Figure 1.5 The collapse of WTC 7 at 5:20 PM (source unknown

report in November 2008. NIST ultimately concluded that the collapse of WTC 7 was the first known instance of the total collapse of a tall building primarily due to fires.

Figure 1.5 The collapse of WTC 7 at 5:20 PM (source unknown).

Concurrent to and since the NIST investigation, many independent researchers have also studied the collapse of WTC 7 and assembled a body of evidence that raises questions about the validity of NIST’s conclusions. That evidence includes the following:

1. WTC 7 experienced between 2.25 and 2.5 seconds of free fall (i.e., gravitational acceleration) during its collapse. This fact was first identified by independent researcher

David Chandler and later corroborated by NIST (Figure 1.6). WTC 7’s free fall is noteworthy because, in a typical building collapse, WTC 7 would be expected to experience a combination of axial rotation and bending of members, resulting in a disjointed, asymmetrical collapse, at less than free-fall acceleration. Asymmetrical, tipping behavior is especially likely because WTC 7 did not have planar symmetry.

2. The debris pile of WTC 7 was contained mostly inside the building’s footprint.

Furthermore, it did not have large pieces of concrete flooring or intact structural framing that would be expected in a building collapse (see Figure 1.7).

3. According to Appendix C of FEMA’s May 2002 report, a steel member recovered from

WTC 7 was found to have experienced corrosion due to a combination of oxidation and sulfidation at 1,000°C, resulting in the formation of a liquid eutectic (see Figure 1.8).

Researchers have hypothesized that the presence of thermate, which is a form of thermite