span of the San Francisco-Oakland Bay Bridge or keep traffic flowing over the old span, the San Francisco Chronicle reported Sunday.

Caltrans will soon tell the commission what it intends to do, but bridge officials say that even with questions about the integrity of more than 2,300 steel rods used in the building of the new span, it is more likely to withstand an earthquake better than the bridge in operation since 1936.

Among the old bridge’s design flaws are its foundation, which Caltrans’ chief engineer on the eastern span project, Brian Maroney, told the newspaper, are too weak to resist a powerful earthquake centered in the San Francisco Bay Area.

Still, even with questions about the steel rods, the new span is more likely to withstand an earthquake than the old bridge.

This old bridge is the biggest problem we’ve got,” Steve Heminger, the transportation commission’s executive director, told The Chronicle

“There is no fix to it, other than moving traffic onto the new span. That’s why we feel such a sense of urgency.”

The old bridge is anchored with treated Douglas fir trees, driven 85 to 120 feet down into clay and mud. Some penetrate the mud and are anchored into three giant, hollow concrete boxes that serve as supports.

The mud can become like Jell-O during a quake, a process known as liquefaction. If that happens, quake forces exerted on nearby solid rock could be amplified by two times against the eastern span’s supports.

That could cause a shift of several inches in the foundations, which would translate into several feet by the time the disruptions traveled 200 feet to the road deck. The likely result would be that the bridge would shake apart, said Joe Nicoletti, a seismic engineer who served for years on Caltrans advisory panels.

“You need very large expansion joints to take care of the displacement of the trusses,” the steel latticework that begins at the western end of the incline, Nicoletti said.

On the new bridge, the skyway is supported by foundation piers that are driven 300 feet down into strong soil. The span’s tower is also built on pilings, secured into bedrock.

In a separate article, The Contra Costa Times reported ( ) Sunday that the new span would be safe after an earthquake, but an earthquake with about the same magnitude as the 1906 San Francisco quake could affect traffic over the bridge for hours or days with lane closures if bridge sections shifted or expansion joints were jammed.

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