Learning Forum

[Sebastien ARRIVET]

Hi,

I'm doing a modal analysis using the perturbation method. In this modal analysis I represent a part of my model using a MASS21 element with KEYOPT(3)=0, meaning I have rotary inertia.

This MASS21 element is connected to many nodes from SOLID187 elements using RBE3, some with Ux and Uy degrees of freedom, others with Ux, Uy and Uz. Thoses nodes are not colinears

When done with the MASS21 element, the modal analysis returns 3 rigid body modes with high mass participation in the rotation degree of freedoms. I understand the the slaves can return rotational moment according to the element referene guide, but I feel like the mass21 element can still freely rotate. Without the MASS21 element those rigid body modes don't appear.

I dont understand how can I link the consequences of the rotation of that mass element to be seen as translation for the SOLID187 element, using the distance between the slave node and the MASS21 node, in order to remove those 3 rigid body modes.

Thanks in advance for your help.

Seb

[dlooman]

It sounds like you are using APDL commands, not the Mechanical gui.  I made a test similar to what you describe with the commands below.  Initially I used the command rbe3,999,uxyz,all and got the three spinning modes, but the rbe3 command below prevents spinning.

nsel,s,loc,x,1  ! Nodes on the face
nsel,a,,,999     ! Node with the mass21
rbe3,999,all,all
celist
nsel,all