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SPECIAL REPORT
MOBILE CRANE STABILIZATION
ACT
SEPTEMBER 2013
THE AUTHOR
David Duerr
, P.E.
is president of 2DM
Associates, Inc., a
consulting engineering
firm in Houston that
specializes in providing engineering and
technical services to the heavy lifting and
transportation industry.
CRANE MAT PAPER
Evaluating the strength, stiffness and load-
spreading ability (the effective length) of a
crane mat requires preparing engineering
calculations. David Duerr has written
a detailed paper on crane mat design
titled “
Effective Bearing Length of Crane
Mats
.” The paper can be downloaded at
.
they can be safely supported by the
ground. This means that crane mats are
needed.
Crane mats serve two purposes. First,
they must spread the load from the
crane over a large enough area that the
bearing pressure to the ground surface
is acceptable. Second, they must provide
support to the crane that is stiff enough
that the crane will not go out of level as
the loads from the crane change during
lifting operations. In order to assure
that these two purposes will be met,
we must know the strength of the mats
and their stiffness. Strength refers to the
ability of the mat to carry the imposed
load without a failure, such as cracking
of the timbers. Stiffness refers to the
ability of the mat to carry the imposed
load without bending or deforming
excessively.
A crane mat will not necessarily spread
the load over its full length. For example,
if a 20-foot (6-meter) long timber mat is
centered under an outrigger or a crawler
track, the load from the outrigger or
track will not be spread out evenly along
the entire length of the mat. Rather, the
pressure under the mat to the soil will be
concentrated in the middle (right under
the outrigger or track) and will taper
off to either end. The true distribution
of this changing pressure is based on
the bending stiffness of the mat and
the compressive stiffness of the soil.
Although it is possible to perform a
detailed analysis of the mat and soil to
determine this pressure distribution, the
information needed to do so often isn’t
available. A more practical approach is
to assume an even pressure under an
effective length of the mat.
The most common types of crane mats
are made from rough-sawn hardwood
timbers. There is also a variety of
synthetic pads for small to medium
outrigger-supported cranes. Ads for
these products regularly appear in this
magazine. Because of the proprietary
nature of these pads, the best information
on their use is that obtained from the
manufacturers.
As with assuring that the right crane
setup information is used to compute the
support loads, the right dimensions and
material properties must be used when
checking the mats.
The bottom line is simple. Without
adequate support, a mobile crane cannot
operate safely. Those responsible for the
planning and performance of the crane
work, regardless of the complexity of the
lifts, must go through the steps necessary
to assure that the crane’s support meets
the demands of the job.
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