Why Galloping Gertie collapsed
The Tacoma Narrows suspension bridge in Washington State was the third longest of its kind when completed. Despite unusual levels of motion during construction, “Galloping Gertie” opened to the public 1940. In November, it began to heave violently as a lone driver, Leonard Coatsworth, drove across it with his cocker spaniel. Unable to continue, and unable to remove “Tubby” from the car, Coatsworth fled on foot. Several unsuccessful attempts were made to rescue Tubby, but eventually the dog, car, and bridge fell into Puget Sound.
The Washington State Highway Department determined that the collapse was not the result of rhythmic wind gusts “exciting” the structure’s natural resonance, as is often argued. Rather, the disaster began with aeroelastic flutter (a vibrational response to air movement), leading to torsional flutter (repetitive twisting). The 2,800' long, 39' wide main span was inherently wind-vulnerable, as its main girders were made of solid steel plates that were only 8' deep. By comparison, an earlier proposal for the bridge used a 25' deep, open-web stiffening truss.
Ten years after the collapse, a replacement bridge was built. “Sturdy Gertie” incorporated the original approach ramps and main piers, but used a 33' deep stiffening truss.
