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What Caused Building 7's Collapse?

This question would appear to be the greatest in engineering history. In over 100 years of experience with steel-framed buildings, fires have never caused the collapse of a single one, even though many were ravaged by severe fires. Indeed, fires have never caused the total collapse of any permanent steel structure.

What was done to answer this most important question? The only official body that admits to having investigated the curious collapse of Building 7 is FEMA's Building Performance Assessment Team (BPAT), which blamed fires for the collapse but admitted to being clueless about how fires caused the collapse.

People who have seen buildings implode in controlled demolitions are unlikely to be as challenged as FEMA's team in understanding the cause of Building 7's collapse. They will notice, upon watching the videos, that Building 7's collapse showed all of the essential features of a controlled demolition.

Despite having the appearance of a controlled demolition, is it possible that Building 7 could have been destroyed by some combination of damage from tower debris, fuel tank explosions, and fires? Let's consider the possible scenarios.

NIST released a photograph in 2005 showing a large gouge in the lower portion of the southwest corner of Building 7, and its collapse scenario deviates significantly from FEMA's in emphasizing alleged structural damage from the collapse of the North Tower. That photograph is notable for its lack of corroboration, and NIST's claims of other regions of damage to the building's south face lack substantiating evidence. Even if NIST's claims about structural damage from North Tower debris were true, it would not begin to explain the precipitous, symmetrical manner in which Building 7 collapsed. Structural damage to the south side -- particularly to the lower stories -- would have made any kind of vertical collapse all the more unlikely.

The idea that diesel fuel stored in Building 7 is to blame for the collapse was promoted by The New York Times in late 2001 and by FEMA's 2002 Building Performance Study. 1 This idea is also untenable. Fires were observed in Building 7 prior to its collapse, but they were isolated in small parts of the building, and were puny by comparison to other building fires. Let's imagine, contrary to the evidence, that debris from the tower collapses damaged Building 7's structure, that diesel fuel tanks exploded, and that incredibly intense fires raged through large parts of the building. Could such events have caused the building to collapse? Not in the manner observed. The reason is that simultaneous and symmetric damage is needed to produce a collapse with the precise symmetry of the vertical fall of building 7. This building had 58 perimeter columns and 25 core columns. In order to cause the building to sink into its footprint, all of the core columns and all of the perimeter columns would have to be broken in the same split-second.

Any debris from the towers impacting Building 7 would have hit its south side, and any columns damaged by it would almost certainly be perimeter columns on its south side. Any fuel tank explosion would only be able to damage nearby structure. The rapid fall-off of blast pressures with distance from the source would preclude any such event from breaking all of the columns in the building.
view of WTC 7 showing entire elevation
Building 7 was about 5 times as tall as it was deep.
(Furthermore the very idea of a tank of diesel fuel exploding taxes the imagination, since diesel fuel does not even begin to boil below 320 F. 2 ) Fires have never been known to damage steel columns in high-rise buildings, but if they could, the damage would be produced gradually and would be localized to the areas where the fire was the most intense.

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. 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. Any asymmetry in the extent and timing of the damage would cause such a building to topple.


1. Engineers are Baffled Over the Collapse of 7 WTC, New York Times, 11/29/01 [cached]

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