The Commerce Department’s National Institute of Standards and Technology (NIST) reported that results from a series of four fire resistance tests conducted this month are part of a 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. No recommendations have yet been made that would influence the future design of structures like the WTC.
Shyam Sunder, lead investigator of the NIST WTC 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. Sunder cautioned, “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.”
“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 said to a group that gathered to watch yesterday’s final test at Underwriters Laboratories (UL) in Northbrook, Ill. “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,” Sunder said.
All four WTC floor system fire 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. The tests were conducted as part of a NIST contract at two separate UL fire test laboratories to take advantage of the different capabilities available at these facilities.
The first two tests, conducted earlier this month at the UL facility in Toronto, Canada, looked at the fire performance of 11-meter (35-foot) floor systems coated with a near-uniform 19-millimeter-thick (0.75-inch) layer of fireproofing material. This is representative of the span size and as-applied average fireproofing thickness of the floor systems in the WTC towers.
One floor system in the Canadian tests was restrained (prevented from expanding due to thermal conditions) while the other was not. Understanding the impact of restraining or not restraining the WTC floor systems during ASTM E119 testing is important. Floor systems tested under ASTM E119 traditionally have been restrained; however, the novel design of the floor systems in the WTC towers did not qualify as either fully restrained or fully unrestrained.
Past experience with the ASTM E119 test method would lead investigators to expect that the unrestrained floor system would not perform as well as the restrained assembly, and therefore, it would receive a lower fire rating. The Canadian tests actually yielded the opposite result: the restrained WTC floor system was fire rated at 1.5 hours while the unrestrained floor system was rated at two hours. NIST investigators will consider this difference when evaluating the performance of the actual WTC floor systems.
For the two experiments at UL in Illinois, 5-meter (17-foot) truss spans – the standard size used in U.S. fire resistance tests – were built. Both were restrained. The test on Aug. 19, 2004, was conducted on a floor system with a fireproofing thickness of 19 millimeters (0.75 inch), the same as the 11-meter assemblies tested in Canada. Yesterday’s final fire test used a 5-meter truss with a fireproofing thickness of 13 millimeters (0.5 inch). This was the thickness of the truss fireproofing originally specified when the WTC towers were built. Therefore, if an ASTM E119 fire resistance test had been conducted on the WTC floor system prior to construction, these would have been the test conditions. NIST has no evidence or record indicating that such a test was ever done.
A fire rating of two hours was determined from the August 19 test with the “as-installed” (19 millimeters) fireproofing thickness. This matches the 1968 New York City building code rating for floor systems in Construction Class IB buildings (the designation assigned to the WTC towers when they were built). A fire rating of one hour was determined from yesterday’s test with the “as-specified” (13 millimeters) fireproofing thickness based on the ASTM E119 standard in place during the 1960s.
One question raised by the data from the four tests is whether or not a fire rating based on the ASTM E119 performance of a 5-meter floor system is “scalable” to a larger floor system – such as the WTC towers assemblies that were 11-meter (35-feet) and 18-meter (60-feet) lengths. This was identified when one of the larger-scale tests in Canada had a lower fire resistance rating than the smaller-scale test in Illinois.