Ground Bearing Pressure Calculator

Lift Parameters

Enter crane, load, and mat details below.

Crane Weight *

Total crane weight in tonnes

Load Weight *

Weight of the load being lifted

Outrigger Mat Dimensions

Mat Length *

Length of one outrigger mat

Mat Width *

Width of one outrigger mat

Ground Bearing Pressure Results

Enter parameters and calculate to see results.

How the Ground Bearing Pressure Calculator Works

Ground bearing pressure is calculated using a simple formula: Load ÷ Contact Area. When a crane lifts a load, the combined weight of the crane, the load, and all rigging is transferred through the outriggers to the ground.

One-outrigger case (Worst Case): This simplified screening case assigns 100% of the entered total load to one outrigger. Use OEM reaction data or an engineered lift assessment for actual support loads.
Two-outrigger case: When lifting over the rear or side, the load is spread over two outriggers. This reduces the pressure because the total contact area doubles.
Mat size matters: Larger mats spread the load over a wider area, reducing the pressure on the ground. Smaller mats increase the pressure.

Note: This calculator provides planning estimates only. Final approval must come from a site assessment, an engineered lift plan, or a competent lifting person.

What Is Included in Total Considered Load

To get an accurate pressure reading, every piece of weight that transfers to the ground must be accounted for. The total considered load includes:

Crane Weight

The total weight of the crane, including all counterweights installed for the lift.

Lifted Load

The actual weight of the object being lifted.

Hook Block & Rigging

The weight of the hook block, slings, shackles, and any spreader bars used.

Outrigger Mats

Large steel or timber mats can weigh several tonnes and add to the total ground force.

Why it matters: Forgetting items like a heavy hook block or spreader beam leads to underestimated ground pressure, which can cause ground failure. Always use actual weights when available.

Common Mistakes

Forgetting to add the hook block or lifting gear weight to the total load.

Using the wrong mat dimensions (e.g., using the outrigger float size instead of the spreader mat size).

Ignoring the one-outrigger worst-case scenario and only planning for shared loads.

Assuming this calculator replaces actual crane manufacturer outrigger reaction data.

Using the pressure results without comparing them to site-approved allowable ground bearing values.

Assuming that using mats automatically makes weak ground safe without a proper site assessment.

Worked Example

Crane weight: 48 t

Load weight: 28.5 t

Hook block: 1.5 t

Lifting gear: 0.54 t

Mat size: 2 m × 2 m = 4 m² per mat

Total considered load48 + 28.5 + 1.5 + 0.54 = 78.54 t

Worst case over one outrigger:

Pressure = 78.54 ÷ 4 = 19.64 t/m²

Load shared over two outriggers:

Pressure = 78.54 ÷ 8 = 9.82 t/m²

The worst-case pressure is 19.64 t/m² when the full load is on one mat. When spread over two mats, the pressure drops to 9.82 t/m² because the total contact area doubles.

Safety Note

For critical lifts, actual manufacturer outrigger reactions or engineer-approved lift data should take priority when available.
This tool provides planning estimates and does not replace a competent lifting person, engineer, or site ground assessment.
Always verify calculations with the crane load chart, lift plan, and actual site ground conditions before commencing any lift.

Screening Assumptions and Site Verification

The displayed cases compare average pressure using simplified load distribution. They do not model actual manufacturer outrigger reactions, mat bending, layered soil response, settlement, slope, drainage, voids or local bearing failure. Use the correct crane data and a competent ground assessment before the lift.

Frequently Asked Questions

What does ground bearing pressure mean?

It is the average load divided by the contact area used in the selected screening case. It is not a prediction of every local stress beneath a mat.

Does the one-outrigger case predict the actual crane reaction?

No. It conservatively places the entered considered load on one mat. Use manufacturer outrigger reactions or engineer-approved lift data when available.

Why can two mats not simply be assumed to share load equally?

Real reactions vary with crane configuration, radius, slew position, load path, setup and ground deformation. Equal sharing is only a simplified comparison case.

What must be included in the considered load?

Include the entered crane and lifted loads plus applicable hook block, rigging, spreader and mat weights. Confirm the required reaction basis for the actual crane.

Can allowable ground pressure be taken from a generic soil table?

Not for a critical decision. Soil layers, fill, groundwater, settlement and local bearing failure require competent site-specific assessment.

Why must underground hazards be checked?

Voids, tanks, utilities, trenches and previously disturbed ground can reduce support or fail locally even when average pressure appears acceptable.

Does a larger mat always make the setup safe?

No. A larger area reduces average pressure but does not by itself verify mat strength, load distribution, edge distance, settlement or underlying ground capacity.

Who should approve the crane support arrangement?

Follow the crane manufacturer, lift plan and site rules. A competent person or qualified engineer should verify the ground and support system where required.