(From the Wikipedia article)
One of the many aspects focused on by these critiques is the collapse of the World Trade Center analysis by Loose Change. The comparison to other notable high-rise fires which did not collapse ignores differences in building design, significant WTC structural damage and compromised fireproofing;[48] as most steel loses over half its strength at 600°C (1112°F).[49] Internet Detectives also found firefighters during those fires were pulled back for fear of collapse and that Madrid's Windsor Building comparison does not mention its steel supported perimeter floors collapsed during the fire. Kevin Ryan from Underwriters Laboratories (UL) was actually employed in a water testing subsidiary,[21] UL does not certify structural steel,[21] and ASTM E119 certification is not meant to predict performance in real uncontrolled fires.[49] UL found no evidence of any firm conducting tests on WTC materials in the past.[49] Another expert quoted, Van Romero, has clarified that he was misquoted by the Albuquerque Journal and he had said it "looked like" explosives took down the WTC. When the misquote was printed he felt like his "scientific reputation was on the line."[49]
And there is this:
World Trade Center 7: The Lies Come Crashing Down
"No evidence of any explosives were ever found, but the conspiracy theory states that this is because the government took away all the debris before it could be independently tested. Since it's normal for debris to be removed following any such destruction, this particular piece of information is too ambiguous to be given serious weight as proof of a conspiracy."
"None of the videos of Building 7's collapse show any minor explosions. They simply show the top of the building begin to gracefully sag, as if it's made of clay, and then the whole thing drops. So while the manner of collapse may look superficially similar to a controlled demolition at first glance, a more careful examination shows critically important (and non-ambiguous) differences."
"Could a building with such little apparent external damage collapse like this? The photos and videos on the conspiracy theory web sites are from other angles, and show only relatively minor, superficial damage to the building; and even the NIST has said the fire alone would probably would not have destroyed the building. But, let's not forget that Building 7 did have damage: Severe damage, a deep gouge cutting a quarter of the way through the building, ten floors high. Yet even if there was such extensive damage, argue the conspiracy theorists, that fact alone would invalidate the government report. Also from WTC7.net:
"The alleged damage was asymmetric, confined to the tower's south side, and any weakening of the steelwork from fire exposure would also be asymmetric. Thus, even if the damage were sufficient to cause the whole building to collapse, it would have fallen over asymmetrically — toward the south."
This claim forgets that nobody has said the damage alone was responsible for the collapse. According to the NIST report, the initial loss of the columns served only to transfer the building load to the remaining columns, thus exceeding their load bearing capacities, which then gave way after being adequately softened by the fire. In such a condition, the building would have insufficient support throughout. The east side, already sagging, dropped first and pulled the rest of the building down in a slightly diagonal collapse. The conspiracy theorists are correct in that the fall was not entirely symmetric, as it strayed enough to do the aforementioned damage to the Verizon and Manhattan Community College buildings. The conspiracy theorists have hardly proven that explosives are the only possible explanation for the collapse."
And I'll repost this again:
Debunking the 9/11 Myths: Special Report - Popular Mechanics
And there's this:
9/11 Controlled Demolition Theory Debunked