In the National Building Code of Canada1 "Fire-resistance rating" is defined in part as: "the time in minutes or hours that a material or assembly of materials will withstand the passage of flame and the transmission of heat when exposed to fire under specified conditions of test and performance criteria…"
Horizontal assemblies such as floors, ceilings and roofs are tested for fire exposure from the underside only. This is because a fire in the compartment below presents the most severe threat. For this reason, the fire-resistance rating is required from the underside of the assembly only.
The fire-resistance rating of the tested assembly will indicate, as part of the listing limitations, the restraint conditions of the test. When selecting a fire-resistance rating, it is important to ensure that the restraint conditions of the test are the same as the construction in the field. Wood frame assemblies are normally tested with no end restraint to correspond with normal construction practice.
Partitions or interior walls required to have a fire-resistance rating must be rated equally from each side since a fire could develop on either side of the fire separation. They are normally designed symmetrically. If they are not symmetrical, the fire-resistance rating of the assembly is determined based on testing from the weakest side.
For a loadbearing wall, the test requires the maximum load permitted by design standards be superimposed on the assembly. Most wood-stud wall assemblies are tested and listed as loadbearing. This allows them to be used in both loadbearing and non-loadbearing applications. Most steel-stud wall assemblies are tested and listed as non-loadbearing because they are used primarily in non-loadbearing applications in noncombustible buildings.
Loadbearing steel-stud wall assemblies typically use studs of a heavier gauge steel than non-loadbearing studs to be able to support the load. The heavier gauge stud reacts differently when exposed to fire and withstands the tendency for studs to twist and distort when exposed to heat. Loadbearing and non-loadbearing steel stud wall listings are not interchangeable because the properties of the studs in these assemblies are not the same. Listings for loadbearing wood stud walls can be used for non-loadbearing cases since the same studs are used in both applications.
Loading during the test is critical as it affects the capacity of the wall assembly to remain in place and serve its purpose in preventing fire spread. The strength loss in studs resulting from elevated temperatures or actual burning of structural elements causes deflection. This deflection affects the capacity of the protective wall membranes (gypsum wallboard) to remain in place and contain the fire. The fire-resistance rating of loadbearing wall assemblies is typically lower than that of a similarly designed non-loadbearing assembly.
Exterior walls only require rating for fire exposure from within a building. This is because fire exposure from the exterior of a building is not likely to be as severe as that from a fire in an interior room or compartment. Because this rating is required from the inside only, exterior wall assemblies do not have to be symmetrical.
The fire-resistance ratings listed have been determined on the basis of tests conducted in accordance with the ULC-S101 standard which is shown in Table 4 - Fire and Sound Resistance of Walls - (Excerpt from Table A-18.104.22.168.A National Building Code of Canada1).
The listings are useful because they offer off-the-shelf solutions to designers. They can, however, restrict innovation because designers use assemblies which have already been tested rather than pay to have new assemblies evaluated. Listed assemblies must be used with the same materials and installation methods as those tested.
The National Fire Protection Research Foundation2 has published a report ,ENGINEERED LIGHTWEIGHT CONSTRUCTION LITERATURE REVIEW & TECHNICAL ANALYSIS, resulting from a study on the fire performance of various types of lightweight construction. The following is a brief description of the report. For availability of the report click here.
ENGINEERED LIGHTWEIGHT CONSTRUCTION LITERATURE REVIEW & TECHNICAL ANALYSIS
by Kirk Grundahl, P.E.
The Literature Review and Technical Analysis -- the first phase of the Engineered Lightweight Construction Fire Research Project -- comprehensively surveys the literature on engineered lightweight construction (trusses, bar joists, prefabricated wood I-joists, light-gauge steel joists, etc.), and analyzes the issues of potential sudden collapse when subjected to fire conditions. There has been widespread concern among the fire service, manufacturing, fire sprinkler and insurance communities regarding the fire performance of construction that relies more for strength on engineering design than on mass. The concern is for firefighter and occupant safety, roof or floor collapse, and fire suppression system adequacy. Many feel that there is inadequate documentation for some current practices, and misapplication of codes. The Technical Analysis measures progress toward answering these questions. It also identifies gaps in knowledge, thus providing a starting point for further research.
Some web sites of wood product manufacturers that contain more information on fire ratings are:
American Wood Council
iLevel by Weyerhaeuser
Structural Building Components Association
- 1. National Building Code of Canada, NRCC No. 38726, National Research Council, Ottawa, ON, 1995.
- 2. Linville, J. et al, SFPE Engineering Guide to Performance-Based Fire Protection, Analysis and Design of Buildings, Society of Fire Protection Engineers, Washington, D.C. and National Fire Protection Association, Quincy, MA, 2000.