Caltrans on tight schedule to repair Bay Bridge broken bolts
03/27/2013 8:35 AM
04/12/2014 7:26 PM
California Department of Transportation officials are developing a work-around to secure vital seismic stabilization equipment after heavy-duty bolts broke on the new eastern span of the San Francisco-Oakland Bay Bridge, project manager Tony Anziano told the Metropolitan Transportation Commission Wednesday.
While the problem should not delay the opening of the bridge, slated for Labor Day weekend, Anziano said he expects the fix to take months to complete. "Time is absolutely tight," Anziano conceded after questions from a commissioner. "Everybody should acknowledge that there is risk" that the bridge opening might be delayed to ensure seismic safety.
Caltrans will replace at least 32 bolts, and perhaps many others, attached to "shear keys," units that allow controlled lateral movement of the bridge in the event of a large quake. The same type of bolts connected to bearings - another seismic control device - have not been found to be defective, but also are considered suspect, according to Caltrans. Overall, officials said they would assess 288 bolts.
Officials said it was too soon to know the cost of repairs, to identify where quality control failed or to place blame, but they attributed the problem to apparently defective steel contaminated by hydrogen during manufacturing. "Hydrogen embrittlement" can cause steel to crack or break.
The bolts, also referred to as rods, attach the four shear keys and four bearings between the easternmost pier of the new suspension span and the roadway above it. The devices were made by the Korean firm Hochang Machinery Industries, but the bolts themselves were made in the United States, according to Caltrans.
A primary challenge, Aziano told commissioners, involves the lack of access to rods for two shear keys located above the vertical sections of the pier. In at least those cases, a workaround - possibly a collar attached to the base of the shear keys, will be used to replace or augment problem bolts.
"We have surmounted far greater engineering challenges than this one in getting this bridge constructed," said Steve Heminger, executive director of the Metropolitan Transportation Commission. "And I have no doubt that we will get through this one as well."
Bearings and shear keys are massive steel devices nearly 6 feet tall. The broken bolts are 3 inches in diameter and up to 24 feet long. At the center of the shear key, a circular joint joins the suspension span to the pier much like a trailer hitch allows a load to sway laterally with some independence, yet remain secured to a pickup truck.
Early this month the bolts were stressed, or placed under tension, to increase overall performance. Among 96 so far examined, 32 broken bolts were found about a week later.
Case Western Reserve University Professor Arthur Hucklebridge, an expert on seismic issues in bridge construction called the problem "extremely unusual" - more so because the bolts snapped after, rather than during stressing.
"If the safety of the bridge is involved, we will investigate fully," said Sen. Mark DeSaulnier, D-Concord, chairman of the California Senate Transportation and Housing Committee.
Call The Bee's Charles Piller, (916) 321-1113. Follow him on Twitter @cpiller.
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