Crane outrigger pad size: the calculation, start to finish

Ground bearing pressure is the contact pressure one support puts into the ground. That is the entire definition, and the arithmetic is a division.

What makes it a real engineering problem is that both numbers resist being pinned down. The load is not the crane's weight and is not shared evenly. The area is not the mat you can see.

The rest of this article is those two numbers, then what the ground is allowed to take, and then the pad itself, which turns out to have a design of its own.

A mobile crane's outrigger loads are published by the manufacturer for the machine, the configuration and the load case. That chart is the primary source, and if you have it you use it.

Three things about those numbers surprise people:

The distribution is nothing like even. In the worked example below the machine grosses 1000 kN and the worst outrigger carries 350 kN. That is 35 percent on one leg, not 25 percent. The counterweight is behind the slew centre, the boom and load are in front of it, and the resulting couple loads one corner heavily while unloading its diagonal opposite.

The governing case is at a slew angle, not at rest. Rotate the superstructure and the heavy corner moves. The worst reaction over the whole slew range is a different number from the reaction in the elevation on your drawing, and a lift plan that checks one angle has checked one angle.

A leg can come off the ground. At the limit, a support unloads completely and the machine is standing on three. Everything downstream of that point is a different structural problem, and a calculation that quietly redistributes load onto a support carrying nothing is telling you something false with great confidence.

Where no chart is available, the reactions can be estimated from a compression-only support model: the machine's own masses, the boom, the counterweight and the hoisted load, resolved onto four supports that can push but not pull. That is a legitimate method and it is not the same thing as a chart. It should be labelled an estimate everywhere it appears, including in the report you hand over.

Take the worst outrigger reaction and divide it by the float area.

In the example that is 350 kN on a 0.5 m square float, so 0.25 square metres, giving 1.40 MPa. Expressed in the units the ground side uses, that is 1400 kPa.

No soil takes that. Firm ground might take 250 kPa; a designed granular platform might take 450 kPa. The float is asking for more than five times the firm ground and more than three times the engineered platform, which is why the float is never the thing standing on the ground.

That fraction is also the sanity check worth doing before anything else on site. If someone tells you the pads are fine, ask what the pressure under the float is. If they answer with the crane's weight divided by four divided by the mat area, three separate errors have already happened.

This is the step that changes answers, and it is the one most often taken for granted.

A mat under a float does not present its plan area to the ground. It spreads the load outwards from the float through its own depth, and how far it spreads depends on how stiff it is relative to the ground beneath it. The conventional model gives a spread angle through the thickness: the loaded square grows by the thickness times the tangent of that angle, on each side, and can never exceed the pad itself.

Run the numbers on a mat that looks generous: a 1.5 m square timber mat, 0.15 m thick, under a 0.5 m square float, so 2.25 square metres of plan area.

That is one fifth of the timber you are looking at.

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The pressure that reaches the ground is therefore 772 kPa, not the 156 kPa a plan-area calculation would have reported. The plan-area answer is optimistic by a factor of three, in the unconservative direction, on a mat that a site would consider well provided for.

If you want to credit the whole plan area, you are asserting the pad is effectively rigid over that span. That is sometimes true and it is a claim you then have to support, by checking the pad in bending over the full overhang at the pressure you are claiming. What you cannot do is take the area for free and skip the check that would justify it.

The pad also has to survive locally. It carries bending as a cantilever out from the float, one-way shear near the float edge, and punching straight through under it. A large thin mat that punches through under the float has delivered exactly the float's area to the ground, which is where this article started.

Two honest routes exist, and one dishonest one.

A declared allowable. Someone with responsibility states a value for this setup position, and the calculation compares against it. This is the normal case, it is perfectly defensible, and the entire weight of it rests on who declared the number and on what basis.

A calculated capacity. Ground parameters are measured or assumed, an ultimate bearing capacity is computed by one of the classical formulations, and a factor of safety is applied to get a working value. The factor of safety is a declared project decision, not a property of the formula, and the ultimate capacity divided by nothing is not an allowable pressure.

A number from the internet. Presumed bearing values published in tables span roughly an order of magnitude between soft clay and dense gravel, and they carry assumptions about drainage, depth, and the absence of a trench three metres away. A site compound frequently meets none of them. Use a presumed value to size a first guess and then get it confirmed for the position the crane is actually setting up in.

And whatever the value, it applies to the ground as it will be on the day. Rain between the survey and the lift changes it. So does the excavation that appeared last week within the zone of influence spreading down and out from the pad, which is the point at which a lift stops being a lift plan and becomes temporary works with a designer attached.

Start with the mat as it is usually specified, and with no spread angle declared. With nothing declared, nothing is credited, and the effective area is the float's.

Now declare a 30 degree spread through the pad's thickness.

To pass, the arithmetic says you need about 1.4 square metres of genuine contact on 250 kPa ground. Neither a thicker mat alone nor better ground alone gets there comfortably, so the realistic answer uses both: a 2.0 m mat at 0.4 m thick, standing on a designed granular working platform good for 450 kPa.

1. Machine weight divided by four. It ignores the couple from boom, load and counterweight, which is the thing that decides which corner is worst. In the example it reports 250 kN where the chart says 350 kN, so every pressure downstream starts out 30 percent light.

2. Plan area credited as contact area. The most expensive error in this subject. On the worked mat the plan area is 2.25 square metres and the spread reaches 0.45, so crediting the plan area overstates the contact by a factor of five, in the unconservative direction, on a mat nobody would have questioned.

3. One slew angle checked. Usually the one in the elevation on the drawing. The governing configuration is found by sweeping the slew, not by picking a picture.

4. The pad checked for bearing only. Bearing on the ground, bearing on the pad, bending, one-way shear and punching are five different checks. The example's final case is comfortable in bending and five times worse in shear, and only one of those two ever gets calculated by hand.

5. Ground conditions taken from the survey and not from the day. Rain, an excavation inside the zone of influence, a buried service, a slope crest nearby: each changes what the ground is allowed to take, and none of them change the number in the calculation unless someone goes back to it.