Failure™ Magazine

Suspended Animation

The collapse of the Tacoma Narrows Bridge.

Written by Jason Zasky   Filed under History, Science & Technology

On November 7, 1940, with the wind blowing steadily at 40-42 mph, one of the extra stiffening cables that had been added to the Tacoma Narrows suddenly came loose and the roadway began to twist back and forth in increasingly violent fashion. The bridge was quickly closed to traffic leaving newspaper editor Leonard Coatsworth, his cocker spaniel Tubby, and an investigating University of Washington engineering professor named F.B. Farquharson on the bridge in its final heaving moments. After Coatsworth lost control of his car he was forced to abandon it with Tubby still inside. At this point, the story becomes the stuff of legend. “I heard two versions back when I was in steel design class,” says Brian McDonald, principal engineer at Exponent Failure Analysis Associates in Menlo Park, California. “In one the professor saved the dog. In the other, when he opened the car door the dog bit him and he came back [without the pooch].”

Moments after the two men crawled off the bridge deck to the relative safety of the toll plaza, a 600-foot section of the center span gave way, plunging upside down into Puget Sound where it lies today as a sort of artificial reef. The side spans (the sections of roadway on the outside of the two towers) held but sagged dramatically in the aftermath, while the steel towers were so disfigured that they had to be removed before a replacement bridge could be built. In 1992, the sunken remains were placed on the National Register of Historic Places to protect them from souvenir seekers.
The collapse of the Tacoma Narrows provided the impetus for civil and structural engineers to begin incorporating aerodynamics into bridge design. “There are now design equations that engineers can use to predict the wind loads on structures,” says McDonald. “If it’s a major project or a unique or exotic structure, they’ll build a scale model that includes the surrounding terrain, then blow wind across it and measure.”

The replacement version of the Tacoma Narrows was unveiled in 1950 and utilizes a potpourri of different design techniques that help control wind behavior. “There’s very little daring about it and you honestly can’t blame them,” says Mark Ketchum, vice president of San Francisco-based bridge engineering firm OPAC. “They were pretty conservative the second time around.” And while its 50-year lifespan has been relatively uneventful, a major incident occurred during its construction when a moderate-sized earthquake flung one of the newly mounted (but as yet unbolted saddles) off the top of one tower, punching a hole in and sinking a barge in the water below. Currently, an architecturally similar bridge is being designed to run parallel to the existing Tacoma Narrows and is scheduled for completion in 2005.

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