From
A Structural Reevaluation of the Collapse of World Trade Center-7:"• Columns 79, 80, and 81 did not fail at the lower floors of the building, as asserted by
NIST. In order to allow for the observed collapse of the east penthouse approximately 7
seconds prior to the collapse of the rest of the structure, these columns needed to have
failed at the upper floors of the building all the way to the penthouse. Yet there were no
documented fires above Floor 30. Therefore, fire did not cause the collapse of Columns
79, 80, and 81 nor the collapse of the east penthouse (Section 4.3).
•The hypothetical failure of Columns 79, 80, and 81 — the three easternmost core
columns — would not trigger a horizontal progression of core column failures. Therefore,
the hypotheses of NIST, Arup/Nordenson, and Weidlinger that the buckling of Column
79 could trigger a progressive collapse of the entire building are invalid, and the collapse
of Columns 79, 80, and 81 high in the building was a separate and distinct event (Section
4.5).
"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.
.
Researchers have hypothesized that the presence of thermate, which is a form of thermite
incendiary that includes sulfur, would explain the sulfidation and formation of a liquid
eutectic (Jones, 2006 and 2007)."