Subgrade Modulus Unit Conversion

[SteelPE]

I have a pretty basic question regarding foundation spring modeling in regards to a slab on grade. I have a subgrade modulus from my Geotech of 200 pci for the soil under my slab. I am trying to model different loadings on the slab using my FEM software. The software allows me to enter the subgrade modulus but it is requesting the units of kip/ft2/ft.

I modeled a 40’x40’ section of slab using 1’-0” square quadrilateral plates (I don’t really want to refine the mat mesh much from here). So it seems like the proper value for the software would be 200pci x (12 in/ft x 12in/ft x 12 in/ft) /1000 = 345.6

This number seems really high to me. Is this the proper unit conversion to get 200 pci into a unit of kip/ft2/ft?

 

[JLNJ]

200pci is toward the top of the range, but still reasonable for slab design.

Mat footings are a little bit different animal and often have a sub grade modulus which is just a fraction of that used for pavement design. Sometimes the Geotech provides this but often you have to request this value.

 

[SteelPE]

JLNJ,

I realize that this isn't the best approach to design the slab and that there are a few holes in my approach. At this point I am just wanting to make sure I am doing the conversion properly. Once I know that then I can reduce the sub grade modulus to see the effects on the slab.

 

[robyengIT]

unit conversion is correct

 

[dik]

kip/ft^3 ?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

MathCAD thinks that you know your 8th grade unit manipulations. I didn't trust myself to do it.

 

[dik]

you're correct... with the order of operations... still k/ft^3

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Lomarandil]

Dik, it's written as /ft2/ft (etc.) to clarify that the parameter is not a density, it is a pressure which increases (linearly) with depth. a pressure per unit deflection, as IDS states below.

Works out the same, but an important nuance for engineers not familiar with the concept of subgrade modulus

----
just call me Lo.

 

[Settingsun]

Lomarandil, it's also the density of a fluid that would exert the same pressure on the base if submerged to the same depth

 

[IDS]

Dik, it's written as /ft2/ft (etc.) to clarify that the parameter is not a density, it is a pressure which increases (linearly) with depth.

?

A pressure increasing linearly with depth would have the same units, but a subgrade modulus is given at a specified depth.

It is a reaction stiffness, expressed as a pressure per unit deflection.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Lomarandil]

Derp! IDS is correct....

----
just call me Lo.

 

[JLNJ]

Apropos of nothing, the density of gold is 0.70 pci.