Natural frequency of structural system and elements

[Philrock]

I am working on a foundation system, with a structure to be rigidly anchored to the foundation system. The issue of concern at the moment is the relationships of the natural frequencies of the elements and the natural frequency of the combined foundation system and structure.

The natural frequency of the foundation system is 9 Hz.

The natural frequency of the combined foundation system and attached structure is 3.5 Hz.

How do I calculate the natural frequency of the attached structure?

 

[FeX32]

How is the system setup?


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