Forget steel, we all know it can bend like crazy. What really amazes me is how the actual ****ing concrete waved all over the place on the Tacoma bridge without cracking. Yeah, that's a classic vid.
Incidentally, the bridge offered a wide (tall) target and a not very aero shape for the wind to attack and it was ridiculously narrow for its length AFAIK. So it was poor design + really bad luck.