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sanity check on calculated spring constants for dynamic analysis of mat foundation

sanity check on calculated spring constants for dynamic analysis of mat foundation

sanity check on calculated spring constants for dynamic analysis of mat foundation

(OP)
I've designed a 14' x 14' x 3' mat foundation to support a jaw crusher & steel support frame, but now I'd like to check the natural frequency in comparison to the operating frequency of the machine (3.8 Hz). I will be using a finite-element model in Staadpro that utilizes spring supports at each node of the foundation.

My main question is whether or not my calculated values for the spring constant are reasonable.

Right, wrong, or otherwise, I cannot convince the customer or my employer (a design/build construction company) to pursue geotechnical services to establish better soil parameters. So, for the sake of furthering the conversation, let's assume the following:

soil poisson ratio = 0.35
void ratio = 0.4

I've been using ACI 351.3R-04 as my main reference. For this application, I was assuming a uniform soil condition and using the Richart-Whitman model as shown in section 4.2.1.1. Since this is a square foundation, the impedance can be found from equations (4-13e) thru (4-13g).

The dynamic shear modulus has been calculated as 1,206,105 psf (per equation 3-34), assuming a probable confined pressure of 300 psf.

Vertical impedance, kv = 54,553 k/ft
Horizontal impedance, ku = 43,311 k/ft

These spring values above just seem high to me. Are they within reason?

*NOTE - I will post this in the foundation design section as well......thanks

RE: sanity check on calculated spring constants for dynamic analysis of mat foundation

This does seem astonishingly high, especially since ACI 351 isn't using some alternate definition of the spring constant - it's still "the amount of force needed to displace the material a given length".

However, the publication SP-78 "Foundations for Equipment and Machinery" by ACI includes some example calculations, and they do use similarly high values for the spring constants. So it appears that high values are "reasonable", though I'd love an explanation from someone with experience in equipment foundation design for how they make any sense at all.

I could upload some scans if you're interested.

Brian C Potter
http://simplesupports.wordpress.com

RE: sanity check on calculated spring constants for dynamic analysis of mat foundation

(OP)
Sure....if you get a chance.....I don't have that reference. Thanks!

RE: sanity check on calculated spring constants for dynamic analysis of mat foundation

(OP)
thanks brian......I'll check it out.

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