Not the first time a steel framed building collapsed due to fire

"Contrary to popular belief, September 11, 2001 was not the first time a steel framed building collapsed due to fire. The McCormick Center in Chicago and the Sight and Sound Theater in Pennsylvania are examples of steel structures collapsing." - debunking911.com

Average building fire temperatures range from approximately 700 Degrees to 900 Degrees Celsius. Steel weakens dramatically as its temperature climbs above 230 Degrees Celsius, retaining only 10% of its strength at about 750 Degrees Celsius.

All steels lose strength with increasing temperature. By 600 Degrees Celsius, most structural steels have lost more than half their strength. At intermediate temperatures the strength is independent of time, but above 500 Degrees Celsius, creep, or time-dependent deformation, further reduces the load-carrying capability. To combat this loss of load-carrying capability, structural steel in buildings is insulated to keep it cool in fire.

NIST is very clear:

"the NIST investigation showed conclusively that the failure of the inwardly bowed perimeter columns initiated collapse and that the occurrence of this inward bowing required the sagging floors to remain connected to the columns and pull the columns inwards. Thus, the floors did not fail progressively to cause a pancaking phenomenon."

"Based on this comprehensive investigation, NIST concluded that the WTC towers collapsed because:
(1) the impact of the planes severed and damaged support columns, dislodged fireproofing insulation coating the steel floor trusses and steel columns, and widely dispersed jet fuel over multiple floors; and
(2) the subsequent unusually large jet-fuel ignited multi-floor fires (which reached temperatures as high as 1,000 degrees Celsius) significantly weakened the floors and columns with dislodged fireproofing to the point where floors sagged and pulled inward on the perimeter columns. This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers.

Both photographic and video evidence—as well as accounts from the New York Police Department aviation unit during a half-hour period prior to collapse—support this sequence for each tower."

"NIST's findings also do not support the "controlled demolition" theory since there is conclusive evidence that:

* the collapse was initiated in the impact and fire floors of the WTC towers and nowhere else, and;

* the time it took for the collapse to initiate (56 minutes for WTC 2 and 102 minutes for WTC 1) was dictated by (1) the extent of damage caused by the aircraft impact, and (2) the time it took for the fires to reach critical locations and weaken the structure to the point that the towers could not resist the tremendous energy released by the downward movement of the massive top section of the building at and above the fire and impact floors.

Video evidence also showed unambiguously that the collapse progressed from the top to the bottom, and there was no evidence (collected by NIST, or by the New York Police Department, the Port Authority Police Department or the Fire Department of New York) of any blast or explosions in the region below the impact and fire floors as the top building sections (including and above the 98th floor in WTC 1 and the 82nd floor in WTC 2) began their downward movement upon collapse initiation.

In summary, NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to Sept. 11, 2001. NIST also did not find any evidence that missiles were fired at or hit the towers. Instead, photographs and videos from several angles clearly show that the collapse initiated at the fire and impact floors and that the collapse progressed from the initiating floors downward until the dust clouds obscured the view."

See:
http://wtc.nist.gov/pubs/factsheets/faqs_8_2006.htm