a couple things.
1. think of a chain. you don't break the strongest link when you break a chain, you break the weakest point. pretty similar to buildings, and probably bridges. It probably wasn't a big structural member failing. I would look at the little pieces holding the big pieces together.
2. most of the "visible damage" you are seeing in the road probably came from the collapse rather than the fire.
3. IMO what likely happened, its what happened down here in Atlanta, is the heat from the fire causes the section to expand, that pushes against the weak points holding it together on either end. those points were also weakened by the fire. It would not take much thermal expansion to create some insane pressures. and there is probably enough expansion from that fire to override whatever built in movement they have. so those weak points at either end, or one end, fail, can't hold the bridge in place anymore, and the thermal expansion eventually pushes it out beyond whatever was physically holding it up, and the section falls. that likely drags down the other end, and explains the crack in the middle with one end giving out first, hitting the ground, the force of the section leaning on that one side found a weak point in the middle, cracked it, and as it gave away again, from the middle, it likely pulled the other end out.
