Set-up, ground conditions and stability

Overturns are among the deadliest MEWP events, and almost all of them begin at ground level. A wheel drops into a hole, an outrigger sinks into soft fill, the machine is raised on a slope beyond its limit, or the platform is pushed outward by wind or by a worker pulling on a structure. This lesson explains why MEWPs tip, how to read ground conditions and the manufacturer's ground pressure data, and how to set a machine up so it stays upright.

Why MEWPs tip over

A MEWP stays upright as long as the combined center of gravity of the machine, the platform and everything on it stays inside the tipping lines. Anything that moves the center of gravity outward, or moves the tipping line inward, reduces stability.

• Load and reach: on a boom, the overturning moment is the weight of the platform and its load times the horizontal distance beyond the tipping line. Doubling the reach roughly doubles the moment. That is why booms have load charts or envelopes and why many have a lower rated load at full extension.
• Slope: tilting the chassis moves the center of gravity toward the downhill side. On a tall scissor lift, a small slope at the chassis becomes a large sideways shift at the platform. A 3 degree tilt shifts the platform of a lift raised 10 m by about 0.5 m.
• Wind: wind pushes on the platform, people, materials and structure, adding a sideways force high up where it has the greatest leverage.
• Side forces: pulling on a cable, pushing on a wall, or using a drill against a structure applies a horizontal force at the platform. Manufacturers state a maximum manual side force, often about 200 to 400 N (45 to 90 lb). Working against a structure can easily exceed it.
• Dynamic effects: driving elevated, braking, hitting a curb or bump, or swinging the boom quickly all add forces beyond the static load.
• Ground failure: a wheel, outrigger or mat that sinks suddenly tilts the whole machine and can start an overturn in a fraction of a second.

Ground conditions

The ground must carry the machine's wheel or outrigger loads without sinking, cracking or collapsing, and it must stay that way for the whole job. Ground that is fine in the morning can soften after rain or thaw, and asphalt softens on hot afternoons.

Reading ground pressure data

Manufacturers publish the maximum wheel load and often the ground pressure for each machine, sometimes as a local concentrated pressure under one tire or pad and an overall pressure under the whole machine. Comparing these with what the ground can carry tells you whether the machine is suitable or whether mats are needed. The bearing capacity of the ground should come from a competent person, such as a geotechnical engineer for critical locations; the values in the example are illustrative.

Mats only work if they are stiff enough. A sheet of plywood under an outrigger bends, so the load goes into the ground through a small patch directly under the foot. Use purpose-made outrigger pads or timber or composite crane mats sized for the load, as HSE-219 describes for cranes.

Setting up the machine

The manual gives the exact set-up sequence. The common steps are:

• position the machine as close as possible to the work, so the platform does not need to reach out further than necessary;
• check the ground under each wheel or outrigger and place mats where needed;
• deploy outriggers or stabilizers fully, as the manual requires, and level the machine within the indicated limits before raising;
• confirm the tilt indicator or level bubble shows level;
• set up the exclusion zone and barriers around the machine and the area beneath the platform;
• check overhead clearance once more before raising.

Dual-rated and multi-rated booms are common. Their envelope charts work like a crane load chart in miniature: the rated load depends on where the platform is. Lesson 10 returns to loading and how to account for materials.

Driving elevated

Many Type 3 machines may be driven with the platform raised, but only within the manufacturer's limits: on a firm, level surface, at reduced speed, with the operator looking in the direction of travel. Driving elevated over rough ground, across ramps, near edges or over cables and hoses multiplies the risk of a jolt, and a jolt is what causes the catapult effect on booms and overturns on tall scissors. The safest rule for supervisors is to allow elevated travel only where the plan says the route has been checked and is clear and level.

Suspended slabs and building floors

Indoor work raises a different ground question: can the floor carry the machine? A slab-on-grade warehouse floor usually can. A suspended slab, such as a mezzanine, a parking deck or an upper floor of an office building, may not, especially for heavy booms or for machines whose wheel loads concentrate on small areas. Building engineers express floor capacity in kilopascals or pounds per square foot of uniform load, and a MEWP's concentrated wheel loads can exceed the local capacity of a slab even when the machine's overall weight looks modest compared with the floor's total capacity.

Before using a MEWP on a suspended slab, ask the building owner or a structural engineer to confirm that the slab can carry the machine's maximum wheel loads and overall weight along the route and at the work location. Give them the manufacturer's data: machine weight, maximum wheel load and tire contact area. The engineer may restrict the route to areas over beams, require load-spreading plates, or recommend a lighter machine.

Similar care applies to elevators and ramps used to move a MEWP between floors. Check the rated capacity of a freight elevator against the machine's weight, and make sure ramps can carry the wheel loads.

Outriggers and stabilizers in practice

Machines with outriggers or stabilizers, such as trailer booms, truck-mounted units and some rough-terrain scissors, rely on them completely for stability when elevated. Interlocks on current machines usually prevent raising until all outriggers are deployed and the chassis is level, but older machines may not have them. Watch for outriggers set on the edge of a curb or a slab, pads placed on top of loose blocking, and outriggers left partly retracted because of a nearby obstruction. Each of these reduces the base the machine depends on.