Sept.
9, 2004
(updated from original statement issued Aug. 26, 2004)
Contact:
Michael E. Newman
(301) 975-3025
michael.newman@nist.gov
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NIST
Tests Provide Fire Resistance Data on
World Trade Center Floor Systems
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 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. Shyam Sunder, lead investigator of
the NIST WTC investigation, cautions, “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 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.”
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 the two separate
UL fire test laboratories to take advantage of the different capabilities
available at these facilities.
The first two
tests, conducted in early August 2004 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. The
test on Aug. 25, 2004, 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 Aug. 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 45 minutes was determined from
the Aug. 25 test with the “as-specified” (13 millimeters)
fireproofing thickness.
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.
The findings
from the ASTM E119 WTC floor system tests are available at http://www.nist.gov/public_affairs/releases/ULTestResults.pdf.
(.pdf;
download Acrobat Reader) These findings
may be revised and additional findings may be included in the NIST
investigation team’s final report, scheduled for release as
a draft document in December 2004. NIST is not making any recommendations
at this time. All recommendations will be made in the final report.
More information on the NIST WTC investigation is available at http://wtc.nist.gov.
As a non-regulatory
agency of the U.S. Department of Commerce’s Technology Administration,
NIST develops and promotes measurement, standards and technology
to enhance productivity, facilitate trade and improve the quality
of life.
![](https://webarchive.library.unt.edu/eot2008/20080924002613im_/http://www.nist.gov/public_affairs/images/wtc_floor_system.gif)
Diagram
of Composite WTC Floor System
(The truss and floor slab sections were the elements tested
in the Canadian and Illinois fire tests)
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Date created:8/27/04
Last updated: 09/09/04
Contact: inquiries@nist.gov
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