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If memory serves me right (from a History channel/TLC show on the subject), the I-beams have such a high melting point because they were/are insulated. During the crash, the isulation was blown off/sheered off the beams on the levels where the plane impacted the tower. The design of the twin towers was such that all the support for the structure was/were the outside beams that went vertically along the side of the building. This allowed for the central core (stairs/elevators/piping) to be minimal since there was no need to have any real structural support there.
When the plane plowed into the building it immediately blew out the support on one side (the side it entered from). The floors were all spot welded I think to the exterior columns. It was these welds/joints that failed. And as they failed one by one they dropped to the level below (as they were still firm on the other side that was attached to the core. Anyhow, as long as the fire raged, enough beams were weakened to allow movement (thats ALOT of weight now being supported by softening metal) and when failure started it was unstoppable.
The only way to have kept the towers from falling were to have the sprinkler systems active which was impossible because the central core had been damaged and all pipes/elevators/stairways at and above the level of impact were either broken or severly damaged.
I really wish I could remember the name of the show that I saw. It was extremely thourgh in its analysis of what caused the towers to collapse.
I hope that in some way helps. If I can find anything more (name of the show and or the channel it was on) I will be back to post more.
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