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Ensuring adequate ground support

Ensuring adequate ground support

23 August 2011 Email this article

With mobile equipment, the ground surface it rests
upon is critical to its strength and stability

Article from IPAF - Elevating Safety 2011,

by Tony Groat , executive vice president, AWPT

 

An old saying in home construction states, “As goes the

foundation, so goes the house.” It is very logical that the base

supporting a building is critical to its strength and stability.

 

That is also true with aerial work platforms.

AWPs are designed very well and are capable of lifting

personnel and their tools to work at height vertically and

some add horizontal reach. But as mobile equipment, the

ground surface they rest upon is critical to their strength 

and stability. It is important to always look down at the surface

that the AWP will be on before moving.

 

It is easy to assume the surface you will drive over is

capable of supporting an AWP. That is an assumption that

has resulted in many machine tipovers, ending in property

damage, serious injuries and even loss of lives.

 

What do you need to know?

How much does the AWP actually weigh? During

numerous operator training sessions, it was found that

many potential operators had no clue as to the actual

weight of these machines. IPAF conducted a non-scientific

test during CONEXPO asking attendees to guess the

weight of various lifts. The majority of participants guessed

at 25 to 50% lower than the actual weight of the machines.

Fortunately, every manufacturer lists the actual weight

of each machine on the machine itself and in the operation

manual. Unfortunately, this simple but important piece of

information is not adequately addressed by many users.

The machine weight must be considered when deciding

where the lift can travel and work. Can the ground

under each tire or outrigger support the load of the lift,

personnel and materials being carried? You need to know

this before operating any AWP.

 

Look down before going up

 

Once aware of the maximum ground pressure per

tire or outrigger of an AWP, a user must ensure the surface

it will travel and work from is sufficient to support

the machine, operator and tools. Ground support for AWPs

can vary significantly from unprepared earth to man-made

structures. A qualified person should ensure all ground surfaces

where an AWP will travel and work from are capable

of supporting the maximum loads.

 

Enlist a competent person

 

The ground surface might look appropriate to support

an AWP to the untrained eye, but a competent person must

determine if the ground surface is adequate to support the

load. Sand, clay, backfill and other materials will provide

different support characteristics that must be considered.

Is the material dry or wet? Is there frost in the ground? Is

it properly compacted?

 

Blacktop and concrete might look adequate, but a competent

person must know the strength of the material to

know its ability to support the weight of the machine. They

might also ‘hide’ underground services — pipe/culverts,

etc. — that could significantly lower the capacity of thesurface

you are driving upon. They may also have visible

utility vaults with covers that could have significantly less

capacity than the main surface.

 

In 2010, a boom lift was driven onto the top deck of a

parking garage and the deck collapsed. In another situation,

a boom lift was driven over a city sidewalk that had a

utility vault beneath. The vault cover could not support the

weight of the lift, collapsed and the lift overturned. These

machine turnovers were caused by inadequate ground

support that should have been addressed prior to the operator

getting into the machine.

 

All aspects of the ground surface that an AWP will travel on
must be known and a competent person should determine
its ability to support the AWP for the weight that will be
imposed in its maximum weight capacity prior to operation.
This will ensure the operator will be safe when the lift is
driven or elevated for the work intended.

 

Hard and level surface


It is sometimes required but always recommended that
AWPs operate on a hard, level surface. At CONEXPO,
IPAF held a demonstration, placing a tire scale under each

of four tires of an AWP. The scales rested upon a fairly level

blacktop parking lot. Each of four separate lifts was weighed.

Even if an operator knew the exact weight of the machine,

simply dividing the weight by four would not provide an

accurate load needed to be supported by the ground surface.

 

When a boom-type lift was observed during operation, the

change in load under each tire drastically changed as the

lift modified its configuration by extending or retracting the

boom, raising or lowering the boom, or rotating around the

turret. As each AWP manufacturer knows this fact, they

provide the maximum tire load for all configurations of the

machine. If you were unaware of this, you have never

adequately addressed the ground load for your

AWP application.

 

This demonstration also revealed another fact. While

most assumed the parking lot blacktop was a hard,

level surface on which to operate the AWPs, the scales

displayed a variance in weight of 100% on opposing

corner tires (i.e., 800 to 1,600 lbs. or approx 362 to 720Kg).

So in addition to the change in tire load due to the change
in machine configuration, you must consider the impact
of the surface being truly level or additional variance might
need to be factored into the support required under

each tire/outrigger.