Notes

Defines a rigid region (link, area or volume) by automatically generating constraint equations to relate nodes in the region. Nodes in the rigid region must be assigned a geometric location before this command is used. Also, nodes must be connected to elements having the required degree of freedom set (see Ldof above). Generated constraint equations are based on small deflection theory. Generated constraint equations are numbered beginning from the highest previously defined equation number (NEQN) plus 1. Equations, once generated, may be listed [CELIST] or modified [CE] as desired. Repeat CERIG command for additional rigid region equations.

This command will generate the constraint equations needed for defining rigid lines in 2-D or 3-D space. Multiple rigid lines relative to a common point are used to define a rigid area or a rigid volume. In 2-D space, with Ldof = ALL, three equations are generated for each pair of constrained nodes. These equations define the three rigid body motions in global Cartesian space, i.e., two in-plane translations and one in-plane rotation. These equations assume the X-Y plane to be the active plane with UX, UY, and ROTZ degrees of freedom available at each node. Other types of equations can be generated with the appropriate Ldof labels.

Six equations are generated for each pair of constrained nodes in 3-D space (with Ldof = ALL). These equations define the six rigid body motions in global Cartesian space. These equations assume that UX, UY, UZ, ROTX, ROTY, and ROTZ degrees of freedom are available at each node.

The UXYZ label allows generating a partial set of rigid region equations. This option is useful for transmitting the bending moment between elements having different degrees of freedom at a node. With this option only two of the three equations are generated for each pair of constrained nodes in 2-D space. In 3-D space, only three of the six equations are generated. In each case the rotational coupling equations are not generated. Similarly, the RXYZ label allows generating a partial set of equations with the translational coupling equations omitted.

Applying this command to a large number of slave nodes may result in constraint equations with a large number of coefficients. This may significantly increase the peak memory required during the process of element assembly. If real memory or virtual memory is not available, consider reducing the number of slave nodes.

Note that under certain circumstances the constraint equations generated by CERIG may be modified during the solution. See Program Modification of Constraint Equations for more information.

As an alternative to the CERIG command, you can define a similar type of rigid region using contact elements and the internal multipoint constraint (MPC) algorithm. See Surface-Based Constraints for more information.

CERIG cannot be deleted using CEDELE,ALL and then regenerated in the second or higher load steps if the LSWRITE and LSSOLVE procedure is used. CERIG writes constraint equations directly into load step files. Deleting constraint equations (CEDELE,ALL) cannot always maintain the consistency among load steps.

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