Beam on elastic foundation

[robdale]

Does anyone know where I can find foundation modulus (k) values for different types of material (not just soils)?

 

[ChipB]

Without a geotech report?

Ugh! Nastiness! Soils part is pretty easy. Just use the published (lowest) values in the "Civil Engineering Reference Manual"

Other material.......
Vesics ugly equation for long beams will probably work out.
k=0.65*((Es*B^4)/(Ef*If))^(1/12)*(Es/(B*(1-v^2))
|this is raised to 1/12| then * this

Es=Modulus of elasticity of what you're bearing on
B= Width of Foundation in inches
Ef=Modulus of elasticity of Foundation material (Conc?)
If =Moment of intertia of foundation cross section
v=poissons ratio of materail you are bearing on

This can be approximated to:
k=Es/(b*(1-v^2))

Use a strengths book for Es and v
Hope it helps and good luck

 

[Lutfi]

In all of my years of practice, I always ask for the k in the geotechnical report. Some times, depending on the site location, I rely on my past experience. Joseph Bowles has an excellent approach on how to compute the sub-grade modulus in his book "Foundation Analysis and Design". I have the fourth edition. I ma sure there is a more recent edition out there.

 

[MrStohler]

Using the lowest values for ks in not always conservative. The lower value for ks does provide higher bearing pressures, but it provides lower bending moments for the element that appllies the pressure to the soil (usually a mat foundation).

It would be more conservative to use the lower values to compute bearing pressures and the higher values to compute moments.

Or better yet, advise your client of the cost savings that could arise from having accurate test results. Not to mention the effect lack of testing has on your liability.

Be very careful.

 

[StephenEd]

What type of non-soil material are you going to be founded upon? Rock?

 

[rccon]

Both chipb and mrstohler suggest that a lower value of the subgrade modulus ks produces a higher bearing pressure, but it seems to me it's just the opposite. A lower ks would permit greater deflection under a concentrated load, resulting in a lower bearing pressure but a higher moment in the footing. Am I missing something?

 

[dlew]

I think that for the same mat, the lower the subgrade modulus would produce more deformation in the mat, and consequently more moment. If the mat was dimensioned properly, and the moment and the settlement of the mat are acceptable, the fact that the bearing pressure under the columns may exceed locally the soil "allowable" bearing pressure is of not consequence.

 

[PhDE]

I would recommend doing a parametric study using 1/2 the furnished k-value and up to 5 times the amount. It is very difficult for the geotech to provide elastic design parameters and subgrade responses.