Six Theories Debunked by NIST
From the good folks at Popular Mechanics:
Claim: "No combination of debris damage, fuel-tank explosions and fires could inflict the kind of simultaneous damage to all the building's columns required to make the building implode," says WTC7.net, a Web site dedicated to conspiracy theories. "The precision of such damage required to bring Building 7 down into its footprint was especially great, given the ratio of its height to its width and depth."
NIST report and press conference: Fire did indeed inflict enough column damage to destroy the building through a previously undocumented collapse sequence of thermal expansion. "Anyone who has run a tight jar lid under water to help loosen it knows that the metal expands when it gets hot," Sunder said. "Heat also causes steel to lose strength and stiffness. Thermal expansion occurs at temperatures much lower than those required to reduce steel strength and stiffness." The report found that as WTC 7's steel beams expanded in the heat, numerous structural connections throughout the building failed. That weakened the structure even before the collapse of any vertical columns.