Fire Resistance Tests of the Floor Truss Systems. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.
Fire Resistance Tests of the Floor Truss Systems.
Federal Building and Fire Safety Investigation of the
World Trade Center Disaster.
(7090 K)
Gross, J. L.; Hervey, F.; Izydorek, M.; Mammoser, J.;
Treadway, J.
NIST NCSTAR 1-6B; 202 p. September 2005.
Keywords:
World Trade Center; high rise buildings; building
collapse; disasters; fire safety; fire investigations;
terrorists; terrorism; fire resistance tests; floors;
trusses; standards; fire tests; test methods; structural
behavior
Abstract:
The National Institute of Standards and Technology
(NIST) review of available documents related to the
design and construction of the World Trade Center (WTC)
towers indicated that the fire performance of the
composite floor system of the WTC towers was an issue of
concern to the building owners and designers from the
original design and throughout the service life of the
buildings. However, NIST found no evidence that fire
resistance tests of the WTC floor system were ever
conducted. As a result, NIST conducted a series of four
standard fire resistance tests (ASTM E 119). In this
series of tests, the effects of three factors were
studied: (1) thickness of sprayed fire-resistive
material (SFRM), (2) test restraint conditions, and (3)
scale of the test. The tests were conducted by
Underwriters Laboratories, Inc. under a NIST contract
and represented both full-scale (35 ft span) and
reduced-scale (17 ft span) floor assemblies constructed
to represent the original design as closely as
practical. For three of the tests, the thickness of the
sprayed fire resistive material was 3/4 in. which
represented the average thickness applied in the
original construction. In the fourth test, the thickness
of applied SFRM was 1/2 in. which was the thickness
specified for the original design. Tests were conducted
in both the restrained and unrestrained condition to
provide bounds on the expected performance of the floor
system under the standard fire exposure. The restrained
full-scale :floor system obtained a fire resistance
rating of 1 1/2 h, while the unrestrained floor system
achieved a 2 h rating. For the unrestrained test
condition, specimens protected with 3/4 in. thick
sprayed fire resistive material were able to sustain the
maximum design load for approximately 2 h without
collapsing; in the unrestrained test, the load was
maintained without collapsing for 3 1/2 h. Past
experience with the ASTM E 119 test method would lead
investigators to expect that the unrestrained floor
assembly would not perform as well as the restrained
assembly, and therefore, would receive a lower fire
rating. A fire rating of 2 h was determined from the
reduced-scale test with the average applied SFRM
thickness of 3/4 in., while a fire rating of 1 1/2 h was
determined from the full-scale test with the same SFRM
thickness. This finding raises the question of whether
or not a fire rating based on the ASTM E 119 performance
of a 17 ft span floor assembly is scalable to a larger
floor system such as found in the WTC towers where spans
ranged from 35 ft to 60 ft.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899