For Type IV-A construction, noncombustible protection shall provide what fraction of the required fire-resistance rating...

The prescribed proportion is two-thirds , making option C correct. The relationship is demonstrated directly by IBC Table 722.7.1(1). A building element requiring a one-hour fire-resistance rating must receive 40 minutes of noncombustible protection; a two-hour element requires 80 minutes; and a three-hour-or-greater element requires 120 minutes. Each of those prescribed values represents two-thirds of the corresponding one-, two-, or three-hour rating.

For Type IV-A specifically, Section 602.4.1.2 requires protection of the interior faces of all mass timber elements. Section 602.4.1.2.1 further states that the protection must contribute at least the time assigned in Table 722.7.1(1) and not less than 80 minutes .

The remaining portion of the required fire resistance is provided by the mass timber element itself. Thus, for a three-hour Type IV-A primary structural element, 120 minutes are provided through noncombustible protection and the timber must supply the remaining required endurance.

This two-component approach is fundamental to the Type IV-A fire-design methodology.

References/Topics: 2021 IBC Sections 602.4.1.2 and 602.4.1.2.1; Tables 601 and 722.7.1(1); protected Type IV-A mass timber.

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