Outriggers, ground and ground pressure — calculate as in the certificate test
The formula from TUR's formula sheet, a worked example and the set-up errors that make you fail the test
Introduction
The crane is no stronger than the ground it stands on. When a mobile crane lifts, a large part of the weight of the crane and the load can end up on one or two outriggers. If the ground gives way, the crane overturns. That is why setting up is just as important a part of the lift as the hoisting itself.
The rule
When a mobile crane is used for lifting, measures must be taken so that it cannot tip, overturn or slide — and it must be checked that they work (BEK no. 428 of 05/04/2022 § 52). BFA Bygge & Anlæg explains further:
- 01The bearing capacity of the ground must be documented before lifting.
- 02Check the manufacturer's instructions for the maximum pressure one outrigger can exert.
- 03All outriggers must stand on ground that can carry the load. Use pads under the outriggers so that the pressure is spread.
- 04If the crane is to travel with the load, the ground must be level and firm — equivalent to a road ready for asphalting — or covered with driving plates.
The formula from the test
TUR's formula sheet for the theory test for mobile cranes 8-30 tonne-metres has two formulas for pads. They work in kg and cm²:
- 01Pad area (cm²) = pressure from outrigger (kg) ÷ ground bearing capacity (kg/cm²)
- 02Ground bearing capacity (kg/cm²) = pressure from outrigger (kg) ÷ pad area (cm²)
Worked example
An outrigger presses with 12,000 kg. The ground can carry 2 kg/cm². How large must the pad be?
- 01Area = 12,000 ÷ 2 = 6,000 cm².
- 02A square plate must have sides of √6,000 ≈ 77.5 cm.
- 03If you only have a pad of 60 × 60 cm = 3,600 cm², the pressure becomes 12,000 ÷ 3,600 ≈ 3.3 kg/cm². That is more than the ground can carry.
What gets you failed
TUR's test guidelines for mobile cranes list these errors as «poor set-up»:
- 01Outrigger pads are left out, are too small or are wrongly placed.
- 02The crane is placed wrongly in relation to the goods.
- 03Safety equipment is not set correctly.
- 04Outrigger bracing is removed with the boom extended too far.
Tasks on unpaved ground must be included when the test is held — but not necessarily for all participants.