Overview
Most people who watched 11 September 2001 unfold do not remember a third building coming down. It did. 7 World Trade Center stood across Vesey Street from the north tower, 47 storeys, completed in 1987, and at 5:20 in the afternoon it collapsed.
Nothing struck it. Debris from the north tower's collapse that morning had gouged the south face and started fires across several floors, and the building's sprinkler system depended on city water mains that had been broken. The fires burned unfought for about seven hours — the fire service had withdrawn from the area, having judged the building unsafe and having more urgent problems.
Then it went down, straight, into roughly its own footprint, in something close to seven seconds.
Why It Became the Centrepiece
Of all the strands of 9/11 argument, this is the one that has held the most attention from people with engineering training, and the reason is that the collapse looks like something.
A building falling symmetrically into its own footprint is what controlled demolition looks like, because that is what demolition is designed to achieve and it is difficult to achieve any other way. Fire damage is asymmetric — it burns hotter in some places than others — and asymmetric damage normally produces asymmetric failure: a lean, a topple, a partial collapse. Building 7 did not do that.
There is also the matter of precedent. Tall steel-framed buildings had burned for far longer and far hotter without coming down. The Windsor Tower in Madrid burned for around twenty hours in 2005 and remained standing, though its steel upper floors did partially collapse.
And the building housed offices of the Securities and Exchange Commission, the Secret Service and the CIA, along with case files, which the theory treats as motive.
What NIST Concluded
The National Institute of Standards and Technology investigated for about seven years and published its final report in November 2008.
The mechanism it described is specific and does not depend on unusually hot fire. Ordinary office fires heated steel floor beams on the east side of the lower floors. The steel expanded — thermal expansion, not weakening — and pushed a girder off its seat at column 79, one of the columns carrying the long floor spans on that side. Floors below then failed in sequence, leaving column 79 unbraced over nine storeys. It buckled. The failure propagated east to west through the core, then the exterior came down as a unit, which is what produced the symmetry.
NIST also said plainly that the building was the first tall building known to have collapsed primarily because of fire, and that its findings did not support a controlled demolition, in part because the explosives required would have produced sound levels audible for miles that were not recorded.
The Alaska Study
In September 2019 a team at the University of Alaska Fairbanks led by Professor Leroy Hulsey, with Zhili Quan and Feng Xiao, published the results of a four-year finite element modelling study.
Its principal conclusion was the opposite of NIST's: that fire did not cause the collapse. Its secondary conclusion was that what happened was a global failure involving the near-simultaneous failure of every column in the building.
The study was funded by Architects and Engineers for 9/11 Truth, which is an advocacy organisation, and that funding is the first thing its critics raise. Its defenders point out that the work was done in a university engineering department, published, and that the modelling files were made available — a level of transparency NIST was criticised for not matching, having withheld some of its own model inputs on the grounds that release might jeopardise public safety.
Where the Argument Actually Sits
The two studies do not disagree about the building. They disagree about what a model shows.
NIST's case rests on a mechanism nobody had documented before, which is a real weakness — a first-of-its-kind explanation for a first-of-its-kind event is not independently corroborated by anything. The UAF case rests on modelling that reaches a conclusion about what did not happen without establishing what did; near-simultaneous column failure is a description of demolition without evidence of the means, and no explosive residue, no charges, no placement crew and no acoustic signature has ever been produced.
The practical difficulty is that the physical evidence was recycled as scrap within months, before any structural investigation had begun — a decision defended at the time as necessary and criticised since by fire investigators as the loss of the only material that could have settled it.
What is left is two models of a building that no longer exists, built by people who started from different assumptions, and a seven-second video that a great many people find difficult to reconcile with an office fire.