Dynamic simulation (normal modes) of a structure with cable in tension with different stiffness in tension and compression

Marco Somà
Marco Somà Altair Community Member
edited April 2022 in Community Q&A

Good afternoon,

I am trying to perform the dynamic simulation of a structure which includes cables in tension and I want to determine the eigenfrequencies and eigenmodes.

This is what I did:

  • Spring (CELAS - PELAS), it works both for linear static and normal modes, but it sets the same stiffness to tension and compression and this is unreal for me
  • Gap (CGAP - PGAP), it works only for non-linear static while it induces a constant stiffness in all the other simulations
  • Bush (CBUSH - PBUSH), I tried but it doesn't allow to set two different stiffness in the same direction

I would like to use something like CGAP - PGAP which is also valid in the dinamic (normal mode) simulation.

All the recommendations are welcome! Thank you very much in advance. 

Marco

 

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Answers

  • Adriano_Koga
    Adriano_Koga
    Altair Employee
    edited April 2022

    hi,

     

    unfortunately the behavior intended is NL, and it is not possible to simulate this using regular linear modal analysis, unfortunately.

     

    You could try running a NL Transient analysis and use a time variant load as your input loading, and in this case, your CGAP will act with different stiffness. BUT this will be computationally more expensive, of course.

  • Marco Somà
    Marco Somà Altair Community Member
    edited April 2022

    Thank you very much Adriano,

    I want to determine the eigenfrequencies of the structure, therefore I was thinking on doing a Frequency responce (direct or modal) simulation, just in order to get the FRF diagram. Thanks to this I should be able to apply a load variable in frequency (vibration).

    But I read on Attacnhment 1, that CGAP can be used only in non-linear static analysis. So I can't model the cable in that way anymore. I tried to model it with PBUSH and PBUSHT by adding the tabel TABLED1 but from what I got it expresses the stiffness in function of the frequency and not in function of the positive or negative displacement. Do you have any ideas on how to model it?

    In addition the preload F0 of the PGAP is only in compression. I would like to apply this preload in tension, I tried to reverse the direction but it doesn't work.

     

    Thank you for your help

    Marco

     

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