What is an outrigger on a crane?

Outriggers are the primary stability system on rubber-tired mobile cranes, including truck cranes, all-terrain cranes, and rough-terrain cranes. Without outriggers deployed, a mobile crane's rated capacity is severely limited because the stability of the machine depends entirely on the tire contact footprint. When outriggers are fully extended and set, they create a much wider base that dramatically increases tipping resistance and allows the crane to operate at significantly higher rated capacities than it could on rubber tires alone.

How Outriggers Work

Each outrigger consists of three main components: the horizontal beam that slides or telescopes out from the crane's carbody, the vertical jack cylinder that extends downward from the end of the beam, and the float or pad that sits on the ground beneath the jack. To set up the outriggers, the operator first extends the beams horizontally to the desired spread, then extends the vertical jacks until the pads contact the ground and the crane's tires are lifted clear of the ground surface. All outrigger pads must be in firm, level contact with the ground before any lifting begins.

The load chart is configuration-specific. A crane's rated capacities vary depending on whether the outriggers are at full spread, mid spread, or retracted. Operating on the wrong load chart column for the actual outrigger spread overestimates capacity and creates a tipping hazard. The operator must identify the exact outrigger configuration before selecting the applicable load chart table and must verify that the outriggers are set in the configuration shown.

OSHA 1926.1402 and Ground Conditions

OSHA 1926.1402 requires the employer to ensure that the ground conditions at the setup location are adequate to support the crane with the imposed loads. The outrigger float transmits the crane's load to the ground through a relatively small contact area. On soft soils, wet ground, or areas with underground utilities or voids, the ground bearing pressure under the outrigger float may exceed the soil's capacity, causing the outrigger to punch through the surface and the crane to tilt or overturn. When ground conditions are uncertain, a qualified engineer must evaluate the soil's bearing capacity and specify the crane mat sizing required to distribute the load over a sufficient area.

The employer's obligation under 1926.1402 is not satisfied simply by deploying the outriggers. The employer must actively evaluate the ground and take corrective action - typically by placing crane mats under the outrigger floats - when the ground is soft, recently disturbed, or of unknown bearing capacity. Ground condition failures are a major contributing factor in mobile crane tip-overs investigated by OSHA.

Outrigger Inspection During Pre-Shift Check

OSHA 1926.1412(e) requires outrigger condition to be checked during every pre-shift inspection. Inspectors should look for hydraulic leaks at the jack cylinders, bent or damaged beam sections, worn or missing float pads, and any structural damage to the outrigger housing. A leaking jack cylinder that slowly retracts under load is a setup condition that can cause the crane to shift during a lift. Any deficiency that affects outrigger stability must result in the crane being removed from service until the deficiency is corrected.