It's right that this is on a "schools of thought" topic. Some explosives experts state that the only, I repeat only, way for a building to fall in that way is fo internal detonations to take place. This would mean contrivance and lead to questions as to the real identity of the professed hijackers (I'm still not sure why they are termed 'bombers' as they did not bomb, but crashed).

The Wiki article on this (http://en.wikipedia.org/wiki/Collapse_of_the_World_Trade_Center) is good. A brief summary -- it was a combination of things. First, there was the simple kinetic energy of the planes impacting the towers. The towers were built to withstand an impact from the largest plane at the time, a 707; however, the impact from a 767 was several times the magnitude. That impact also delivered a _full_ load of jet fuel into the core of the building -- a core which was lighter and more open than the core of most skyscrapers, and which was not fireproofed against that sheer volume of high-temperature fuel. A couple of things made this worse. Some of the fireproofing, which existed as cladding around the steel beams, had actually been blown off by the impact of the planes, and the open-core design helped carry burning fuel through the core of the building sort of like a chimney. Many load-bearing girders were severed by the planes; more were softened by the heat of the fires. As the result of this combination, the external building started to bow outwards, and the internal floors to sag; the weight of the building was redistributed to girders and columns which weren't designed to carry that load, especially not from certain angles. Eventually it simply collapsed catastrophically -- but they did collapse nearly straight down, although the tops of the buildings tilted according to which support structures had been physically severed and which were still attached. Read the Wiki, though. It has a lot of additional, exact detail. Edit to add: the NIST data on temperature are quite clear; steel loses much of its strength at 1000 degrees, and that was easily reached in ten minutes of the fire. Look, if you aren't interested in what engineers have to say on the matter and are just looking for confirmation of your own prejudices, don't bother asking, ok?

There was weakening of the steel girders where the jet fuel burned. That weakening of those girders warbed the girders to the point that the floors above the fire fell straight down, hitting the floors under the top floors. The impact of the top floors was so great that as each floor was hit the very small pieces of that floor spread out- the clouds seen in the videos. The clouds were made up primarily of concrete that had been returned to it's original size before being mixed with water when the towers were made. A worker using a jackhammer to break up old concrete sends up a small cloud(normally)of dry concrete like when the top floors hit eah of the bottom floors. Also, I have seen videos of big buildings demolished-the same big clouds of concrete dust spreads out as each floor hits the floor beneath it. The weight of the top floors left nothing to be pancacked, except for themselves and a portion of the bottom 6 floors(because the top floors fell slightly of center.