Determining the probable collapse sequence for WTC 7, NIST found that the impact of debris from the collapse of WTC 1 ignited fires on at least 10 floors of WTC 7, and the fires burned out of control on six lower floors. The heat from these uncontrolled fires caused thermal expansion of the steel beams on the lower floors of the east side of WTC 7, damaging the floor framing on multiple floors. Eventually, a girder on Floor 13 lost its connection to a critical interior column that provided support for the long floor spans on the east side of the building. The displaced girder and other local fire-induced damage caused Floor 13 to collapse, beginning a cascade of floor failures down to the fifth floor. Many of these floors had already been at least partially weakened by the fires in the vicinity of the critical column. This collapse of floors left the critical column unsupported over nine stories.
“When this critical column buckled due to lack of floor supports, it was the first domino in the chain,” Sunder explained. “What followed in rapid succession was a progression of structural failures. Failure first occurred all the way to the roof line—involving all three interior columns on the most eastern side of the building. Then, progressing from east to west across WTC 7, all of the columns in the core of the building failed. Finally, the entire façade collapsed.”
The investigation team considered the possibility of other factors playing a role in the collapse of WTC 7, including the possible use of explosives, fires fed by the fuel supply tanks in and under the building, and damage from the falling debris of WTC 1.
The team said that the smallest blast event capable of crippling the critical column would have produced a “sound level of 130 to 140 decibels at a distance of half a mile,” yet no noise this loud was reported by witnesses or recorded on videos.
As for fuel fires, the team found that they could not have been sustained long enough, could not have generated sufficient heat to fail a critical column, and/or would have produced “large amounts of visible smoke” from Floors 5 and 6, which was not observed.
Finally, the report notes that “while debris impact from the collapse of WTC 1 initiated fires in WTC 7, the resulting structural damage had little effect in causing the collapse of WTC 7.”
A chain of probable causes was discovered and laid out in detail by the NIST investigative team. The bottom line: the evidence did NOT point towards controlled demolition.
This parallels the situation of ID: how can we determine if design is a legitimate explanation? At first glance, it may look like design, but peer deeper into the details, and the design may show itself to be an illusion.
Mike Gene has laid out how we may investigate our suspicions in The Design Matrix, but he leaves it at the investigation stage. This is perhaps due in part to ID being "a nascent protoscience".
So let's open the floor for comments. The theme is "By purpose or by accident: how can we tell?"