Equivalent Modulus of Subgrade Reaction for EPS Insulation under Slab

[HeavyCivil]

I'm curious if anyone has approximated a modulus of subgrade reaction for rigid foam insulation beneath concrete slabs.

I see regular foam board used in residential construction frequently - not the 40-100 psi hi-test variety that is preferred for under slab use. I am curious how you would compensate in design, other than assuming some arbitrary and low value for k.

 

[rowingengineer]

I assume the residential building have footings?

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[HeavyCivil]

Typically... though this is somewhat irrelevant to the design of a slab on grade.

 

[rowingengineer]

but not irrelevant to your question, slab on ground rely heavily on the sub-grade for support, hence why the insulation is generally a high spec items with the ability to take high loads. Conversely a residential slab has footing which do provided the main support for the building. generally these slabs do not use a k value for design. If you were to use a low grade insulation in the slab area I would suggest you need to span the slab from footing to footing.

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[vato]

Seems like a good question to me. I'm about to present a slab on grade design to a contractor tomorrow and I sure didn't design it on a foam based modulus.

 

[HeavyCivil]

Rowingengineer: That seems very conservative. I've used a 'structural slab' in that manner in industrial buildings when extremely poor soil is present (slab spans between grade beams and caissons) but that floor had a live load of 1000psf and very large post/rack loads.

I think that when dealing with 40 or 50psf LL the insulation likely will 'react' to enough of an extent that the slab still performs like a slab on grade, and does not develop enough flexural stress to justify spanning across footings.

My 4" slab with #4's at 16" o/c is good for little more than 1k'. A passenger vehicle would induce a moment many times higher than that if I consider my slab spanning 20'.

 

[ajk1]

The maker of Styrofoam (is it Dow Construction Products?) has published values of the the foundation factor for their Hi 40, HI 60 and HI 100 insulations. Their engineering department can be quite helpful if you can get past their sales department who at first claim they don't have an engineering department!You have to be persistent and forceful. Their engineering department can check the stresses on the insualtion and tell you if it is ok. thye do require large safety factors, mostly because of creep, I believe.

 

[BAretired]

The compressive stress for a particular strain in the rigid foam is known, so if you know the properties of the soil below, it should be possible to calculate a modified coefficient of subgrade reaction for the combined materials, i.e. foam and soil.

http://www.excellence-in-insulation...materials_made_of_rigid_polyurethane_foam.pdf

BA

 

[ajk1]

1/K equiv. = 1/K insulation + 1/K soil