Formwork Lateral Pressure Calculator
Smaller of the two GB 50666 formulas, plus effective head height.
Inputs
Result
Formula
| F1 = 0.22·γc·t0·β1·β2·V^(1/2) | t0 = 200/(T+15) |
| F2 = γc·H | hydrostatic from pour height |
| F = min(F1, F2) | smaller value |
| h = F/γc | triangular below h, constant above |
25 ℃, 2 m/h pour rate, 3 m form height, 110-150 mm slump, no retarder: t0 = 5 h, β1 = 1.0, β2 = 1.15. F1 = 42.9 kN/m², F2 = 72 kN/m², so 42.9 governs with an effective head of 1.79 m - pressure increases only over the bottom 1.79 m.
FAQ
Why does doubling the pour rate only raise pressure by 1.4?
Because formula 1 uses the square root of the rate. Doubling V takes √V from 1 to 1.414. Faster pouring stacks fresh concrete on layers that have not set, but those layers are also stiffening and absorbing pressure, so the effects partly cancel. Low temperature (longer t0) and high slump raise pressure far more.
What is the effective head height for?
It defines the pressure profile: triangular growth over the top h metres, then constant maximum below that. Tie bolts and walers are designed for that one maximum, so no need to densify the lower rows. Designing for full hydrostatic over the whole height (72 kN/m² at 3 m) wastes a lot of steel.
How do I pick parameters for layered mass pours?
Use the worst case. If the lower layer has set, treat each layer separately by its own height and rate. If it is still plastic, add the residual pressure from below. In practice the allowable pour rate is back-calculated from layer thickness and the shortest setting time - that decision belongs to the site technical lead.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.