Most commercial and multi-family residential buildings built in accordance with the International Building Code (IBC) are required to provide fire resistant assemblies at floors and roofs. The code dictates the level of fire resistance, often described in a length of time, based on the following facts:
– The building construction
– Whether it is equipped with a fire suppression system.
The most common way for a designer to comply with a fire rating is to specify an assembly that has been previously tested to achieve the required fire rating.
For wood construction, the assembly often consists of non-combustible layers of materials including gypsum board, cement plaster, insulation, and low density concrete (gyp-crete). However, in some situations, the wood itself can be used as the fire rated assembly. When wood burns, it forms a layer of char that in turn insulates the remaining wood from burning. With the wood providing the fire rating, the additional layers of material can be omitted.
The most common subfloor used in wood construction is plywood or OSB sheathing. However, in some situations, wood decking is chosen as the subfloor because of its aesthetic and span advantage over plywood or OSB. The typical width of a corridor in a multi-family residential building is 6 to 8 feet which is optimal for decking. By using decking in lieu of traditional 2x framing, designers have a few more inches to fit utilities while maintaining the desired floor-to-floor and ceiling heights. Additionally, the fire resistance of decking may be utilized to achieve a fire rating, eliminating the need for a gypsum board (hard lid) ceiling. In order to qualify for the rating, the designer may need to apply for an appeal to the authority having jurisdiction (AHJ). The appeal should include the opinion of a fire protection engineer. In the City of Portland, this appeal has been granted in Type III and Type V construction for 1 and 2 hour fire ratings.
When a fire rating appeal is pursued, it’s recommended that a fire protection engineer is retained early in the design to ensure the proper decking is selected since the char rate of decking is dependent on the wood size, jointing, and species (Janssens). Decking is available in several different thicknesses between 1.5 to 3.5 inches that can span 4 to 16 feet when subject to typical loading. The fire protection engineer can provide the structural engineer with the thickness of char at the required rating. The structural engineer can use the reduced section to ensure the charred decking is adequate to support the weight of the floor or roof and the occupants. With the approval of the AHJ, the structural engineer may use a reduced live load when evaluating the decking during a fire.
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