MEWP Ground Pressure: Why Machine Weight Is Not Enough

A six-tonne boom lift is parked on a concrete slab. Is it enough to divide its weight by the area it occupies and compare the result with the floor’s rated capacity?

No. A mobile elevating work platform (MEWP) transfers its load through tyres, tracks or outriggers, not evenly across the rectangle outlined by its chassis. Machine weight tells you the scale of the load. It does not tell you how much force reaches a particular wheel or outrigger pad. That distinction matters on suspended floors, basement slabs, backfilled ground and any site where a machine relies on outriggers.

1. Three figures that are often confused

A six-tonne machine does not necessarily place 1.5 tonnes on each of its four tyres. Platform loading, boom position, counterweight arrangement and machine configuration can all redistribute the load.

IPAF’s guidance on ground conditions and supporting structures addresses changes in individual support loads as boom position and operating conditions change.

2. A real Genie example: 13.98 kPa versus 689 kPa

Genie’s Floor Loading Specifications contains a useful example for the Z-45/25 DC, before serial number 22051.

Source: Genie, Floor Loading Specifications, Part No. 97897, Rev J (March 2017), PDF page 5. These figures apply to the specified serial-number range, not every Z-45/25 variant.

The contrast is striking: 13.98 kPa of average pressure over the machine’s occupied floor area versus 689 kPa at the tyre contact area.

The occupied-floor figure uses the chassis footprint. But the machine does not bear on the floor across that entire rectangle; its tyres carry the load at much smaller contact areas. Treating the footprint average as the local floor pressure can therefore badly understate the concentrated loading.

One qualification is essential. Genie defines these figures under different conditions: occupied-floor pressure is a stowed-position average with rated load; maximum tyre load refers to a working configuration; and the listed tyre contact pressure uses the manufacturer’s stated contact-pressure condition. They are not a single simultaneous load distribution and should not be combined as though they were.

3. Why does the load on each wheel change?

On a boom lift, the boom and counterweight rotate around the slewing axis. As the boom extends or slews, moments change and loads are redistributed between support points.

The tyre closest to the platform is not necessarily the most heavily loaded one. The counterweight also generates a moment, so another support point may experience the higher reaction in a particular configuration.

Scissor lifts do not have the same rotating boom arrangement, but an extended deck carrying people or materials can still shift loads between the front and rear wheels.

This is why the manufacturer’s maximum wheel-load data are more informative for a site assessment than simply dividing machine weight by the number of wheels. Always check the load case and tyre configuration behind the figure.

4. What do outrigger pads actually change?

Spider lifts, truck-mounted platforms and some boom lifts transfer substantial loads through outrigger feet. A spreader pad can distribute a reaction over a larger effective area.

See the guide to spider lift setup space for the relevant checks.

In a simplified calculation:

Average bearing pressure = outrigger reaction ÷ effective bearing area.

Suppose an outrigger applies 40 kN. If the load is distributed uniformly over 0.10 m², the average pressure is 400 kPa. Over an effective area of 0.40 m², it falls to 100 kPa.

These are hypothetical numbers to illustrate the relationship—not pad-sizing instructions. Real pads do not necessarily distribute load uniformly over their full footprint. Pad stiffness, thickness, material, foot position and ground behaviour all matter.

IPAF’s spreader-pad guidance makes the same point: the pad must have adequate strength and stiffness for the actual support reaction and ground conditions.

A pad can spread the force. It cannot make an inadequate slab or weak underlying structure adequate by itself.

5. “The floor is rated for 500 kg/m².” Is a three-tonne lift acceptable?

That statement, by itself, cannot answer the question.

A stated uniform floor load and the concentrated action of a tyre are different load cases. Even if machine weight divided by the chassis footprint is below the quoted uniform-load allowance, the floor may still need checking for local punching, bending, structural load paths and supporting elements.

The same applies to hard-looking ground above basements, drainage structures, service trenches or recent backfill. A sound surface finish says little about what is underneath.

For a suspended slab or uncertain ground, provide the machine’s load information to the person competent to assess the structure or ground. A website article—and one average-pressure number—cannot replace that assessment.

6. What should a buyer request from the supplier?

A concise request is usually enough:

Weight tells you how much load exists. The load path and the area through which it reaches the ground tell you why that weight can be critical at a particular point.