Prestess normal modes and force impact

[algpi]

Dears,

I'm wondering why force applied in prestress normal modes hasn't got the same impact as concentrated mass on results.

As You can see structure has almost the same freq. without and with applied forces.

Can anyone tell me why this is like that?

(I'm I missing something in the theory?)

Please see attachments.

Thank You in advance for Your help

[John_Holtz · Accepted solution]

Hi @algpi 

If you are applying mass to the model, that changes the mass that has to vibrate. That will change the natural frequency. (If the mass is large compared to the original mass, then the natural frequency will change a lot.)

A force does not change the mass that vibrates; it only changes the stiffness. If the change in stiffness is small (which is the usual condition), the natural frequency will not change very much.

[algpi]

Dear John,

Thank You again for Your help.

I've blocked myself, but thanks to You it's obvious.
Now it's clear, and it can't be different.

I really appreciate such a People like You and Your colegues from Autodesk that I can rely on.

Thank You once more time for Your help.