Detached Garage Foundation 1

[HDStructural]

Hello,

I am working on a foundation for a detached garage, which will be unheated. It is in a frost prone region with a frost depth of 38".

The foundation will consist of exterior wall footings (going down past frost depth), a slab-on-grade and a center column foundation.

For the center column foundation, I have some concerns with thickening the slab in that area and adding some rebar as one would normally do. Since it is unheated, I am concerned it may frost heave.
Do you think it is necessary to have the middle column foundation extend down below frost depth?

For the SOG, I've read a lot of opinions about rebar vs mesh vs unreinforced. I am planning on doing a 5" slab with control joints, the question is whether or not to specify #3's @ 12", WWM, or nothing at all. Any thoughts?

Thanks!

 

[HTURKAK]

Unheated but closed bldg. The frost depth inside the bldg. would be around 15 in. if the open air frost depth is 38".You may use frost free material ( sand or gravel ) under the sog.
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[XR250]

Can you insulate the inside of the frost walls and underside of the slab to capture the sub-grade heat?

 

[Alistair_Heaton]

Is it a commercial job or for yourself?

I have built something similar which is in the hobbies forum.

I have slightly deeper frost line with down to -30 deg C air temp.

I heated mine though for when I get forced into getting an EV

 

[jhnblgr]

If under a roof and not in a wet area the walls should prevent water or moisture from getting into the slab in theory. Perimeter drainage to daylight at footing and clean stone backfill. If concerned look at shallow foundation design guidelines by ASCE, IBC I think also has this now. Use high load foam insulation and use clean stone without fines for fill.

 

[HDStructural]

This is for a friend, not a commercial job.

Insulation is a good option. HTURKAK seems think it isn't required though. Do you have any resources for an effective frost depth for interior footings in unheated spaces HTURKAK?

I am not sure about the cost breakdown of adding insulation vs dropping the interior foundation below frost depth. I can give him options that he can price.

The building will remain unheated year round.

 

[Alistair_Heaton]

Well Scandinavian and Baltic countries it's pretty much standard.

It's definitely cheaper than extending all the way down to frost line.

I did a "floating slab" down to 500mm 400mm wide outside perimeter with rebar at the bottom hooked up to mesh in the 100mm thick slab 1/3 up from its base. 100mm foam under the lot with moisture barrier on top then the concrete.

 

[Alistair_Heaton]

This is my building. Clay soil no bedrock.

 

[HTURKAK]

Insulation would be necessary at exterior perimeter to avoid cold bridges heat loss . My suggestion is based on frost region experience .
I will suggest you to look ASCE 32 ( Design and construction of frost-protected shallow foundations ).
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[lexpatrie]

I don't quite follow the logic of insulating something that's not going to have any heat. Over a prolonged winter, it feels like it will eventually all reach freezing. If you are excavating for the perimeter, you're going to disturb a fair bit of soil for the perimeter toward the interior of the building as well, some option to try to provide frost resistant soil / sand cushion.

If you wanted to discuss with owner, present options and let them select, either slap the load onto the slab, thicken accordingly, or design a pier and a spread footing. To me frost heave is more of a documented phenomenon on isolated exterior footings (deck footings), and the exterior perimeter of the building. An interior column and footing (for a normal [heated] building) wouldn't usually be likely to heave. It's also possible frost heave is specific to some identifiable soil characteristic and either your fill or the native soil will pass this criteria.

 

[Alistair_Heaton]

I think it more to do with having a flexible pad that the frost heave can work against and then recover afterwards.

To be honest the way these electric vehicles are going having ready for heating is more future proofing.

If they are low charge and get parked up in the cold without charger on, the voltage drops as they cool and then the battery locks out. This needs a trip to the dealer and 24hrs to do a battery cycle test and unlock.

Where I am all foundations get the footings insulated be the building heated or unheated. It gets down to -30 C in the winter. The Polish do the same as well

Oh and the price of wood where I am it worked out significantly cheaper to build mine out of aerated concrete blocks which meant no insulation required on the walls either. If its wood framing you would still need to insulate a bit otherwise you would get condensation on the walls and roof with a wind blowing.

 

[XR250]

I don't quite follow the logic of insulating something that's not going to have any heat. Over a prolonged winter, it feels like it will eventually all reach freezing.

Seems to me if you get your perimeter below the frost line (above freezing), then you are kinda creating a "hot air balloon to trap the above-freezing heat within the perimeter.

 

[lexpatrie]

It's unheated XR250. Plus it's a garage so you have to open the doors to get the vehicle in and out during normal use in winter?

You should get some heat flow from beneath from the earth, but is that enough to keep the interior frost-free? or the soil below the slab? Sounds like a nice ASHRAE research project. Until then, I don't consider passive heat flow from underneath as significant.

 

[EdStainless]

The interior won't be frost free, but the frost depth in the interior of an unheated building is less than at the outside walls.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[XR250]

It's unheated XR250

Yea, I get that. I am talking about sub-grade heat rising from below the frost line.

 

[lexpatrie]

That makes physical sense, I'm just unaware of any published research on the subject to justify it.

I'd be tempted to design this as a freeze situation, similar to early foundation drawings in freeze-thaw areas where you drop the interior footings to frost depth and change to exterior concrete spec for winter construction.

This isn't your standard project, since it's for a friend and all, but that's not really a variable we're supposed to use.

 

[dauwerda]

lexpatrie, there is definitely research and published information, go check out ASCE 32.

 

[CarlB]

As mentioned, ASCE 32 provides methods for insulating below an unheated building to prevent frost penetration. There are no allowances/reductions for interior footings. Where is this energy coming from that may lessen the frost depth? The air temperature inside and out should be essentially the same over the winter. The radiation balance would be different; less heat loss on a clear night, less heat gain on a sunny day, overall no real difference.

Many Eng-tips threads on this. My 2 cents at

https://www.eng-tips.com/viewthread.cfm?qid=492593

 

[lexpatrie]

Isn't that for heated structures? Ah, wait, it does say unheated. Cheers.

 

[Alistair_Heaton]

seems the research on this topic was done by Norwegians in the 1950's

The papers and reports that I can find are behind pay walls or you have to buy them.

And its valid for heated and unheated.

Shallow raft foundation seems to be the term used this side of the pond.

And some mention ASCE 32.2

It was done because 1 meter frost lines are not uncommon in Scandinavia and substantial savings are made not going down that deep with the normal depth being around 400mm ground penetration when done.

They even do the same for garden retaining walls for terraces etc.