If the Bridge is a Rockin... RUUUUN!

Moving this video to lucky760's personal queue. It failed to receive enough votes to get sifted up to the front page within 3 days.

rottenseed

says...

Could be resonance from foot traffic?

charliem says...

Pedestrian traffic is too erratic to cause resonance on a bridge.

You have to also consider those that are not inputting energy on the same harmonic frequency as the bridge, are actually creating a damping effect.

Mythbusters (and I cant beleive im citing that show as evidence here) tried the myth and failed. The only reason they failed however, was because they did not take into consideration the harmonic frequency. In order for resonance to occur, their needs to be an input of energy into the system as it reaches the min/max of the swing, any energy input outside of that ideal, very short range, acts as a damper.

oileanach

says...

This is a relatively new pedestrian bridge in London, and the bridge was closed and modified to eliminate the problem. Note that wobbling isn't the same as a structural failure. It's quite possible that it would wobble for years with no catastrophic failure, or that it would go out of control and tear itself apart. Either the builders didn't feel lucky, or they got tired of wiping up all the vomit.

While charliem is correct that you need force applied at a resonant frequency (similar to the way pumping one's legs at the proper moment allows one to make a swing go higher, but pumping at a different moment has the opposite effect), it's interesting that in this case this lateral wobble itself actually encourages people to shift their weight, thereby exacerbating the problem.