Shane Rosie

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Finite Element Project Tower Crane Shane
Rosie Brett Aldridge
Shane Rosie Brett Aldridge
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Tower Cranes are a common fixture at many
construction sites. They are widely used to lift
construction materials with their extreme height
and reach.
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Dimensions
Mechanical slewing unit

• The working arm or jib has a length of 54 m with
  a triangular cross-section
• The mast is 78 m in height and is comprised of a
  3x3 m truss section
• A counterweight is needed to counteract the
  moment generated by the load applied to the
  working arm. This section comprises of a 3.5x3.5
  m truss section with a plate on the bottom to
  support the required counterweight.
• Mechanical slewing unit is comprised of gears and
  motors which allows the cranes arm to rotate.

Jib or working arm
counterweight
Mast or Tower
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How it was Modelled
Strand 7 was chosen over Microstran to analyse
the structure due to the 2-D plate elements
needed to successfully and accurately model all
elements. The tower and arms are mainly comprised
of trusses of varying shapes and dimensions. Each
of these truss members where modelled as beam
members giving each member 4 dof (2 at each
node). This obviously complicates the overall
stiffness matrix of the structure and requires
the computer to use a lot more memory (increased
solution time). The Counterweight section was
modelled using plate/shell elements as we have
forces acting perpendicular to the major
plane. The slewing unit has been simplified to a
plate/shell element for reasons explained above.
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Critical Sections
Tower section
D 150 mm T 9 mm
3m
3m
3m
D 100 mm T 10 mm
3m
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Concrete base
The Tower is fixed through a concrete base in
this case and therefore can be analysed by fixing
these points in all directions in both
translation and rotation. As shown above.
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Slewing unit
A plate/shell element of thickness 15 mm was used
to model this rotational unit. You can see the
face pressures applied through the overlying
truss. This is where a change in cross-sectional
area occurs. Less weight at higher altitudes.
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Jib or Working Arm
3 m
2.4 m
2.4 m
16 mm bar as tension member
D 100 mm T 10 mm
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Counterweight
This section has been modelled using a 3.5x3.5 m
cross section to allow for the large concrete
counterweights. The concrete counterweights can
be applied by use of a face pressure on the
plates of thickness 10 mm.
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Materials
Structural steel was used for all members
including the plates for its high strength and
relatively low weight. Also we must consider
transport and the ease of assembly on site. The
crane cannot be transported in one complete
section and therefore the crane must be assembled
on site in sections as shown in the picture.
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Load Cases
1.Original Model
This case is for a maximum lifting weight of the
crane. A 120 KN (12 tonnes) acting at 15.4 m from
the tower. This creates a moment of 1872 KN.m
which must be counteracted by the counterweight.
Plate pressure of 2.63 KPa was used to model the
concrete counterweight
A acceleration of -9.81m/s2 was entered into this
case to model the self-weight of the structure.
120 KN total
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Displacement scale at 5.
1.Results
Maximum displacement at end of working arm. Too
much displacement at 4.57 m in -y direction.
There is too much weight at end of the jib. Can
see that the tower is rigid and doesnt need any
remodelling. Operator comfort is essential.
Can see stresses are too high in the tension
members. Up to 630 Mpa. We need stresses below
0.9500 450 Mpa.
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1.Results (cont.)
Stresses in the plate (slewing unit) are at an
acceptable level, maximum of 320 Mpa in
compression.
Displacement contour of the shape reveals the
following. The maximum displacement is 0.23 m in
the y direction. This is unacceptable and
therefore we need to increase the thickness of
the plate
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Counterweight section
1.Results (cont.)
The stress in the plate is acceptable at a 400
Mpa but this will be a constant force and
therefore we require a larger factor of safety.
We need to increase the thickness of the plate
but we must be mindful how these changes effect
the weight and therefore the stress in the
critical tension members.
The maximum displacement in the counterweight
plate is in the y direction at approx 1 m.
Another reason to increase the plate size.
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1.Discussion
Optimally for this case we would only make the
working arm 15.4 m long. This would decrease the
weight to a level where the tension members would
satisfy the stress requirements. One of the
cranes main features is its long reach and
therefore this modification would defeat the
purpose of using a crane in the first place.
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Load Cases
2.Revised model with tension cable
A cable of diameter 20 mm was used in the
modelling
The revised model incorporates the use of a
tension cable to lessen the deflection in the
working arm and alleviate the stress in the
tension members. It also incorporates changes to
the plate thicknesses.
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2.Revised model with tension cable
We increased the thickness of the tower plate to
30 mm. This is the maximum we could increase the
plate due to the fact that the thickness cannot
be more than 10 of the other dimensions. If the
thickness was increased beyond this brick
elements would have to be used to gain adequate
results.
The thickness of the counterweight section was
increased to 15 mm.
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2.Results
The same loads were applied to the structure as
in the original case.
The stresses experienced in the cable alone where
in the region of 52 GPa. This is an enormous
stress. The rest of the structure performs just
as poorly having tensile and compressive forces
also in the magnitude of the stress experienced
above.
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2.Results (cont.)
The deflection of the plate in the y direction
are huge. In the worst case we are getting values
of 50 mm. This is due to the extra weight added
by the tower to connect the cable. This would be
unsafe for the operator and serviceability
demands.
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2.Results (cont.)
The counterweight deflection is roughly 10 mm in
the worst case. Problems were encountered when
checking the results for deflection for this
section. The deflection is measured using the
global co-ordinate system and this section is
already displaced. Therefore the results can only
be used when relative to a close node.
Unacceptable and inefficient design. Back to our
original design.
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Load Cases
3. Crane with optimum weight distribution
For this design we are making modifications to
the original. In order to counteract the problems
with the original stresses we need to distribute
the weight of the members more efficiently.
Basically by having the tension member with less
weight furthest away from the tower. The plate
elements have be left the same for the tower and
counterweight sections. 30 and 15 mm respectively.
16 mm bars
12 mm bar
To start with we varied the section once across
the working arm. As shown.
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3.Results
This deflection is sufficient
We are still getting 614 Mpa in the tension
members. The weight on the end is still too
extreme.
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3.Results (cont.)
This deflection is sufficient due to the fact
that most is produced by the compression and
movement of the overall structure.
The counterweight deflection is sufficient but we
will increase the thickness to 18 mm.
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3.Weight Distribution ii)
Now we will have 5 member sizes along the length
of the jib.
16 mm
12 mm
22 mm
14 mm
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19 mm
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3.Results
We have the critical members below the 450 Mpa
max yield stress. As this is the critical case it
would not occur very frequently. The members
would not experience such loading conditions on a
regular basis.
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3.Results ii) (cont.)
The deflection is satisfactory. 1.28 m in 50 m.
This may seem high but at this height and length
the deflection is satisfactory. As long as the
tower remains rigid there is no concern.
The deflection of both plates are satisfactory
with the thicknesses specified.
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3.Results (different load cases max lever arm)
For this case the force was placed at the
greatest distance from the tower (max lever arm).
This is best utilising the cranes reach. The
force was adjusted to induce a moment of 1872
KN.m. 36.3 KN with a lever arm of 51.6 m.
The maximum stress generated was 433 Mpa which is
in the acceptable range. The maximum displacement
was 1.67 m which we deemed to be acceptable.
Displacement in the plates where satisfactory.
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3.Results (different load cases only
counterweight)
This occurs before and after the crane lifts the
load from its original to its final position. The
stress is small in comparison with 335 MPa
maximum. The counterweights must be restrained
due to the displacement of this section (1.1 m
max). The tower with this loading condition sways
considerably (0.6 m in the x direction). The
operator must be able to withstand this effect.
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3.Results (different load cases self-weight only)
The maximum stress due to self-weight is 225 Mpa.
We can see from this that the self-weight of the
structure is the major design consideration. In
terms of our model the stress does not overly
vary when the load is increased. Indicating that
the crane can be used over a wide range of load
and lever arm conditions. The max displacement is
1.1 m (- y direction). Compare this with max
deflection of the critical case (1.28 m) shows
loading conditions have little effect.
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3.Results (different load cases wind loading)
This is using our original load conditions but I
am adding a wind load which in practical
situations would be applied through a banner. The
banner was modelled using a 1mm thick aluminium
plate. The idea is just to distribute the load to
the tower section.
The force added was calculated using height
factors and site wind speeds from the relevant
wind code. A 2.24 KPa face pressure was added.
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5 displacement scale
3.Results (different load cases wind loading)
(cont.)
As we can see the stress is similar to the one
experienced in the first case as the stress is
applied in a section where minimal stresses where
originally experienced.
The deflection of the tower here is a critical
design consideration. The tower deflects a
maximum of 1.6 m. Over a height of 78 m and
considering this is designed for a ARI of 100
years it is within the tolerance limits of our
design.
20 displacement scale
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3.Results (different load cases wind loading)
(cont.)
There are large compressive and tensile forces in
the base. The concrete base might need to be
redesigned to withstand the shear and bending
effects generated by the load and transferred
through the bolts to the concrete base.
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Conclusions

• We need to vary the size/weight of the tension
  members over the length of the working arm.
  Decreasing size as we get further from the tower
  hence distributing the stresses in the tensile
  members more efficiently. The more variations the
  better. Constructibility.
• The load must be applied on the working arm
  within 15.4 51.6 m from the tower.
• The moment generated by the load or the
  counterweight must not exceed 1872 KN.m.
• The counterweights must be spread over the entire
  length of the plate and the concrete weights must
  be restrained. As the plates have been designed
  to carry a maximum of 2.63 KPa
• Only tension members were used in the jib and
  counterweight sections to decrease the weight.
• The plate for the tower section must be at least
  30 mm thick.
• The tower must be rigid in order to decrease sway
  and keep the operator safe at all times.

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This concludes our presentation Thankyou for
your time
Shane Rosie Brett Aldridge