Beam formwork = soffit (width × span) + two sides (depth below slab × span). A 300 × 600 mm beam spanning 6 m under a 150 mm slab needs 1.8 + 5.4 = 7.2 m². With props at 1.2 m that is 6 props carrying about 6.7 kN each.
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Beam Formwork Calculator
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Formwork area (soffit + sides)
7.20m²
Soffit
1.80 m²
Sides below the slab
5.40 m²
Sheets
3 sheets, 10% waste incl.
Props
6 pcs
Load per prop (approx.)
6.7 kN
Side clamps / kickers
11 sets
Concrete in beam below slab
0.81 m³
Compare the prop load with the prop’s safe working load at its extended height, from the supplier’s table.
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Line load = (concrete 0.600 m deep + 0.5 kPa forms + 2.4 kPa live) × b + side forms = 5.56 kN/m
Load per prop = 5.56 kN/m × prop spacing = 6.7 kN (single row of props under the beam)
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Beam Sizes at a Glance (150 mm slab, props at 1.2 m)
Beam b × D
Formwork per metre
Line load
Load per prop
230 × 450 mm
0.83 m²
3.40 kN/m
4.1 kN
300 × 600 mm
1.20 m²
5.56 kN/m
6.7 kN
300 × 750 mm
1.50 m²
6.77 kN/m
8.1 kN
400 × 900 mm
1.90 m²
10.38 kN/m
12.5 kN
450 × 1200 mm
2.55 m²
15.06 kN/m
18.1 kN
Assumptions
Formwork self-weight 0.5 kPa (10 psf) and construction live load 2.4 kPa (50 psf), per ACI 347R-14 minimums.
Load from the slab next to the beam is carried by the slab falsework, not the beam props.
Not a falsework design: check props, heads and soffit bearers with the supplier’s data.
Frequently Asked Questions
How do I calculate beam formwork area?
Add the soffit (beam width × span) and both sides (depth below the slab × span × 2). Only form the depth below the slab; the slab soffit forms cover the rest.
How far apart should beam props be?
Often 0.9–1.5 m, depending on the beam size, the soffit bearers and the prop capacity. Work out the load per prop and check it against the prop's safe working load at its extended height.
What load does a beam prop carry?
The weight of the wet concrete over its tributary length, plus formwork self-weight and a construction live load. For a 300 × 600 mm beam at 1.2 m prop spacing, that is about 6.7 kN per prop.
Do deep beams need two rows of props?
Wide or heavy beams usually do, with a cross-head under the soffit. The calculator assumes one row; split the load if you use two.
References
ACI 347R-14 §4.1: construction live load ≥ 2.4 kPa (50 psf); dead + live ≥ 4.8 kPa (100 psf)
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