UC Davis Professor Mark Rashid says highway construction wasn’t the problem after a tanker truck carrying a load of gasoline overturned and erupted in a huge fire yesterday:

With temperatures around 2000 degrees, basically any normal structural material is going to lose its load carrying capacity.
 
He says that’s exactly what happened to the steel supports at the MacArthur Maze near the Bay Bridge.  They softened, causing about 250 feet of the freeway above to collapse.  Rashid says Caltrans could have encased the supports in concrete, which transmits heat more slowly than steel, but the cost would be prohibitive.  He also says eventually the fire’s heat would have made it through to the steel bolsters anyway.