# Example 1 – RBE3 used to prevent rigid body displacement

In this example we restrain an externally pressurized cylinder that is free-free, i.e. not constrained at the ends. Coord 1 (cylindrical) is used as the analysis coordinate frame.

To create a free-free condition, we create RBE3 MPC’s to constrain 4 nodes at one end of the cylinder.

Node 150 is located at 0,0,0 in the cylindrical coordinate system 1. One end of the cylinder is located at Z=0, the RBE’s are defined are at this end. Four(4) different MPC’s are defined using node 150 as the “independent” and 4 nodes on the cylinder (one node per MPC) are the “dependents”.

Node 150 is constrained along all 6 DOFs and with the expection of external pressure, the shell elements are unconstrained.

Individual MPC’s are defined as follows:

Elements
CREATE
MPC
EXPLICIT (or RBE3, note that the same form will come up)

Turn off Auto Execute
Select a node on the shell then pick UY,UZ, RX,RY,RZ for the dependent DOF (note that we allow the radial direction (UX) to move).
“Apply”

Click “Create Independent”
Select the independent node (node 150 in this case)
pick UX,UY,UZ, RX,RY,RZ for the independent DOF
“Apply”

Repeat until all MPCs are created

Figure 9, is a snapshot of the MPC(modify) form. Figure 10 illustrates the displaced shape.

Figure 9

Figure 10

Multiple nodes on flange from 3-D tet elements need to be coupled to 3-D beam element.
Use RBE3 – calculates node motion based on weighted average.

Node on beam end is independent, flange nodes are dependent.

DOF for flange nodes UX,UY,UZ
DOF for Beam UX,UY,UZ, RX,RY,RZ

Elements
CREATE
MPC
EXPLICIT