Subgrade Modulus for Mat foundations

[HZStruct]

Hello all,

I'm analyzing a mat foundation in STAAD and would like to get input on the subgrade modulus being used. I'm using plate mat foundation supports in the Y-direction in STAAD. The geotech report stated a unit subgrade modulus of 150 pci and suggested that the subgrade modulus be adjusted for the mat foundation geometry using the following equation.

K = k ((B+1) / 2B)^2; where B is the foundation width.


My question is, if I am using 1'x1' plates, can I use the unit subgrade modulus or do I need to adjust the subgrade modulus for the overall geometry of the foundation?

 

[Smoulder]

Whole thing. Reason is load spreads out a bit. For small footing, effective area of ground providing resistance is much bigger than foundation width. For big footings the areas are pretty similar so less improvement.

 

[ads0221]

What he said. If you're using 1'x1' plates use the same for the entire foundation.

 

[TRAK.Structural]

Agree with the above responses.

Additionally, I've gotten in to the habit of checking some standard upper and lower bounds (100 pci and 10 pci). While this does change the stresses, for large mat foundations I find that this rarely changes the thickness/reinforcing design.