more :ducktales: from NIST
Why did WTC 7 collapse, while no other known building in history has collapsed due to fires alone?
The collapse of WTC 7 is the first known instance of a tall building brought down primarily by uncontrolled fires. The fires in WTC 7 were similar to those that have occurred in several tall buildings where the automatic sprinklers did not function or were not present. These other buildings, including Philadelphia's One Meridian Plaza, a 38-story skyscraper that burned for 18 hours in 1991, did not collapse due to differences in the design of the structural system.
Factors contributing to WTC 7's collapse included: the thermal expansion of building elements such as floor beams and girders, which occurred at temperatures hundreds of degrees below those typically considered in current practice for fire-resistance ratings; significant magnification of thermal expansion effects due to the long-span floors in the building; connections between structural elements that were designed to resist the vertical forces of gravity, not the thermally induced horizontal or lateral loads; and an overall structural system not designed to prevent fire-induced progressive collapse.
What are the major differences between "typical" major high rise building fires that have occurred in the United States and the fire in the WTC 7 building on September 11, 2001?
There are more similarities than differences between the uncontrolled fires that burned in WTC 7 and those that occurred in the following buildings: First Interstate Bank Building (1988), One Meridian Plaza Building (1981), One New York Plaza (1970), and WTC 51 (2001).
The following factors describe the fire events that occurred in both WTC 7 and the referenced buildings: 1) the fuel for the fires was ordinary office combustibles at ordinary combustible load levels; 2) there was no use of accelerants; 3) the spread of fire from combustible to combustible was governed by ordinary fire physics; 4) fire-induced window breakage provided ventilation for continued fire spread and growth; 5) there were simultaneous fires on multiple floors; 6) the fires on each floor occupied a substantial portion of the floor; 7) the fires on each floor had passed the point of flashover and the structure was subjected to typical post-flashover temperatures; 8) the sprinklers were inoperative or ineffective; and 9) the fires burned for sufficient time to cause significant distortion and/or failure to the building structure.
There were some differences between the fires in WTC 7 and those in the referenced buildings, but these differences were secondary to the fire factors that led to the collapse of WTC 7: 1) Fires in high rise buildings typically have a single point of origin on a single floor, whereas the fires in WTC 7 likely had a single point of origin on multiple (10) floors; 2); fires in other high rise buildings were due to isolated events, whereas the fires in WTC 7 followed the collapse of WTC 1; 3) water was available to fight fires in the other high rise buildings, but the water supply to fight fires in WTC 7 was impaired; and 4) while the fires in the other buildings were actively fought by fire fighters to the extent possible, in WTC 7, no efforts were made to fight the fires.
The differences in the fires were not meaningful for the following reasons. By the time that WTC 7 collapsed, the fires in WTC 7 had advanced well beyond the likely points of origin on multiple floors (i.e., south and west faces) and originating points of fire origin had no bearing on the fire conditions when the building collapsed (i.e., in the northeast quadrant). Additionally, in each of the other referenced buildings, the fires burned out several floors, even with available water and fire fighting activities (except for WTC 5). Thus, whether the fire fighters fought the WTC 7 fires or not is not a meaningful point of dissimilarity from the other cited fires.
