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The program implements NDS Chapter 16 for fire design.
Input
For each member in the structure, you can indicate the required fire resistance rating in minutes, which sides of the member are exposed, and whether gypsum fire protection is used.
Whether fire calculations are applied to Concept Mode, and the default fire duration rating are set in the Design Settings.
Fire Protection
The program takes into account the protection from fire afforded by materials covering the exposed surfaces of the affected members. This is not specified in either the NDS or the AWC documents; it comes from Fire Resistance of Wood Members with Directly Applied Protection (Robert H. White 2009). You are limited to the choices of one- or two-ply 5-8” fire rated (Type X) gypsum wall board for as they were the ones tested by White. The duration of fire used in the charring calculations is reduced from the required fire rating by the amount of time afforded by the Protection type selected. For one ply 5-8” fire rated (Type X ) gypsum wall board it is 30 minutes; for two ply of this material it is 60 minutes
Char Rate and Residual Section
Fore each exposed side, a char depth achar is calculated using the 16.2.1.2, equation 16.2-2 for sawn lumber, SCL, and glulam, and 16.2.1.3 , equation 16.2-3 for CLT.
This char depth is based on a nominal char rate βn of 1.5 in./hr. as recommended by NDS.
The achar depth is then multiplied by 1.2 as per 16.2.1 to get the effective char depth, aeff .
A residual section size is then determined by subtracting the effective r each exposed surface in the char depth.
CLT Char Rate and Residual Section
Equation 16.2-3 calculates the number of layers that will fully burn during the exposure time, using an increased char rate for the fully burned layers to account for the effect of lamination, then adds the char depth of the final partially charred layer using the usual char rate.
For fire-reduced sections, transverse layers that are the final partially charred layer and are thus the outermost layer on the exposed side of the panel are not included in the residual section for the following calculations
Loads Analysis
The member is analysed using the stiffness EI of the full section. Since stiffness is constant over the length of the member, the resulting reactions and stresses are the same as for the reduced section. Only load combinations involving Dead, Live and Snow are evaluated at reduced cross section for fire design criteria.
As per the 2011 AWC FIre Design Standard, 3.1.4.1.4 l oad combination factors used are always 1.0, even if live and snow appear in the same combination, in other words, D + L + S is used instead of the ASD combination.
Design
Design Criteria
Several design criteria used for non-fire design are calculated using the residual cross section as the member breadth b and depth d values. The member design is considered to have failed if one of them fails, and the program passes over a candidate section when designing for unknowns if it fails one of these criteria.
The design criteria checked for fire design are given in NDS 16.2.2 and Table 16.2.2:
For beams, columns, and all CLT panels
For columns and wall panels only
In addition to these criteria from the NDS, the following criterion from the AWC Technical Report 10 is checked
For beams, columns and all CLT panels
Technical Report 10 describes an identical procedure as in NDS 16 for the other criteria.
Adjustment Factors
The adjustment factors shown in Table 16.2.2 are the only modification factors applied. Each design criterion has a "design stress to member strength" factor that is specific to fire design; the other factors are those used for non-fire design.
The size factor CF glulam volume factor Cv, and flat use factor Cfu are calculated using the full cross section of the member ; the stability factors CL and CP use the fire-reduced section.
The the "design stress to member strength" factor for shear design are given in AWC Technical Report 10; it is 2.75.
CLT Design
For CLT, following values are calculated differently for fire design to account for the modified geometry of the section due to charring, for example because of a shifted neutral axis:
Output
Fire endurance results for each design criterion are output in a similar manner to other design criteria. In addition, the program outputs the fire protection time, size of the residual section, the char rate, and the Design Stress to Member Stress factor from Table 16.2.2