I'm trying to optimize a tunnel model in Siemens NX 11.0. I'm applying the loads stepwise in time. Between 0s. and 1s., only thrust force is acting on the tunnel. However, a small clockwise rotation appears in the results and I have no idea where it comes from.
In order to suppress the small rotation, I tried to add a RBE3-connection between nodes at the circumference of the tunnel (source) and a node at the center of the tunnel ring (target). I'm supposing to constraint DOF1, DOF2 and DOF6 of the central node, as no rotation around the longitudinal z-axis are allowed.
Still I have errors in my model set-up. I tried different ways to define the boundary conditions but none of them has worked. In attachment, you can find the model.
Can anybody help me in solving this problem?
Solved! Go to Solution.
You can't directly constrain the dependent node of an RBE3 because it's displacement is already prescribed by the RBE3.
You may be able to achieve the effect that you're looking for by constraining a new node that is connected to the dependent node by a spring (CBUSH) with high stiffness.
You can apply constraints to the reference node of an RBE3 by using one of the following methods:
easy method = use param,autompc,yes(Nastran will internally modify the dependent/indepent dof's to avoid the conflict
more difficult method = use the "UM" input on the RBE3 entry to switch dependent/independent dof's to avoid the conflict
See the QRG for these entries for details.