Crane Outrigger Loads, Explained

What an outrigger reaction is

On outriggers, the whole crane — carrier, superstructure, counterweight, boom, hook block, rigging and load — is carried by the outrigger pads. For a static lift, the outrigger reactions add up to that total weight. What they do not do is share it equally.

Why the load is not shared equally

• The load moment tips the crane towards the load, adding force to the outriggers on that side and taking it off the far side.
• The crane’s own centre of gravity is rarely at the centre of the outrigger base — counterweight at the rear pulls it backwards.
• As the crane slews, the most heavily loaded outrigger changes. The highest single-leg load often occurs with the boom over or close to one outrigger, but the exact position depends on the crane.
• A shorter outrigger spread gives the crane less leverage against the load moment, which generally increases the load on the legs nearest the lift.

The load moment

Load moment is the part of the outrigger calculation that needs no crane data. It grows in direct proportion to radius: the same load at 30 m puts 50% more moment on the crane than at 20 m.

1. Lifted mass — A 10,000 kg load with 1,000 kg of rigging and hook block is lifted at a 20 m radius. These numbers are illustrative.
2. Convert to force — 11,000 kg × 9.80665 ÷ 1,000 = 107.873 kN.
3. Multiply by radius — 107.873 kN × 20 m = 2,157.5 kN·m.
4. Move the load out — At 30 m the same load gives 107.873 × 30 = 3,236.2 kN·m.

Why “total weight ÷ 4” is unsafe

Dividing the total weight by four gives an average, not the maximum. Take an illustrative crane and load totalling 60 t: the average is 15 t per leg. If the manufacturer’s data shows the leg nearest the load carrying 25 t, the real load on that leg is 67% higher than the average — and every ground pressure and mat size based on 15 t is wrong by the same margin.

Where a verified outrigger load comes from

• The manufacturer’s outrigger or pad load data for the crane, in its operator documentation or supplied by the manufacturer.
• Manufacturer planning software. For example, Manitowoc publishes a ground bearing pressure estimator for its lattice-boom cranes and an outrigger pad load calculator for Grove cranes, and Liebherr’s Crane Planner 2.0 reports ground bearing pressures and ground forces.
• A calculation by a competent engineer using the full crane data: self-weight and centre of gravity, counterweight, outrigger geometry, boom configuration and the lift itself.

Whichever source you use, the inputs must match the lift: the same counterweight, outrigger spread, boom length, radius, slew range and load.

What changes the outrigger load

• Load, rigging and hook block weight.
• Radius — the further out, the larger the load moment.
• Slew angle relative to the outriggers.
• Counterweight configuration.
• Outrigger extension: full, intermediate or retracted.
• Boom length and any jib or attachments.
• Ground that is not level, wind on the boom and load, and dynamic effects from slewing, starting and stopping.

From outrigger load to ground pressure

Once you have the verified maximum single-leg load, the ground check is simple: divide it by the bearing area and compare the result with the allowable ground pressure for that position. The ground bearing pressure and crane mat sizing guides walk through both steps.

Common questions