Wow! Quite a response!
I think that if you only wanted to stop traffic over the bridge, you could do it with less than a truck-full of explosives. I bet a car-full of t reaction-causing material (sorry if this is too oblique - I feel guilty posting this much, though) might compromise the roadway sufficiently to require months of inspection and repair work. Of course, it would not cause a big death-toll, which seems to be a requirement. An engineer who knows something about bridges could comment about the feasibility