Originally posted by smarteye
I'm trying to wrap my brain around this thermal expansion thing. Sounds official
Here's the deal Mr. Scientist, we need you to come to your best conclusion as to why that building fell. Here's a list of possibilities we have already ruled out, because these just can't be true.

Here's another article: MSNBC

Although virtually all of the structural steel from the Twin Towers and Building 7 was removed and destroyed, preventing forensic analysis, FEMA's volunteer investigators did manage to perform "limited metallurgical examination" of some of the steel before it was recycled. Their observations, including numerous micrographs, are recorded in Appendix C of the WTC Building Performance Study. Prior to the release of FEMA's report, a fire protection engineer and two science professors published a brief report in JOM disclosing some of this evidence. 1

The results of the examination are striking. They reveal a phenomenon never before observed in building fires: eutectic reactions, which caused "intergranular melting capable of turning a solid steel girder into Swiss cheese." The New York Times described this as "perhaps the deepest mystery uncovered in the investigation." 2 WPI provides a graphic summary of the phenomenon.