How the ground bearing pressure calculator works
A crane hands its weight and the load to the ground through a few outrigger floats. Each float pushes down on a small patch of ground, and the pressure on that patch is the force divided by the area it touches:
- Pressure = outrigger load ÷ contact area. In US units, lb ÷ sq ft gives psf. In metric, kN ÷ m² gives kPa.
- Minimum area = outrigger load ÷ allowable pressure. Any pad with at least this much area keeps the pressure at or under the value you entered.
- Minimum square pad side = √(minimum area). Because area goes with the side squared, doubling the side cuts the pressure to a quarter.
The calculator assumes the pad is stiff enough to spread the load evenly over its whole face. Real pads flex, soil is not uniform, and a load that sits off center presses harder on one side, so the minimum side is a floor and not a design.
Two inputs matter most, and both are yours to supply. The outrigger load is not the crane's weight divided by four: with a heavy load near one side, a single outrigger can carry far more than a quarter of the total, so use the figure from the crane's own load information. The allowable bearing pressure comes from the ground assessment for the site. Check whether the number you were given already includes a safety margin, because some values quote the pressure the ground can take before it fails. OSHA 1926.1402 covers ground conditions and who is responsible for them.
Worked example 1: 40,000 lb on a 24 in float
A 24 in square float is 4 sq ft, so 40,000 lb gives 40,000 ÷ 4 = 10,000 psf. The allowable value in this example is 3,000 psf, so the float alone is over by more than three times. Minimum area = 40,000 ÷ 3,000 = 13.333 sq ft, and the side is √13.333 = 3.651 ft, about 43.8 in. A 48 in square pad is 16 sq ft, which gives 40,000 ÷ 16 = 2,500 psf, four times lower than the float alone and under the 3,000 psf allowable.
Worked example 2: 120 kN on a 300 mm round float
A 300 mm round float has an area of π × 0.15² = 0.0707 m², so 120 kN gives 120 ÷ 0.0707 = 1,698 kPa. With an allowable value of 150 kPa, the minimum area is 120 ÷ 150 = 0.8 m² and the minimum square side is √0.8 = 0.894 m, about 894 mm. A 1,000 mm square pad is 1 m², so the pressure falls to 120 kPa.
These are original numbers for teaching, not exam questions and not a crane or soil rating.
Outrigger pad questions
Is the outrigger load just the crane weight divided by four?
No. The load on one outrigger changes with the boom position, the radius and where the load sits, and it can be much higher than a quarter of the total weight. Use the figure from the crane's own load information for the setup you are checking.
Where does the allowable ground pressure come from?
From the ground assessment for the site, by the person responsible for the ground. This calculator does not guess a value for you, because soil type, moisture, fill and compaction can change it a lot. Ask whether the figure already includes a safety margin.
If I double the mat width, does the pressure halve?
No, it drops to a quarter. Area follows the side squared, so a 4 ft mat has four times the area of a 2 ft mat under the same load. A bigger mat cannot fix a void or weak layer below it, which is why the ground still has to be assessed.
Site is about 22% of the Core exam, and mats and ground pressure are part of it. For the reasoning behind outriggers, mats and cribbing, read ground conditions, outriggers and mats.