Recent tests have resolved a longstanding debate among designers over the amount of fire protection appropriate for a steel floor assembly.
Revisions to UL Design No. D982 in the Underwriter Laboratories Fire Resistance Directory, based on fire tests sponsored by the American Institute of Steel Construction and the American Iron and Steel Institute (AISI), mean steel assemblies qualify for half the customary fire-protection thickness, regardless of the design detail in the building, says Charles J. Carter, AISC’s vice president and chief structural engineer. The appropriate thickness had long been argued, he adds.
AISC and AISI paid UL to do the first U.S. fire tests, using a 14-ft x 17-ft replica of a steel floor, built in an unrestrained condition. “Unrestrained” leaves a gap between the test specimen and the furnace walls; the gap allows the unrestrained beam to expand when heated. Previously, U.S. tests had been done only on restrained assemblies with no gap.
In the test, the specimen had the fire protection-thickness of only a restrained assembly—half the amount typically used for construction of unrestrained assemblies. The specimen performed acceptably, says Carter, who on Dec. 5 replaces the retiring Roger E. Ferch as AISC president. “Steel can be considered restrained and qualify for half the fire-protection thickness,” Carter adds, which translates to half the material and labor costs.
The test results did not surprise Farid Alfawakhiri, AISI’s senior engineer for construction codes and standards. During tests in other places around the world, steel-framed floors had demonstrated “good performance,” he notes.
D982 now covers all common steel-framed floor configurations and provides two-hour assembly ratings for unprotected steel-deck and spray-applied fire-protection materials on steel beams with thicknesses sufficient to obtain a one-hour unrestrained-beam rating, says AISC. D982 can be used in the U.S. and Canada with fire-protection materials bearing the UL Classification Mark and works with a range of steel-deck products.