Research: Structures – Fire and Steel Stud Walls

Stud spacing and type of insulation are just two of the factors that will affect how long a typical residential loadbearing steel stud wall assembly can last in a fire.

Steel framing has been widely used in residential buildings in recent years. Loadbearing steel stud wall assemblies form part of steel framing and are often used as party walls in townhouses and as party and corridor walls in multi-unit low-rise construction. Consequently, in Canada, they are required to meet fire resistance requirements.

Because there is very limited information on the fire resistance of loadbearing steel stud wall assemblies in the literature and in building codes, the National Research Council of Canada’s Institute for Research in Construction conducted a series of fire resistance experiments on full-scale wall assemblies in an attempt to generate data. The tests were part of a major collaborative research project initiated with nine industry partners to develop fire resistance ratings for these and various other types of wall assemblies.

The test parameters included stud-spacing, stud rows, shear bracing, load intensity, gauge thickness, gypsum board layers, resilient channel installation and type of insulation. Fourteen complete assemblies (whole systems, not just materials) were tested to get a complete picture of how the various parameters affect fire performance. Parallel acoustical tests were also conducted, as the achievement of good acoustic performance may have negative implications for fire performance, or vice versa.

Systems tested were replicas of wall assemblies commonly used in North America and were 3,048 mm high by 3,658 mm wide. Thirteen of the assemblies were provided with steel cross-bracing to enhance lateral resistance, while the fourteenth assembly was provided with an OSB (oriented strand board) shear membrane. All assemblies were protected with Type X gypsum board on both fire-exposed and unexposed sides.

The assemblies were exposed to heat in a propane-fired vertical furnace. The tests were conducted in accordance with the requirements of CAN/ULC-S101 (which is similar to ASTM E119). Fire exposure continued until the assembly failed — structurally, or by exceeding a specified temperature on the unexposed surface, or by permitting the penetration of flames or gases. All the wall assemblies failed structurally and the unexposed surface temperature (average of single reading temperatures) at the time of the structural failure was below the criteria temperature for failure.

Stud spacing and insulation

The most significant factors influencing the fire performance of the wall assemblies were the number of gypsum board layers, the presence of resilient channels, the stud spacing, the number of stud rows, and the type of insulation in the wall cavity.

Figure 1 shows the effect of stud-spacing on the fire resistance of the single row steel stud wall assembly. The wall assembly with a stud-spacing of 610 mm failed at 74 minutes, while the one with a spacing of 406 mm failed at 59 minutes. Aside from the stud spacing, the two assemblies were of similar configuration, and were loaded with corresponding specified loads. The higher fire resistance at 610-mm spacing could be attributed to factors such as a redistribution of load to the two end studs that occurs during the later stage of fire exposure.

The type of insulation also has a major influence, as illustrated in Figure 2. The uninsulated wall assembly provided the highest fire resistance, at 77 minutes. For the insulated assemblies, the highest fire resistance was provided by the cellulose fibre assembly at 71 minutes, followed by rock fibre at 59 minutes and glass fibre at 56 minutes. All four single-row stud walls, with two layers of gypsum board protection on each side, were of similar configuration except for the type of insulation. These results suggest that maximum fire resistance in a steel stud wall assembly can be obtained with no insulation in the cavity.

Other influences

The tests also indicated the following about steel stud wall assemblies:

Number of stud rows — double-stud walls have higher fire resistance

Number of gypsum board layers — two layers of gypsum board provide higher fire resistance

Replacing one gypsum board layer with an OSB shear membrane in double layer protected walls decreases fire resistance

Installing resilient channels for enhancing acoustical performance decreases fire resistance.

As a result of this project and the parallel acoustic tests, IRC and its industry partners are preparing a proposal for updating the fire and sound resistance ratings of wall assemblies in Part 9 of the NBC. The number of listed steel stud wall assemblies is likely to be increased in the next issue of the code. The information gained in the project will assist builders and regulators to provide suitable wall assemblies, particularly for multi-family dwellings.

Note that the fire resistance of wall assemblies is dependent on a number of other factors such as fastener spacing, type and thickness of gypsum board, and load level. Detailed information on these specifications, as well as details on the assemblies tested and all the major results, including temperatures and deflections, are given in internal reports available from IRC.

Dr. V.K.R. Kodur, P.Eng., is a research officer in the Fire Risk Management Program at the National Research Council’s Institute for Research in Construction in Ottawa. Dr. M. A. Sultan is manager of the Fire Resistant Construction sub-program.