The Bridge Twisted Apart
On November 7, 1940, Washington State’s Tacoma Narrows Bridge began twisting violently in winds of roughly 40 mph. Sections of roadway eventually tore loose and plunged into Puget Sound. The spectacular collapse became one of engineering’s most famous disasters, and its fundamental cause changed the way engineers thought about suspension bridges.
It Was Already Moving
The bridge had opened only four months earlier, on July 1, but its strange behavior wasn’t entirely unexpected. Workers and motorists had already watched the roadway rise and fall in relatively modest winds, earning it the nickname “Galloping Gertie.” Engineers attempted several measures to control the movement, but none solved the underlying aerodynamic problem.
Screenshot of video of Tacoma Narrows Bridge collapse (modified); Public domain, Wikimedia Commons
Eight Feet Made Trouble
One seemingly small design decision proved enormously important. An earlier proposal by engineer Clark Eldridge called for a traditional 25-foot-deep stiffening truss beneath the roadway. The final design associated with engineer Leon Moisseiff instead used sleek, solid plate girders only eight feet deep. That made the bridge lighter, slimmer, and much more flexible.
The solid girders created another problem. Rather than allowing air to flow freely through an open truss, the eight-foot steel sides presented broad surfaces to the wind. Investigators later concluded that the deck and solid plate girders interacted with the wind aerodynamically, producing lift and contributing to the bridge’s dangerous instability.
The Wind Found Weakness
November 7 was windy, but the bridge wasn’t simply blown over by an extraordinary storm. Its vulnerability was more complicated. The long, narrow roadway possessed relatively little resistance to twisting, and aerodynamic forces could feed energy into its motion. That morning, its familiar vertical movement eventually transformed into something much more destructive…