Error #725. RBE2 has rotational rigid body modes in all three directions.

AshwaniMittal
AshwaniMittal Altair Community Member
edited June 2021 in Community Q&A

Hello all,

I am working on a model and while running it on optistruct the following error occurs:

*** ERROR # 725 ***
An invalid rigid element.
This RBE2 has rotational rigid body modes in all three directions.
RBE2 element id = 436463
independent grid id = 83349
Note: If this rigid element is also connected with other rigid elements,
then this error means that there is rigid body mode or mechanism
in this rigid element chain due to the lack of connected structural elements.

Can anyone tell me why is this error occurring and what can I do to correct it?

I have also attached the HM file of my model for refrence.

Thanks

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Answers

  • Adriano A. Koga
    Adriano A. Koga
    Altair Employee
    edited June 2021

    the error message says that you have rigid elements connected to other rigids, creating rigid body modes.

    If you take a look at this particular element, and its surroundings, you will notice that the node 95816 is independent node in 2 different elements, meaning that it is the master node for them. The dependent nodes will follow its movement. But, in your case, the independent node is "flying" free to move, thus this causes the error.

    Try checking your Independent/dependent nodes in the Rigid, and avoid the same node being independent in 2 elements, or even worse, the same node being dependent of 2 different independent nodes.

    You can use the 'switch' option under update to change who is indep and dep node.

    image

  • AshwaniMittal
    AshwaniMittal Altair Community Member
    edited June 2021

    Hello @Adriano A. Koga,

    Thanks for your reply. Can you please tell me how can I find out which node for which rigid element is acting as dependent/independent?

    Thanks in Advance.

  • Adriano A. Koga
    Adriano A. Koga
    Altair Employee
    edited June 2021
    AshwaniMittal said:

    Hello @Adriano A. Koga,

    Thanks for your reply. Can you please tell me how can I find out which node for which rigid element is acting as dependent/independent?

    Thanks in Advance.

    if you go to the 'rigids' panel, click 'review', and then click your rbe2 element, and it will show just like the picture before. I and D

  • AshwaniMittal
    AshwaniMittal Altair Community Member
    edited June 2021

    Hello @Adriano A. Koga,

    As suggested, I have switched the nodes of elements where 2 dependent or independent nodes of 2 different rigid elements were occurring at same node. After running my model again the same error occurred at different rigid element but that rigid element is not connected to any other rigid element as shown in the image, therefore no 2 dependent/independent nodes are occurring at the same node this time.

    Please help to resolve this issue.

    I am also attaching the HM file of the updated model.

    Thanks 

  • Adriano A. Koga
    Adriano A. Koga
    Altair Employee
    edited June 2021
    AshwaniMittal said:

    Hello @Adriano A. Koga,

    As suggested, I have switched the nodes of elements where 2 dependent or independent nodes of 2 different rigid elements were occurring at same node. After running my model again the same error occurred at different rigid element but that rigid element is not connected to any other rigid element as shown in the image, therefore no 2 dependent/independent nodes are occurring at the same node this time.

    Please help to resolve this issue.

    I am also attaching the HM file of the updated model.

    Thanks 

    the problem now is that the solid elements have translational dofs only, and no rotations.

    So your RBE2 is connected to the solid through only 1 node, and it has no stiffness for rotation.

    You could anchor the rb2 in at leat 3 nodes to make it stabnle.

     

    Also always check (F10) for 1d>>dependency

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