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36 csengineermag.com august 2019 For example, a 6-3/4-inch x 13-1/2-inch glulam beam with an unadjusted allowable bending stress of 2,400 psi would first be checked for all structural loading conditions and limit states (bending, shear deflection, vibration and others as applicable) using the full cross-sectional dimensions and adjustment factors per NDS Chapter 5. If this beam were required to have a 1-hour fire-resistance rating (perhaps as a floor beam in a Type V-A structure) then its effective char depth on all three exposed sides would be 1.8 inches (per NDS Table 16.2.1A). Its cross- sectional dimensions under fire conditions would be: Width = 6.75" – (2)(1.8") = 3.15" Depth = 13.5" – 1.8" = 11.7" Calculation-Based Method As referenced in IBC Section 722.1, NDS Chapter 16 can be used to calculate the structural fire-resistance rating of vari- ous wood products, including solid sawn, glulam, SCL, and CLT. As noted by Douglas and Smart in Structure magazine (July 2014), "The design procedure allows calculation of the capac- ity of exposed wood members using basic wood engineering mechanics. Actual mechanical and physical properties of the wood are used, and member capacity is directly calculated for a given period of time—up to 2 hours. Section properties are computed assuming an effective char depth, βeff, at a given time, t. Reductions of strength and stiffness of wood directly adjacent to the char layer are addressed by accelerating the char rate by 20 percent. Average member strength properties are approximated from existing accepted procedures used to calculate design properties. Finally, wood members are de- signed using accepted engineering procedures found in NDS for allowable stress design." The American Wood Council's (AWC's) Technical Report 10 – Calculating the Fire Resistance of Wood Members and Assemblies (TR 10) provides an in-depth explanation of the concepts and background associated with exposed wood fire design. This document also includes a number of design examples for exposed structural wood members utilizing the provisions of NDS Chapter 16. Structural Design Calculations under Fire Conditions When utilizing the char calculation option of NDS Chapter 16 to demonstrate fire-resistance ratings, a structural design check must also be done to determine structural adequacy of framing members under fire conditions. One of the main benefits of the char calculation method is that it accounts for the ability that heavy and mass timber have to form a char zone, which insulates the remaining wood cross-section, al- lowing it to retain structural capacity. NDS Section 16.2.2 states that, under fire design conditions, the average member strength can be approximated by mul- tiplying reference design values such as Fb by the adjustment factors specified in Table 16.2.2. As indicated in Table 7, an increase in allowable design stresses by a factor of 2.03 to 2.85 is allowed, depending on the stress under consideration.

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