COMPARISON BETWEEN RBE3 AND CERIG -Ritesh Dungarwal

Essence: In order to simulate the physics of problem into the mathematics of ANSYS, it often becomes unavoidable to associate a set of nodes with a particular behavior. The reasons for such associations may be the assumptions of the basic theory used for the solution algorithm, or, the limitations of ANSYS while accepting the actual problem (an example for the latter: ANSYS does not provide any direct means to apply torque loads and hence calculate the torsional stresses introduced while such applications.). RBE3 and CERIG are the commands which are used for the associations referred to, above. Although the applications of both the commands traverse the same domain, the basic underlying concept and the way either work is different. Various important criterion in which the two commands differ have been tabulated here.

Criteria CERIG RBE3 Syntax CERIG, MASTE, SLAVE,

Ldof, Ldof2, Ldof3, Ldof4, Ldof5

RBE3, Master, DOF, Slaves, Wtfact

Concept forms a directional rigid zone by linking a set of nodes to a master node in the two/ three dimensional space (using constrained equations).

distributes the load (force/ moment) on a master to the slave nodes (using constrained equation).

Number of constrained equations

=A*B. where, A=dof used in “Ldof” field(s), B=no. of slave nodes.

=1.

DoF of the slave nodes used in constrained equation

all (transational and rotational) used.

Only transational used.

Terms in any constrained equation

DoF of master node and 1 slave node

Dof of master node and all slave nodes.

Additional Stiffening introduced in the direction of tie-up.

not introduced.

Load distribution cannot be defined by user. Can be defined (using “Wtfact” field)

Set definition uses node numbers and component names.

Uses array parameter names

Applicability: As tabulated above, the usage of either of the commands has its own limitations, more importantly, in terms of stiffening effects and constrained equations. While, using CERIG may lead to a more determinant set of equations, it may result in directional model stiffening; something which can be avoided by using RBE3 (with a possible compromise on the accuracy of resulting solution). The use of CERIG is generally avoided due to the stiffening effects.