Oh boy... The above is TOTALLY incorrect. NIST never claims that the beam thermally expanded and pushed upon column 79 causing it to fail. ARUP never claims the beam sagged and pulled on column 79 causing it to fail. What NIST says happened is that the beam beneath the concrete floor thermally expanded and walked off the seat at column 79. The floor and beam then impacted the floor below which caused THAT floor to fail and fall. That cascading failure of floors REMOVED lateral support of column 79 that the floors/beams provided. With that lateral floor support gone, column 79 buckled/failed. http://www.nist.gov/el/disasterstudies/wtc/faqs_wtc7.cfm, bullet point 5. ARUP shows that the beam below the floor was PULLED off it's seat, thus starting the same cascading floor failure which removed lateral support of column 79. This is why ARUP calls the beam being pulled off its seat a "collapse initiating event" in their report.