[Legbreaker]

While I agree that the supporting structure are the preferred elements to attack, I rather doubt anything much less than a truckload of explosives is going to do the job without may hours of preparation (boring holes, etc).
I would also be rather hesitant to plan for just damaging the bridge and hoping the weight of subsequent vehicles bring it down. Chances are that unless very lucky or the calculations and application of charges is spot on, not enough will be inflicted or the damage will be so great even a blind man would be able to spot how unsafe the structure is.
The aim really should be to drop as much of the bridge into the river as possible whilst leaving as little as possible for the following enemy engineers to work with. Ideally as has been previously mentioned, even starting repairs should need significant effort to simply clear the debris.
With only the explosives a small team of SF troops are able to carry (remembering they're probably already loaded down with 40+ kgs of weapons, ammo and other vital equipment) the best that could be hoped for is cratering of the decking, and damaging supporting beams. With luck/planning, some beams mad be dropped across the decking creating more work for the engineers (both clearing and ensuring the structure is still able to carry tanks, etc).
The more debris that can be created, even without significant damage to the structural components the better - more locations to place booby traps and anti-handling charges.
While obviously not ideal, slowing the enemy by causing them to spend time on the bridge may be enough.

Unfortunately one has to remember that modern armies possess a much greater ability to cross water obstacles than even just a few decades ago. Many armoured vehicles are either capable of swimming, or in the case of some heavier AFVs able to use snorkels. A dropped, or even badly damaged bridge may not hold up the enemy advance for long....