Didn’t we have some OWS types plotting to take down a bridge in Ohio? And a few years back there was a group called Green Anarchy promoting the destruction of bridges and other infrastructure.
It would not be hard to mess with bridges - if not bring them down, at least drive a whole bunch of trucks and cars on them, douse with gasoline, and set them afire. Even I, a non-terrorist, can think up stuff like that.