Concrete Support Wall Frequency Calculation

[Grant M]

Hello,

I need to design three reinforced concrete supports (9" thickness, 6'10" length, 2'6" height) to hold up a centrifuge, the supports are spaced 50" and 140" apart. The resonance frequency of the centrifuge is in the 3-8 Hz range and I need the substructure to have a rating in which the natural frequencies of the substructure are at least 30% above the resonance frequency.

So far, I have been using ASCE 26.9.3 - Approximate Natural Frequency, and have been treating the supports as concrete shear walls. However, using this method I am getting an approximate lower bound frequency of 650 Hz. I was wondering if this is the correct method to use and a reasonable result to get, or if not what steps to take.


Thanks

 

[WARose]

I would think the section of ASCE you are talking about isn't appropriate because it is calculating the frequency based on this acting as a shear wall. Likely, the lowest natural frequency will be a translational frequency in the out-of-plane direction of the wall.

My approach with this would be a FEA model that included dynamic spring constants to represent the foundation.

And 650 Hz is outrageously high. I would think something like that would have it's lowest frequency be 30 Hz or less.

 

[Grant M]

Okay, thanks for the help!

 

[Grant M]

I failed to mention in my original post that these supports are to be located on the 2nd floor of a large, existing building. So I am wondering if I would have to determine the frequency of the supporting slab/beams/columns or possibly even of the entire building?


Thanks

 

[WARose]

I failed to mention in my original post that these supports are to be located on the 2nd floor of a large, existing building. So I am wondering if I would have to determine the frequency of the supporting slab/beams/columns or possibly even of the entire building?

Hard to say without seeing it.....but you are (at the least) talking about modeling a lot of the building (if not the whole thing) to have an idea as to what is going on.