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Where a continuous airspace separates multiple wythes of the wall or partition, the total fire-resistance rating shall be determined by the formula:

R = The fire-resistance of the assembly (hours).
R1, R2 and Rn = The fire-resistance of the individual wythes (hours).
as = Coefficient for continuous airspace.

Values for Rn0.59 for use in Equation 7-10 are given in Table 722.4.1(3). The coefficient for each continuous airspace of 1/2 inch to 31/2 inches (12.7 to 89 mm) separating two individual wythes shall be 0.3.

Related Code Sections

721.3.3 Fire-Resistance-Rated Construction, Multiwythe Masonry Walls
The fire-resistance rating of wall assemblies constructed of multiple wythes of masonry materials shall be permitted to be based on the fire ...