The requirement applies to Type IV-C construction. Under the 2021 IBC provisions governing Type IV-C, combustible construction forming concealed spaces must be protected with noncombustible protection having an assigned protection time of not less than 40 minutes. Therefore, option C is correct.
The assigned protection time is determined using the protection-time values prescribed by the code, including the applicable values for gypsum board and other approved noncombustible materials. The provision is intended to delay ignition and involvement of combustible mass timber surfaces located within concealed spaces, where fire growth might otherwise remain undetected and become difficult for fire-suppression personnel to access.
Type IV-A and Type IV-B are subject to more extensive noncombustible protection requirements because those construction types support taller buildings and require greater protection of mass timber elements. Type IV-HT uses the traditional heavy-timber concealed-space provisions and may employ alternative methods, such as complete filling with noncombustible insulation.
During inspection, the special inspector must confirm that the installed layers, thicknesses, fastening patterns, joints, penetrations, and transitions collectively provide the required 40-minute assigned protection time.
References/Topics: 2021 IBC Section 602.4.3.5; Table 722.7.1(1); Type IV-C concealed-space protection.
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