Outrigger pad and ground pressure

R 03 mHow it works

F = manufacturer's outrigger force, or
F ≈ 0.65 × (truck kg + load kg) × 9.80665 ÷ 1000   (kN)
A = a²  |  a × b  |  π × d² ÷ 4   (m²)
P = F ÷ A   (kPa)        P ÷ 9.80665   (t/m²)
A needed = F ÷ allowable pressure;  square side = √A

Force on the pad. Use the manufacturer's maximum outrigger force if you have it. It overrides the estimate, and it is the better number: the manufacturer knows where the mass sits and how the outrigger loads change as the boom slews. Without it, the tool uses the approximation in Queensland's mobile crane code, a non-Canadian reference: the maximum pressure under one outrigger is about 0.65 × (total crane mass + lifted load) divided by that outrigger's pad area¹. Masses become forces with standard gravity, 9.80665 m/s².

Pressure and pad size. The same code gives outrigger pressure as force divided by pad area, and the minimum pad area as force divided by the allowable pressure (non-Canadian reference)¹. With force in kilonewtons and area in square metres, the result is in kilopascals. The tool also shows it in tonnes per square metre, the unit Queensland's table uses, by dividing by 9.80665.

A check you can repeat. Queensland's worked example, a non-Canadian reference, puts a 40 t crane lifting 20 t on 0.64 m² of timber at about 60.9 t/m² under one outrigger¹. Enter 40,000 kg, 20,000 kg and a 0.8 m square pad and the tool returns the same 60.9 t/m², which is 597.6 kPa.

The allowable pressure is the input that matters most. The pad size you need changes in direct proportion to it. Halve the allowable pressure and the pad area doubles. That is why the default option in the picker is the value from your engineer or a geotechnical report, and why every other option carries its source in the label.

R 06 mAssumptions and sources

For orientation, the picker offers the typical maximum permissible ground pressures from Queensland's code, a non-Canadian reference: hard rock 200 t/m², shale rock and sandstone 80 t/m², compacted gravel with up to 20% sand 40 t/m², asphalt, compacted sand or dry stiff clay 20 t/m², and dry soft clay or loose sand 10 t/m² (wet clay is less)¹. The kPa figures beside them are converted by the tool, not published.

The BC Building Code's Part 9 table is shown under the calculator for comparison and cannot be selected. It lists, for example, 150 kPa for dense or compact sand or gravel⁴ and 50 kPa for loose sand or gravel⁴. Those are allowable pressures for house footings. Do not use them to size an outrigger pad.

• Timbers under outriggers should be at least 200 mm wide and 75 mm thick, with gaps of no more than 25 mm in the top and bottom layers (Queensland code, non-Canadian reference)¹.
• IHSA states that chart ratings apply only on ground firm enough to keep the crane level, and that timber or steel mats must be used on soft ground or backfill⁵.
• Queensland's code calls for geotechnical certification of ground bearing capacity before heavy lifts such as loads of 50 t or more (non-Canadian reference)¹.
• The pressure is treated as evenly spread over the pad. A pad that is not stiff enough, or ground that is not flat, concentrates the force.

R 09 mReading the result

The big number is the average pressure the pad puts on the ground under the most heavily loaded outrigger, on the force you entered or the 0.65 estimate. Compare it with the allowable pressure for the ground. If it is higher, the tool says the pad is too small and gives the area that would bring the pressure down to the allowable value, as a square side and, for round pads, a diameter.

If the pad is big enough on paper, three things can still make it wrong. The ground may not be what you assumed: mixed ground takes the poorer value, and a buried service, a void or a backfilled trench changes everything. The worst outrigger may carry more than the estimate when the boom slews over it. And the empty hook can be the governing case, so a light lift does not mean light outriggers. Those cautions sit under the calculator because they apply to every answer it gives.

Near an excavation, the CICA/CANZ rule of thumb, a non-Canadian reference, keeps the nearest outrigger at least as far from the edge as the trench is deep, assuming a 45° zone of influence². For the provincial rules on trenches themselves, CivilConstruction.ca has a guide to trench safety and shoring by province.