Idle Nutt

I first saw video of the Tacoma Narrows Bridge collapse in college and to this day I’m still amazed by it. You can really see the waves starting at about the 0:25 mark.

A common explanation given in introductory physics classes for the collapse is resonance or forced resonance - that the frequency of the wind pressure coincided with the natural frequency of the bridge so that the energy kept building up until the bridge couldn’t handle it anymore. (A more common example of forced resonance is when you push someone on a swing - when your pushing frequency coincides with the swing’s moving frequency, the swing moves higher and higher.)

But the real explanation is a lot more complicated than that. Forced resonance from wind pressure probably had little to do with the collapse because the wind’s rhythms are random and unlikely to be regular enough to cause forced resonance. Instead, the forced resonance probably came from something called vortex shedding (vortex shedding is why a sheet of paper flutters as it falls to the ground, or why traffic lights hanging on cables bob vertically when the wind passes over them horizontally).

Another cause is something called self-excitation or negative damping. Basically, the twisting frequency from the bridge itself weakened it further (instead of an external force from the wind) because it coincided with the bridge’s natural torsional frequency.

If you’re comfortable with technical papers, you can read more about the theories for the collapse here, here, and here.

Wow. This is probably my nerdiest post yet. ;)