CSG - lets you create craters and cracks on-the-fly.
To get it running with good performance might take some tweaking.
But if you manage to pre-damage your bridge and save it, performance should be okay.
Just save versions: bridge_100_percent.j3o, bridge_75.j3o, bridge_50.j3o, bridge_25.j3o
(just some idea, not implemented or tested yet, but should work - in threory)