Bridge design in question as cracked beam repaired

Steel beams on a bridge should not rip open after less than a decade — and the one that did that on the San Mateo-Hayward Bridge raises grave concerns from an engineering expert that a retrofit project was seriously flawed.

An engineer who performed an analysis of the bridge prior to the 2001 retrofit said the failure was likely caused by one of two things: either a major design flaw in the earthquake retrofit project or that the materials used for the bridge were defective.

Westbound traffic across the bridge was expected to stay slow through at least midday today while Caltrans workers attempt to shore up the broken beam.

The northernmost lane has been closed since Wednesday, when the 10-inch crack through the 12-inch beam was discovered during a “routine inspection” of the bridge, the first in two years, Caltrans spokesman Bob Haus said. The crack was found on a 30-foot-long wishbone-shaped beam under the East Bay portion of the bridge, about 100 feet before it rises over the shipping channel. The beam, which cracked at its Y, was installed as part of a retrofit intended to make it less vulnerable in an earthquake.

The crack does not pose a critical threat to the bridge, Caltrans said, so all other lanes are open.

To support the broken beam, two 6-foot-long, 1.5-inch-thick steel plates were fabricated and were being affixed to the top and the bottom of the beam. Haus said that would likely be a permanent fix.

“The one silver lining in this is the inspections work,” he said. “The inspections are meant to determine if there are any problems, so that was the success here. This is the first time we’ve ever found any problem on this bridge [since it was retrofitted].”

But UC Berkeley engineering professor Abolhassan Astaneh-Asl, who in the late 1990s provided Caltrans with an analysis of the San Mateo-Hayward Bridge and its earthquake vulnerabilities, said no problems should have been found so soon after the retrofit. He said it’s a sign there were either flaws in the design or defects in the steel, bolts or welds.

“Steel on a bridge should not crack in nine years. Forty years, yes, 50 years, yes. But not nine years,” Astaneh-Asl said.

It’s not unheard of for an earthquake retrofit to actually do more damage to a bridge than it prevents, he said. Astaneh-Asl pointed to the Bon Aire Bridge in Larkspur, which began to fall apart several years after a seismic retrofit. Later, a jury determined the culprit was a bad design and it forced the designer to reimburse the city about $8 million.

Astaneh-Asl said Caltrans owes the public an explanation about the crack.

kworth@sfexaminer.com

Bay Area NewsLocalNEPSan Mateo BridgeTransittransportation

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