I imagine that the story of the Tacoma Narrows bridge will be familiar to most people. For those that aren’t, the bridge was a suspension bridge in Washington State built in the late 30’s. On the 7th of November, 1940, it began to be shaken by strong gusting winds. Most disturbingly it began to resonate with the winds, in several modes including tortionally, and increasingly violently.
The situation was described by Leonard Coatsworth who drove onto the bridge.
Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car… I jammed on the brakes and got out, only to be thrown onto my face against the curb… Around me I could hear concrete cracking… The car itself began to slide from side to side of the roadway.
On hands and knees most of the time, I crawled 500 yards [457 m] or more to the towers… My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb… Toward the last, I risked rising to my feet and running a few yards at a time… Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows.
The story is a cautionary tale for engineers, and a classic example of the power of resonance on a structure. Fortunately, other than Tubby the Dog stranded in one of the cars, no lives were lost. If you want to know more read the wikipedia entry.
The pictures and video of the event are always worth a look – the reason for the post! I really can’t see this enough times.
The most destructive tortional mode, can be clearly seen. There is a story about a physicist walking out along the center of the bridge quite freely during it to prove a point about the centre being the node of vibration, and hence stationary.
Finally we have the destruction.
Fortunately the bridge took some time to blow itself apart so we have a movie of the event. Even this small sample is worth a look.
Update: The longer video.