This article describes the concept of the Rigid Elements in NASTRAN. The name rigid element is often misleading. The R-type elements include both rigid and interpolation elements.
Rigid elements consist of RBAR, RBE1, RBE2, RROD, and RTRPLT. Of these elements, the RBAR and RBE2 are the most commonly used.
Interpolation elements consist of the RBE3 and RSPLINE. The RBE3 is a linear interpolation element often used to distribute either loading or mass, and the RSPLINE is an interpolation element that uses the beam equations to write MPC relationships. The RSPLINE is often used to model mesh transitions.
Each of these R-type elements generates internal MPC (multipoint constraint) equations in MSC NASTRAN. The simplest description of an MPC equation is that it is used to describe the motion of one (dependent) DOF in a model as a linear combination of the motion of one or more other (independent) DOFs.
Simple examples of MPCs are to describe the motion of one DOF as the average of two other DOFs or to impose a rigid connection between two points.