Please read The Fire, The Collapse from:
http://www.tms.org/pubs/journals/JOM...agar-0112.html
Here is a highly technical article about the physics of the collapse
http://www.tam.uiuc.edu/news/200109wtc/
Here is a less technical article
http://www.civil.usyd.edu.au/wtc.shtml
And one new thing I learned today, WTC was one of the first building to be built with the aid of computers, before that time, steel buildings were built with wide margins for error, usually doubling the capacity for the load needed. If the floor had to support 100 tons, it would be built to hold 200 tons. WTC was the first with the aid of computers to be precise and not have to be over built, if a floor had to hold 40 tons, it could only hold 40 tons, there was not margin for error because none was needed. We must also remember that the building was contracted with tubular steel, extremely strong when static, but easy to bend with enough force, and once bent, loses most of its structural integrity. Think of a soda can, even when I weighed 300lb I could carefully stand on an empty can of soda, but if I malformed it slightly, even a pin hole, it would collapse as soon as I stood on it, it lost all structural integrity when it is damage, the steel tubing is not quite this severe, but the plane smashing through the supports would have removed a great deal o there integrity.
http://vincentdunn.com/wtc.html