CBUSH to solid element connection

[kanraj]

I'm using FEMAP-nxnastran. I came across an old model where a CBUSH is directly connected to a node in TET element. The load that CBUSH sees is not only axial and also CBUSH nodes are not lying on one plane. I think coordinates can explain it better.

ID Def CS Out CS X1 X2 X3 Color PermBC> Layer SEID #Elem #Load #BC
__________________________________________________________________________________________________________________
21778 0 0 745.501 21.7999 72.5519 46 ------ 21 0 5 0 0
22949 0 0 745.499 21.7999 71.3569 28675 ------ 24 0 17 0 0

This is CBUSH property used.
NASTRAN PBUSH Properties
Stiffness,K1 5000. K2 5000. K3 5000.
K4 1. K5 500. K6 500.
Damping, C1 0. C2 0. C3 0.
C4 0. C5 0. C6 0.
My question is, with no rotational stiffness to latch on to on the solid element side, how did this model run successfully (it did run successfully, no bailout)..Just by looking at the CBUSH coordinates, CBUSH property and the kind of connection, i expected the model to see a “RUN TERMINATED DUE TO EXCESSIVE PIVOT RATIOS IN MATRIX KL" due to instability in R5 direction. Could someone explain how this model ran successfully? or am i overlooking something? Thanks

 

[Tuw]

Interesting topic. I hope someone can answer too.

Personally, I think the unstable condition could be prevented if you have multiple cbush elements in various orientation. If you have only 1 cbush element that restrain only 1 DOF of the Tetra elements, then I agree the error would appear. But If you have cbush elements at several points, and several DOF (even though no rotational DOF), maybe the model could surpass the error.

Tuw