jet fuel doesn't need to melt steel beams when an airplane takes out more than half the core supports
also one tower leaned as it fell
the towers were actually designed to stay standing even with a hole in the side.
most of the load bearing part of the structure was on the outside and connected to pillars nearer to the inside of the building
if you look at the history behind the buildings the engineers actually took into account the chance that a plane would hit the buildings and desinged them to withstand it.
jet fuel can weaken it tho
also the way they fell was because a loving airliner hit the building, taking out supports and causing the floors to eventually collapse and the weight of the floors on top of the next fell and then the next and so on
most of the jet fuel on each plane burned away immediatley
its the huge fireball you see during each impact.
the small amount of fuel left wouldnt be enough to burn for several hours and cover a large enough area to weaken that much steel
the arguement could be made that other combustibles ie paper, drywall, desks etc continued to burn, but the smoke was black, meaning that it was oxygen starved. fires deeper inside the building would be very unlikely to burn hot enough or large enough to do any serious damage
