[Looking for Diogenes]

The momentum of the plane (velocity and mass) was so great that if it collapsed enough of the columns at the level it hit or at a lower level, the weight of the structure above is such that once it started falling on the collapsed level, its inertia and weight would have collapsed anything beneath it. That's one problem I have with these forums, there's too much uninformed speculation out there, and then others repeat this as fact.

I'm no engineer, but the source for the article is Gregory Fenves, a professor of Civil Engineering at the University of California at Berkeley.

The collapse mechanism you suggest does not account for the hour or so that passed between collision and collapse.

[aquawrench]

Im guessing that the heat from the uncontrolled fire weakend the structure to the point of failure. The jet impact probably took out the sprinkler system by causing a massive pressure loss when the mains burst and poured out water in an inefficient manner.

[Maelstrom]

When the heat of jet fuel changes the crystalline structure of the steel, this is exactly what happens.

[yooper]

"The collapse mechanism you suggest does not account for the hour or so that passed between collision and collapse."

The hour which passed was the time needed for the steel members to heat up and transfer heat to other steel members and flammables. Once the crash level floors lost their structural integrity, the building was doomed to cascade failure.