What Outriggers and Ground Bearing Actually Decide

Why this matters

An outrigger does not make a crane stable. It takes the overturning moment the machine is already generating and concentrates it into a contact patch a couple of feet square, and whether the ground under that patch can take it is a completely separate question from anything on the load chart. The chart assumes firm level support and says nothing about your site. This card walks the arithmetic from a published outrigger reaction to a mat size, prints every correction as its own line, and names who owns each number you cannot derive, because three of the numbers in the worked example are not yours to invent.

The duty exists before the arithmetic does

For construction work, 29 CFR 1926.1402 requires that ground conditions be adequate before the equipment is assembled or used, meaning firm, drained and graded to a sufficient degree, with supporting materials such as blocking, cribbing, pads and mats provided as necessary, and it places the duty to ensure that preparation on the controlling entity for the site. That is a real allocation of responsibility, and it means the ground question has a named owner before your crew arrives. If nobody on the job can tell you who that is, that is a finding you raise before the outriggers come out, not a detail you work around.

Outrigger extension itself is a manufacturer procedure, and 1926.1417 requires operations to comply with manufacturer procedures. Full, intermediate and retracted extension are separate chart tables with separate reactions. An intermediate setting is never an average of the other two, and "as far as it would go before it hit the fence" is not a setting at all.

What the outriggers are actually carrying

The reaction at one outrigger is not the load. It is the machine's own weight, its counterweight, the boom, the rigging and the load, resolved through the geometry of the outrigger box at the boom position you are at. Three consequences follow, and each of them surprises somebody at least once.

The critical corner can carry far more than a quarter of anything. When the boom is over a corner, most of the resisting moment goes through two floats and the reaction at the loaded corner can exceed what a simple share of the total weight would suggest. Reactions also change continuously as the machine swings, so the critical corner is a function of the swing path, not a fixed corner.

The reaction changes with radius even when the load does not. Reaching further raises the overturning moment, which raises the reaction at the leading floats, which is why an outrigger reaction figure is quoted against a configuration and a radius the way a chart capacity is.

Nobody computes it from scratch at the tailgate. Outrigger reaction charts, or the manufacturer's lift planning software, own this number. A rigger deriving reactions from statics on a notepad is doing engineering without the machine's internal weights, and the error runs in whichever direction the assumptions did.

Pressure is reaction divided by an area you have to justify

The float on the end of an outrigger beam is a machine part sized for the machine, not for your soil. Its area is small, deliberately, and the pressure under it is correspondingly high. Cribbing, pads and mats exist to turn that small area into a larger one.

The catch is that the area you get is not automatically the area you laid down. A mat spreads load through its own bending stiffness. A stiff mat spreads a reaction over most of its plan area; a flexible one deflects under the float and much of the plan area carries very little. The relationship you can rely on is directional: effective bearing area is always less than or equal to the geometric area, and the ratio improves with mat stiffness and worsens as the float gets smaller relative to the mat. The actual factor belongs to the mat manufacturer or to the engineer who sized the support, and there is no field rule of thumb that is safe to substitute for it.

        outrigger beam
     =======[  ]=======
             ||
            float
          +------+
   +======|======|=======+   mat, plan area larger
   +=====================+   than the float
   ///////////////////////   prepared subgrade
      .    .    .    .       load spreads with depth,
        .    .    .          and the mat only spreads it
                             as far as its stiffness allows

The allowable pressure is a soil property, and it already has a factor in it

An allowable bearing pressure is not a strength. It is a value derived by a geotechnical engineer or a registered professional engineer from the soil at your specific location and depth, already reduced by a factor against a bearing capacity failure and usually also limited so that settlement stays within tolerance. That factor is already inside the number, which means two things: you do not get to add a comfort margin on top and then re-spend it somewhere else, and you certainly do not get to round it up.

Where no such value exists for the site, that is the finding. There is no visual soil class you can call from a truck window that turns into a number. Recently placed fill, a backfilled utility trench, a vault or basement below, a slope crest, the zone behind a retaining wall, and anywhere within the influence of an excavation are all conditions where the surface tells you nothing about what is three feet down. For an excavation specifically, 29 CFR 1926 Subpart P governs in construction, and it has no general-industry counterpart; 1926.651 requires that surcharge loads be kept back from the edge and that adjoining structures endangered by the excavation be supported. A crane float is a surcharge load, and the setback is a determination for the competent person and, where a protective system is involved, for the registered professional engineer who designed it.

Worked example: from a published reaction to a mat that works

Configuration set, outriggers on full extension, and the manufacturer's outrigger reaction chart for this machine, boom length, radius and quadrant gives a critical corner reaction of 62,000 lb. Two of the figures below come from documents and are labelled as such; the rest is arithmetic you can follow.

Re-basing line, printed first. That 62,000 lb already contains the machine's own weight, its counterweight, the boom, the block, the rigging and the suspended load. It is the total reaction at that float, not the load's share of it. You do not add machine weight to it, and you do not add the load to it a second time.

Pressure on the bare float. The float is 24 in by 24 in, which is 2 ft by 2 ft, or 4.0 ft2.

• 62,000 lb / 4.0 ft2 = 15,500 lb/ft2

Allowable pressure. From the geotechnical report covering this area of the site, illustrative here, the allowable bearing pressure on the placed fill at float depth is 2,000 lb/ft2. That number is the engineer's, not yours.

Required effective area.

• 62,000 lb / 2,000 lb/ft2 = 31.0 ft2

First mat attempt, and the correction that kills it. An 8 ft by 4 ft mat has a plan area of 32.0 ft2, which clears 31.0 ft2 on paper.

• Nominal pressure, geometric area: 62,000 / 32.0 = 1,938 lb/ft2
• Effectiveness factor for this mat and this float, from the mat manufacturer, illustrative here: 0.80
• Effective area: 32.0 ft2 x 0.80 = 25.6 ft2
• Corrected pressure: 62,000 / 25.6 = 2,422 lb/ft2

2,422 is above the allowable 2,000, so the 8 ft by 4 ft mat fails. Note that the correction is a re-basing of the area, not an extra load applied on top: nothing got heavier, the denominator got smaller because it was never the whole mat in the first place. Note also which direction it ran. Every honest correction in ground bearing runs against you, and any arithmetic that comes out in your favour deserves a second look at what you assumed.

Second mat. A 10 ft by 5 ft mat, plan area 50.0 ft2.

• Effective area at the same 0.80 factor: 50.0 x 0.80 = 40.0 ft2
• Corrected pressure: 62,000 / 40.0 = 1,550 lb/ft2

1,550 is below 2,000, so this one works, with the effective area 29 percent above the 31.0 ft2 minimum rather than 17 percent below it.

What the arithmetic does not cover. The float has to sit fully on the mat and near its centre, the mat has to bear on prepared subgrade over its whole underside rather than bridging a rut, and the same check has to be repeated for every corner that becomes critical as the machine swings, not just the one you calculated. A mat sized correctly and laid across a soft spot is a mat that will find the soft spot.

What getting this wrong looks like in the field. It rarely looks like a mat splitting. It looks like a float that is a hand's width lower after the load has been up for a few minutes, on ground that took the setup weight without complaint. Bearing failure under a crane is a settlement problem before it is a collapse problem, and the settlement is what puts the machine out of level and moves the radius. That progression is the subject of the soft-ground card in this library and is not re-derived here.

How to verify the support before the boom comes up

Every check below is made with the boom stowed or stationary, the crew outside the swing radius, and nobody standing between a float and the machine while outriggers are being extended, because an extending outrigger beam is a crush hazard with no operator sightline.

• Get the outrigger reaction figures for your actual configuration, from the manufacturer's chart or planning software, at the radius and quadrant you will work in, not at the one you set up in.
• Get an allowable bearing pressure from the party who owns the ground, in writing. Where the answer is "it looks fine", you do not have a number and the lift is not planned.
• Ask what is buried. Vaults, tanks, basements, backfilled trenches and utilities under a float are a void problem, not a pressure problem, and no mat size solves a void.
• Confirm the outrigger extension matches the chart table you used, by looking at the marked extension positions on the beams rather than at the operator's memory.
• Confirm the float bears fully and centrally on the mat, and that the mat bears fully on prepared grade, before any load is picked.
• Mark the mat elevation with a level reference at setup, and re-check it after the first pick from outside the swing radius, so settlement is measured rather than noticed.
• Set back from every excavation edge by the distance the competent person for that excavation specifies, and treat the float as the surcharge it is.

References

• 29 CFR 1926.1402, ground conditions for cranes and derricks in construction, which requires adequate ground preparation and supporting materials and assigns the preparation duty to the controlling entity
• 29 CFR 1926.1417, operation, which requires compliance with the manufacturer's procedures, including outrigger extension positions
• 29 CFR 1926 Subpart P, excavations, including 1926.651 on surcharge loads and the protection of adjoining structures; this subpart has no general-industry counterpart
• ASME B30.5, mobile and locomotive cranes, in the edition adopted by your employer's program, your contract or your authority having jurisdiction
• Manufacturer outrigger reaction charts and mat manufacturer load distribution data, plus the site geotechnical report or a registered professional engineer, which own the reaction, the effectiveness factor and the allowable bearing pressure respectively