Heavylift Tip – Cranes

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Ground Bearing Pressure under Crane Mats

A question that appears time and again in the heavy-lift industry is how to calculate the ground’s allowable bearing pressure to determine its ability to withstand the force exerted under crane mats or outriggers. It has been estimated that around 40 per cent of crane accidents happen due to under-estimated outrigger loads, or the ground failing due to the load imposed upon it. Therefore is it essential that the outrigger load is determined accurately, the acceptable ground bearing pressure is known, and using those values, ensure the load is adequately spread into the ground.

While this issue can be complex, one of the main factors to ensure a safe lift is to establish the maximum load:

· For a mobile crane, the maximum load occurs when the loaded jib is aligned over one of the outriggers (assuming they are all equidistant from the centre of rotation).

· For a crawler crane, there is a different distribution based upon a trapezoidal distribution:
o This distribution is affected by the size and radius of load, but not in the obvious way. The position and size of the load affects the load distribution (as opposed to the load intensity)
o This distribution is further altered by use of a super-lift

Effective management is crucial for the safety of any lifting operation and will only succeed if all parties are aware of their individual roles and responsibilities on the job. On the basis of a contract lift, the crane hire companies will supply the maximum outrigger load for any given crane and load configuration. It is then the responsibility of the site owner to inform the crane supplier of the acceptable ground bearing capacity that it can withstand. Once these two values are determined, it is a fairly straightforward calculation to determine the size of the outrigger pad to ensure the ground is not unduly loaded.

Inexperienced people commonly assume that if there is, for example, a requirement for 1m2 loadspreading under an outrigger, then the 1m diameter mat that comes with the crane will be big enough. This is simply not the case. Accurate calculations are crucial for a successful lifting operation and should be properly prepared and executed by a professional.

Improper crane set-up and lack of expertise increases the chance of lifting accidents, so ensure you have a highly-skilled team who can assess the ground’s capability to withstand the load in question.

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