Fire might be the rarest cause of bridge collapses, but fire has brought a few bridges down in the past. In fact, it used to happen much more often, when bridges were made out of wood. Train bridges were especially susceptible to fire, because the steel wheels of the train on the steel rails of the track frequently sent sparks shooting onto the bridge. If it was very dry or the wind fanned the sparks, the bridge could catch fire and burn completely down [source: Letchworth].
Bridge fires aren't a thing of the distant past, however. Several modern bridges have also collapsed or been severely damaged due to fire. The cause is typically the crash of a tanker truck carrying a large amount of a highly flammable substance like gasoline. The crash triggers an explosion and a blaze so intense it melts the steel used to build the bridge. Eventually, the softened steel can no longer hold up the structure, and the bridge falls.
This is exactly what happened in 2009 when a tanker truck on I-75 near Detroit suddenly burst into flames directly under a bridge. The resulting inferno destroyed the bridge completely and forced the closing of I-75. Amazingly, no one was killed [source: Guthrie].