How did resonance destroy the Tacoma Narrows Bridge in Washington State?

The Tacoma Narrows Bridge, or “Galloping Gertie” as it was often called, was built in 1940 and was known for its unusual, undulating movement. All bridges vibrate to some extent, but to many motorists, the suspension bridge in Tacoma felt more like an amusement park ride than a bridge.

On the morning of November 7, 1940, four months after the bridge opened, the wind was blowing at approximately 42 miles per hour. This moderate wind hit the solid bridge deck and caused the deck to vibrate back and forth as it did almost every day since the bridge had opened. But the bridge began to vibrate more dramatically than ever before. It appeared as though a standing wave had formed between the two towers of the bridge. There was one distinct node in the center of the bridge and an anti-node on each side of the node. After several hours of dramatic vibrations, the bridge deck collapsed into the river below, along with its only casualty, a dog named “Tubby,” left in a car by its owner, who narrowly escaped death himself.

There is still an active controversy about the exact cause of the collapse of the bridge. Was it the steady wind, or variable winds? What factor did the solid deck and the solid, high sides to the deck play? Was the deck just not stiff enough? Suffice it to say that more recently built bridges use perforated decks and lower, open sides.


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