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Crane Hire and Outrigger Pads
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Read moreGround Bearing Pressure for Cranes: How to Calculate It (Outriggers and Crawlers)
07 October, 2026 | Admin
Short answer: ground bearing pressure for cranes is the load on the ground divided by the area carrying it, in kPa. It must stay below what the ground can safely bear. A pad or mat works by increasing that area, so the same load produces a much lower pressure.
The Formula
Ground pressure (kPa) = load (kN) ÷ contact area (m²)
Load in kN = tonnes × 9.81. Note that 1 kPa = 1 kN/m².
Why Pads Matter: The Same Load, Three Footprints
Take an outrigger load of 15 tonnes, which is 147.15 kN.
| Footprint under the leg | Area | Pressure |
|---|---|---|
| Outrigger float only, 400 × 400 mm (illustrative) | 0.16 m² | about 920 kPa |
| Saferig SR-10107, 1000 × 1000 × 70 mm | 1.00 m² | about 147 kPa |
| Saferig SR-12128, 1200 × 1200 × 80 mm | 1.44 m² | about 102 kPa |
The float size is an illustrative figure. Check your own crane’s float dimensions.
The Pad Is Not the Only Limit: The Ground Is
Use the same 1000 × 1000 mm pad (147 kPa, unfactored) on four ground types:
| Ground | Allowable | Result |
|---|---|---|
| Rock or compacted aggregate | 300 kPa | Passes |
| Compacted fill | 200 kPa | Passes without a safety factor. With a 1.5 factor (221 kPa) it fails. |
| Sandy | 120 kPa | Fails |
| Sabkha, soft or reclaimed | 75 kPa | Fails |
The pad’s own rating (SR-10107 has a rated capacity of 100 tonnes) is nowhere near its limit. The ground is what runs out first. On sand, this load needs a larger pad.
Crawler Cranes: Tracks, Not Outriggers
For a crawler, the contact area is the area of both tracks on the ground.
Example (illustrative): a crawler crane with its load totals 120 tonnes (1,177.2 kN). Each track is 0.9 m wide and 7.0 m long, so the total track area is 2 × 0.9 × 7.0 = 12.6 m².
Average pressure = 1,177.2 ÷ 12.6 = about 93 kPa.
That is the average. Pressure is not uniform. When the load is off-centre, it concentrates toward one end of each track, and the peak can reach roughly twice the average as a planning rule of thumb. Ask the manufacturer for the maximum ground pressure figure for your configuration.
If the ground is sabkha (75 kPa): with a 1.5 safety factor, the required effective area is 1,177.2 × 1.5 ÷ 75 = 23.5 m², almost double the track area. That is a job for crane mats or a prepared platform, designed by an engineer. See our ground protection mats.
Typical Reference Values
| Ground | Indicative allowable pressure |
|---|---|
| Rock or compacted aggregate | 300 kPa |
| Compacted fill | 200 kPa |
| Sandy ground | 120 kPa |
| Sabkha, soft or reclaimed | 75 kPa |
Reference values only. Use a geotechnical report for anything critical.
Practical Checklist
- Get the worst-case outrigger reaction or total crawler weight, in tonnes.
- Identify the ground and its allowable pressure. If unsure, take the weaker category.
- Apply a safety factor of 1.5 to 2.0.
- Compute the minimum area and choose a pad or mat that exceeds it.
- Check that the pad’s rated capacity covers the load.
- Note changes in conditions: rain, fresh fill, buried services and trenches nearby.
To work out the leg load itself, read our Crane Outrigger Load Calculation guide, which includes a free Excel template.
Related Guides
Frequently Asked Questions
What is ground bearing pressure?
The pressure a load applies to the ground, equal to the load divided by the contact area, usually in kPa.
How do I reduce ground pressure under an outrigger?
Increase the contact area with a larger pad. Doubling the area halves the pressure.
Is crawler crane pressure the average or the peak?
Both matter. The average is load over total track area. The peak can be roughly double that when loading is off-centre. Use the manufacturer’s figure.
Does a stronger pad reduce ground pressure?
No. Only a larger contact area does. A stronger pad prevents the pad itself from failing.
Related guides: Crane Outrigger Load Calculation · Outrigger Pad Sizing Guide · Crane Mat vs Outrigger Pad · Outrigger Pad Size Calculator
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