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As defined in last section, constraints are the restrictions applied externally to the model in order to avoid some movements or prescribe some of these movements. We define connections as a way to change the relative movements between different parts of the model.
By default, all the elements and parts of the model are attached together as completely rigid. This condition is used to disconnect some degrees in one or several nodes between different parts of the structure. In this way, it is easy to define Rotules, that permmit free rotation between several parts of the structure.
The disconnection of several degrees of freedom can be made in several ways. The simple ones are Rotules and Beam rotules , that are only useful for beams (not shells) and do not permmit to make several groups of beams or just disconnect some degrees:
All nodes that have this condition assigned will permit a free rotation between all the beams that are joined in that node. The rotation in every beam that arrives to the node is independent of the rotation of all the other beams that arrive to the same node. The rotation is free in both, the Y’ and Z’ directions. Note that the rotules are only applicable to beams and not to shells and solids.
This is a condition similar to the last one, but it is assigned to the lines that define the beam. It is possible to define if only the initial node of the beam or the end node of the beam or both of them are rotules.
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Note 1: Do not mesh more than one beam element per beam. Every beam element will have its own rotules. (Check function Meshing->Structured->Lines->1 and select the lines).
Note 2: To know which point of the line is the beginning and which one is the end, use Utilities->Draw normals->Lines and select the line.
The more general solution, that permmits all the possibilities of disconnection, either with beams or with shells is to use a combination of Disconnect Id and Disconnect Degrees .
This condition defines a group, identified by a name, that work together as a part. This group will have the marked degrees desconnected of the rest of beams or shell elements that share the same node.
The elements that belong to the group must be marked with condition Disconnect Id and the same Identifier .
If local axes are defined, the degrees disconnected are related to that local axes. If not, they are related to the global axes.
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The special identifier –ANY- , means that every element connected to that node is an individual part with all the marked degrees desconnected from the other elements.
This condition is used in collaboration with the condition Disconnect Degrees and is used to mark the elements that belong to a common part with a common Identifier .
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The special identifier –ANY- , means that this element is an individual part with all the marked degrees in the node desconnected from the other elements.
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In this example, one part, called Id1, is defined with the two bars that have a rigid connection between them. Degree y rotation is disconnected from the rest of the bars. So, the third bar has free rotation related to these two.
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Typical results in momentum for that node.
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In this example, one part, called Id1, is defined with all the elements that belong to one of the two surfaces. Degree z rotation and x displacement are disconnected for all the elements that belong to one of the surfaces and contain one node or 2 on the connection line. So, the elements of the other surface have free rotation related to the elements of this surface