How to Calculate Crane Outrigger Cribbing

Most mobile crane accidents result from improper setup. The most common of those mistakes, according to materials-handling trainers, is inadequate cribbing under crane outrigger pads. "Cribbing " is a term used to describe pads that fit under the round or rectangular feet that keep cranes stable during operation. Crane manufacturers describe proper cribbing procedures for each crane you will operate. But to determine that proper amount of cribbing, you will need to calculate two variables: the compressibility of the soil under the pads and the total weight of the crane and its load.

Cranes can collapse when they are improperly supported.

Step 1

Estimate how much weight the soil at your job site will support without compressing. Rock can support about three tons per square foot. Silt will only support about 2,000 pounds per square foot, and the most common soil type, Type A or "cohesive soil," will support about 3,000 pounds per square foot.

Step 2

Calculate the maximum total weight of your fully loaded crane. A 50-ton crane, capable of lifting 100,000 pounds, itself weighs 86,000 pounds. It will, therefore, have a total loaded weight of 186,000 pounds.

Step 3

Divide the pounds per square foot of soil resistance (for example, 3,000 pounds per square foot for Type A soil) by 144, which is the total number of square inches in a square foot. For type A soil this works out to 20.9 pounds of soil resistance per square inch.

Step 4

Divide the weight of your loaded crane by the weight the soil under your crane can support. A loaded crane that weighs 186,000 pounds on Type A soil that will support 20 pounds per square inch needs a total of 9,300 square inches of support under each crane outrigger.

Step 5

Calculate the square root of this total support area to determine the size of the cribbing pads you must use under each outrigger foot. The square root of 9,300 is about 96.5 inches. So you must use a square cribbing pad that measures at least 96.5 inches on each side to support the weight of a 186,000-pound crane on Type A cohesive soil. The weaker the soil, the larger the cribbing pads must be.