Two-Way Slab Calculator
Design two-way reinforced concrete flat plate slabs using the Direct Design Method per ACI 318.
Panel Parameters
Span Ratio
1.25
Two-way action governs
Thickness OK (min: 7.27")
Total Static Moment (Mo)
Long Direction
179.20 k-ft
Short Direction
143.36 k-ft
Column Strip Moments (k-ft/ft)
Long: -M interior
11.760
Long: +M midspan
4.704
Short: -M interior
9.408
Short: +M midspan
3.763
Required Reinforcement
Long Direction
0.402 in²/ft
#4 @ 6.0" o.c.
Short Direction
0.310 in²/ft
#4 @ 7.7" o.c.
Column strip width: 8.0 ft
Factored load: 224.0 psf
Direct Design Method Requirements
DDM Applicability
- Minimum 3 continuous spans each direction
- Span ratio (long/short) ≤ 2
- Successive spans differ by ≤ 1/3
- Columns offset ≤ 10% of span
- Uniform loading, LL/DL ≤ 2
Moment Distribution
- Interior negative: 65-70% of Mo
- Positive: 35-52% of Mo
- Column strip: 60-75% of panel moment
- Middle strip: remainder