or... (taking a few guesses here) the fire melted the asphalt base (looks like asphalt to me) which in turn gave up its structural integrity, which added dead weight to the structure, which also took away some of the steels heat protection, after the steel got hot its structural integrity started to fade (a property that of metals that some seem to neglect, they dont have to melt completely to fail, a little heat will make them maleable), once the steels structural integrity started to go, it was incapable of supporting the dead load on the interchange, which lead to its ultimate failure.
metals dont have to melt in order to fail //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif . when heat is added they can become pliable, they can stretch, lose their shape, and completely lose all of their expansion/compression stability.
just a guess, dont know if this was mentioned in the article or if anyone else had mentioned it. just some food for thought. and no, im not debating 911 at all on this one, just something else to consider in this case //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif