Machine foundation modal analysis

[rauno.verbitskas233CE]

Hello!

I have made a machine foundation model with volumetric structure design. I am trying to make modal analysis with four rigid links with machine loads. I have supported foundation with planar support and added elastic springs and damping. Problem that I have is with soil springs damping. It doesn't effect foundation frequency-s. At the moment I have first mode frequency about 58,41 Hz but they should be about 18,35 Hz, so the difference is about 3 times.

I have added screenshots with loads, support conditions, soil conditions, and results that the calculations should be.

I also added my Robot model.

Thank You,

Rauno

[Krzysztof_Wasik]

Hi @rauno.verbitskas233CE 

Results of modal analysis do not depend on defined damping. Damping is considered while seismic, spectral, harmonic or time history analysis.

Frequencies caluclated for modal analysis depend on mass (mass distribution) and defined supports elasticity.

Since it seems that structure mass is defined correctly , check support elasticity defined in Robot model. Probably you have defined too big Kx, Ky and Kz values.

[rauno.verbitskas233CE]

Hi,

Thank you for answer.

A little bit confusing is information on autodesk Robot help where there is said that damping could be used during modal analysis.

https://knowledge.autodesk.com/support/robot-structural-analysis-professional/learn-explore/caas/Clo...

Thank You,

Rauno

[Krzysztof_Wasik]

Hi @rauno.verbitskas233CE 

As far as I know this specific damping type is used only when simultaneously French code PS92 is used in model for seismic analysis