[f5fstop]

I should also mention, the expert I saw a TV answered the question on temps of burning jet fuel, max of around 800 degrees, versus temp to melt steel, around 1600 degrees.
The towers had numerous tunnels underneath. The fires drew in colder air from the bottom tunnels, thus causing a bellows affect, creating temps higher than 800 degrees.

In addition, he made mention that it does not take long for metal to become brittle at temps above 400 degrees, and at around 800 the process is faster. So when the floors that were hit burned for a while, the metal became brittle, collapsing the floors above and that massive weight, started the domino affect.