ANSI - American National Standards Institute
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On Eve of 9-11 Anniversary, More Details Emerge on Structural and Fire Elements of WTC Disaster

New York, Sep 10, 2004

In its ongoing building and fire safety investigation into the World Trade Center (WTC) disaster of Sept. 11, 2001, the National Institute of Standards and Technology (NIST) recently reported on results from testing to determine the fire resistance of the floor systems typical of those used in the WTC towers. A series of four fire resistance tests conducted in August 2004 on composite concrete-steel trussed floor systems showed that the test structures were able to withstand fire conditions for between 45 minutes and two hours, in compliance with standards at the time of the buildings’ construction.

However, fire conditions in the towers far exceeded those found in a traditional office fire. “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,” said Shyam Sunder, lead investigator of the NIST WTC investigation.

Further complications emerge when considering that the towers had two different levels of fireproofing on the day of the attack. According to the New York Times, the south tower had only three-quarters of an inch of fireproofing on its upper floors (the amount installed during the original construction), while the north tower had 2.5 inches of fireproofing on the same floors. The discrepancy is due to a 1995 effort by the New York/New Jersey Port Authority to gradually begin upgrading the fireproofing thicknesses in the towers.

The tests were conducted at Underwriters Laboratories (UL) facilities in Northbrook, Ill., and Toronto, Canada; NIST and UL are both ANSI members and accredited standards developers. The tests were conducted as part of a NIST contract at the two separate UL fire test laboratories to take advantage of the different capabilities available at these facilities.

All four WTC floor system fire tests used the standard procedure ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, for rating the fire resistance of a building structural unit such as a floor system, column or beam under prescribed conditions. The tests took into account the differing fireproofing levels, as well as examined fire resistance on different lengths of flooring. According to NIST, 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 these accepted test procedures.

Sunder cautions that “These tests alone cannot be used to determine the actual performance of the floor systems in the collapse of the WTC towers. However, they are already providing valuable insight into the role that the floors may have played in causing the inward bowing of the perimeter columns minutes before both buildings collapsed.”

“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.”

A link to the findings from the ASTM E119 WTC floor system tests and the NIST announcement regarding the testing can be found at

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