LegalEagle is correct about the weight of the debris demolishing the lower floors.
Weight of debris? Where did the debris come from? You have to first blow the truss to pieces to attain the debris. And how exactly could debris fall on top of the center columns when they ran continuous top to bottom?
The construction of the WTC was part of the reason why it collapsed in the way that it did. Each floor was designed to only hold the weight of whatever was on that floor and that load was then transferred to the outside structure which was responsible for holding up the entire building. The impact of the planes destroyed both the integrity of the outer structure and the floor supports. They also knocked the remaining floor supports out of alignment. The fire only had to heat the outer structure to the point where it started to bend under the weight of the upper floors. Since the bending could not go inwards it had to go outwards instead. That bending pulled the remaining floor supports out and that floor then collapsed onto the floor below which wasn't strong enough to support 2 floors which then collapsed onto the floor below. Essentially it was like vertically stacked dominoes from that point onwards.
The construction of the towers was a vertical truss. The strongest central cores ever built by mankind was connected by heavy steel beams, the kind found only on maintenance floors in regular skyscrapers (to hold up the heavy elevator equipment) to walls constructed with heavy steel tubes. All three components far exceeded normal skyscraper design. The truss is itself a support structure, we hold bridges up with trusses, we add trusses to weak designs to strengthen them. The WTC towers were ridiculously strong, the architect wanted to provide the first office towers free of the connecting beams in a normal skyscraper design; all the floors at the World Trade Center were completely open space from the center elevators to the windows. This was achieved by building a vertical truss; every one of those floors were, in fact, stronger than the a-typical maintenance floor located at mid-height and roof levels of normal skyscrapers.
The impact of the jets punched holes in the curtain wall, and this has been likened to sticking a pencil through a screen, you have to punch a lot of holes before you destroy the " integrity of the outer structure". Neither did the jet impacts collapse the floors. There's pictures of people hanging out the gashes of those jet impacts; obviously the floor leading to the holes in the curtain wall had to be intact.
Tower 2 fire was not a hot fire. There has been hotter fires in weaker structures, and yet, none have ever collapsed. The sprinkler system functioned just fine on both towers on 09/11/2001. Some stairwells had as much as 3 inches of water running down them in spots. Also, firemen entered the towers and fought the fires, Tower 2 was very close to extinguished when it was demolished... that was what they call a "cold fire", under 1000 degrees. There was no bending or buckling from a cold fire. The fire did not collapse any floors, not even the floors of impact... how hot could that steel be if people were hanging onto it with their bare hands? Not hot at all, warm maybe, but try this, heat your oven to 300 degrees and try hanging onto the steel with your bare hands. (don't really try that) The fire was originally hottest at impact, reaching 1500 degrees but that lasted ten minutes, tops. Fire burns up and then the floors of impact were below the fire, cold enough to hang onto what had been the hottest steel in the fire. The steel that was in the active fire heats up, but then the fire travels and the steel cools. No floors collapsed from the fire.
Pancaking only happened at the very onset of demolition. Very quickly, the demolition wave moved down and up the tower, faster than the floors could drop. The towers collapsed first on the impact floors... which were already cool floors by the time they pulled the buildings. Visually, they gave the imagery of having been caused by the fires, when in fact, the fires were stone cold on those floors, with people hanging onto the steel openings with their bare hands. The only loss of structural integrity would have been the holes caused by the jet impact... and the building was designed to handle that. You are still stuck on the discredited pancaking theory... pancaking didn't happen, if it had happened, the cores and very likely the walls would still be standing, and the floors would still be stacked up, 100+ high; that's the result of pancaking, no one is saying it was pancaking, that was an early Lie from the government meant for consumption by the very foolish. Pancaking does not reduce a structure to a pile of debris, and the WTC design was not susceptible to pancaking; the floors were too strong.
