Crane Outrigger Calculator
This crane outrigger calculator estimates the worst-case reaction under a single outrigger, the pad (mat) area needed to keep ground bearing pressure within the soil capacity, and a recommended square pad size. It applies a conservative load-concentration factor to the combined gross load and counterweight, then divides by the allowable bearing pressure for your selected soil to size the pad. The result flags whether the chosen pad keeps bearing pressure at or below the soil limit.
Formula
Rmax = 0.75 · (gross + counterweight) ; A = Rmax / qa ; pressure = Rmax / side²
- Rmax
- Worst-case reaction under the most heavily loaded outrigger (lbs)
- 0.75
- Load-concentration factor assigning ~75% of total weight to one outrigger during a worst-case lift
- qa
- Allowable soil bearing pressure for the selected soil type (psf)
- A
- Required pad bearing area (sq ft)
- side
- Recommended square pad side, rounded up to the nearest 0.5 ft
How it works
- Enter the gross load on the crane (boom, rigging, and lifted weight) and the counterweight, both in pounds. The tool sums them as the total weight to be supported.
- Select the soil type, which sets the allowable bearing pressure (psf) from presumptive values aligned with IBC Table 1806.2.
- The calculator takes 75% of the total weight as the worst-case single-outrigger reaction, divides by the allowable pressure to get the required pad area, rounds the pad side up to the nearest 6 inches, and reports the resulting actual bearing pressure and utilization.
Worked example
A crane with an 80,000 lb gross load and 20,000 lb counterweight set on medium dense sand with 3,000 psf allowable bearing.
- Total weight = 80,000 + 20,000 = 100,000 lb.
- Worst-case reaction Rmax = 0.75 × 100,000 = 75,000 lb.
- Required pad area = 75,000 / 3,000 = 25 sq ft, so the pad side = √25 = 5.0 ft (already a 0.5-ft increment).
- Actual bearing pressure = 75,000 / (5 × 5) = 3,000 psf; utilization = 3,000 / 3,000 = 1.0.
Rmax = 75,000 lb, required area 25 sq ft, a 5 ft × 5 ft pad gives 3,000 psf at exactly 1.0 utilization — adequate but with no reserve, so a larger pad is prudent.
Frequently asked questions
- Why is 75% of the total weight used for one outrigger?
- During a lift the center of gravity shifts toward the load side, so the reactions are not shared equally among the four outriggers. Assigning about 75% of the total weight to the most loaded outrigger is a common conservative assumption for pad sizing.
- Does the outrigger spread change the reaction?
- In this simplified model the reaction is driven by the total weight and the concentration factor, not the spread. A wider spread improves stability and reduces tipping risk, but a detailed reaction analysis would require the load geometry and crane chart.
- How do I know the allowable soil bearing pressure?
- The calculator uses presumptive values by soil type (for example, dense sand or stiff clay) consistent with IBC Table 1806.2. For an actual project a geotechnical report giving site-specific allowable bearing should govern.
- Why round the pad up to the nearest half foot?
- Crane mats and timber pads come in standard sizes, so the tool rounds the computed side up to the nearest 6 inches. This guarantees the chosen pad area is at least the required area and keeps bearing pressure within the soil limit.