Two-way slab calculator
The static moment, the panel split, the column-strip share and the punching check at the column — the four numbers a two-way slab design turns on. The direct design method from §8.10, with the punching coefficients from §22.6 named.
| Quantity | Value | Unit |
|---|---|---|
| Clear span | 5600 | mm |
| Total static moment M₀ | 235.2 | kN·m |
| Negative moment | 152.9 | kN·m |
| Positive moment | 82.3 | kN·m |
| Column strip negative | 114.7 | kN·m |
| Middle strip negative | 38.2 | kN·m |
| Steel required at column strip | 1.032e+4 | mm²/m |
| Critical perimeter | 2280 | mm |
ACI 318-19 §8.10 direct design method: valid for three or more spans, a long/short ratio ≤ 2 and live load ≤ 2× dead.
M₀ = wu · l2 · ln² / 8 · Total moment in the panel, both directions.
vu = Vu / (bo · d) · bo is the perimeter at d/2 from the column.
vc = min(0.33, 0.17(1+2/β), 0.083(2+αs·d/bo))√f'c · ACI 318 §22.6.5.2; φ = 0.75.
A clear path from inputs to results.
A two-way slab carries load in both directions, and the design turns on four numbers: the static moment, how it splits between negative and positive, how much of it the column strip takes, and whether the column punches through. This gives all four.
How to use
- Enter the panel spans, thickness and effective depth.
- Set the ultimate load and the column size and position.
- Read the moments, the column-strip share and the punching utilisation.
The formula
Reference: ACI 318-19 §8.10 (direct design method) and §22.6.5.2 (punching).
Worked example
6 × 6 m panel, 200 mm slab, 10 kPa
- 400 mm square interior column, d = 170 mm, f'c 30 MPa.
- 1
ln = 5.6 m, M₀ = 235 kN·m. Negative 153 kN·m, positive 82 kN·m, column strip 115 kN·m. bo = 2,280 mm, vu = 0.93 MPa against φvc = 1.36 MPa.
M₀ 235 kN·m; punching at 69% — a 200 mm slab works at this span.
Interpreting the result
Punching, not flexure, usually sizes a flat plate. The perimeter grows linearly with the column size while the reaction grows with the square of the span, so long spans fail in punching first.
Assumptions & limitations
- Three or more spans, long/short ≤ 2, live ≤ 2× dead.
- Interior-panel coefficients 0.65 and 0.35.
- Column strip takes 75% of the negative moment.
- Minimum thickness for deflection is not checked.
Frequently asked questions
When can I not use the direct design method?
With fewer than three spans, an aspect ratio above 2, or a live load more than twice the dead load.
What if punching fails?
Increase the depth, the column size, or add shear reinforcement — in that order of cost.
Why does the column strip take 75%?
Because the slab is stiffest over the column line and sheds moment there; the middle strip takes the rest.
Does this design the reinforcement?
It gives the area required at the column strip; detailing and bar selection follow the code rules.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.