Working with RBE2 and RBE3 spider elements

With the Nastran solvers, RBE2 and RBE3 elements behave differently:

• RBE2 — The active degrees of freedom of the single core node (independent) enforce the active degrees of freedom of the leg nodes (dependent). Therefore, the active degrees of freedom of all nodes included in an RBE2 element are considered rigidly connected.(1) independent node (2) dependent nodes

• RBE3 — The weighted average of the active degrees of freedom of the leg nodes (independent) enforces the active degrees of freedom of the single core node (dependent). This weighting is set to 1 and cannot be changed using this software; all nodes contribute to the motion equally. Unlike the RBE2, the RBE3 element is flexible (not infinitely rigid), due to the allowable freedom of the leg nodes. Common uses for this element type include adding and distributing mass without adding stiffness, and distributing a load from multiple points to a single point.(1) dependent node (2) independent nodes

Understanding double dependencies

Double dependencies occur when two elements share a dependent node. If your model contains double-dependencies, the solver may not be able to correctly resolve the degrees of freedom in your model.

Double-dependent nodes in RBE2 (left) and RBE3 (right) elements.

To check for double dependencies within your model, you can use the DOF Dependence Check command. The DOF Dependence Check command also allows you to resolve existing double dependencies by editing or deleting each element with conflicts.

For more information, see DOF dependence check.

Additionally, if you are using the Simcenter Nastran solver, you can set the AUTOMPC parameter to Yes to automatically resolve most double dependencies.

To set the AUTOMPC parameter, create a Solution Parameters modeling object and select it on the Parameters page in the Solution Options dialog box for the current solution. For more information see Define Nastran solution parameters and Solution dialog box (Nastran), Parameters page.

Define the degrees of freedom

Depending on the element type (for example, RBE2 or RBE3), you can define the degrees of freedom of the leg nodes or both the core and leg nodes by editing the Mesh Associated Data for the elements. For more information, see Mesh-associated data.

Note:

With RBE3 elements, DOF4, DOF5, and DOF6 of the leg node degrees of freedom are set to Off by default. In most cases, you should not change the default degrees of freedom for the leg nodes in RBE3 elements. For more information on setting the degrees of freedom, see the Simcenter Nastran Quick Reference Guide description for RBE2 and RBE3 elements.

Deleting spider elements

When you delete a spider element, the software also deletes the underlying node on which the core node is defined. If you want to keep the core node, use the Element Delete command to delete the spider element, and make sure the Delete Orphan Nodes option is not selected.

Keep in mind that if your geometry changes (for example, if you change the diameter of a hole on which a spider element is defined) and your geometry-based mesh is updated, the leg nodes of the spider element may no longer be connected appropriately and the spider element may need to be recreated.

How do I

Create a node-to-node connection

Create an element edge to element face connection

Create a point-to-point connection

Create a point-to-edge connection

Create a point-to-face connection

Create an edge-to-edge connection

Create an edge-to-face connection

Create a face-to-face connection

Learn more

1D Connection

Edge-face 1D connections

Point-to-point and node-to-node connections

Spider element connections

1D mesh to face connections

Understanding Samcef RBE and MEAN elements (Simcenter Samcef)

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Working with RBE2 and RBE3 spider elements, Simcenter 3D 2021.1 Series

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