http://www.nist.gov/public_affairs/factsheet/wtc_fire_resistance_data.htm
Title:
NIST Tests Provide Fire Resistance Data on
World Trade Center Floor Systems
Partial excerpt:
The Commerce Department's National Institute of Standards and Technology (NIST) recently reported that results from a series of four fire resistance tests conducted in August 2004 on composite concrete-steel trussed floor systems typical of those used in the World Trade Center (WTC) towers showed that the test structures were able to withstand standard fire conditions for between 45 minutes and two hours. The tests--which were conducted at Underwriters Laboratories (UL) facilities in Northbrook, Ill., and Toronto, Canada--are part of NIST's building and fire safety investigation of the WTC disaster on Sept. 11, 2001.
The 1968 New York City building code--the code that the towers were intended but not required to meet when they were built--required a two-hour fire rating for the floor system.
...
"The fire conditions in the towers on 9-11 were far more extreme than those to which floor systems in standard U.S. fire rating tests are subjected," Sunder says. "Our investigation's final assessment of how the floor system performed in the WTC fires also must consider factors such as the combustible fuel load of the hijacked jets, the extent and number of floors involved, the rate of the fire spread across and between floors, ventilation conditions, and the impact of the aircraft-damaged towers' ability to resist the fire."
It's my understanding that the fire might have not been as bad if the Towers had been built using asbestos.