STUD SPACING AND INSULATION AFFECT BEARING WALL ASSEMBLY FIRE RESISTANCE

Stud spacing and type of insulation used were two of the factors that most affected the performance of load-bearing steel-stud wall assemblies in studies conducted by the Institute for Research in Construction (IRC), National Research Council Canada. The tests were conducted under the IRC’s Fire Risk Management Program and in conjunction with nine “industry partners.”(1)
The IRC conducted 14 full-scale standard tests to assess the effects of various parameters on the fire resistance of load-bearing steel-stud wall assemblies. Among the test parameters were the spacing of steel studs, the number of stud rows (single vs. double-stud walls), the number of gypsum board layers (one layer vs. two layers of protection), the use of cross-bracing, replacing one layer of gypsum board with an oriented strand board shear membrane, using resilient metal channels to support the gypsum board, and the type of insulation used in the wall cavity.
Each test assessed an entire wall system of the size that is commonly used in North American buildings and did not focus on individual materials so that fire performance could be assessed in various scenarios.
Each assembly was exposed to fire in a propane-fired vertical furnace until it failed structurally, exceeded a specified temperature, or was penetrated by flame or gas.
In all cases, the assemblies failed structurally. The unexposed surface temperature was below the temperature criteria for failure.
The results indicate that steel-stud wall systems with 610 mm stud spacing provided higher fire resistance than those with stud spacing at 406 mm. According to the IRC, the difference in fire resistance can be explained in that the wall assembly with wider spacing has fewer studs than the wall assembly with closer spacing and, therefore, carries a smaller load.
Test protocol was based the load on the number of studs?fewer studs mean a smaller overall load. During the fire tests for both assemblies, the load shifted to the end studs, which are less exposed to fire than the inner studs. Since the end studs must carry a greater load for the wall assembly with closer spacing, this assembly fails more quickly than the assembly with wider spacing, explains the IRC.
The test also showed that uninsulated wall assemblies provided a higher fire resistance than those with insulation. Insulation reduces the fire resistance, explains the IRC, because it keeps the gypsum board facing the fire hot, which causes it to crack and fail more quickly than it would if the cavity were empty. The insulation allows the heat to build up and become trapped in the cavity, hastening the structural failure of the studs. Once the assembly has failed, the insulation and studs are exposed to the heat of the fire. Effects on fire resistance differed with the type of insulation.
Specific questions about this project may be directed to Dr. Venkatesh Kodur, e-mail venkatesh.kodur@nrc.ca. Source: Construction, NRC-CNR, 7:2, June 2002, 6-7.

Endnote
1. Participants were the Canadian Home Builders Association, Canadian Sheet Steel Building Institute, Canadian Steel Construction Council, Canadian Wood Council, Cellulose Insulation Manufacturers Association of Canada, Forintek Canada Corporation, Gypsum Manufacturers of Canada, Owens-Corning Canada, and Roxul Inc.

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