Structure on a stiff foundation 1

[izax1]

My structure has the first natural frequency at approx.15 Hz. The structure is mounted on a foundation with stiffness approx. 30 Hz. The manufacturer of the foundation claims he can measure the 15 Hz. frequency at the foundation. I would assume that if you have the 15 Hz at the structure and the 30 Hz at the foundation, the separation should be enough to not see any significant accelerations at 15 Hz at the foundation. So where could the 15 Hz. come from? Do I need more separation between the structure and the foundation?

 

[Twoballcane]

I think your thinking of frequency seperation so that your structure will not be affected (or coupled into)by the 30 Hz. But, I think the manufacute is seeing the frequency of the combined wieght of the fondationa and structure. If the if the foundation is 30hz, when you add moremass, the Fn will drop. May be enought to get you to 15 hz.

Tobalcane
"If you avoid failure, you also avoid success."

 

[GregLocock]

Look at your spring mass SDOF system. Do you seriously think the foundation sees none of the spring force?



Cheers

Greg Locock


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[Tmoose]

What "forcing" frequencies are going to be generated by equipment, processes, weather conditions, etc? Any impacts?

 

[izax1]

Thanks for feedback.
Twoballcane: The 30 Hz. frequency is with the mass of the structure.
Greg: I agree it will see the spring force, but not as a dynamic load. If they are decoupled as we want to achieve, it will be like a static force, and no dynamic amplification. But the foundation manufacturer measures a clear amplification.

 

[Twoballcane]

Where is he taking the measurements?

Tobalcane
"If you avoid failure, you also avoid success."

 

[saplanti]

I am not an expert on the issue. I am just guessing that the clear multiple of 2 between the natural frequencies may be effecting the foundation.

Regards,

Ibrahim Demir

 

[izax1]

The measurements are taken right next to the attachment bolts of the structure.

 

[saplanti]

Please excuse my mistake in my previous post. The corrected sentence is to be :

"I am just guessing thet the clear multiple of 2 between the forcing frequency and the natural frequency may be effecting the foundation."


Reagrds,

Ibrahim Demir

 

[Twoballcane]

So I'm confused,

"The structure is mounted on a foundation with stiffness approx. 30 Hz" or "The 30 Hz. frequency is with the mass of the structure."...The foundation is at 30hz with or without the structure?

Tobalcane
"If you avoid failure, you also avoid success."