Ensuring adequate ground support
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
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