How to get normal modes WITH Gravity load?

Rahul_P1
Rahul_P1
Altair Employee
edited October 2020 in Community Q&A

Hi,
the picture showes the first two eigenmodes of a steel ruler, analysed with normal mode.
The model is attached (rename the second file to xxx.hm).

In a hardware test the low frequencies will be influenced by the gravity.
Example: different frequencies in horizontal an vertical position of model.

How can I calculate Eigenfrequencies/ Eigenmodes WITH Gravity?
(Eigenmodes of a prestressed structure)

ThankX, Dirk
51u16-lineal_moden1und2.JPG

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Answers

  • Rahul_P1
    Rahul_P1
    Altair Employee
    edited November 2014

    Hi,

    to add gravity,
    1) first create a linear static subcase with load as Gravity.
    2) create a normal modes subcase and reference the linear static subcase in normal modes subcase under STATSUB(PRELOAD)

    es7e4-Capture.JPG

    However, you may not observe noticeable change in the frequencies when the gravity is acting normal to the surface of the elements. The reason is that the loading from is distributed equally among all its nodes. As the nodes react to the constraints and degrees of freedom, they expand and contract radially in relation to the mid surface.

    you may observe change in the mode shapes and frequencies when you change the direction of the gravity.

  • Rahul_P1
    Rahul_P1
    Altair Employee
    edited November 2014

    Great, thanks,
    easy to do, I checked it with the ruler:
    Radioss FEM solution (with different GRAV load) correlates well with my hardware test ( 500mm steel ruler )
    Result: movement depends on ruler position
    # slowly --> vertical, free end on top
    # middle --> horizontal
    # fast --> vertical, free end on bottom
    Next I will add a single mass to the free end (-: (-:

    Best regard, Dirk

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