Failure™ Magazine

Suspended Animation

The collapse of the Tacoma Narrows Bridge.

Written by Jason Zasky   Filed under History, Science & Technology

In the past 15 years, the futuristic-looking cable-stayed bridge has replaced the suspension bridge as the favorite of engineers and public officials, owing to its aesthetically pleasing appearance and cost-efficiency. A cable-stayed bridge (such as the Sunshine Skyway in Tampa, Florida), is supported by cables that run down to the deck from one or more towers. Ironically, the post-construction behavior of these bridges has been eerily reminiscent of suspension bridges in the 1930s, except that the issue is cable oscillation rather than movement of the roadway. In short, the cables are demonstrating an alarming tendency to vibrate, behaving much like a garden hose does when you lay it on the ground and “whip it.”

“The largest cable-stayed bridge is Tartara in Japan,” says Ketchum. “If you stand at the top of its towers you can see these traveling waves coming up and down and you feel this thump as the energy pulse hits the tower.” Ketchum has also visited the cable-stayed Rama IX bridge in Bangkok and reports: “It shakes constantly at 10 to 15g vertical acceleration, which is kind of like what the 1989 [San Francisco] earthquake was except it’s doing this all the time.” Recently, when Ketchum drove out onto Rama IX he found that his car was bottoming and topping its suspension. “Bang! Bang! Bang! It was shaking that hard,” he recalls.

Henry Petroski, professor of civil engineering at Duke University and author of “Engineers of Dreams” and “To Engineer is Human” concurs that cable-stayed bridges are demonstrating behavior not anticipated and being built longer and more slender than is wise—the same ingredients that contributed to the Tacoma Narrows disaster. “There are these warning signs,” he says. “There’s a long history of bridges that have failed, and they generally fall in 30-year cycles. We’re about due . . . and cable-stayed bridges bear careful watching.”

Ketchum relates the cycle to alcoholism skipping generations. “My mom’s old saying is that if you see your parent as an alcoholic you won’t become one yourself because you see how nasty it is. It skips that generation but then the next generation doesn’t see the danger so they fall into that trap. There hasn’t been a horrible collapse for a while and maybe its skipping generations of bridge design,” he concludes.

Page 3 of 4 pages « First < 1 2 3 4 > Last »