Ignoring mass for specific subcase

Gimmicks
Gimmicks Altair Community Member
edited January 2022 in Community Q&A

Hello,

I am trying to do a static simulation for multiple loading cases for one model. I use mass CONM2 and acceleration in some of those cases. Is it possible to ignore some mass points in a single subcase? Or do i have to create separate model with those elements removed? 

 

 

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  • Ben Buchanan
    Ben Buchanan
    Altair Employee
    edited January 2022

    One way to do this is with MPCs and SPCs.  Connect your CONM2s with MPCs and only add those MPCs to the loadcases that need them. Also put SPCs on the CONM2s and call those out in the other cases so that the mases don't fly around when not attached to the structure.

  • Gimmicks
    Gimmicks Altair Community Member
    edited January 2022
    Ben Buchanan said:

    One way to do this is with MPCs and SPCs.  Connect your CONM2s with MPCs and only add those MPCs to the loadcases that need them. Also put SPCs on the CONM2s and call those out in the other cases so that the mases don't fly around when not attached to the structure.

    Thanks for answer, but can you please explain MPCs to me? I have my masses connected with structure using RBE2 and 3. What should i input to make  MPC behave like RBE? 1 everywhere for RBE2? Also wont constraining a CONM2 node connected with already constrained node using RBE2 cause change in reaction forces?

  • Ben Buchanan
    Ben Buchanan
    Altair Employee
    edited January 2022 Answer ✓
    Gimmicks said:

    Thanks for answer, but can you please explain MPCs to me? I have my masses connected with structure using RBE2 and 3. What should i input to make  MPC behave like RBE? 1 everywhere for RBE2? Also wont constraining a CONM2 node connected with already constrained node using RBE2 cause change in reaction forces?

    MPCs can be be added to your current model between the rigid elements and the CONM2.  You need to be careful that the dependent node from the MPC is not attached to a dependent node of the rigid.  So for the cases where you call the MPCs it should act exactly as it did before and for the cases where you don't call them (as long as the CONM2 doesn't cause issues by being free, see below) it should act as if no weight is attached.  

    Concerning the constraints, you would have separate SPC collectors and only call the SPCs constraining the CONM2s in the loadcases you don't call the MPCs attaching them to the structure.  Otherwise those CONM2s aren't connected to anything and may cause problems if they are just floating in space.

    Here are some links from the help so if my explanation doesn't clear things up hopefully they help:

    https://2021.help.altair.com/2021.2/hwsolvers/os/topics/solvers/os/mpc_bulk_r.htm?zoom_highlight=mpc

    https://2021.help.altair.com/2021.2/hwdesktop/hwx/topics/pre_processing/model_build_and_assembly/mpc_create_edit_review_c.htm?zoom_highlight=mpc

    https://2021.help.altair.com/2021.2/hwsolvers/os/topics/solvers/os/mpcadd_bulk_r.htm?zoom_highlight=mpc

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