Any man made structure can be brought down but to bring down the Big Mac would require a mighty big explosion. I would be more inclined to think another of a jetliner attack severing cables or a ship passing near the towers than vehicles crossing the bridge.
I am no engineer, nor am I educated with regard to explosives, bombs, etc. It's just that those losers seem to get lucky sometimes. Bottom line is a lot of people could die, even if the bridge isn't totally destroyed.
So even if the intent is not to bring down the bridge, a few detonations at various points on the bridge while it is filled with 60,000 or so people will kill a few hundred people outright, BUT, it will create a stampede of panic in a semi-confined area that would cause at least 10 times the deaths and injury. In a large crowd like the picture posted elsewhere in this thread even 10 explosions spread across the bridge would cause the herd mentality to kick in and the people trampled to death, pushed off of the bridge and injured in the confusion of which way to run would be substantial.