SECTION 721 CALCULATED FIRE RESISTANCE

721.4.2 Multiwythe walls. The fire-resistance rating for walls or partitions consisting of two or more dissimilar wythes shall be permitted to be determined by the formula:

  where:

R = The fire-resistance rating of the assembly (hours).

RI R2 and Rn = The fire-resistance rating of the individual wythes (hours).

Values for Rn0.59 for use in Equation 7-11 are given in Table 721.4.1(3).

721.4.2.1 Multiwythe walls of different material. For walls that consist of two or more wythes of different materials (concrete or concrete masonry units) in combination with clay masonry units, the fire-resistance rating of the different materials shall be permitted to be determined from Table 721.2.1.1 for concrete; Table 721.3.2 for concrete masonry units or Table 721.4.1(1) or 721.4.1 (2) for clay and tile masonry units.

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