In response to the World Trade Center tragedy, the National Institute of Standards and Technology is conducting a series of building and fire safety investigations to determine the factors that caused the towers to collapse.

NIST's results from a series of four fire-resistance tests conducted on composite concrete-steel trussed floor systems typical of those used in the towers showed that the test structures were able to withstand standard fire conditions for one to two hours.

The 1968 New York City building code, which the towers were intended but not required to meet when they were built, required a two-hour fire rating for the floor system.

Shyam Sunder, lead investigator of the NIST World Trade Center investigation, explained that the four laboratory tests provide only a means for evaluating the relative fire-resistance rating of the floor systems under standard fire conditions and according to accepted test procedures.

“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 told a group of observers gathered Aug. 27 to watch the final test at Underwriters Laboratories in Northbrook, Ill.

The 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 aircraft-damaged towers' ability to resist the fire, he said.

All four tests used the standard procedure known as ASTM E119 for rating the fire resistance of a building structural unit such as a floor system, column or beam under prescribed conditions.