Insulated Slabs on Grade

[sponcyv]

I am designing an insulated slab on grade at cooler/freezer rooms on an industrial project. We have rack loads of 20 kips max. I am looking for good design examples taking in consideration the properties of the insulation i.e. subgrade modulus, compressive modulus, flexural modulus. I have attached a decent breakdown in a document that I found published by owens corning and wanted others opinions. If there are any industry design guides that I could download for free, please let me know.

Thanks!

 

[dik]

If a freezer, then you have to provide heat to the subgrade to keep the frost from penetrating the soil in the long term.

There are several threads on eng-tips... do a search

Dik

 

[sponcyv]

dik,

That's not my call. I'm simply designing the concrete slab.

I found the following, which answered my concerns about using the insulation's subgrade modulus or the soils subgrade modulus:

ACI 360R-44 section 13.2.1 - Insulation Modulus - "For slab-on-ground design, the strength of the soil support system directly below the slab is considered. In the case of a floor in a refrigerated building, consider the strength of the insulation in a similar manner. Some design methods use the modulus of subgrade reaction to account for soil properties. Insulation has a similar modulus."

 

[HeavyCivil]

Were you able to quantify that Insulation's support modulus though? How? I have come across this recently with no good solution other than to reduce the actual soil K by an arbitrary and conservative amount and design for that...

 

[sponcyv]

VTEIT,

Do you mean the insulation's subgrade modulus? This is given by the manufacturer. Owens Corning gives design examples. Freezermate gives design calculations as well, but doesn't do as good of a job in examples. Both list the subgrade modulus values on their websites. The values are much higher than the typical soil subgrade modulus values. For my design, the subgrade modulus for each layer was 610 pci. The subgrade modulus for the two layers that we're using is 305 pci. you typically divide the values they give by the number of layers because each layer will deflect. I was worried that the soil subgrade would need to be considered, but I think this is typically ignored. I'm assuming that as long as the stresses on the insulation are lower than the allowable stresses, the modulus of subgrade for the insulation is valid.

 

[HeavyCivil]

Thanks - I was able to find those, as well as a reference to design of refridgerator slabs in PCA's "Concrete Floors on Ground" text (p. 124.

I am surprised how high the values are. For 4" of a low grade insulation (Foamular 150) you still get 300 pci - about double that of a decent subsoil.

PCA suggests using 75 pci conservatively in lieu of manufacturers data.

 

[sponcyv]

WOW! That's what I was worried about. The question is can the combined concrete slab, insulation, and mud slab (in my case) spread the load out to where we will not get localized settlement or depression in the soil. I run a finite element analysis on the slabs that I design to check the tensile stresses, compressive stresses, and base pressures. I check my base pressures against the insulation manufacturer's allowable stresses. Keep in mind they are using safety factors of 3 for dead load allowable stresses and 5 for live load allowable stresses. Then, we use a safety factor of 2 on plain slabs with concentrated rack loads, not to mention the rack loads that I have been given are the highest possible loads that could be placed on the racks. I can sleep at night knowing that we have all of these safety factors especially since it is slab on grade which ultimately we can't guarantee won't crack no matter what precautions we take.