"The structure below the level of collapse initiation offered minimal resistance to the falling building mass .... The potential energy released by the downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that .... Since the stories below the level of collapse initiation provided little resistance to the tremendous energy released by the falling building mass, the building section above came down essentially in free fall .... As the stories below sequentially failed, the falling mass increased, further increasing the demand on the floors below, which were unable to arrest the moving mass." NIST's Final Report
Okay, so according to this theory...once the fire had damaged the structure enough to initiate the collapse...at this point - the falling mass and potential energy it released onto the lowers floors is ultimately what caused the pulverization of the building. I think we can all agree, that without the falling mass (upper floors) above the impact point....the lower floors obviously would NOT have just crumbled to the ground, since there would have been a clear absence of the energy it would have taken to power such an event. So again - NIST's theory is that this falling building mass (upper floors above the impact) smashed down upon the lower floors causing them to pulverize into dust. The problem we have with this theory, is that we lose sight of the entire building about half way through the collapse due to the enormous cloud of dust that engulfs each tower during it's downward descent. So, how can NIST even be all that certain, that the upper mass remained intact during the entire collapse? They can't...and it's just another critical point that is left up to complete speculation. I, for one - will at least present this video, which shows the collapse of WTC 2:
http://www.youtube.com/watch?v=_u_k217RkUo
In this clip, you can clearly see that the critical upper mass of WTC 2 begins to fall over and to the side, just shortly after the collapse begins. A simple lesson in one of the fundamental laws of physics i.e. inertia - states that...A body moving on a level surface will continue in the same direction at a constant speed unless disturbed. So, following...SCIENTIFIC law, the upper mass will not and cannot move itself back to it's original position i.e. directly above the lower floors. (that is - barring an attempt by superman to swoop in and push it back over there) So, what does this tell us? Well, a lot. Since the scientifically measurable force created by the sine qua non (upper floor mass backed with potential energy) of the collapse has now been certifiably shifted, we can easily deduce that according to NIST's theory - there is insufficient data to conclude whether or not the upper floor mass from WTC 2 exerted the potential energy (required in a FULL collapse) down upon the entire lower structure of WTC 2. A car doesn't run without gas, and steel structured floors don't pulverize into the ground without a driving energy. The upper mass on WTC 2 was moving away from the lower floors at the time of its disappearance into the dust cloud, so we can reasonably conclude (backed with the LAW of inertia) that the critical upper mass was NOT a sufficient force in the collapse of WTC 2.