Load and crane
Outrigger mat ground bearing pressure
- where
- average ground bearing pressure under the mat (kPa)
- maximum outrigger load (t)
- kN per tonne
- mat length and width (m)
- utilisation of allowable bearing pressure (%)
- allowable ground bearing pressure (kPa)
- minimum mat area needed (m²)
Explain this equation
Checks the average pressure under a crane outrigger mat against what the ground can safely carry, and gives the smallest mat area that would work. Use it when setting up mobile cranes, MEWPs or piling rigs on outriggers.
F is the largest outrigger load in tonnes, from the crane maker's data. Multiplying by 9.81 turns it into kN. L and B are the mat length and width. P is the average pressure in kPa, which is the same as kN/m². q is the allowable bearing pressure from the ground assessment. U is P as a percentage of q, and A_req is the minimum mat area.
Pressure is force divided by the area it acts on, so doubling the mat area halves the pressure. Dividing the force by the allowable pressure gives the area needed to stay at or below it.
Sense check: a 50 t outrigger load is about 490 kN. On firm ground good for 100 kPa that needs about 4.9 m², for example a 2.5 m × 2 m mat.
Watch out: this assumes the mat is stiff enough to spread the load over its whole area. A thin or flexible mat concentrates the load under the outrigger foot.
Checks the average pressure under a crane outrigger mat against the allowable bearing pressure of the ground, and gives the minimum mat area needed.
Enter your values to see the result.
Good to know
Take the maximum outrigger load from the crane manufacturer’s software or data for the worst slew position, not the lift weight. The allowable bearing pressure must come from a site investigation or a competent assessment of the ground; never guess it. kPa = kN/m². Loads in tonnes are converted at 9.81 kN/t.
Common mistake
Using a mat that is too flexible for its size: the load concentrates under the outrigger foot and the real pressure is far higher than load ÷ total area. Mats must be stiff enough to spread the load across their full area.
Limits
Average pressure assuming a rigid mat on uniform ground; no allowance for voids, services, slopes, edges of excavations or soft spots. For planning only: this does not replace a lift plan prepared by a competent person under LOLER 1998 (reg. 8) and BS 7121, and the manufacturer’s WLL markings, load charts and data always govern over this result.
Sources
- Brilliant Ideas Ltd, How we size a crane mat: the load spread calculation (area = force ÷ allowable bearing pressure; 50 t × 9.81 = 490.5 kN ÷ 100 kPa = 4.905 m²)
- CPA / Strategic Forum Plant Safety Group, Ground conditions for construction plant (guidance)
- BS 7121-1:2016 Code of practice for safe use of cranes. General (lift planning, appointed person, multiple-crane lifts, ground conditions, wind)
- The Lifting Operations and Lifting Equipment Regulations 1998 (SI 1998/2307), reg. 8: every lifting operation properly planned by a competent person, appropriately supervised and carried out safely
- HSE L113 Safe use of lifting equipment. Lifting Operations and Lifting Equipment Regulations 1998. Approved Code of Practice and guidance
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.