CPS technical guide · reviewed 19 July 2026
Calculate bearing area from verified lift and ground data
Correct pad sizing starts with the highest verified outrigger reaction for the planned lift and a competent assessment of the ground or supporting structure. This guide explains the calculation and the checks that must follow it.
Manufacturer-listed pad data remains the source of truth for each CPS product. Final selection belongs in the lift plan.
What you need before calculating
1. Verified maximum reaction
Use the highest design reaction at any outrigger for the exact machine configuration and planned operation. Record whether the value is factored or unfactored.
2. Assessed ground capacity
Use a permissible value for the actual surface, supporting layers and nearby hazards. A surface description alone does not establish capacity.
Walk every outrigger position and identify soft spots, recent excavation, backfill, drainage, cellars, ducts, trenches, slopes and suspended structures. Conditions can differ between legs and can change with weather or site activity.
The calculation
Verify the maximum reaction
Use the planned lift data—not gross vehicle weight and not a simple average across the outriggers.
Confirm the ground input
Use a permissible ground or structural capacity supplied or accepted by the competent person responsible for the assessment.
Convert tonnes to kilonewtons
Example: 15 tonnes × 9.81 = 147.15 kN.
Calculate the strict minimum area
At an assessed 150 kN/m²: 147.15 ÷ 150 = 0.981 m².
Apply the project method
Determine whether the reaction and ground values are already factored. Apply only the factors required by the lift plan, engineering method or applicable guidance.
Verify the exact pad
Check effective area, manufacturer-listed pad capacity, foot size, centring, stiffness, condition and positioning. Passing the area calculation alone does not approve the pad or the lift.
Ground inputs used by the CPS screening calculator
These figures are selectable examples for early screening. They are not confirmed site capacities and must not replace ground investigation, engineering information or an assessment by a competent person.
| Screening description | Example input | Required verification |
|---|---|---|
| Concrete or hardstanding | 400 kN/m² | Verify the slab, construction, support and condition. |
| Dense gravel or hardcore | 400 kN/m² | Verify compaction, depth and supporting layers. |
| Compact sand or stiff clay | 300 kN/m² | Obtain a site-specific value. |
| Loose gravel or firm clay | 150 kN/m² | Obtain a site-specific value. |
| Medium sand | 100 kN/m² | Obtain a site-specific value. |
| Soft clay, silt or topsoil | 75 kN/m² | Specialist assessment may be required. |
| Loose sand or made ground | 50 kN/m² | Specialist assessment may be required. |
| Unverified ground | 50 kN/m² placeholder | This is not a confirmed capacity and cannot approve a lift. |
Pad bearing areas for quick reference
Geometric area is only the first check. The effective area can be lower if a pad is not sufficiently stiff, is poorly supported, is damaged, or the foot is not positioned correctly.
| Format | Geometric area | Format | Geometric area |
|---|---|---|---|
| 400 × 400mm square | 0.16m² | 800mm round | 0.50m² |
| 500 × 500mm square | 0.25m² | 900mm round | 0.64m² |
| 600 × 600mm square | 0.36m² | 1000mm round | 0.79m² |
| 800 × 800mm square | 0.64m² | 1200mm round | 1.13m² |
| 1000 × 1000mm square | 1.00m² | 1500 × 1500mm square | 2.25m² |
| 1200 × 1200mm square | 1.44m² | 2000 × 1200mm rectangle | 2.40m² |
| 1800 × 1800mm square | 3.24m² | 3000 × 2000mm rectangle | 6.00m² |
Worked screening examples
15-tonne reaction on an assessed 150 kN/m² input
Reaction: 15 × 9.81 = 147.15 kN. Strict minimum area: 147.15 ÷ 150 = 0.981m². At the CPS calculator’s 80% utilisation limit, the screening area is 147.15 ÷ 120 = 1.226m².
A 1200 × 1200mm pad has 1.44m² geometric area and clears the area screen. The exact pad must still have sufficient manufacturer-listed capacity for the verified reaction and satisfy the positioning and support checks.
Area screen clears18-tonne reaction on an assessed 150 kN/m² input
Reaction: 18 × 9.81 = 176.58 kN. Strict minimum area: 1.177m². At the CPS 80% utilisation limit, the screening area is 1.472m².
A 1200 × 1200mm pad provides 1.44m², so it does not clear the CPS screen. A larger candidate must be assessed against both area and its exact manufacturer-listed pad capacity.
1.44m² does not meet 1.472m²3-tonne MEWP reaction on a 400 kN/m² screening input
Reaction: 3 × 9.81 = 29.43 kN. Strict minimum area: 0.074m². At the CPS 80% utilisation limit, the screening area is 0.092m².
A 300 × 300mm pad has 0.09m² and sits just below that screen. A 400 × 400mm pad provides 0.16m² and clears the area screen, subject to manufacturer-listed capacity and site verification.
400 × 400mm clears the area screen22-tonne reaction on a 75 kN/m² screening input
Reaction: 22 × 9.81 = 215.82 kN. Strict minimum area: 2.878m². At the CPS 80% utilisation limit, the screening area is 3.597m².
A 2000 × 1200mm pad provides 2.40m² and is below both the CPS screen and the strict minimum. The answer may require a larger engineered bearing system, ground improvement or a revised lift arrangement—not merely a higher-rated pad.
Additional engineering requiredCommon mistakes to avoid
Averaging the machine weight
Boom position and configuration can put a far greater reaction through one leg. Always use the verified maximum reaction.
Treating a surface label as proof
“Concrete”, “hardcore” or “firm clay” does not confirm the capacity of the supporting structure or layers beneath it.
Double-applying a factor
First establish whether the supplied load and capacity values are factored or unfactored. Follow the project method.
Checking area but not pad capacity
The candidate must pass both the ground-pressure calculation and the exact manufacturer-listed pad-capacity check.
Ignoring stiffness and centring
A flexible, poorly supported or eccentrically loaded pad may not spread the reaction over its full geometric area.
Using one ground value everywhere
Assess every outrigger location and consider excavations, services, edges, slopes, drainage and changing conditions.
Guidance and source position
This CPS guide is an early-screening aid. The following industry sources explain why verified plant loads, ground assessment, pad stiffness and competent engineering judgement matter:
Screen the pad, then verify the lift
Use the CPS calculator with a verified reaction and assessed ground input. Then check the recommended candidate against its manufacturer-listed data and the final lift plan.
Screening limitation: This article and the CPS calculators do not approve a lift, confirm ground conditions or replace manufacturer instructions, a lift plan, competent ground assessment or project-specific engineering. If the input data is uncertain, stop and obtain competent advice.
