7.2 Ground Bearing Pressure, Outrigger Setup & Cribbing Sizing

Quick Answer: Ground failure under outriggers is a frequent cause of mobile crane tip-overs. When a crane swings a heavy load over a corner or over the side, most of the machine and load weight can concentrate on a single outrigger pad. To prevent catastrophic ground punching or turnover, operators must calculate the maximum outrigger vertical force ($F_{\text{outrigger}}$), verify allowable soil bearing capacity ($P_{\text{allowable}}$), and size blocking mats using $A_{\text{required}} = F_{\text{outrigger}} / P_{\text{allowable}}$. Cribbing should be sound hardwood or engineered mats, outrigger beams set to the position the chart requires, and the tires clear of the ground when using on-outrigger charts.

A mobile crane is only as stable as the ground beneath its outriggers. 230 CMR 6.08(2)(c)17 requires outriggers or stabilizers to be used with adequate blocking, according to the manufacturer's specifications or surface conditions. OSHA 29 CFR 1926.1402 requires ground that is firm, drained and graded so that, with supporting materials if necessary, the manufacturer's specifications for support and degree of level are met. The controlling entity must provide the ground preparation and must tell the crane user and operator about voids, tanks, utilities and other hazards beneath the setup area that it knows of. If the A/D director or the operator finds the ground inadequate, their employer must work out the needed preparation with the controlling entity. Inadequate cribbing, soft backfill and improperly extended outriggers are frequent contributors to crane turnovers.

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1. Outrigger Load Distribution Physics: The Corner Concentration Effect

When a mobile truck crane or rough-terrain crane is set up on four outriggers, its total static dead weight (carrier, upperworks, counterweight, and boom) is initially distributed across all four support points. However, the moment a load is lifted and the upperworks rotates, the center of gravity of the entire system shifts dramatically toward the tipping axis.

How Much Can One Outrigger Carry?

When the boom swings over a corner outrigger pad or directly over the side at maximum working radius:

• The stabilizing moment of the rear counterweight shifts directly toward the forward tipping fulcrum.
• The overturning moment of the boom and gross suspended load acts directly over that specific outrigger jack.
• As a result, a large majority of the combined weight of the crane plus the suspended load can become concentrated on that single outrigger float. The real figure depends on the crane, counterweight, boom position and load, so planners take it from the manufacturer's outrigger-load data, the crane's computer readout or an engineering calculation. A widely taught conservative rule of thumb assumes one outrigger carries 75% of the crane weight plus the load.

For example, a crane weighing 70,000 lbs lifting a 20,000 lb precast beam has a combined weight of 90,000 lbs. The rule of thumb gives $0.75 \times 90{,}000 = 67{,}500$ lbs on a single float that may be only about 2 feet across.

If the soil beneath that single pad yields, shears, or punches through by just a few inches, the crane tilts. A tilt of only 1 to 2 degrees creates massive out-of-level side loading on the boom, increases the operating radius, and can initiate an uncontrollable, catastrophic turnover.

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2. Mathematical Formulas for Ground Bearing Pressure

Ground bearing calculations ensure that the vertical compressive stress applied by the outrigger pad to the subgrade remains well within the safe structural limits of the soil.

Formula 1: Applied Ground Bearing Pressure

Applied ground bearing pressure is defined as the vertical outrigger force divided by the effective contact surface area of the outrigger pad or cribbing mat:

$$P_{\text{applied}} = \frac{F_{\text{outrigger}}}{A_{\text{pad}}}$$

where:

• $P_{\text{applied}}$ is the applied ground bearing pressure in pounds per square foot (psf),
• $F_{\text{outrigger}}$ is the maximum outrigger vertical reaction force in pounds (lbs),
• $A_{\text{pad}}$ is the contact surface area of the outrigger pad, steel float, or timber cribbing in square feet (sq ft).

Formula 2: Safety Verification Criterion

To guarantee site stability and prevent ground punch-through, the applied pressure must never exceed the allowable soil bearing capacity:

$$P_{\text{applied}} \le P_{\text{allowable}}$$

Formula 3: Sizing Required Pad or Cribbing Mat Surface Area

To determine the minimum contact surface area required for a timber cribbing mat or steel spreader pad, rearrange the equation:

$$A_{\text{required}} = \frac{F_{\text{outrigger}}}{P_{\text{allowable}}}$$

For a square cribbing mat of side length $S$:

$$S = \sqrt{A_{\text{required}}}$$

For a circular pad of diameter $D$:

$$D = \sqrt{\frac{4 \times A_{\text{required}}}{\pi}}$$

3. Allowable Soil Bearing Capacities Reference Table

Soil bearing capacity varies widely depending on soil composition, moisture content, compaction density, and geological history. The following table gives typical presumptive allowable bearing values ($P_{\text{allowable}}$) used for preliminary planning. Site-specific values from a geotechnical report or a qualified person govern:

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4. Step-by-Step Worked Mathematical Sizing Calculation

Work through this realistic jobsite cribbing sizing problem:

Problem Scenario

A 50-ton hydraulic truck crane is assigned to lift a heavy rooftop mechanical unit. The total crane gross weight (carrier, upperworks, counterweight, and boom) is 70,000 lbs, and the suspended load with rigging weighs 15,000 lbs. During maximum working radius slewing over the front-right outrigger, engineering data confirms that 85% of the total system weight ($85,000\text{ lbs}$) is concentrated onto that single outrigger jack:

$$F_{\text{outrigger}} = 0.85 \times 85,000\text{ lbs} = 72,250\text{ lbs}$$

Geotechnical test reports establish that the jobsite subgrade consists of medium-stiff clay with an allowable bearing capacity of $P_{\text{allowable}} = 2,500\text{ psf}$.

Step 1: Calculate Minimum Required Mat Area ($A_{\text{required}}$)

Using Formula 3:

$$A_{\text{required}} = \frac{F_{\text{outrigger}}}{P_{\text{allowable}}} = \frac{72,250\text{ lbs}}{2,500\text{ psf}} = 28.9\text{ sq ft}$$

Step 2: Determine Dimensions for a Square Timber Mat

Calculate the required side length $S$:

$$S = \sqrt{28.9\text{ sq ft}} \approx 5.38\text{ ft}$$

Because timbers are supplied in standard whole-foot increments, round up to the next safe standard dimension: construct a 6 ft × 6 ft square cribbing mat:

$$A_{\text{actual}} = 6\text{ ft} \times 6\text{ ft} = 36.0\text{ sq ft}$$

Step 3: Verify Applied Ground Bearing Pressure

Check the actual applied pressure with the 36.0 sq ft mat:

$$P_{\text{applied}} = \frac{72,250\text{ lbs}}{36.0\text{ sq ft}} = 2,006.9\text{ psf}$$

Since $2,006.9\text{ psf} \le 2,500\text{ psf}$, the 6 ft × 6 ft hardwood timber mat safely reduces ground bearing pressure below the subgrade's allowable limit, establishing an acceptable safety margin.

5. Outrigger Engineering Rules & Cribbing Construction Protocols

Safe outrigger support follows the manufacturer's specifications (230 CMR 6.08(2)(c)17; OSHA 1926.1402) and these widely used practices:

1. Carrier Tires Completely Clear of the Ground

OSHA 1926.1404(q)(2) requires outriggers to be set so they remove the equipment weight from the wheels (locomotive cranes excepted), and on-outrigger load charts are based on the tires being clear of the ground. ASME B30.5's 85% stability rating for wheel-mounted cranes on outriggers assumes the tires are off the supporting surface. If tires remain in contact with the ground, a portion of the machine's weight is borne by the pneumatic tire rubber and truck suspension springs. As the crane swings, tire deflection causes unpredictable shifting of the tipping axis, inducing severe oscillation and dynamic instability.

2. Full Extension of Outrigger Beams

OSHA 1926.1404(q)(1) requires outriggers to be either fully extended or, if the manufacturer's procedures permit, deployed as specified in the load chart. Each outrigger must be visible to the operator or a signal person while it is extended and set, floats must be attached to the outriggers, and blocking goes only under the float or pad (1926.1404(q)(3)–(5)). Operating on partially retracted or mid-position outriggers is therefore allowed only when:

• The crane is equipped with manufacturer-certified intermediate outrigger load charts.
• Outrigger beams are mechanically locked into the designated intermediate pin positions.
• The intermediate load chart is explicitly selected on the crane's Rated Capacity Indicator (RCI/LMI) system.

3. Size Blocking by Calculation, Not Just a Ratio

Some manufacturers and company procedures add a minimum mat-to-float area ratio, and 3:1 is a figure often quoted. Treat any such ratio only as a minimum. The governing check is the bearing-pressure calculation above, with a mat stiff enough to spread the load over its full area:

$$A_{\text{cribbing}} \ge 3 \times A_{\text{float}}$$

For example, if a crane float has an area of 3 square feet, the minimum cribbing mat area placed beneath it must be at least 9 square feet (e.g., 3 ft × 3 ft).

4. Timber Material Quality & Species Selection

• Hardwood Only: Cribbing timbers must be constructed from dense, sound hardwood species (such as oak, beech, maple, or birch). Hardwood possesses the compressive perpendicular-to-grain fiber strength required to resist crushing.
• Unsuitable Materials: Do not use hollow concrete blocks, scrap plywood, shipping pallets, or damaged or rotting timbers. Blocking must be sized and in condition to sustain the loads and keep the crane stable (OSHA 1926.1404(q)(5) applies the blocking rule in 1926.1404(h)(2) to outrigger and stabilizer blocking). Cinder blocks can crush suddenly under point loading.

5. Cribbing Geometry & Construction Rules

• Perpendicular Layering: Timbers must be arranged in interlocking, perpendicular layers (cribbing box) to distribute loads evenly in both horizontal axes.
• Solid Decking: The top layer beneath the outrigger float must be completely solid (no gaps between timbers) so that the entire float base is fully supported.
• Centered Float Placement: The float must be placed precisely in the center of the cribbing mat. Placing the float off-center creates eccentric bending moments that can cause the cribbing to roll, buckle, or tip.
• Keep Stacks Low and Wide: A common conservative practice keeps the stacked height of timber cribbing no greater than its shortest base dimension (1:1) so the stack cannot topple sideways. Follow the manufacturer's or a qualified person's limits.

6. Excavation Setback Distances

When setting up outriggers next to trenches, excavations or embankments, keep well back from the edge. A common rule of thumb (not a Subpart P requirement) keeps the outrigger pad back from the edge by at least the depth of the excavation ($Setback \ge Depth$) in firm soil, and about $1.5 \times Depth$ in loose soil. Subpart P itself requires equipment to be at least 2 feet from the edge and protective systems that account for surcharge loads (Section 6.3).