Commercial Building Inspector B2Wall Construction and Wall CoveringsHard
A commercial building inspector is reviewing the plans for a new building with interior nonbearing walls constructed with 8-inch nominal concrete masonry units (CMU). The design calls for a 2-hour fire-resistance rating. What is the minimum equivalent thickness of solid masonry required for this wall to achieve the rating?
- A5.0 inches
- B4.4 inches
- C4.0 inches
- D3.6 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 4.4 inches
According to IBC Table 722.3.2.1(1) (Equivalent Thickness for Fire-Resistance Rating of Concrete Masonry Walls), for a 2-hour fire-resistance rating with typical aggregates (e.g., calcareous or siliceous gravel), the minimum equivalent thickness is 4.4 inches.
Why the other options are wrong
- A. This equivalent thickness is typically required for a 3-hour rating or higher.
- C. This equivalent thickness might be for a 1.5-hour rating or specific aggregate types, not a general 2-hour rating.
- D. This equivalent thickness is typically for a 1-hour rating, not 2 hours.
CMU Equivalent Thickness for Fire Rating
The 'equivalent thickness' of a concrete masonry unit (CMU) wall is the solid thickness of masonry that would provide the same fire resistance as the hollow unit, and is used to determine the fire-resistance rating based on aggregate type.
- Accounts for hollow cells in CMU.
- Calculated by dividing net solid volume by wall area.
- IBC Table 722.3.2.1(1) specifies requirements.
Memory trick: CMU walls need thickness to fight fire, their equivalent solid form is higher.