Whether the airliners alone caused the collapse or whether it was helped along by explosives, I think the obvious result proves that a high-rise structure can collapse upon itself. If you watch the videos, the failure happened up high, around where the planes hit. The buildings failed from the top down, all the way to the ground.
Even if you postulate the existence of explosives, I don't think al Qaeda managed to hide explosives in each and every floor of the WTC, nor could they sequence the explosions to make it appear that the buildings fell down from their own weight. Whatever happened to start the failure, it continued on its own due to...gravity.
exactly. The planes took out several of the lessor supports immediately on impact, the heat from the fire weakened the remaining pillars. Lets not forget how buildings are built. Pillars are arranged in a way which uses each slab to dissapate the load from the level above it, that way pillars don't have the weight of the entire building on them. It's kind of hard to explain, but that's about as simple as I can without diagrams.
Anyways, once those first main remaining supports heated, they collapsed, bringing the entire weight of the 20 or so floors above it onto the floor below. Millions of tons falling 10 feet onto a design meant only to hold the weight of one floor above. The rest was chain reaction all the way down.
Air gets compressed as this happens, which blew out windows, making it look like little explosions just before the floors above pancaked into the next one below, and so on.