Crane Outrigger Pad & Ground Bearing Calculator

How it works

Outrigger load ≈ share % × (crane weight + gross load), 85% for a corner pick; pressure = load ÷ bearing area; effective mat area ≈ (float + 2 × mat thickness)² for timber; required area = load ÷ allowable soil pressure

When a crane swings a load over one corner, most of the machine's weight and the load transfer to that single outrigger — the industry rule of thumb is up to 85% of the total for a corner pick on a four-outrigger truck crane, and manufacturers' outrigger-load calculators give exact figures per configuration. That force on a two-foot float produces pressures of tens of thousands of pounds per square foot, far beyond what any soil or pavement can take, so mats spread it out; but a timber mat only spreads load at about 45° through its thickness unless it is stiff enough to act as a plate, which is why a big thin mat can be less effective than a smaller thick one. Comparing the resulting pressure with the allowable bearing pressure of the actual ground — soft clay, fill, asphalt over unknown base — is the step most often skipped before a crane tips.

Worked example

A 120,000 lb crane picking a 20,700 lb gross load can put 85% of the 140,700 lb total — 119,600 lb, nearly 60 tons — on one outrigger. On a 24″ float that is 38,000 psf, over 12 times a 3,000 psf soil; a 12″ thick timber mat spreads it over about a 4 × 4 ft area (7,500 psf), still too much, so the plan needs about 40 ft² of stiff mat — a 4 × 10 ft steel or engineered mat, or two layers of timber.

Frequently asked questions