How is the fire-resistance rating of multiwythe masonry walls calculated as per the 2018 International Building Code?

The fire-resistance rating of multiwythe masonry walls is calculated using a formula that considers the fire-resistance rating of each wythe and the continuous airspace between them. The formula is RA = (R1^0.59 + R2^0.59 + ... + Rn^0.59 + A1 + A2 + ... + An)^1.7, where R1, R2, ..., Rn are the fire-resistance ratings of the wythes and A1, A2, ..., An are factors for each continuous airspace.

In the 2018 International Building Code, the fire-resistance rating of multiwythe masonry walls is determined by a specific calculation method. The formula used is RA = (R1^0.59 + R2^0.59 + ... + Rn^0.59 + A1 + A2 + ... + An)^1.7. Here, RA represents the fire-resistance rating of the assembly in hours. R1, R2, ..., Rn are the fire-resistance ratings of each individual wythe, and A1, A2, ..., An are factors for each continuous airspace between the wythes, with a value of 0.30 for each airspace that is at least 1/2 inch deep. This method allows for the integration of the fire-resistance properties of each wythe and the airspaces to determine the overall fire-resistance rating of the wall assembly.