Formulas
Bearers = ⌈W ÷ bearer spacing⌉ + 1 (each L long)
Joists = ⌈L ÷ joist spacing⌉ + 1 (each W long)
Props = bearers × (⌈L ÷ prop spacing⌉ + 1)
Soffit load = ρ·g·t + 0.5 kPa + 2.4 kPa (≥ 4.8 kPa)
Load per prop = soffit load × bearer spacing × prop spacingWorked example
A 6 × 5 m slab, 200 mm thick, joists at 400 mm, bearers at 1.2 m, props at 1.2 m along each bearer:
- Sheets = ⌈30 × 1.1 ÷ 2.977⌉ = 12
- Bearers = ⌈5 ÷ 1.2⌉ + 1 = 6 × 6 m = 36 m
- Joists = ⌈6 ÷ 0.4⌉ + 1 = 16 × 5 m = 80 m
- Props = 6 × (⌈6 ÷ 1.2⌉ + 1) = 36
- Soffit load = 2,400 × 9.81 × 0.2 + 500 + 2,400 = 7.61 kPa → 11.0 kN per prop
Load per prop at 1.2 × 1.2 m grid
| Slab thickness | Soffit load | Load per prop |
|---|---|---|
| 150 mm | 6.43 kPa | 9.3 kN |
| 200 mm | 7.61 kPa | 11.0 kN |
| 250 mm | 8.78 kPa | 12.6 kN |
| 300 mm | 9.96 kPa | 14.3 kN |
Assumptions
- Interior props carry a full tributary area; edge props carry less, so the figure is conservative.
- The calculator counts material; it does not check plywood, joist or bearer bending and deflection, or prop buckling.