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Thursday, August 2, 2007

Carquinez Bridge Shares Design of Minneapolis' I-35, Collapse Unlikely, Local Expert Says

Posted By on Thu, Aug 2, 2007 at 2:58 PM

Expert Opinion: S.F. Bridge Engineer Analyzes Minneapolis Bridge Failure


When Mark Ketchum saw the images of the grotesquely twisted beams of green steel, the perforated roadway and the frantic men and women thrashing about the chilly Mississippi River, he had the same feelings as anyone else: What a nightmare come to life.

That was his human reaction to Wednesday's collapse of the Interstate 35 Bridge in Minneapolis, Minn. As a structural engineer specializing in bridges, however, his professional reaction went a bit deeper.

And for Bay Area residents who have sworn off crossing bodies altogether in the wake of the disaster, the San Francisco engineer has some good news: A Minnesota-type, out-of-the-blue bridge collapse is exceedingly unlikely here in the Bay Area.

"Honestly, California is much more benign with respect to fatigue than a place with sub-freezing temperatures. Steel becomes marginally more brittle in very cold temperatures and you just don't get those here. Metal fatigue is just less of a problem in coastal California than Minnesota," said Ketchum, whose day job is at OPAC Engineers in the City.

The rapid demise of the I-35 Bridge leads Ketchum to suspect "fatigue and related brittle fracture" as the rationale behind the fatal collapse. He does not see the 2001 reports about the bridge's metal fatigue as a smoking gun but "20/20 hindsight — metal fatigue is not an unusual problem on bridges made of steel in the 1960s).

In media reports nationwide, the ill-fated I-35 Bridge (seen here in happier days) has been described as a "arched truss bridge." Ketchum feels this description is inaccurate.

In actuality it was a "haunched-truss" bridge, meaning the trusses were somewhat deeper over the piers than in the middle of the span, so as to accommodate more stress at those locations.

This is significant because there is a haunched-truss bridge in the Bay Area — The Carquinez Bridge. What's more, Ketchum notes that "metal fatigue" was detected on the span in the mid- to late-1980s. But before you swear off traveling to Benicia (at least because of the bridge), remember that the span was repaired in 2001. A second span of the Carquinez, replacing the 1927 bridge, was completed in 2003. It's not a truss bridge but a suspension structure.

You don't need to be a bridge expert to know that they don't collapse spectacularly in America very often. Ketchum can only recall two instances in the modern era: The Schoharie Creek Bridge disaster in upstate New York in 1987 and the famous hula dance of death of the Tacoma Narrows Bridge in 1940.

"The practice of structural engineering is not an exact science. We learn, experiment and adopt modern techniques," said Ketchum.

"There have been other bridges from [I-35's] era that have had fatigue problems. But all the ones I know of in California have been corrected."

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David Downs


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