Cobra Effect · Engineering failures
The bridge that twisted itself apart
How a steady wind twisted a brand new bridge until it fell.
7 cards, read aloud in 2:45, with a test and sources.
In July 1940, a slender new suspension bridge opened across the Tacoma Narrows.
The Narrows is a strait of Puget Sound, in Washington State. Its main span stretched more than eight hundred metres, but its deck was narrow and very shallow. The final design, from the consultant Leon Moisseiff, was lighter and slimmer than the state’s own earlier plan. Slim bridges looked elegant, and cost less to build.
Even before it opened, its deck rose and fell in waves in moderate winds.
People nicknamed it Galloping Gertie. Engineers studied the motion and tried several fixes. None of them stopped it.
On a November morning in 1940, a strong, steady wind blew through the Narrows.
It reached more than forty miles an hour. At around ten o’clock, a cable band near the middle of the span slipped. The motion changed. Instead of rising and falling, the deck began to twist.
A newspaperman named Leonard Coatsworth abandoned his car on the bridge.
His daughter’s dog, Tubby, was still inside. A university engineer, Frederick Farquharson, tried to rescue the dog, but the frightened animal bit him. Just after eleven, the centre of the span tore away and fell into Puget Sound, taking the car with it. Tubby was the only life lost.
Cameras were rolling, and the film became one of the most famous in engineering.
Most of the footage was shot by Barney Elliott and his colleagues from a Tacoma camera shop. Farquharson, who had been studying the bridge for months, filmed it too. Generations of students have watched it since.
Textbooks often call it resonance, but that explanation is misleading.
Resonance needs a push that repeats in time with a structure’s natural rhythm, and a steady wind has no such beat. Instead, the twisting deck changed how the wind flowed around it, so each twist drew more energy from the wind. Engineers call this flutter. The motion fed itself.
So when a design looks elegant, ask what forces it has never been tested against.
The replacement bridge, opened in 1950, was wider and far stiffer. After Tacoma, engineers began testing models of bridge decks in wind tunnels. A slim design is not a flaw in itself, but how it behaves in wind must be understood.
Sources
- The collapse of Galloping Gertie, Washington State Department of Transportation. A minute by minute account of the day the bridge fell.
- Lessons from the failure, Washington State Department of Transportation. What engineers learned about wind and bridge design.
- Tacoma Narrows Bridge failure, Tom Irvine, Vibrationdata. A short engineering note on why resonance does not fit.
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