Commercial Building Inspector B2Wall Construction and Wall CoveringsHard
A client is proposing a new commercial building with a 2-hour fire-resistance-rated masonry wall. The wall will be constructed using 8-inch (203 mm) concrete masonry units (CMU). What is the minimum equivalent thickness of solid masonry required for this wall assembly to achieve the 2-hour rating, assuming standard weight concrete masonry?
- A3.6 inches (91 mm)
- B5.3 inches (135 mm)
- C4.8 inches (122 mm)
- D4.0 inches (102 mm)
Show answer & explanationAnswer & explanation
Correct answer: C. 4.8 inches (122 mm)
According to IBC Table 722.3.2.2(1) for concrete masonry walls, a 2-hour fire-resistance rating requires a minimum equivalent thickness of 4.8 inches (122 mm) for standard weight concrete masonry (120 pcf). The nominal 8-inch CMU must achieve this equivalent solid thickness.
Why the other options are wrong
- A. 3.6 inches is for a 1.5-hour rating, not 2 hours.
- B. 5.3 inches is for a 3-hour rating, not 2 hours.
- D. 4.0 inches is for a 1.5-hour rating or specific lightweight units, not standard weight 2-hour.
CMU Fire-Resistance Equivalent Thickness
The minimum equivalent solid thickness of concrete masonry units required to achieve a specific fire-resistance rating.
- Equivalent thickness accounts for the hollow spaces in CMU.
- Fire-resistance rating depends on concrete type (e.g., normal weight, lightweight) and aggregate.
- IBC Table 722.3.2.2(1) provides these values.
Memory trick: CMU's fire strength: thickness, weight, and the clock's tick.