Found a good excerpt that refutes the notion that the planes and fires brought down the twin towers:
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The Strength of Steel Frame Structures
For those who want us to imagine that the Towers "collapsed", the assumption that they "fell" on their own is a critical part of the story. But to anyone who is familiar with the performance of modern steel-frame structures, it should be obvious that they cannot simply collapse on their own, with or without an office fire, or even from the impact of a falling portion of the same building. If impacted from above, the Towers might bend or distort, but they wouldn't explode, disintegrate in mid-air, or collapse like a house of cards!
Anyone who's ever played with an Erector Set knows that as long as the structural members remain well-connected, a framework may become twisted and distorted if it falls to the floor, but it will never just collapse into pieces under any scenario involving self-related and self-proportional forces. Buildings that have fallen in earthquakes demonstrate this resistance to disintegration.
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http://www.truememes.com/semantics.html
For a more detailed analysis of why collapses had to have been demolitions, feel free to visit my web page on the subject:
http://scott3x.tripod.com/911/cd/