What is ground bearing pressure for crane outriggers?

Ground bearing pressure is a critical engineering parameter in mobile crane setup. Every crane outrigger float or crawler track transmits the crane's weight and load moment to the ground through a defined contact area. The ratio of force to area is the ground bearing pressure, expressed in pounds per square foot or kips per square foot. If this pressure exceeds the soil's ability to resist it, the ground will compress, displace, or fail, causing the outrigger to punch through and the crane to tilt or overturn.

How Ground Bearing Pressure is Calculated

For a mobile crane on outriggers, the calculation begins with the maximum outrigger load at each pad, which depends on the crane's own weight, the load weight, and the crane's geometry. The load chart may provide maximum outrigger loads for various configurations, or the crane manufacturer may provide them separately. The outrigger load is then divided by the contact area of the crane mat or outrigger float to determine the bearing pressure. If this pressure exceeds the soil's allowable bearing capacity, a larger crane mat is required to spread the load over a greater area and reduce the pressure to a safe level.

The allowable bearing capacity of soil varies significantly by soil type. Dense, dry, undisturbed gravel can support relatively high bearing pressures. Saturated clay, fill soil, or recently excavated and backfilled areas may have very low bearing capacity, often less than what an outrigger float without matting would exert. Underground utilities, voids, or disturbed pavement can create localized weak zones that are not visible from the surface.

When Engineering Involvement is Required

OSHA 1926.1402 requires the employer to ensure that the ground conditions are adequate to support the crane, and for conditions that are soft, uncertain, or near excavations, a qualified engineer must be involved in evaluating the ground and specifying the mat configuration. The engineer's evaluation may involve soil borings, a review of site grading records, or analysis of as-built utility drawings. On many commercial construction sites, the geotechnical report prepared for the structural foundation includes soil bearing capacity values that the crane planner can use directly.

Crane Mats as the Standard Solution

Timber crane mats or structural composite mats are the standard method for distributing outrigger loads over a larger ground area. The mat distributes the outrigger point load across its full bearing area. Larger mats reduce ground bearing pressure proportionally. The mat must be placed on a firm, level surface and must not span soft spots or voids. After placement, the outrigger is extended onto the mat and the mat's position is checked for level and stability before the crane is loaded. Post-set inspection during the lift is also required, as ground conditions can change during the lift particularly in wet conditions.