Going so soon? What about all those really great questions I asked you?
Let's perform a thought experiment, shall we? Have you ever played the game Jenga? A group of people construct a wooden tower using alternating patterns of 3 wooden blocks, and then they take turns removing pieces. The winner is the last person to successfully remove a piece.
Jenga is an interesting sidebar into a branch of newtonian mechanics known as statics. The tower has some interesting properties. First, each floor (except for the top floor) must be in static equillibrium in order to prevent the tower from collapsing, second, there are two possible equillibria states. One involves equillibrium with two blocks and one with only one block.
The Jenga tower is a good model for progressive collapse because it follows the two conditions that real towers must posess. Each floor must be in static equillibrium and each floor has two states of equillibrium: the determinate and the indeterminate. Consider that the single beam Jenga floor is the determinate case wherein the entire weight of the structure above rests on one beam. In the determinate case, the stress on the beam is equal to the weight of the structure above the beam.
Consider the indeterminate case of two beams. Despite the fact that each beam now carries half of the load of the above structure, the removal of either of the beams will cause the structure to collapse. Thus, despite the very true assertion that the tower is redundantly supported (indeed, only one beam is necessary, as proved by the determinate case), that has no bearing in its collapse. As soon as one beam is removed, collapse becomes inevitable. Notice that we do not make the assumption that the lower levels of the Jenga tower will support the load. Indeed, the Jenga beams are capable of supporting extremely heavy loads, but they are not capable of stopping the collapse.
This is a very important point: the load bearing capacity of the floors below is not a measure of their ability to stop collapse. Indeed, stopping a collapse would require a damping ability; the ability to slow down and dissipate all of the potential energy.
Like the wooden blocks of the Jenga tower, the WTC towers had rigid supports. Had they the structural capacity to bear the weight of one extra floor, the fact remains that Newton's 3rd law must be obeyed. All of the energy that the upper portions of the WTC tower had as kenetic energy during their decent was reflected back to them by the portion of the tower that remained fixed to the ground. This force simply exceeds the material strength of the towers.
Consider that, for the multiple redundancies in the Jenga tower, only one beam needs to be removed in order to make collapse inevitable. The bowing of columns in the WTC tower was indeed the final block necessary to make the tower collapse. It is possible to examine and pinpoint the sequence of events that lead to the collapse initiation state in the same way that it is possible to determine the winner of a game of Jenga.
To summarize:
NIST made the assumption that collapse became inevitable at some point based on the very valid premise that the towers were rigid structures with no ability to absorb and dissipate the energy present in the falling sections of the towers. The WTC towers were incapable of stopping any form of collapse after it started.
Furthermore, the purpose of the NIST investigation was not to determine who was at fault for the WTC collapse. Rather, the genesis of the investigation was to determine what, if any, building codes needed to be changed in order to prevent another WTC type collapse. Indeed, it is far wiser to determine methods that prevent the lead up to a precise point in collapse initiation than to attempt to make any skyscraper capable of stopping progressive collapse. In the case of collapse initiation, NIST has outlined a very specific series of events leading to collapse, and it follows that an interruption of any of the events would prevent collapse.
