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The Tacoma Narrows bridge collapsed.



There's a video clip at this link:

http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/

This is a perfect example of "forgetting the little things".

In this case the little thing is called: "Sympathetic Vibration." It's the same principle that allows a singer to shatter a champagne glass with their voice. Have you ever blown across the top of a half-full coke bottle? That tone you hear is the sympathetic fequency of that bottle. If you supply a sound wave at a great enough amplitude at that frequency, the bottle will also shatter. (Although it will take more power than a champagne class.)

So here's a great physics lesson, the length of the bridge was exactly equal to the length of the sympathetic sound wave.

As the wind blew down the canyon, the sound of the wind began to pile up energy on the bridge at it's sumpathetic frequency.

You can see the bridge actually turning itself into a sound wave!

Wow thats amazing. Should it have had more supports to stop the wind moving it like that? or would it have cpollapsed anyway?

there were a few things that could have been done:

1. Make the bridge shorter. Now that wouldn't be a real solution, would it?

2. Make the bridge longer. That could have been done, but would have been costly.

3. Add mass to the bridge to change it's resonant frequency. That would have been the best way to do it. Just some weights, or baffles on the bottom to change the frequency at which it vibrated, so that it didn't match the frequency of the sound of the wind in the canyon.

Jives wrote:

3. Add mass to the bridge to change it's resonant frequency. That would have been the best way to do it. Just some weights, or baffles on the bottom to change the frequency at which it vibrated, so that it didn't match the frequency of the sound of the wind in the canyon.


Does this mean that the frequency of the sound of the wind in the canyon is always the same?

At a certain MPH velocity, yes, the sound of the wind blowing across the bridge is always the same.

Worse yet, this movie was taken on the day of dedication for the bridge, the wind was at just the right velocity to generate this show.

How embarrassing for the engineers.

BTW this movie is now shown in every engineering school when teaching resonant frequency.

Engineering is cool.

Far Rider wrote: I vaguley remember the army testing directional sound weapons. I'll have to look and see what I find. Very interesting Jives. Thanks for the explaination.


I heard about that too. I think they tried it out on mythbusters, to find if the mythical "brown note" actually exists. Unfortunatly, or perhaps not, the experiemnt was a failure.