Application of Subgrade Modulus in Mat Footing Computer Design

[rlewistx]

I've searched the archives and found 249 threads dealing with subgrade modulus. I haven't read all of them, just enough to get frustrated. I don't see the question I have discussed. It has to do with the application of the subgrade modulus in a finite element computer analysis.

I've worked with two main stream programs for mat foundations. They each request the subgrade modulus for the analysis. My question is, do I put in the value for the 1'x1' plate, or do I put in the value for the large mat dimension? I understand that for cohesive soil the mat adjusted subgrade modulus is K'=K/B where B is the width of the mat footing. If I have a K=150 PCI and a mat footing width of 30'x30', do I enter in the computer program K=150 PCI or 3 PCI? The finite elements are going to be smaller than the 1'x1' plate of the modulus test, but the overall footing is much greater than the 1'x1' test plate.

I understand the moments and shears are dependent on the deflection and the deflection is greatly influenced by the subgrade modulus. There is a big difference between a K of 150 verses a K of 3. What should be the standard practice is using the modulus of subgrade reaction in a computer analysis?

Thanks for your assistance and insight.

Rich

 

[JoshPlumSE]

I think it depends on the program you're using. The documentation should make it clear what you enter.

In RISA, for example, we want you to enter in the finalized / adjusted subgrade modulus. That means after you have adjusted for the size and shape of the footing. Now, we don't really get too much into how you want to adjust the 1'x1' value. There are some fairly standard equations in Geotech books. But, you might get a geotech requesting a different procedure. Or, the procedure is only valid for rectangular or circular footings and you've got something else.

Other program may try to automate this process, I suppose. But, to me, it makes most sense for the user to determine the final value (based on geotech guidance or engineering judgment or such) and for the programs to merely do a simple trib area calculation to calculate spring constants at individual joints. Programs are great at math, but not so great at engineering judgment for odd cases that inevitably arise.

 

[rlewistx]

Josh,

Thanks for your response. I appreciate your insight.

So as I understand your response, and I use my specific example, for RISA the recommended value for subgrade modulus for the computer analysis is K=3 PCI in my above illustration. I understand what you said about engineering judgment, but if I use the published equation of K'=Kv/B where B is the width of the mat footing, then K' = 3 PCI. That give large deflections and consequently, large shears and moments. With the given parameters of above, that is how you see it?

Rich

 

[rlewistx]

Okay, I just noticed my math is off above. Should have had an adjusted value of K=5 PCI. Dumb math error.