Outrigger Mat Size & Ground Bearing Calculator
Work out the maximum outrigger load, the minimum bearing area the ground needs and the pressure under the mat you plan to use. It follows the Temporary Works Forum guidance, Assessment and management of outrigger loading (TWf2022:02), and was built by a CPCS A61 Appointed Person.
Outrigger mat calculator
Step 3: is the mat strong enough?
Area is only half the check. The mat must also have a rated capacity at least equal to the maximum outrigger load, shown on an outrigger mat capacity certificate or by a temporary works assessment, and the outrigger must sit in the centre of it. A mat that is big enough can still bend or break and concentrate the load.
A preliminary check, not a ground or temporary works assessment. Buried services, slopes, excavation edges, made ground and soft layers below the surface all need a competent engineer. Need the outrigger loads in a lift plan?
How the calculation works
Outrigger loading is temporary works. The Temporary Works Forum sets out three steps, and a mat is only right when all three are satisfied.
Step 1: the maximum outrigger load
The most reliable figure is the one from the crane or plant supplier for the actual configuration, radius and slew range, or the maximum outrigger load published by the manufacturer. For all-terrain mobile cranes up to 160 t capacity, the 75% rule gives a quick, usually cautious estimate:
Maximum outrigger load = 0.75 × rigged crane weight + load + hook block and accessories
The rigged crane weight includes all the counterweight fitted for the lift. The rule is not for lorry loaders, MEWPs, concrete pumps or cranes over 160 t, and where it gives more than the values in CIRIA C703 the loads should be worked out by specific calculation, which is often smaller and saves an oversized mat.
Step 2: the minimum bearing area
Minimum bearing area (m²) = maximum outrigger load (kN) ÷ allowable bearing pressure (kN/m²)
The allowable bearing pressure comes from the ground or temporary works assessment for that position. That is why this calculator has no "ground type" list: presumed values from tables such as BS 5975 Table 18 are written for buried foundations and the guidance says they are unsuitable for most outrigger assessments.
Step 3: a mat strong enough to spread the load
The mat must have a rated capacity at least equal to the maximum outrigger load and an area at least equal to the minimum bearing area, shown on an outrigger mat capacity certificate or by a temporary works assessment, with the outrigger set in its centre. Timber sleepers, plastic pads and steel mats all bend; a mat that is large enough but too flexible puts most of the load under the foot.
Worked example
A 60 t all-terrain crane, rigged for the lift at 48 t with its counterweight, lifts a 4.2 t load with 0.5 t of hook block and lifting accessories. The ground assessment gives an allowable bearing pressure of 200 kN/m².
- Maximum outrigger load: 0.75 × 48 + 4.2 + 0.5 = 40.70 t, or 399.1 kN
- Minimum bearing area: 399.1 ÷ 200 = 2.00 m², so at least 1.42 m square or 1.60 m diameter
- A 1.5 m × 1.5 m mat weighing 300 kg gives 178.7 kN/m² over 2.25 m², within the 200 kN/m² allowable, provided its certificate shows a rated capacity of at least 40.7 t
Illustrative figures only. Use the weights and ground information for your own lift.
Questions
What is the 75% rule for outrigger loads?
It is a rule of thumb, taught on the CPCS A61 Appointed Person course, for estimating the maximum load on one outrigger of an all-terrain mobile crane: 75% of the gross weight of the rigged crane, including all counterweight, plus 100% of the hoisted load, including the hook block and all lifting accessories. The Temporary Works Forum recommends it is not used for cranes over 160 t capacity, and that for cranes up to 160 t the result is compared with the values in CIRIA C703; where the rule gives more, the loads should be worked out by specific calculation.
Can I use the 75% rule for a lorry loader, MEWP or concrete pump?
No. TWf2022:02 says equivalent rules of thumb for other cranes and plant are not well developed and should only be used by competent temporary works designers. Use the maximum outrigger load published by the manufacturer (ALLMI and several manufacturers publish them, and it is often marked on the machine).
Where does the allowable bearing pressure come from?
From a ground or temporary works assessment for the actual position, usually by a competent civil, structural or geotechnical engineer. Published values can be used only in simple, low-risk situations by a competent Appointed Person or Temporary Works Coordinator. BS 5975 Table 18 is for buried foundations, and the guidance says it is unsuitable for most outrigger loading assessments.
If the mat is big enough, is it safe?
Not on its own. The mat must also be strong and stiff enough to spread the maximum outrigger load over its whole area, shown by a rated capacity on an outrigger mat capacity certificate or by a temporary works assessment, and the outrigger must sit in the centre of the mat. The guidance warns that it is wrong to assume a mat only has to be large enough.
What is the difference between kN/m² and kPa?
None: 1 kN/m² is 1 kPa. One tonne of load is 9.81 kN, so 10 t on 1 m² is about 98 kN/m².
Source: Temporary Works Forum, Assessment and management of outrigger loading, TWf2022:02, September 2022.
Want the outrigger loads worked out for you?
Every RMT mobile crane lift plan includes the outrigger loads for the actual configuration, the mat specification and an exclusion zone drawing, written by a CPCS A61 Appointed Person.