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Crane Safety Guide

The Critical Role of Outrigger Mats in Preventing Ground Failure

Why proper mat sizing is non-negotiable for safe lifting operations and how to calculate it correctly.

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Table of Contents
01

Why Outrigger Mats Matter

Every crane operation begins with a single critical decision: can the ground support the load? Outrigger mats (also called outrigger pads or crane mats) are not optional accessories. They are engineered safety devices designed to distribute the immense point load from crane outriggers across a larger surface area, preventing ground subsidence, sinking, and catastrophic tip-overs.

Crane outrigger deployed on ground with safety markings
Properly deployed crane outrigger with visible load distribution base

When a crane outrigger makes direct contact with soil, asphalt, or unprepared ground, the pressure exerted can exceed the ground bearing capacity. This leads to ground failure, which accounts for approximately 45% of crane accidents related to instability. The mat acts as a bridge, spreading that concentrated force over a wider footprint.

02

Consequences of Inadequate Mats

45% Of crane tip-overs caused by ground failure
100% Of max load can shift to one outrigger
0 Safe rules of thumb exist per OSHA

Operating without properly sized mats exposes crews to severe hazards:

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Ground Collapse Hazard

When bearing pressure exceeds soil capacity, the outrigger sinks, tilting the crane and shifting the center of gravity outside the stability footprint. This often results in total crane overturn.

Heavy pipes resting on crane outrigger mat
Heavy pipe loads demonstrate the extreme point pressure exerted on outrigger bases
OSHA 29 CFR 1926.1402 - Ground Conditions Equipment must be assembled and used only on ground that is firm, drained, and graded sufficiently. When necessary, supporting materials such as blocking, cribbing, pads, or mats must be used to provide adequate support and levelness.
03

Mat Size Calculation Formula

Determining the correct mat size is a mathematical process, not a guess. The following formula calculates the required mat dimension based on total load and ground bearing capacity:

Mat Size Equation
Mat Size = (CW + CWt) x F + HB + FJ + Aux + LG GBP
Where Mat Size equals the side length if the mat is square (meters or feet)
CW Counterweight - Crane counterweight massUnit: kg / lb / ton
CWt Crane Weight - Total weight of the craneUnit: kg / lb / ton
F Factor - Load distribution factorUnit: Dimensionless (e.g. 1.15 to 1.25)
HB Hook Block - Weight of the hook block assemblyUnit: kg / lb / ton
FJ Fly Jib - Weight of the fly jib attachmentUnit: kg / lb / ton
Aux Auxiliary - Weight of auxiliary equipmentUnit: kg / lb / ton
LG Load - Gross weight of the lifted loadUnit: kg / lb / ton
GBP Ground Bearing Pressure - Allowable soil pressureUnit: kN/m2 (kPa) / psf / t/m2
Important Note Always verify CW and CWt values from the crane manufacturer's load chart or lift plan. Never estimate critical weights. The factor (F) adjusts for dynamic forces and outrigger reaction distribution during slewing operations. Ensure all weight units match the GBP unit before calculation.
04

4-Step Calculation Process

1

Step One - Calculate Total Force

Sum the crane weight, counterweight, hook block, fly jib, auxiliary gear, and the gross load being lifted. Apply the distribution factor to determine the maximum outrigger reaction force.

2

Step Two - Determine Ground Bearing Pressure

Obtain the allowable ground bearing capacity for your specific soil type. Refer to geotechnical reports or standard tables (e.g. BS 8004) for dense gravel, clay, or compacted sand values.

3

Step Three - Compute Required Area

Divide the total outrigger force by the allowable ground bearing pressure. This gives the minimum mat area required to keep ground pressure within safe limits.

4

Step Four - Find Mat Dimensions

Take the square root of the required area to determine the side length of a square mat. If using rectangular mats, ensure the area meets or exceeds the calculated value. Always round up.

05

Pressure Distribution Diagram

The diagram below illustrates how an outrigger mat reduces ground pressure by spreading the concentrated point load over a larger surface area:

Without Mat vs With Mat - Pressure Spread Visualization
Without Mat Soil / Ground Outrigger HIGH PRESSURE Point Load With Mat Soil / Ground Outrigger Mat Outrigger LOW PRESSURE Distributed Load
High Concentrated Pressure
Distributed Low Pressure
Outrigger Mat
06

Essential Safety Guidelines

Check for cracks, deformation, or material degradation. Damaged mats can fail catastrophically under load. UHMW and steel mats should be free of gouges that compromise structural integrity.

Outrigger pads must rest on solid, continuous ground. Do not use them to span trenches, manholes, or uneven surfaces. Full contact between mat and soil is mandatory.

After deploying outriggers and placing mats, perform a dry run. Check for any sinking, sliding, or tilting. Re-level and add cribbing if any movement is observed.

Wood degrades when exposed to moisture or chemicals. UHMW polyethylene offers superior durability and is 1/7th the weight of steel. Select material rated for your maximum point load.

An unloaded crane at minimum radius can sometimes exert higher outrigger loads than a loaded pick. Always calculate based on the worst-case scenario from the manufacturer's software or load chart.

#CraneSafety #OutriggerMats #LiftingOperations #GroundBearingPressure #ConstructionSafety #OSHA #HeavyLifting #CraneRigging #SiteSafety #Engineering

Safety Starts with the Ground

Proper outrigger mat selection is engineering, not estimation. Calculate precisely, inspect thoroughly, and lift safely.

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