Foundations · Rafts & slabs on ground

Slab on ground (Westergaard) calculator

Flexural stress for interior, edge and corner wheel or post loads on a slab on grade.

Standard: Westergaard (1926) · AS 3600 f'ct,f · AS 2870 context

Inputs in the worked example

The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.

Result	Value	Unit
Radius of relative stiffness ℓ	782	mm
Equivalent radius b	140	mm
Interior stress	1.58	MPa
Edge stress	2.36	MPa
Corner stress	2.01	MPa
f'ct,f = 0.6√f'c	3.39	MPa
Allowable	1.70	MPa

Diagram for the worked example

ℓ = [E h³ / (12(1 − ν²) k)]^¼
b = √(1.6a² + h²) − 0.675h when a < 1.724h, else b = a
Interior σ = 0.316 P/h² [4 log(ℓ/b) + 1.069]
Edge σ = 0.572 P/h² [4 log(ℓ/b) + 0.359]
Corner σ = 3P/h² [1 − (a√2/ℓ)^0.6]
Residential slabs on reactive sites are designed to AS 2870 (site classification, stiffened rafts); this checks concentrated loads.

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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.