Jack Moehle -- a U.C. Berkeley engineering professor and one of the Bay Area's acknowledged experts on freeway and bridge failure -- said that Caltrans' initial claims that high winds contributed to yesterday's rupture on the Bay Bridge is "a credible explanation." Still, he's taken aback that the failure occurred just weeks after the installation of the parts in question.
"Wind can result in resonance of a cable or rod, which can amplify stresses," wrote Moehle in an e-mail. "Normally, this is a longer-term problem associated with high-cycle fatigue (steel can fail at a stress below its one-cycle strength if millions of cycles are applied). What is surprising is that it happened just six weeks (or so) after initial installation."
Here's the professor's explanation in more layman-friendly terms: Wind makes cables and rods vibrate, which induces wear and tear ("wear and tear" is the blue-collar way of referring to the engineering term "fatigue"). "One-cycle strength" is, essentially, steel's breaking point. But steel can actually fail at less
than its breaking point, if minor stresses are persistently applied over time. The analogy that springs to mind is "death from one thousand small cuts."
Moehle, being the responsible engineer that he is, would not speculate
further. But it stands to reason that the failure could be the result
of three factors (independently or combined): Faulty engineering, faulty
manufacture, or faulty installation.
There are nebulous ways to conjecture about the latter pair, and the former could be the result of -- and, again, this is speculative -- underestimation of design loads or other analysis errors.
Caltrans has not yet made any announcement other than the aforementioned initial statement that high winds were likely a factor. Obviously, we'll keep you posted.
Photo | Larfo