Effect of Assembly Size, End Restraints, and Fireproofing Thickness on Fire Endurance Testing of Floor Systems

As part of the National Institute of Standards and Technology's building and fire safety investigation of the World Trade Center disaster, four standard fire resistance tests were conducted on the composite floor system used in the towers. The fire resistance tests were conducted to study three factors, the effect of (1) fireproofing thickness, (2) scale of the test, and (3) test restraint conditions. Two full-scale tests, 35 ft (10.7 m) span, and two reduced-scale tests, 17 ft (5.2 m) span, were conducted. Three tests were conducted under restrained conditions and one under unrestrained conditions. Additionally, two fireproofing thicknesses were applied, the specified thickness and the average applied thickness. Results showed that 1) a fire rating of ¾ h was determined from the reduced-scale restrained test with the specified fireproofing thickness which would not have satisfied 1968 New York City building code rating requirement of 2 h for the WTC towers, 2) the full-scale restrained floor system obtained a fire resistance rating of 1 ½ h, and 3 h) the full-scale unrestrained floor system achieved a 2 h rating. Past experience with the ASTM E 119 test method would lead one to expect that the unrestrained floor assembly would not perform as well as the restrained assembly, and therefore, would receive a lower fire rating.