7.2 Spreader Plates, Outrigger Mats & Setup

Spreader Plates, Outrigger Mats & Setup

Quick Answer: The steel foot (shoe) on an outrigger is rarely large enough to spread its load safely onto natural ground or paving. Spreader plates (outrigger pads) enlarge the contact area so the applied pressure stays below the ground's allowable bearing pressure ($q_{\text{allowable}}$). Size them with $\text{Area } (m^2) = \frac{\text{Outrigger Point Load } (kN)}{\text{Ground Bearing Capacity } (kN/m^2)}$, use purpose-made pads rated for the load, centre the foot on the pad, and set the outriggers as the manufacturer instructs (on most vehicle mounts, until the wheels are clear of the ground).

Proper outrigger setup is the single most critical procedural defence against MEWP overturns. A failure to size, position, or level outrigger pads correctly can cause rapid structural instability, resulting in catastrophic loss of life and equipment destruction.

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Purpose & Physics of Spreader Plates

A standard manufacturer-fitted outrigger shoe typically has a diameter or side length of 200 mm to 350 mm, providing a contact area of merely $0.03\text{ m}^2$ to $0.09\text{ m}^2$. When a 60 kN outrigger load is transferred through an unaugmented 300 mm diameter shoe ($A \approx 0.071\text{ m}^2$), the ground pressure exerted is:

$P = \frac{F}{A} = \frac{60\text{ kN}}{0.071\text{ m}^2} \approx 845\text{ kN/m}^2$

As established in Section 7.1, very few soils on construction sites—with the exception of solid bedrock—can withstand $845\text{ kN/m}^2$. On typical firm ground ($100\text{ kN/m}^2$), this unaugmented shoe will punch straight through the surface.

Spreader plates (also termed outrigger pads or mats) bridge this mechanical divide. By introducing a rigid, wide-area pad between the metal outrigger shoe and the ground, the concentrated point load is dispersed across a much larger surface area, driving the applied pressure below the ground's allowable bearing threshold.

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The Sizing Formula & Worked Engineering Calculation

To determine the minimum surface area required for an outrigger spreader plate, operators and planners use the basic pressure equation (IPAF's online Spreader Pad Calculator applies the same principle):

$\text{Required Area } A\text{ (m}^2\text{)} = \frac{\text{Maximum Outrigger Point Load } P_{\max}\text{ (kN)}}{\text{Allowable Ground Bearing Capacity } q_{\text{allowable}}\text{ (kN/m}^2\text{)}}$

Once the required surface area ($A$) is calculated, the physical dimensions of the pad are derived based on pad geometry:

• Square Spreader Plate: Side length $L = \sqrt{A}$
• Circular Spreader Plate: Diameter $D = \sqrt{\frac{4A}{\pi}}$

Step-by-Step Worked Sizing Example

An operator must set up a Category 1b truck-mounted boom on an unpaved grass verge adjacent to a commercial building.

• Step 1: Identify Maximum Outrigger Point Load ($P_{\max}$) Consult the manufacturer's specification decal or operator handbook. For this 7,500 kg gross weight unit, the maximum calculated corner outrigger point load under full outreach and payload is specified as $65\text{ kN}$ (~6.6 tonnes force).

• Step 2: Determine Allowable Ground Bearing Capacity ($q_{\text{allowable}}$) The site ground assessment confirms the subsoil is firm, naturally consolidated cohesive clay with an allowable bearing capacity of $100\text{ kN/m}^2$ (~10 t/m²).

• Step 3: Calculate Minimum Required Pad Area ($A$) $A = \frac{P_{\max}}{q_{\text{allowable}}} = \frac{65\text{ kN}}{100\text{ kN/m}^2} = 0.65\text{ m}^2$

• Step 4: Determine Pad Dimensions For a square spreader plate: $L = \sqrt{0.65\text{ m}^2} \approx 0.806\text{ m} = 806\text{ mm}$

• Step 5: Select Commercial Spreader Plate Always round up to the next commercially available size. In this scenario, standard $800\text{ mm} \times 800\text{ mm}$ ($0.64\text{ m}^2$) pads are slightly undersized; the operator must select $900\text{ mm} \times 900\text{ mm}$ ($0.81\text{ m}^2$) or $1,000\text{ mm} \times 1,000\text{ mm}$ ($1.0\text{ m}^2$) spreader mats.

$\text{Actual Applied Pressure with } 900\text{ mm Pad} = \frac{65\text{ kN}}{0.81\text{ m}^2} = 80.2\text{ kN/m}^2$

Because $80.2\text{ kN/m}^2 < 100\text{ kN/m}^2$, the ground will safely support the machine without shear failure or excessive settlement.

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Spreader Mat Materials: Approved vs. Prohibited

The material composition of an outrigger pad dictates its ability to transmit bending moments and resist punching shear.

Approved Spreader Mat Materials

1. High-Density Polyethylene (HDPE) / UHMW Composite Mats:
   • Modern engineered plastic mats are the industry benchmark for powered access operations.
   • Manufactured from ultra-high-molecular-weight polyethylene, they are virtually unbreakable, impervious to water, oil, and chemicals, and will not rot, crack, or splinter.
   • They maintain consistent flexural modulus across extreme temperature ranges (-40°C to +80°C) and feature integrated carrying handles.
2. Engineered Hardwood Timber Mats / Sleepers:
   • Seasoned dense hardwoods such as Oak, Ekki, or Greenheart provide exceptional compressive strength perpendicular to the grain.
   • Hardwood timbers must be structurally sound, free from rot, splits, or insect infestation, and bolted together with steel tie rods into a unified load-distributing mat.

Materials Not to Use

Industry guidance and site rules are clear that improvised packing must not be used under outriggers. Never use:

• Softwood Timber Scaffold Boards: Scaffold boards are manufactured from European whitewood/redwood designed for distributed pedestrian loads in tension and bending. Under high concentrated compressive point loads, softwood crushes, splits along the grain, and snaps instantly.
• Cinder Blocks, Bricks & Concrete Masonry: Concrete blocks and building bricks possess high compressive strength only when uniformly loaded across an entire wall mortar bed. Under a localised outrigger point load, masonry fractures brittly and shatters into rubble.
• Wooden Pallets & Timber Offcuts: Slatted wooden shipping pallets contain huge internal voids. Placing an outrigger jack on a pallet results in instantaneous collapse as the thin pine slats snap.
• Plywood Sheets & OSB Offcuts: Thin sheets of standard shuttering plywood or oriented strand board (OSB) delaminate rapidly when wet and lack the flexural stiffness required to spread heavy point loads.

Spreader Mat Material Comparison

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Pad Thickness, Stiffness & Cantilever Point Loading

A common and dangerous misconception among untrained operators is that simply placing any thin plate under an outrigger foot guarantees safe load distribution. In reality, pad thickness and bending stiffness are just as critical as surface area.

          INCORRECT (THIN / FLEXIBLE MAT)             CORRECT (RIGID / THICK MAT)
                 Outrigger Shoe                              Outrigger Shoe
                    │      │                                    │      │
                    ▼      ▼                                    ▼      ▼
               ┌──────────────┐                            ┌──────────────┐
             ══╡              ╞══                        ████████████████████
            ╱  └──────────────┘  ╲                       ████████████████████
           ╱   ▲              ▲   ╲                             │      │
          ▼   ─┴──────────────┴─   ▼                            ▼      ▼
      Edges Curl Up; Point Load Concentrated         Uniform 45° Stress Dispersion Across
        Directly Under Centre Shoe                         Entire Ground Area

The Bending Mechanism (Cantilever Deflection)

When an outrigger shoe pushes down into the centre of a wide spreader plate, the ground pushes back upward against the outer edges of the plate. This creates a severe bending moment (cantilever flexure) acting from the edge of the steel shoe to the outer perimeter of the pad.

• If the pad is too thin or flexible, it deflects elastically. The outer edges of the pad bend upward, losing contact with the ground entirely. As a result, the entire 60 kN load remains concentrated in the exact centre beneath the shoe, completely defeating the purpose of the spreader plate.
• If the pad is sufficiently thick and rigid, the load disperses outward through the material at approximately a 45° angle of distribution, transferring uniform compressive stress across the entire bottom surface of the pad.

Rules for Pad Thickness & Centreing

1. Rated Pads Only: Use pads that their manufacturer rates for the outrigger load and pad size. Thin or flexible sheets bend under the foot and concentrate the load in the centre, so thickness and stiffness matter as much as area.
2. Centreing the Shoe: The outrigger foot must always be positioned dead-centre on the spreader plate. Placing an outrigger shoe near the edge of a pad creates extreme eccentric loading, causing the pad to tilt, gouge into the subsoil, and flip upward, leading to an immediate machine tilt.

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Full Extension Interlocks & Setup Sequence

Modern Mobile Elevating Work Platforms incorporate sophisticated electrical and hydraulic safety interlocks governed by BS EN 280 to prevent unsafe operation.

Outrigger Beam Extension Interlocks

Many Category 1b machines feature variable outrigger configurations (e.g., full extension, partial extension, or narrow jacking). Each outrigger beam is fitted with internal microswitches or linear transducers that continuously monitor horizontal extension. If an outrigger beam is not fully extended to its locked detent—or if the control system has not been programmed for a narrow-jacking envelope—the electrical safety circuit isolates the boom lift hydraulics, preventing boom elevation from the stowed position.

Wheel Clearance & Suspension Isolation (Category 1b)

For many static boom platforms (truck mounts, trailer mounts and spider lifts), the manufacturer's set-up procedure lifts the wheels or tracks clear of the ground so that tyres and suspension are not part of the support.

Hydraulic Pilot Check Valves (Counterbalance Valves)

Every outrigger hydraulic jack cylinder is fitted with an integral, steel-bodied pilot-operated check valve (counterbalance valve) flanged directly to the cylinder port. If a high-pressure hydraulic supply hose bursts or leaks while the platform is elevated, the check valve locks the hydraulic oil inside the cylinder, preventing the jack leg from collapsing.

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Levelling Verification: Bullseye Spirit Level & Digital Displays

A MEWP elevated on an unlevel chassis experiences severe eccentric side-loading on its boom structure and slews unevenly, massively increasing the risk of structural failure and tipping.

Tolerance Limits

• Outrigger-supported machines (Categories 1a and 1b) typically have an extremely strict allowable levelling tolerance—usually less than 1° (often 0.5°) from true horizontal.
• Wheeled self-propelled machines (Categories 3a and 3b) typically allow from about 1.5° (slab scissors, side to side) up to 5° (rough-terrain booms) before the tilt alarm cuts out functions.

Levelling Instrumentation

1. Bullseye Spirit Level (Circular Bubble): Mounted directly on the chassis frame. The bubble must be adjusted until it rests precisely within the central target circle. When viewing a bullseye level, the operator must look directly from above to eliminate parallax viewing error.
2. Electronic Digital Level Displays: High-specification truck mounts and spider lifts feature electronic dual-axis inclinometers that display precise pitch and roll angles in degrees on the ground control console. Automatic outrigger auto-level systems can deploy and level the chassis at the touch of a button, but the operator must manually verify pad contact and ground integrity.

Step-by-Step Levelling Protocol

Step 1: Conduct Ground Survey & Position Rated Spreader Pads Beneath All Outriggers
Step 2: Lower Outriggers in the Sequence the Manufacturer Specifies
Step 3: Keep Each Outrigger Foot Centred on Its Pad
Step 4: Extend Until the Wheels Are Clear of the Ground Where the Manual Requires
Step 5: Adjust to Centre the Level Indicator Within the Manufacturer's Tolerance
Step 6: Visually Verify All Pads Bear Solid Ground Contact with No Voids
Step 7: Confirm Interlock Status Indicators Before Operating the Boom