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Structural Design Calculations under Fire Conditions 1 :X dt " i=l,! 'r ri5 When utilizing the char calculation option of NDS Chapter 16 to demonstrate fire-resistance ratings,a structural design Beduclamm Mara"Wiefff Depth pus? 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, • allowing it to retain structural capacity. • """'" "_"'"' : ° a s NDS Section 16,2.2 states that, under fire design conditions, , l £ i the average member strength can be approximated by multiplying reference design values such as Fb by the `-""t adjustment factors specified in Table 16.2.2. As indicated .-' r in Table 7,an increase in allowable design stresses by a factor ranging from 2.03 to 2.85 is allowed, depending on the stress 'Fn 'u I9 "' ' Saw�xN pBC under consideration. +t"� 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 TABLE 6: applicable)using the full cross-sectional dimensions and Effective Char Rates and Char Depths adjustment factors per NDS Chapter 5. If this beam were (tor 8, = 1.5 inches/hour) required to have a 1-hour fire-resistance rating(perhaps as n +e Ex a 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 tr NDS Table 16.2.1A). Its cross-sectional dimensions under • a fire conditions would be: 1 215 .1 Width = 6.75•-(2111.8') =3.15' 1-1/2 .1 2 2.6 Depth = 13.5'-1.8" = 11.7' Source NOS Table 162IA,AWC This reduced cross-section would then be checked under fire conditions,with allowable design stresses increased by the factors given in NDS Table 16.2.2. For example, a TABLE 7: 2.85 increase factor could be applied to allowable bending Member Strength Adjustment Factors stresses.AWC's TR 10 provides design examples for a number of exposed timber applications under fire conditions The stress adjustment factor, K,to increase the reference design stress is for use when performing a structural capacity Bending strength 2.85 check under the fire load condition with allowable stress Beam bucking strength 2.03 design (ASD) load combinations le.g., D + L,etc.). This stress Tensile strength 2.65 adjustment factor is not intended to be used with load and resistance factor design ILRFD)load combinations,including Compressive strength 2.58 those intended for extraordinary events such as in ASCE 7-16 Column buckling strength 2.03 Section 2.5. Shear strength 2.75" Source:NOS Table 16.1.1 AWC *From TR 1O Table 4.1.2,not the NOS J 5