You need to think of the physics of gravity, Vulcan.
We see gravity as it normally appears to us, we drop something, it falls. Cause, then effect. The fact is gravity is is a force upon objects at all times. Thing of Superman pushing against a car's bumper. The tires are spinning and burning rubber, but the car isn't going to move. The split second that Superman lets go, the car will speed off in the direction it's facing.
OK, now for the next little bit. Have you ever tried to hammer in a nail by just resting the hammer on the head of the nail, then pushing down with all your weight? Of course the nail isn't going to go anywhere. But hit the nail from a few inches abouve, and it will go down.
So now, take the force of gravity on the top of the building, and have one floor pancake. With ~20 floors above, you are talking about 100s of tonnes of force that has been pushing down for 30 years; waiting for a weakness to allow it to go down. Using my nail analogy, it would be like hitting that nail with a 500 pound sledge hammer.
As for the heat- keep in mind the heat was not so intense to melt the steel. It was just simply hot enough to weaken the steel. We all know steel workers heat steel to bend it. That's because when it gets hot it weakens.
And there's proof I watch entirely too much Discovery Channel.
