Engineers Cite Vibrations, Wind in Bay Bridge Failure

(Photo by Noah Berger / San Francisco Chronicle)

Engineers working on San Francisco's ill-starred Bay Bridge have fingered a culprit in the repair job that went awry Tuesday evening, forcing the bridge's closure: metal fatigue caused by harmonic resonance. The phenomenon, in which small vibrations at key frequencies gradually build up until they are large enough to damage a structure, is the scourge of bridge architects everywhere. At first, many experts had fingered wind as the culprit in the accident. Heavy winds lashed the Bay Area late Tuesday afternoon. At 5 pm that day, several massive pieces of metal crashed 100 feet down from the bridge superstructure onto the roadway, striking a truck and damaging a car.

Heavy winds continued into the following day, gusting to 50 mph at times, hampering repair efforts. But while the winds may have contributed to the problem, officials say, they did not cause it in the first place. "The wind could have helped develop the harmonics on the rods, which contributed to the fatigue, but [this] was not a sole factor in and of itself," says Lauren Wonder, a representative for Caltrans, the state transportation department. Rather, she says, vibrations from traffic had built up over time, gradually weakening welds in a previous repair job.

The bridge's latest problem first came to light in early September, when Caltrans closed the bridge over Labor Day weekend in order to connect a new on-ramp to the bridge. At the time a bridge inspection crew discovered a crack in an eyebar, a diagonally slanting tension member that helps hold up the roadway. With less than 70 hours to go before the bridge reopened for traffic, Caltrans engineers raced to diagnose the problem and design a solution, then build the necessary hardware and install it. In the end, the bridge was opened after a delay of just a few hours. The hurried fix involved building a kind of brace for the fractured eyebar. Large metal brackets were fabricated to fit around each end of the damaged members, and then joined together by four 80-foot-long, three-inch-diameter cables called tie rods. These tie rods were tightened to relieve the load on the eyebar. On Tuesday, welds holding the upper bracket together came undone, sending a 5000-pound piece of the bracket crashing down along with two lengths of tie rod.

As of Thursday morning, Caltrans has not yet said when the bridge will reopen. At present its plan is to repair the repair job by gouging out the welds within the brackets and then fasten the pieces together with deeper, stronger welds so that they will be better equipped to withstand vibrations. Four larger tie rods will then be installed. As the work progresses, engineers are continuing to analyze the state of the project, and if necessary may decide to modify their plan. The 73-year-old bridge, a crucial transportation link between San Francisco and Oakland across the San Francisco Bay, has suffered a string of high-profile mishaps. Most significantly, on Oct. 17, 1989, a 7.1-magnitude earthquake hit San Francisco, collapsing a 50-foot section of the span and killing a motorist. In the aftermath of that incident, plans were laid to build replace the structure with a new bridge, which is currently under construction alongside the existing span.

The scheduled opening of the new bridge in 2013 means that the repairs Caltrans is making on the damaged eyebar don't need to endure for all time—they just need to get the time-worn structure through another couple of years, when it is scheduled to be demolished.

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