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Effective Use of Joint Elements
Only one element division should be defined in
a point joint mesh attribute assigned between points. For an
interface joint
line/surface mesh attribute, assigned between lines/surfaces, the number of
divisions required in the joint mesh attribute will set the mesh
divisions on the 'Master' assignment line/surface, and this should
match the number of divisions for the existing mesh assigned to the lines/surfaces
which will be assigned as the 'Slave' assignment.
All joint elements operate in a local
coordinate system. If a local coordinate system is not manually
used with the
joint mesh, Modeller will attempt to automatically define the
coordinate system. There are two main cases that Modeller
considers when carrying this out. as follows:
Case 1:
nodes 1 and 2 are coincident
Case 2:
nodes 1 and 2 are not coincident
-
Node 3 is
positioned on the line defined from node 1 to node 2
extended beyond node 2
-
Node
4 , first the most stable z-axis direction is found from a
choice of the 3 global axes. Node 4 is then positioned on
the cross-product of the x-axis from node 1 to node 3 and
this z-axis.
However, the recommended method
is to explicitly define a local coordinate dataset in a known
orientation and specify this when assigning the joint mesh. In
this way the direction in which the stiffnesses operate will be
known definitively.
Strain is measured in the local axis system of
the joint elements as
[displacement of node 2 - displacement of
node 1]
The sign of the displacements of each of the
nodes being measured in the local coordinate system. A negative
strain implies compression whilst a positive strain indicates
tension. In Modeller the first joint node is the one which is
closer to the joint z symbol. To determine which node is the first/second depends on
which method of assigning joints has been used...
- If assigning to individual lines, the order
of the nodes originates from the definition of the
underlying line - the feature line local x-axis originates
at node one and points to node 2
- If assigning using the interface joint
method, then the node order is from the assigned 'Master' line/surface
to the 'Slave' line/surface. Node
1 will be indicated by a “Z” in the displayed mesh and
this is on the Master assignment side.
Note that the 'Slave' assignment is taken from
the second selected feature or the selection in the selection
memory if the option "Mesh from master to slave"
is chosen on assigning the mesh.
Search for "Joint Material
Properties" in Help > Help Topics > Search for more information on the
Force-Displacement relationships associated with each of the
nonlinear joint material models.
Results
When investigating the results from an
analysis using joints, look at the joint strains using the
values layer. This will show in which direction the joint is
actually displacing. Once all the joint strains appear
reasonable, investigate the normal/shear joint forces this will
point out any problems with specification of the joint material
parameters. If an analysis refuses to run, revert to the linear
stiffness joint material and run the analysis again, in this way
a results file will be created (assuming that the joint
materials were the reason for the solution difficulties) that can be
investigated as above.
Friction Joint Material Model
For the Friction Joint Material Model, there are a few additional comments…
- The normal force necessary to establish
friction force is assumed to be in the local
x-direction. The local y-axis (also the z-axis in 3D) is the
frictional plane(s)
- The classical Coulomb friction law is used
as the basis for this material in which frictional force is
entirely dependent on the development of normal force. It is
necessary, therefore, to ensure that analyses using this
model apply the normal force prior to any shear loading
- The elastic stiffness in each of the joint
axes is the stiffness used when the joint force is still
below the sliding friction threshold. Normally this would be
a reasonably large number to simulate the limited shear
strain that is experienced prior to sliding (stick to slip
conditions).
- The joint acts as a lift-off joint in
the local x direction.
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