Appendix C of FEMA’s 2002 Building Performance Study documented the existence of several beams from World Trade Center Building 7 that had experienced a “severe high temperature corrosion attack.”1 The New York Times called this “perhaps the deepest mystery uncovered in the investigation.”2 The Worcester Polytechnic Institute Journal, Transformations, described one piece of steel, saying:3

“A one-inch column has been reduced to half-inch thickness. Its edges–which are curled like a paper scroll–have been thinned to almost razor sharpness. Gaping holes–some larger than a silver dollar–let light shine through a formerly solid steel flange. This Swiss cheese appearance shocked all of the fire-wise professors, who expected to see distortion and bending–but not holes.”


The authors of Appendix C hypothesized that a eutectic formed in the steel at approximately 1000° C due to a slow sulfidation process in the debris pile after 9/11, but concluded by stating, “No clear explanation for the source of sulfur has been identified.” Independent researchers challenge the argument that such a reaction could have resulted from the building’s normal contents:4

“[T]o form a molten iron-oxygen-sulfur eutectic at about 1000° C would require a very high concentration of sulfur… The fact that sulfur evaporates at a low temperature, 445° C, along with the very low levels of elemental sulfur in office buildings appears to preclude the possibility that the eutectic could have formed as a result of a slow sulfidation process in the debris pile.”

Unfortunately, the National Institute of Standards and Technology did not follow up on the FEMA Building Performance Study’s call for further analysis of this steel in its subsequent multi-year investigation into the destruction of Building 7.

[1] Federal Emergency Management Agency (FEMA), “World Trade Center Building Performance Study,” Washington DC.  May 1, 2002, Appendix C, p.1-13.

[2] JKM, “The ‘Deep Mystery’ of Melted Steel,” WPI – Transformations

[3] Ibid.

[4] Jones, Ferrer, Jenkins, Legge, Gourley, Ryan, Farnsworth, Grabbe, “Extremely high temperatures during the World Trade Center destruction.” Journal of 9/11 Studies. January 19, 2008.