The Tacoma Narrows Bridge opened to traffic on 1 July 1940. It quickly earned its nickname “Galloping Gertie” from its rolling, undulating movements.
The bridge had been built with the aim of creating an elegant and artistic bridge and its fragile structure made it very vulnerable to the wind. Motorists crossing the bridge sometimes felt as if they were riding a giant rollercoaster, watching the cars ahead disappear completely for a few moments as if they had been swallowed up by a huge wave.
The structure of the bridge caught the wind instead of allowing it to pass through. On 7 November 1940, strong winds caused the bridge to undulate.
The motion grew stronger and stronger and as the bridge began to vibrate torsionally, it started to move with a twisting motion. The wind-induced vibrations resonated with the natural frequency of the bridge, increasing the twisting motions and finally causing it to collapse. This happened just four months after its opening.
Eyewitnesses have reported how the bridge swayed violently from side to side and how they heard the sound of concrete cracking, as they desperately crawled on their hands and knees towards the bank.
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Links and references:
See a mathematical model on the collapse:
A film on the twisting bridge: