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Worth Knowing
Whenever a mobile crane or a hook and ladder fire
truck needs to extend its boom or ladder out a long
distance or with a heavy weight, it requires additional support
to prevent it from toppling over. This support is provided by
retractable outriggers.
Outriggers are sturdy metal legs that
extend the footprint of the mobile equipment out to cover a
larger area. They can either extend directly from the truck to
the ground at an angle, or extend horizontally outward from the
truck with a vertical leg that makes contact with the ground. In
either case, the outriggers must lift all the weight of the
truck up off the tires to provide maximum stability. Without the
outriggers, the equipment is considered to be "on rubber" and
with them it is "on outriggers". The operator must obey
different loading restrictions in each case.
Stability is dependent on the equipment's
footprint and center of mass. The footprint is the total area
enclosed by the support structures of the equipment. The center
of mass is the point at which the equipment would balance if it
were set on top of a single point of support. If the center of
mass is inside the footprint, the equipment is stable. If the
center of mass is off the edge of the footprint, the equipment
will topple.
So it's not hard to imagine how a crane
could become unstable when extending its boom with a heavy
weight hanging from it. Since not extending the boom isn't
really an option, there are only two ways to provide extra
stability: Use a counterweight to help shift the center of mass
back over the footprint, or make the footprint bigger. All
cranes have counterweights, but there is a limit to how heavy
they can get. Too much and the crane wouldn't be able to move,
and the ground would have trouble supporting it. Simply building
the crane with a wider footprint would prevent it from traveling
on public roads. Using retractable outriggers to extend the
footprint therefore is essential to the crane's stability.
Some potential problems with outriggers:
No insulation from the ground - Outriggers
are made of steel, and lift the rubber tires off the road. This
provides a good electrical connection to ground. Particularly
dangerous when working underneath overhead power lines.
Small contact patch - The outriggers are
generally less than a foot squared each, meaning that there is a
great deal of weight sitting on a relatively small surface. Soft
surfaces such as dirt, mud, and even asphalt (for very heavy
weights) will be damaged by the pressure. Not only would this
damage need to be fixed, but outriggers sitting on damaged
surfaces can't provide the support that they need to. Plywood
sheets, two-by-fours, or other lumber is generally used in these
cases to spread out the weight.
Forgetting to use them - It happens.
Especially when the crane is being used in a narrow passage,
such as a bridge, and the outriggers are in the way of other
traffic. Some accidents have been attributed to the operator
pulling in the outriggers to allow a truck to go through, and
then forgetting to put them back out before resuming operation.
Outriggers not fully extended - Sometimes
there is not enough room to extend the outriggers all the way.
Sometimes a malfunction causes one or more outriggers to be
deployed only part way or not at all. Sometimes the operator
simply didn't extend them all the way for whatever reason. In
any case, the footprint is not as big as it should be. The rated
load for a crane on outriggers is calculated for when they are
fully extended, and it can be difficult and dangerous to
estimate what is safe for anything less than full extension.
Overloading - Overloading can mean two
things. First, if the boom is extended too far with too much
weight, the crane can still topple despite the extended
footprint. Second, if the crane is loaded with too much weight
but the center of mass is still inside the footprint, the
outriggers themselves may collapse. If this happens, between the
load now being shifted at an angle due to the collapse and the
shortened footprint due to the broken outrigger, the center of
mass may now be outside the footprint and topple the crane.
Cranes are carefully designed and their
safe limits of operation are calculated by engineers who
(hopefully) understand the responsibility they are entrusted
with. Furthermore, crane operators are trained and skilled
professionals, and licensing in the United States is available
through various state agencies and The National Commission for
the Certification of Crane Operators (http://www.nccco.org).
Between good design practices and knowledgeable operators, crane
accidents are thankfully rare events. |
