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THE Top Tip – Ground Bearing under Trailers

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Checking the suitability of the ground under modular trailers is formed of two aspects, ground bearing pressure (GBP) and surface composition. There are different ways to calculate GBP, the two most common of which are shadow area and wheel footprint.

Shadow area
This is simply a case of taking the area of the trailer and dividing it by the gross weight acting on the trailer. Quite often engineers will divide by the payload weight, forgetting about the weights of the trailer, powerpacks and any packing steel. There are also various interpretations of what the “shadow area” is. These can range from end axle to end axle and bare trailer width, to overall trailer plan + 200mm.

For the shadow area to be valid the ground surface needs to capable of receiving the loads and transmitting it further down into the subsoil without any deformation or failure. This leads into the topic of surface composition and suitability of the surface to transmit the loads in this way. Therefore the typical calculation of shadow area is of the length of the trailer axle units x basic width. Anything additional to this could need further justification with respect to surface composition.

Individual Wheel Footprint
If we look at a tyre under no load it is round, whereas under full load it has a flat spot. The pressure inside the tyre will remain constant. The GBP local to the tyre will depend on the load on the trailers, the pressure in the tyres and consequently the size of the flat spot. The GBP would then be found in much the same way as the shadow area, dividing the total load by the total area provided by the flat spots under each wheel.

For a thin surface, such as a barge deck or similar, where there will be no distribution of load, the wheel footprint is a more realistic choice to check loadings. For typical ground, the shadow area is more likely to be valid and offers a simpler check. Always compare the applied GBP against the supplied ground bearing capacity to ensure the ground is capable of taking the loads.