The previous section on fire-resistance ratings deals with the determination of fire-resistance ratings from standard tests. Alternative methods for determining fire-resistance ratings are permitted as well.
The alternative methods of determining fire-resistance ratings are contained in the NBCC, Division B, Appendix D, Fire-Performance Ratings. These alternative calculation methods can replace expensive proprietary fire tests. In some cases, these allow less stringent installation and design requirements such as alternate fastener details for gypsum wallboard and the allowance of openings in ceiling membranes for ventilation systems.
Section D-2 in NBCC, Division B, Appendix D includes methods of assigning fire-resistance ratings to:
- wood-framed walls, floors and roofs
- glue-laminated timber beams and columns
Component Additive Method
The most practical alternative calculation method, includes procedures for calculating the fire-resistance rating of light wood-frame wall, floor and roof assemblies based on generic descriptions of materials. This component additive method (CAM) can be used when it is clear that the fire-resistance rating of an assembly depends strictly on the specification and arrangement of materials for which nationally recognized standards exist.
The assemblies must conform to all requirements in NBCC, Division B, Appendix D for the rating to be valid.
Calculating Fire Resistance of Glulam Timbers
NBCC, Division B, Appendix D also includes empirical equations for calculating the fire-resistance rating of glue-laminated (glulam) timber beams and columns. These equations were developed from theoretical predictions and validated by test results.
Large wood members have an inherent fire resistance because:
the slow burning rate of large timbers, approximately 0.6mm/minute under standard fire test conditions
the insulating effects of the char layer which protects the unburned portion on the wood
These factors result in unprotected members that can stay in place for a considerable time when exposed to fire. The NBCC recognizes this characteristic and allows unprotected wood members including floor and roof decks, which meet the minimum sizes for heavy timber to be used both where a 45 minute fire-resistance rating is required and in many noncombustible buildings.
Provisions for calculating the fire-resistance rating of glulam beams and columns are based on data from tests on timber beams and columns. This calculation method determines a fire-resistance rating for beams and columns based on exposure to fire from three or four sides. Using this approach, the fire-resistance rating (FRR) in minutes of glulam beams and columns is
f = the load factor shown in Figure 1 to compensate for partial loading
B = the full dimension of the smaller side of the beam or column in mm before exposure to fire as shown in Figure 2
D = the full dimension of the larger side of the beam or column in mm before exposure to fire as shown in Figure 2
|Figure 1: Load factor for glulam fire-resistance calculations (NBCC, 2010)|
The formula for columns or beams which may be exposed on three sides applies only when the unexposed face is the smaller side of a column; no experimental data exists to verify the formula when a larger side is unexposed. If a column is recessed into a wall or a beam into a floor, as shown below, the full dimensions of the structural member are used in the formula for exposure to fire on three sides.
|Figure 2: Glulam exposure cases (NBCC Appendix D, 1995)|
Comparisons of the calculated fire-resistance ratings with experimental results show the calculated values are very often conservative.
A designer may determine the factored resistance for a beam or column by referring to CSA Standard O86-09 Engineering Design in Wood or the 1995 Canadian Wood Council's Wood Design Manual.
An example of fire-resistance calculation of glulam beam is shown below.
Determine the fire-resistance rating of a glulam beam exposed on three sides having dimensions of 175 x 380mm and with a factored bending moment equal to 80 percent of its bending moment resistance.
B = 175mm D = 380mm
From Figure 1, f=1.075 for a beam designed to carry a factored load producing 80 percent of factored bending moment resistance.
Total fire-resistance rating = 66.6 minutes
This beam could be used to support a one hour fire-resistance rated wood frame floor assembly.
Further information on the calculation of fire resistance of heavy timber members is available in the American Wood Council's publication Calculating the Fire Resistance of Exposed Wood Members. Note that this document is not currently referenced in the NBCC.