Science Busts The Biggest Myth Ever About Why Bridges Collapse

Ethan Siegel in Forbes:

The collapse of the Tacoma Narrows Bridge on the morning of November 7, 1940, is the most iconic example of a spectacular bridge failure in modern times. As the third largest suspension bridge in the world, behind only the George Washington and Golden Gate bridges, it connected Tacoma to the entire Kitsap Peninsula in Puget Sound, and opened to the public on July 1st, 1940. Just four months later, under the right wind conditions, the bridge was driven at its resonant frequency, causing it to oscillate and twist uncontrollably. After undulating for over an hour, the middle section collapsed, and the bridge was destroyed. It was a testimony to the power of resonance, and has been used as a classic example in physics and engineering classes across the country ever since. Unfortunately, the story is a complete myth.

Every physical system or object has a frequency that's naturally inherent to it: its resonant frequency. A swing, for example, has a certain frequency you can drive it at; as a child you learn to pump yourself in time with the swing. Pump too slowly or too quickly, and you'll never build up speed, but if you pump at just the right rate, you can swing as high as your muscles will take you. Resonant frequencies can also be disastrous if you build up too much vibrational energy in a system that can't handle it, which is how sound alone at just the right pitch is capable of causing a wine glass to shatter.

It makes sense, looking at what happened to the bridge, that resonance would be the culprit. And that's the easiest pitfall in science: when you come up with an explanation that's simple, compelling, and appears obvious. Because in this case, it's completely wrong.

More here.