Structural steel loses strength the longer it is heated which in this case was 7 hours. This was NOT enough to cause failure in the columns, but it WAS enough to cause expansion and deformation of the floor supports on the lower level which did. (These were fabricated Truss supports NOT solid I beams.)
Excepts from the NIST report on the collapse of WTC 7. I am going with these structural engineers and their report.
NIST Final report on the Building 7 Collapse
From page 100:
"The structural design did not explicitly evaluate fire effects, which was typical for engineering practice at that time and continues to remain so today. Many of the shear connections in WTC 7 were not capable of resisting lateral loads resulting from thermal expansion effects in the steel floor framing when the floor beams were heated."
"The ASTM E119 test does not capture critical behavior of structural systems, e.g., the effect of thermal expansion or sagging of floor beams on girders, connections, and/or columns. The thermal expansion of the WTC 7 floor beams that initiated the probable collapse sequence occurred primarily at temperatures below approximately 400 °C (750 ºF). Thus, to the extent that thermal expansion, rather than loss of structural strength, precipitates an unsafe condition, thermal expansion effects need to be evaluated. The current fire resistance rating system, which does not include thermal expansion effects, is not conservative"
"The primary water supply for fire protection for the low zone, floors 1 through 20, was a direct connection to the city water mains. The secondary supply was from an automatic fire pump, which was connected to the city water main as well. • Since the city water main had been compromised as a result of the collapses of the two towers, there was no water supply to control the fires on the 7th through 13 th floors. By contrast, the early fires on the 22nd , 29th , and 30th floors may have been limited by the sprinkler system on the upper floors, whose primary water supply was from the storage tanks on the 46 th floor."
"Due to the focus on rescuing people trapped in the debris field, providing aid to the injured, and the loss of water in the hydrant system, FDNY was not able to consider the possibility of fighting the fires in WTC 7 until approximately 1:00 p.m. At approximately 2:30 p.m., FDNY gave the order to forego firefighting activity and for personnel to withdraw to a safe distance from the building"
Additional information. (Don't like his use of "conspiracy theory" term!)
Discussion of Building 7 "conspiracy theory"
Gif of trusses for WTC 1,2, and 7.
WTC 7 ; The Roof structure (containing the AC cooling and sprinkler water tanks) collapses first, but the lower levels collapse is at the bottom and and progressively to the top. (WTC 1 and 2 pancaked down...fires at top.)
GIF WTC2 showing the inward lean as the floor supports pull and collapse the exterior frame towards the center of the building. This also happened in WTC7, but in the lower levels.
Note that the additional information report discusses the contention of explosions in the building .
Note that it takes a long time to set up and initiate a controlled collapse. The wires that send the signals to the detonation charges in the explosive are running all over the place. There were fires in all three buildings but somehow the wires running to the explosive charges in 3 buildings were NOT destroyed in the heat and the explosives supposedly used were not burned or destroyed. (C4 burns...not sure about PETN or RDX). This discusses some of the explosives used to take down a CONCRETE building.
https://www.livemint.com/news/india/noida-twin-tower-demolition-know-all-about-the-powerful-explosives-used-to-demolish-the-supertech-buildings-11661678663096.html
If there was a conspiracy it was in bringing and training the people who hi jacked the planes. (Oily Islamic people, The Saudis yes, Bush and the CIA...Hum...maybe.)