In reply to this comment by oileanach:
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.

That, my good man, is called a positive feedback system.
Just like in lectures when the dude at the front is trying to talk over everyone else, and everyone else in turn is trying to talk over him, shit just gets louder and louder and louder etc. till finally its no point anymore.

Ohh yeah, im slightlyloldrunk atm

Got it

In reply to this comment by MarineGunrock:
Don't do it - That's considered self linking and is a bannable offense.

In reply to this comment by charliem:
So tempting to post video's of my kitten.