What this calculator does
Falsework is the one structure on site that carries load it was never designed to carry, is assembled by people who are not engineers, and is often reused several times before anyone checks it. Two questions decide whether it is safe: how much load reaches each leg, and how much that leg can take. Both are worked here, step by step, in an order a site engineer can check by hand.
How to use it
- Enter the slab area, thickness and the concrete density you are actually placing — lightweight structural concrete changes the whole answer.
- Enter the imposed construction load for the pour. This includes the concrete placing equipment and the men on the deck, and it is a figure you take from your own loading plan.
- Enter the prop capacity and the sole board bearing area. These are the two numbers that decide whether the table stands up at all.
- Read the governing step rather than the pass or fail alone. If the sole boards are at 95 % you have no margin for uneven ground, and that is a different conversation from the props at 40 %.
Governing formula
W_slab = A · h · γ · g → F_prop = (W_slab · γ_p + W_imposed) / n Where:
-
A= Slab area [m²] -
h= Slab thickness [m] -
γ= Concrete density [kg/m³] -
γ_p= Permanent load factor applied to the concrete -
n= Number of props
Derived Equations:
σ_bearing = F_prop / A_sole span_max = √(8 · σ_allow · I / w) Worked example — 100 m² slab at 200 mm on 100 props
The figures below were produced by running this page's own engine with the example inputs (Slab area supported (m²), Slab thickness (m), Concrete density (kg/m³), Imposed construction load (kPa), Number of props (no), Prop cross-sectional area (mm²), Allowable capacity of one prop (N), Sole board bearing area (mm²), Allowable bearing pressure (N/mm²), Bearer strength (N/mm²), Bearer span between props (m), Permanent load factor on concrete (–)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Total load reaching the falsework | 785688 N |
|---|---|
| Load per prop | 7857 N |
| Every step within capacity | — |
| Prop utilisation | 39.28 % |
| Sole board / soil utilisation | 15.71 % |
| Bearer utilisation | 90.26 % |
| Bearing pressure | 0.08 N/mm² |
| Maximum bearer span | 1.99 m |
| Slab self-weight | 470880 N |
| Imposed construction load | 150000 N |
| What this means | The table stands up: 100 props carry 7.9 kN each, which is 39 % of prop capacity, 16 % of the bearing pressure, and the bearer is spanning 1.80 m against a 1.99 m limit. The total reaching the falsework is 785.7 kN: 470.9 kN of concrete at a factor of 1.35 plus 150.0 kN imposed. Those factors are YOUR inputs \u2014 check them against your own code rather than taking them from here. This is the simple load path. It does not redistribute load through the table, model continuity between props, or account for the stiffness of the ply-and-bearer assembly. Concrete is not placed in a single lift unless the plan says so. |
- The slab weighs 100 × 0.2 × 2400 × 9.81 = 470.9 kN, which at a factor of 1.35 becomes 635.7 kN.
- The imposed load adds 1.5 kPa × 100 m² = 150 kN, giving 785.7 kN total.
- Across 100 props that is 7.86 kN per prop — 39 % of a 20 kN prop.
- The sole boards see 7.86 kN over 0.1 m², which is 0.079 N/mm² against 0.5 allowable, so 16 %.
Assumptions & limitations
Engineering calculation. The simple vertical load path: slab self-weight at a stated permanent-load factor plus imposed construction load, shared to the props, then each prop, sole board and bearer checked against a capacity you supply.
- Concrete density, imposed load, prop capacity, prop spacing, bearing pressure, bearer strength and every load factor are USER inputs. This tool holds no timber table, no design code and no load schedule, and looks nothing up.
- The bearer check derives a second moment of area by treating the entered prop area as a solid square. That is a stand-in for a real bearer section, NOT a section property for the bearer you are actually using. Size the bearer properly before relying on it.
- This is the SIMPLE load path. Load redistribution through the table, continuity between props, the stiffness of the ply-and-bearer assembly and creep of the concrete are not modelled.
- It assumes a single uniform lift of concrete. If the slab is to be placed in bays or in layers, the imposed load and the sequencing have to be reconsidered for that plan.
- A pass here is not a permit to strike the falsework. The striking time is set by the concrete strength and the span, and that check is not in this tool.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS 5975 — falsework — design, construction and operations, as the framework for this load path.
- EN 1992 — design of concrete structures, for the imposed construction loading.