Loads
Base plate and sole board ground bearing pressure
- where
- sole board area (m²)
- sole board length (mm)
- sole board width (mm)
- pressure under the sole board (kPa)
- standards on each sole board
- leg load per standard (kN)
- bearing check (1 or less passes)
- allowable ground bearing pressure (kPa)
- minimum sole board area (cm²)
- pressure under the base plate (kPa)
- base plate size (mm)
Explain this equation
Checks that sole boards spread the scaffold legs' load enough for the ground, and works out the minimum sole board area.
Dividing millimetre dimensions by a million gives square metres. Pressure is load divided by area; kN per m² is kPa. Comparing that with the allowable bearing pressure for the ground gives a pass or fail. Turned round, load divided by allowable pressure is the area needed, times 10,000 to show it in cm². The base plate alone spreads the load over a much smaller area, which is why it needs a sole board on anything but hard ground.
Pressure equals force over area.
Sense check: a 7.1 kN leg on a 500 × 225 mm sole board puts about 63 kPa on the ground, fine on firm ground at 100 kPa. On the 150 mm base plate alone it would be over 300 kPa.
Watch out: soft, made-up or frozen ground, and ground near excavations, can have very low bearing capacity. Get it assessed.
Checks that the leg load spread through a sole board does not exceed what the ground can safely carry.
Enter your values to see the result.
Good to know
Pressure = force / area: 1 kN on 1 m² is 1 kPa. Standard practice is a timber sole board of at least about 1000 cm² per standard (for example 220–225 × 500 mm, 35 mm or more thick), more on soft or disturbed ground, with the base plate centred and not near the end. The allowable bearing pressure must come from your own site assessment, the TG20:21 ground-condition guidance or a temporary works engineer; the 100 kPa shown is only an example. On a slab or paving, check the slab and any voids or drains below.
Common mistake
Putting a base plate straight on to soil or tarmac: a 150 mm plate concentrates the load about five times more than a sole board.
Limits
Assumes the load spreads evenly over the whole sole board, which is only true for a stiff board on level, even ground. No allowance for slopes, frozen or saturated ground, excavations nearby or settlement. For planning and estimating only. This does not replace TG20:21 compliance (a TG20 compliance sheet or eGuide output) or a scaffold design by a competent person.
Sources
- Health and Safety Authority (Ireland), Code of Practice for Access and Working Scaffolds (sole boards ≥ 1000 cm², ties ≥ 6.1 kN, façade bracing at least every six bays, tower height to least base ≤ 3, toe-boards ≥ 150 mm)
- NASC TG20:21 Good practice guidance for tube and fitting scaffolding (Operational Guide, Design Guide, User Guide and eGuide compliance sheets)
- BS EN 12811-1:2003 Temporary works equipment. Scaffolds. Performance requirements and general design (Table 3 load classes; 5.5 side protection; Annex A cladding force coefficients)
- BS EN 74-1 Couplers, spigot pins and baseplates for use in falsework and scaffolds. Couplers for tubes
Results are calculations from the published methods and figures shown, and depend on the data you enter. For planning and checking only. Scaffolds must be erected to NASC TG20 or a bespoke design by competent persons, and lifting operations planned under LOLER by a competent person. Never rely on these results instead of a design, a lift plan or manufacturer ratings.