Adjacent elements whose stiffness differ by several orders of magnitude or more can produce an ill-conditioned stiffness matrix, which leads to problems when solving the model. For example, if you were to use a CBAR element with extremely large values of I1 and I2 to simulate a rigid connection, an ill-conditioned stiffness matrix would likely arise.
The RBE2 (Rigid Body Element, type 2) element on FEMAP with NX NASTRAN defines a rigid body whose independent degrees of freedom are specified at a single point and whose dependent degrees of freedom are specified at an arbitrary number of points. The RBE2 element uses constraint equations to couple the motion of the dependent degrees of freedom to the motion of the independent degrees of freedom. Consequently, RBE2 elements do not contribute directly to the stiffness matrix of the structure and ill-conditioning is avoided. The RBE2 provides a very convenient tool for rigidly connecting the same components of several grid points together
In the other side, the RBE3 element on FEMAP with NX NASTRAN is a powerful tool for distributing applied loads and mass in a model. Unlike the RBAR and RBE2 elements, the RBE3 doesn’t add additional stiffness to your structure. Forces and moments applied to reference points are distributed to a set of independent degrees of freedom based on the RBE3 geometry and local weight factors.
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Blas.
www.iberisa.com
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