So your're saying that the collapse of several stories was caused by a combination of weakened steel (from the heat of the fire), and structural damage from the impact.
Fair enough, but even if thats correct this still wouldnt pulverize the entire building:
http://www.attackonamerica.net/proofofcontrolleddemolitionatwtc.htm
"Steel frame towers are built very strongly. They need to withstand the pressure of gale-force winds, the violent rocking motion of earthquakes, and the ravages of time. For this reason, they are almost impossible to destroy.
Airplane strikes do not destroy skyscrapers. A bomber strike to the Empire State Building during World War II did not harm that building. The World Trade Center towers were designed to survive a strike by a Boeing 707. The 767 is more massive, so the building was stressed near its design limits. But if a failure had occurred at that moment, it would have been at the point of highest levered stress, near the base of the tower, and the tower would have fallen over like a giant tree in a forest windstorm. That, of course, did not happen.
Fires do not destroy skyscrapers. Never in the history of steel frame structures has a single one been destroyed by fire.
How to destroy a skyscraper. So, how do you destroy a skyscraper? Suppose you need the vacant land to build another one, for example.
A nuclear bomb is very effective, but it can be difficult to get permits from the city.
An early invention was the wrecking ball. A huge lump of steel and lead is swung from a massive chain at high speed. With the benefit of momentum, it is able to bend or break a few girders at a time. But it would be a hopeless task to destroy a tower the size of the World Trade Center, using a wrecking ball.
The most effective, cleanest, safest way to destroy a skyscraper is known as controlled demolition. The trick is to distribute explosives at key points throughout the structure. The explosives are detonated simultaneously, destroying the integrity of the steel frame at key points, such that no part of the building is supported against the force of gravity. The entire mass is pulled swiftly to earth, where gravity does the work of pounding the structure into tiny fragments of steel and concrete. The gravitational potential energy of the structure is converted smoothly and uniformly into kinetic energy, and then is available very efficiently to pulverize the fragments of the building as they impact against the unyielding earth. Controlled demolitions have a striking and characteristic appearance of smooth, flowing collapse.
As your eyes will tell you, the World Trade Center collapses looked like controlled demolitions. Here's the proof.
The proof. According to the law of gravity, it is possible to calculate the time it takes for an object to fall a given distance. The equation is H=(1/2)at2, where H is the height, a is the acceleration of gravity (10 meters per second squared) and t is time in seconds. Plug in the height of the building at 1350 feet (411 meters) and we get 9 seconds. That is just about the length of time it took for the very top of the World Trade Center to fall to the street below. According to all reports, the whole thing was over in just about ten seconds.
It is as if the entire building were falling straight down through thin air. As if the entire solid structure below, the strong part which had not been burned or sliced or harmed in any significant way, just disappeared into nothingness. Yet this (within a small tolerance) is what we would expect to find if there had been a controlled demolition, because the explosions below really do leave the upper stories completely unsupported. Like the Road Runner after he runs off the edge of the cliff, the entire building pauses a moment, then goes straight down.
Any kind of viscous process or friction process should have slowed the whole thing down. Like dropping a lead ball into a vat of molasses, or dropping a feather into the air, gravitational acceleration cannot achieve its full effect if it is fighting any opposing force. In the case of the World Trade Center, the intact building below should have at least braked the fall of the upper stories. This did not happen. There was no measurable friction at all.
This proves controlled demolition.
We have been lied to. We have been lied to about this, at multiple levels. The first lie was that the load of fuel from the aircraft was the cause of structural failure. No kerosene fire can burn hot enough to melt steel. In point of fact, most of the fuel in the jets was contained in their wing tanks. The thin aluminum of the tanks was pierced or stripped as the airplanes penetrated the walls of the towers, and the result was the huge fireball which was seen on national TV, where most of this fuel was burned.
A hot, vigorous fire would have blown out many windows in the building and would have burned a red or white color. This was not what happened. The fire in the World Trade Center was an ordinary smoldering office fire.
But let's suppose that the fire was hot enough to melt steel. What would have happened in that case? Before it breaks, hot steel begins to bend. This redistributes the forces in the structure and puts elastic stress on those parts that are still cool. The process is asymmetric, so that the structure should visibly bend before breaking. But of course, no steel skyscraper has even bent over in a fire.
Let's suppose the structure were sufficiently weakened that it did fail catastrophically near the point of the airplane strike. In this case, the intact structure below would exert an upward force on the base of the upper story portion of the building (the part that has been broken loose), while any asymmetry would allow the force of gravity to work uninhibited on the tip of the skyscraper. Thus, the top section of the skyscraper would tip and fall sideways.
If it did not tip, it would have ground straight down through the building below. The gravitational potential energy of the upper stories would be coupled into the frame below, beginning to destroy it. The frame below would deflect elastically, absorbing energy in the process of deflecting. At weak points, the metal structure would break, but the elastic energy absorbed into the entire frame would not be available to do more destruction. Instead, it would be dissipated in vibration, acoustic noise and heat. Eventually this process would grind to a halt, because the gravitational potential energy of a skyscraper is nowhere near sufficient to destroy its own frame.
If the World Trade Center towers had been built entirely out of concrete, they might have stood for awhile before toppling in the wind. But in that case, if they had collapsed straight downwards, the energy required to pulverize the concrete would have slowed the downward progress of the upward stories. The gravitational potential energy of the World Trade Center was barely sufficient to convert its concrete into powder, and for that to happen in an accidental collapse would have been impossible, but would have taken a lot longer than 10 seconds in any case."