It's not really about melting, it more that sufficient heat made the beams unable to do the job of holding up a building which creaked in high winds. After a failure close to the aircraft impact, floor after floor would pancake. That being said Building 7 cannot be explained that way. It's loss was perhaps opportune given its content.
The twin towers were designed to creak, or sway slightly, in high winds. All buildings move, to some extent, depending on temperature mostly, hence the need for expansion joints to accommodate this, especially in larger buildings.
There were 100,000 tonnes of structural steel in each of those towers. The fires from the burning jet fuel were oxygen starved, hence the volumes of smoke. This meant that they were burning at low temperatures. Steel doesn't even weaken at such temperatures.
The impact of the planes, would have had no effect whatsoever of the strucural integrity of those buildings. The planes were basically hollow tubes of alumimium, a much softer metal than steel. Whatever effect they had at the impact points would have been contained by the sheer force of the remaining steel.
But let's say that the planes, by some miracle, did indeed cause the top floors of the towers to collapse. For every action there is an equal and opposite reaction, isn't there? If, say, the top 20 (or so) floors collapsed downwards, this would create a downward force on the floors below. But as Newton pointed out, the downward force exterted by the falling floors would be met by the upward resisting force of the 80 (or so) floors below.
The only way the towers could have collapsed at free fall speed is if the upward resisting force of the lower floors, which were most of both towers, was removed. And the only way this could be achieved was by controlled demolition.
The Architects and Engineers for 911 Truth video explains all this.
