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Fire Resistance Tests of the Floor Truss Systems. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.


pdf icon 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.