Weakened metal due to the impact, weakened further due to the heat, wasn't necessarily melted, but softer, the floors above weight causes the supports to give, all that weight is then suddenly dropped onto the floor beneath it and so on, it's sort of like having a human pyramid, and then one level of people just give, and the sudden weight put on the level below causes it to collapse as well.

Where in my comment did I say that the steel did melt?
I just said that jetfuel burns at higher temperatures.
You must feel real proud of yourself for telling me something I know