Commercial Building Inspector B2Floor ConstructionHard
A commercial building inspector is reviewing the structural plans for a new office building. The plans show a precast concrete floor system with openings for mechanical ducts. According to ACI 318, as referenced by the IBC, what is the maximum permissible size for an opening in a two-way slab without beams if it is located in the middle-strip region and does not contain reinforcement specifically designed for the opening?
- ANot exceeding 4 times the slab thickness.
- BNot exceeding 16 times the slab thickness.
- CNot exceeding 8 times the slab thickness.
- DNot exceeding 24 times the slab thickness.
Show answer & explanationAnswer & explanation
Correct answer: C. Not exceeding 8 times the slab thickness.
ACI 318-19, Section 8.7.4.2.1(a), states that openings in two-way slabs without beams, not containing reinforcement specifically designed for the opening, and located in the middle-strip region, shall not exceed 8 times the slab thickness in any direction. The total area of such openings in a middle strip shall not exceed 1/20 of the total middle strip area.
Why the other options are wrong
- A. 4 times the slab thickness is too restrictive; this might apply to openings in column strip regions or near concentrated loads.
- B. 16 times the slab thickness is excessively large and would require specific reinforcement and analysis.
- D. 24 times the slab thickness is far too large for an unreinforced opening in a two-way slab.
Slab Opening Limits (ACI 318)
ACI 318 provides specific limitations on the size and location of openings in concrete slabs, especially two-way slabs, to maintain structural integrity without requiring additional reinforcing.
- Limits depend on slab type (one-way, two-way).
- Limits depend on location (column strip, middle strip).
- Limits depend on whether specific reinforcement is provided.
- Unreinforced openings in middle strips are limited to 8 times slab thickness.
Memory trick: Holes in the slab, by ACI's grab, middle-strip's max, 8x thickness, no lax.