1/N In honor of the recent 80th anniversary of the collapse of the Tacoma Narrows Bridge (Nov 7, 1940), a thread on why it fell

The TNB was a giant wind instrument, like a clarinet. A constant wind was converted into large amplitude oscillations at the natural frequency

2/N How did this happen? The wind created a feedback mechanism where any oscillation would grow exponentially, like when you bring two microphones too close together.

Vortices formed at the front of the bridge and drifted across. At high enough speeds, the vortex does work.

3/N These vortices are formed in part because of the shape of the bridge. The H-shaped cross section made it easy to form the vortices. As the bridge oscillates, the vortex then drifts across.

At low wind speeds, the vortex drifts too slowly and does no work (averages out)

4/N At higher wind speed, the vortices drift faster and get to the midway point before it starts oscillating back down. At this point, the vortex does work and causes the oscillation to get bigger (feedback)

5/N In one of the original videos, the cement breaks up and you can even see one of these vortices drift over the bridge

6/N These vortices are not to be confused with the von Karman vortex street. These form off the back of the bridge and have a frequency unrelated to the natural frequency (much higher frequency in the simulations above)

7/N This is also not an example of resonance. Resonance only produces large oscillations when an external force is tuned very precisely to the natural frequency. The constant wind doesn't provide such a force.

For details see the paper: https://aapt.scitation.org/doi/abs/10.1119/1.2201854