Engineers Baffled By Claims Bay Bridge Suffering From 75-Year-Old Design Flaw
|Amazing scenes were witnessed today...|
But, reading the article, the premise appears to do the very same thing a hastily assembled fix to an aging eyebar did last year on the bridge: It falls apart. The situation isn't that the Bay Bridge has been plagued by a design flaw since its inception. It's that eyebars are obsolete technology -- and have been for half a century -- and the eyebars on the Bay Bridge are nearly 80 years old.
Eyebars are "technology that's been superseded. And that's one reason we're replacing the bridge instead of retrofitting it," said engineer Mark Ketchum, one of the Bay Area's foremost experts on bridges. "We're basically at the end of the 'design life' for the bridge. That doesn't mean the bridge [suddenly] collapses. It means you need major reinvestment. With the Bay Bridge, the choice has been to replace it instead of upgrading."
When asked if the Bay Bridge's eyebars were flawed in their design or simply obsolete technology that was growing old, Bill Zanetich, Caltrans' senior bridge engineer, also indicated it was the latter.
|State of California|
|The eyebar in question|
As far as eyebars go, they've been outmoded for half a century -- and no American bridges have used them for 40 years. As the Examiner article even noted, legislation mandating eyebar bridges be inspected every two years has been the law of the land since 1967, following the collapse of the Silver Bridge, which killed 46 people.
Locally, the 1927 span of the Carquinez Bridge -- connecting Vallejo and Crockett -- was plagued by eyebar ruptures in the early 1970s, and shut down while engineers conducted extensive repairs. It's no coincidence the 1958 Carquinez span was constructed using newer techniques than the archaic eyebars. In 2003, a third span replaced the problematic 1927 bridge, which has been scrapped.
That this technology ages poorly and with potentially disastrous consequences was well established by the time of the Johnson administration. That didn't stop the Examiner's eye-catching headline from, well, catching eyes. Sen. Mark Leno, who successfully pushed for increased oversight of the bridge, this morning tweeted out "Bay Bridge eyebar woes date to 1930s" with a link to the Ex story.
What, exactly, is the problem with eyebars? 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.
And there you go. Eyebars are obsolete. And any capital project begins to show its age after 75 years.
"What we're seeing is the end-of-life blues," says Ketchum. And, yes, it's a life that "dates to the 1930s."