Question
I would like to know the precautions when using Rigid Link and Elastic Link.
Answer
The Rigid Link makes the degree of freedom (d.o.f.) of arbitrary nodes (dependent nodes, slave nodes) dependent on the master node.
1. Inactivating nodes which consist of boundary conditions during construction stage analysis
This error occurs when Elastic Link and Rigid Link are activated while the nodes consisting of Elastic Link and Rigid Link are not activated during construction stage analysis.
The nodes that constitute Elastic Link and Rigid Link should be activated either in the same stage or in any of the previous stages.
2. Proper boundary conditions for Rigid Link + Support
For the first model, the boundary conditions are correctly defined as Rigid Link + Elastic Link + Support.
In the second model, a support boundary condition is applied at the master node of the rigid link. However, it is inappropriate, as the d.o.f. of the slave node also follow the support boundary conditions.
In the third model, Support boundary condition is applied on the slave node of the Rigid Link. The d.o.f. of the salve node is dependent on the master node, so this is an inappropriate boundary condition, as it releases the d.o.f. of the Support on the salve node.
3. Proper boundary conditions for Elastic Link + Support
'Singular Error' was caused in the above model because no Support boundary condition was assigned to the Elastic Link. If no support boundary condition is assigned to the elastic link, then the elastic link is recognized as a beam with the same stiffness.
Support boundary conditions should be defined at the end of the Elastic Link, or boundary conditions should be changed to Point Spring Supports.
4. Elastic Link's 'Rigid Type' feature and Rigid Link
Although both the Elastic Link's 'Rigid Type' feature and Rigid Link represent rigid body behavior, caution is required when using them. It is because the way they are reflected in the program is different.
The Elastic Link's 'Rigid Type' automatically processes a stiffness 100,000 times the maximum stiffness of the element in the model. Therefore, if there is an element that has a very large stiffness, in the frame structure of the model, multiplying that stiffness by 100,000 may cause numerical errors due to excessively large stiffness.
Therefore, if there is a member that has been given a large stiffness to represent rigid body in the frame structure of the model, it is better to use a Rigid Link rather than a Elastic Link's 'Rigid Type'.
The Rigid Link is a boundary condition that mutually restricts the geometric relative behavior of the master node and the slave nodes regardless of the magnitude of the stiffness, so it is not affected by the use of other rigid members.
5. Stiffness of Elastic Link for Seismic Analysis
When the superstructure and substructure are modeled simultanueously, Elastic Link is used to applied the moving direction of the support. The stiffness of the Elastic Link is generally applied with a large value such as 100,000,000 kN/m, but the stiffness of the Elastic Link varies depending on the stiffness of the elements that make up the model.
Therefore, through several trial and error, you need to find the appropriate stiffness value that effectively performs the role of fixed support for each direction.