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Fire Resistance Tests of the Floor Truss Systems. Federal Building and Fire Safety Investigation of the World Trade Center Disaster (NIST NCSTAR 1-6B) ***DRAFT for Public Comments***



John L. Gross, F Hervey, M Izydorek, J Mammoser, J Treadway


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.
National Construction Safety Team Act Reports (NIST NCSTAR) - 1-6
Report Number


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
Created September 1, 2005, Updated February 19, 2017