How to get normal modes WITH Gravity load?

Rahul_P1

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

Rahul_P1

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)

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.