You need to understand that jet fuel does not burn at a temperature that can melt steel.
http://en.wikipedia.org/wiki/Jet_fuel
Steel weakens at about 1000 degrees and melts at about 2000 degrees. Even if the steel was weakened, it would not cause a footprint collapse of a skyscraper at near freefall speed. It would more likely twist or topple. Even so, the steel would take longer than one hour to weaken or melt from a jet fuel fire.
The bulk of the explosion occurred outside of the tower during impact. Firefighters reported the fire in the building was under control. The survivors in the tower were standing in the hole where the plane hit, waiting to be rescued. Yet the fuel would have to be hot enough to melt steel and continue burning in order to destroy the other floors and cause it to collapse without any resistance from those floors. If the fire had weakened the steel and simply died down before collapse, then the resistance of the floors below the impact would have slowed down the collapse, taking much longer to fall, and likely toppling to one side as the rubble from each floor piled underneath.
Instead, you have to believe the tower can collapse as if all floors below the impact simply weren't there. Jet fuel cannot melt through and destroy 100+ floors leaving molten steel at the bottom. If the floors naturally collapsed, they would provide resistance, and yet they didn.t
Bomb charges would cause large explosions to erupt if they were meant to destroy the tower. But more likely, they were used to set off thermite. Thermite has the ability to burn at over 2000 degrees and can easily melt through steel and any number of floors, as it goes straight down and melts through everything in its path.
Learn how a thermite reaction works.
http://en.wikipedia.org/wiki/Thermite