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Tuesday, November 3, 2009

Bridge Expert: I'll Still Drive Bay Bridge -- But Break Is Baffling

Posted By on Tue, Nov 3, 2009 at 8:30 AM

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Whenever something goes wrong with a bridge here or elsewhere, one of the first people we call is Mark Ketchum -- one of the Structural Engineers Association of Northern California's designated experts on bridges.

So, for what it's worth, Ketchum said he will unhesitatingly drive his car across the jury-rigged Bay Bridge -- though he's a regular BART commuter now rejoicing he can get a seat once again. Still, when asked what questions he had about the alarming failure of the bridge's Labor Day repairs, he answered "all of them."

"There's really no data out there on what broke, why it broke, how they tested the design and the new fabrication," said the engineer. "I think all bridge engineering professionals would be well-advised to be interested in that." Still, with the incredible scrutiny Caltrans was facing as a result of the internationally reported bridge failure, he's confident the bridge is as safe as the organization says it is: "In the light of this break and the copious quantities of egg everyone has to deal with -- I believe they really pulled out the stops on a conservative approach to the second round of repairs."

Oh, something else -- maybe we should get used to this. When Cassandras wail about America's crumbling infrastructure -- this is what they're talking about.

"The public has gotten used to a very low-risk environment. And one of the reasons we live in a low-risk environment is because we put adequate resources into keeping things in good condition," he says. "This underscores the need not only to build new stuff -- which is what makes constituents happy with their politicians -- but to maintain our old stuff.

"I had a mentor professor at Cal who made a statement on the order of 'the true sign of an advanced civilization is not necessarily just what they build but how they maintain what they build,'" continued Ketchum. "We made wise investments in the 20s and 30s on the Golden Gate Bridge, Bay Bridge, dams, power plants and the like. These investments are reaching the ends of their design lives."

That doesn't mean they'll come crashing down the moment they reach the end of their anticipated life expectancies. But it does mean that significant investments may be in order to "maintain the integrity" of our aging transportation infrastructure, which was built generations ago.

"Those big bridges in Japan on the same scale of the Golden Gate and Bay Bridge have tolls several times higher than what we're paying here because they paid recent money to build them instead of old money," he notes. Our bridges "are paid off. We made wise investments and we've been reaping the benefits of them." But the time to pay appears to be here.

Incidentally, a generation or two down the road, engineers may not even know how to patch up decaying eyebars of the sort that plagued the Bay Bridge. Eyebars haven't been used in bridge design for at least 40 years and were outmoded more than 50 years ago (the 1958 Carquinez span, for example, didn't use them; the since-demolished '27 span did -- and was plagued by ruptures in the early 1970s).

Eyebars have given way to welded plate steel -- a stronger and more versatile system. For what it's worth: Eyebars can be used for tension -- pulling -- but not compression -- pushing. "So, near the pier on a cantilever bridge on the tower, you can use eyebars on the top but not on the bottom," explains Ketchum. Still confused? Okay, take a pen and hold it in front of you, horizontally,  with one hand on either end. When you pull, that's tension. Push, and that's compression. When eyebars are forced to withstand compression, you get what engineers call "buckling," which leads to failure.

In any event Ketchum won't go out of his way to drive on the bridge -- again, he's a happy BART commuter -- but he won't avoid it either. "I think the risk is low enough that I'm there."

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About The Author

Joe Eskenazi

Joe Eskenazi

Joe Eskenazi was born in San Francisco, raised in the Bay Area, and attended U.C. Berkeley. He never left. "Your humble narrator" was a staff writer and columnist for SF Weekly from 2007 to 2015. He resides in the Excelsior with his wife, 4.3 miles from his birthplace and 5,474 from hers.


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