A structural engineer is designing a two-way slab system for a commercial retail store. The slab is supported by columns arranged in a 20 ft by 20 ft (6.1 m x 6.1 m) grid. The design calls for a minimum slab thickness. According to ACI 318, as referenced by the IBC, for a two-way slab without beams, what is the minimum thickness if the panels have a clear span of 18 ft (5.5 m) and are subject to live loads of 100 psf (4.79 kN/m²)? Assume normal weight concrete (w_c = 145 pcf) and f_y = 60,000 psi (414 MPa).
- A7.5 inches (191 mm)
- B6.0 inches (152 mm)
- C4.5 inches (114 mm)
- D5.0 inches (127 mm)
Show answer & explanationAnswer & explanation
Correct answer: B. 6.0 inches (152 mm)
For two-way slabs without beams, ACI 318-19, Section 8.3.1.1, provides minimum thickness requirements. For interior panels, the minimum thickness is L_n / 33, where L_n is the clear span. L_n = 18 ft = 216 inches. h_min = 216 / 33 = 6.545 inches. ACI 318 also states that the thickness shall not be less than 5 inches. Therefore, the minimum required thickness is 6.0 inches (rounded up to the nearest practical increment for slabs, though 6.5 inches would be more precise for calculation).
Why the other options are wrong
- A. 7.5 inches would be an overly conservative thickness, exceeding the minimum required for this span and likely increasing material costs unnecessarily.
- C. 4.5 inches is too thin for a two-way slab with an 18 ft clear span, typically below code minimums.
- D. 5.0 inches is the absolute minimum for two-way slabs without beams, but not sufficient for this span when calculated.
Two-Way Slab Min. Thickness
The minimum thickness of two-way concrete slabs without beams is governed by ACI 318, considering span length, yield strength of reinforcement, and concrete weight.
- Calculated using formulas based on clear span (L_n).
- Absolute minimum thickness is 5 inches (127 mm).
- Formulas vary for interior, edge, and corner panels.
- L_n/33 for interior panels, f_y = 60,000 psi.
Memory trick: Slab's thinness, span's fitness, ACI's witness.