“How the hell do you burn down an overpass or elevated highway? It’s concrete and steel. There’s not much there to burn.”
The sustained high temperature affects the steel yield point. IOW, it weakens the steel I-beams and trusses that support the huge concrete load that makes the road part in the bridge span.
Case in point, the WTC collapse in 911 was more of jet fuel burning that weakened the I-beams and subsequently, the pancaking.
That's one reason I hate to drive on overpasses or underpasses. Or go through tunnels...but that's another story.