Do not completely write off the possibility of taking out the bridge. Certainly a typical IED or truck bomb would not destroy the bridge but the terrorists are able to think along big lines.
Giant sausage rolls of explosives places in the right place, blown, reapplied, and blown again could eat through nearly anything. The FMLN terrorists in El Salvador took out both main suspension bridges in that country. Of course the bridges were much smaller but they were destroyed with modest charges applied to just the right places.
Even terrorists who cheated their way through sophomore courses can still read an explosive charge chart.
You don't have to destroy the WHOLE BRIDGE, just one section and the rest will take care of itself.
Thermite (high school chemistry) in the right place....