Heavylift Tip – Trailers

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Ground Bearing Under Trailers

Ground bearing capacity checks under modular trailers are formed of ground bearing pressure (GBP) and surface composition. In our industry, there are two keys methods of calculating GBP: shadow area and wheel footprint.

Shadow area
The ‘shadow’ area is determined by taking the area of the modular trailer – the length of the trailer axle units multiplied by the basic width – and dividing it by the gross weight of the cargo it carries. Quite often, engineers will divide this figure by the payload weight, forgetting to also consider the weight of the trailer, powerpacks and any packing steel. There is also the variable of what is the “shadow area”. The options range from end axle to end axle and bare trailer width, to overall trailer plan plus 200mm.

For the shadow area to be considered effective, the ground surface needs to be capable of receiving the loads and transmitting it further down into the subsoil without causing any deformation or failure. This leads into the topic of surface composition which is beyond the scope of this short technical piece.

Individual Wheel Footprint
While the tyres of a modular trailer carrying no load will be round, a trailer that is under full load will have a flat spot on the tyres due to the additional load. However, what is constant in both scenarios is the pressure inside the tyre. It can be shown that for any given tyre pressure (say 10 bar) the flat spot area generates a similar GBP in sold ground to the shadow area calculation. Therefore for solid ground, this means that the shadow method is normally sufficient.

However, for a thin surface, such as a barge deck or similar, where there will be no load spreading, the wheel footprint is a more realistic choice to use in order to check the capacity of the deck.

In order to minimise issues while on site, ground loading or deck loading capacity should be determined and checked against the applied loading.

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