Frost Protection with ASCE 32 1

[SteelPE]

Is anyone familiar with ASCE 32?

I have a client who wants me to design the foundations for a storage facility (Risk Category I). The building is located in an area where the frost depth has been determined to be 4’-0” by the AHJ. The client is looking to turn the slab down 16” into the ground and then place 32” for granular soil under the footing to provide frost protection.

In accordance with chapter 18 of IBC 2015 you either need to construct the footings below frost line or construct in accordance with ASCE 32. Section 4.2 of ASCE 32 appears to allow for this type of construction (see attached). I have never done this type of construction before and I am wondering if I have interpreted this paragraph correctly (allowing me to use the detail proposed by the client).

 

[jittles]

We've done many frost-protected foundations per ASCE 32, but have never used this provision. Color me a bit surprised, but it appears that you're correct...

4.1 clearly lays out your three options - use an NFS layer, insulate, or engineering analysis.

I'd be concerned about making absolutely sure that layer under the footings is actually NFS, and that it will stay NFS over time, i.e. is there any chance of fines getting in there and compromising the NFS status.

We get recommendations from Geotechs all the time for these systems, and never have I seen one recommend this option. Just an anecdote for you, I couldn't tell you why they lean away from it.

 

[SteelPE]

I understand the concern with regards to fines getting into the clean compacted fill area. It might be something where we would recommend using a filter fabric between the compacted fill and native soil to prevent infiltration of fines.

My next question would be in regards to the bearing walls in-between the units...... would you think these interior wall in a non-heated structure would required the same detail? That is the direction that I am leaning, but I am not sure if there is a way to get out of the requirement.

 

[BAretired]

If the native soil is a well draining material, there is no problem with the method proposed. If the soil is an impermeable clay, the granular material could fill with water during periods of heavy rainfall or runoff. In that case, frost heave would still be a problem. Better to use low strength concrete fill.



BA

 

[jittles]

If the structure is non-heated, then you definitely need that same detail at the interior.

 

[SteelPE]

jittles,

Just playing devil's advocate..... if you need water and cold to create frost, are you not eliminating interior water from the equation?

I agree with the idea of requiring frost depth at the interior locations, but the client isn't currently thinking about dropping those locations. I did tell him I may require it.... and if he doesn't want to do it then we will just go our separate ways.

 

[jittles]

How are you eliminating water from under the interior?

Perhaps by... providing NFS soil under the footings?

And another note, frost depth is usually deeper for cold footings versus heated footings. Is the 48" frost depth for cold, unheated footings?

Here where I practice in a cold climate, we have 42" frost depth for heated footings but that goes down to 120" completely unheated, or we can typically reduce that to 60" WITH NFS under the footings.

 

[SteelPE]

I have had the argument proposed that you are eliminating water by capping the site with the building... and in this instance, the paving against and around the building.

Now, I have reviewed ASCE 32 a little further. In the instance you are using insulation for frost protection on an unheated structure, ASCE 32 requires the insulation to be placed under the entire structure. So you could infer that the fort mitigation techniques used on the perimeter should be extended to the interior for unheated buildings.

I am no leaning towards convincing the client to place insulation under the entire structure. Use 12" of NFS soil on top of 6" of extruded polystyrene. We could then thicken the slab at interior structure bearing locations. The insulation would need to extend 49" beyond the face of the building (based upon F100 = 1500 and MAT = 45).

Yes, the frost depth here is 48" in all instances. I don't know where you are (I am guessing Canada), but I don't think I would like the winters there.

 

[jittles]

In the USA, just to the left of Canada

 

[SteelPE]

Well, now I understand why you have a 10' frost depth.... and I also know why you are an expert on ASCE 32.