48
INTERNATIONAL
ANDSPECIALIZED TRANSPORT
■
MARCH 2015
THEKNOWLEDGE
2.)A swaying load canmake a variety
ofmovements. It can sway from left
to right, dependingon the angle of
the crane’s superstructure andboom
compared to the carrier; however, this
does not necessarilyhave to result in an
increase in groundpressure. The load
can also sway away from the crane and
towards the crane; thismotiondoes have
an impact on the groundpressure.How
much impact, however, againdepends on
the angle of the crane and the amplitude
of the load. The load, for example, can
sway in a circularmotion; this canhave
an impact inonedirection (left and right)
anddoes have an impact in the other
direction (to and from). Last but not least,
aswe know, where there arewind turbines
there iswind. The load can sway in an
ellipse-shapedmotion. The constantwind
coming fromone side of the loadmay
push the load away from its centreline, but
maynot allow the load to fully swayback.
This is kindof unpredictable in terms of
groundpressure.
Figure 5 is taken from the article of
Søren Jansen andKlausMeissner. It shows
only the last distribution case, thenon-
symmetrical loading. The groundpressure
under the tracks is shown in red and can
be calculated for a stable environment.
When the load sways, however, (as is
shownby the circle around the original
Centre ofGravity) the groundpressure
changes. The blackoutline represents the
load swinging to the left; the blue outline
represents the load swinging to the right.
A significant increase in groundpressure
is causedby a seeminglyharmlessmotion.
Without going into calculations, the
FEM5.016document advises a rule of
thumbon groundpressure increases.
It recommends taking into account
20 to35% groundpressure increase
compared to groundpressure givenby the
manufacturer.
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inpoint B andC. The load is now lifted
tohalf its elevation, let us say to a 50m
elevation; thehoistwire above the load
is also50m, aswe startedwith a 100m
initial length (seeFigure 3).
Theperiodof the loadhas nowbeen
reduced to14 seconds. The kinetic energy
inpointAdidnot change aswehavenot
addedor taken away any energy. Since the
mass has not changed, the velocity inpoint
A cannot have changed either.
As the load approaches point B, all of
the kinetic energy is transformed into
potential energy equal to “m gh”. As stated
before, themass (m) and the gravity (g)
didnot change. Therefore, theheight (h)
didnot change either.However, as the
lengthof thehoistwirehas been shortened
from100m to50m the swaying load
makes apartial circlewith a smaller radius
(seeFigure 4). According to the formula,
the loadwill reach toheight (h) but in
order to reach that height itwill sway to a
larger amplitude angle θ (Theta).
Goingback toFigure 1, we can easily
see that a larger angle θ (Theta) results
in larger (near) horizontal components
“mg sinθ”. This larger force, as explained
earlier, transfers all thewaydown to
ground level and can result in an increase
in groundpressure. And it getsworse. As
wehave seen, the velocity inpointAhas
not changedbetween the initial liftwith
100m and50mhoistwire. Theperiodof
time (T) however has decreased from
20 seconds to14 seconds. Thismeans that
the velocity (v) is nowdecreasing from
maximum value (inpointA) to zero value
(inpoint B andC) in a shorter time
(20 sec vs 14 seconds for the full period).
Lifting the load another 25mwill reduce
theperiod (T) to10 seconds. The load
sways increasinglymore violent. This
canonlybe achievedwith an increased
deceleration force. Any increase in any
of the forces on the load automatically
transfer to a force onto the boom aswell,
and any force on the boom transfers to
ground level and canmean an increase in
groundpressure.
In summary, whenwe lift a load from
ground level to a certain elevation it can
result in an increase in groundpressure.
This increase canhavedevastating results
if not understoodor accounted for.
Twoquestions should come tomind
now: 1.)Why is this only applicable for
wind turbine components? 2.)Whydo
I keep reading “can increase ground
pressure” insteadof “will increase ground
pressure”?
1.)This theory is not only applicable to
wind turbine components; accidents just
happenmore frequentlywhile erecting
turbinesmore than in anyother industry.
Why is that?Ask yourself, howoften is an
average crane taken to the limit in terms
of boom length and, or, elevation and
capacity combined?While youmaynot be
able to answer that question, I can inform
you thatwhile erectingwind turbines it is
almost 100%of the time.
Figure2
Figure3
Figure4
Figure5
h
h
h
h
h
B
B
B
B
C
C
C
C
A
A
A
A
A
