Frost depth for interior footings within basement parking

[Pelotoner2]

We have a project (3 stories over parking) in the northern mid-west with a drive-in parking garage on the ground floor, surrounded on 3 sides by retaining walls. The grade slopes up to the first floor with a steep slope. The garage is un-heated and un-conditioned. We placed all of the interior foundations such that the bottom of the footings is a minimum of the frost depth below the slab on grade, which is sloping.

The GC is claiming they "have not seen" the interior footings dropped to meet frost; we need to affirm our stance. One item to consider, the building is a long-sided rectangle, with the garage entrance on a short side. Are there any methods to calculate a reduced temperature or frost depth in an enclosed garage, although un-heated?
Does anyone agree with the GC?

 

[jayrod12]

If it truly is open to being as cold as the outdoor air in there, I'd agree with your assessment. However if the parking area is generally closed off to the outside, but is unheated, I could potentially play ball with the contractor. I have an attached garage on my house that is un-insulated and unheated except for the heat loss through the wall from the main house. It is consistently 10-15 degrees Celsius higher in the garage in the winter than the outside air. That included when it was 40 below (which is the same for Fahrenheit and Celsius). So if this basement is mostly buried, and generally closed off from the exterior, I wouldn't think that frost depth is as much of a concern.

A good person to ask would be the project geotechnical engineer. Likely not the first time they've come across it.

 

[CarlB]

I don't know of any rationale that explains why frost depth would be less on the interior of a structure that has essentially same temperature as the outside. In some cases it might be greater, as it doesn't have insulating effects of snow cover. The risk of frost heave may be less, as typically surface water infiltration cannot reach interior footing like a perimeter or exposed footing.
Have you considered insulating the footing? Frost penetration calculations show that 1" of insulation (moisture-resistant polystyrene)is about equivalent to 2 feet of soil cover, in regards to reducing frost penetration. So you could raise the footing 2' to 4', place 2" horizontal insulation over top of footing, and extend beyond footing 2' to 6' (or along side of footing). The specifics depend on frost depth/freezing index.

 

[le99]

Remember that frost-heave needs 3 ingredients to occur - freezing temperature, water, and capillary path. We don't usually have control over the first two ingredients, but we surely can do something to cut off the third.

 

[BigH]

le99: I gather by capillary path you mean "low" capillary rise? - clean sands and sands and gravel and highly plastic clays offer little frost heave potential - the former due to low ability of water to rise and the clay because it is such low permeability (if uniform and not layered like varved clay)

 

[dik]

In some jurisdictions, if you have approx 25% open 'wall' you can use natural ventilation and don't require mechanical ventilation; this would ensure freezing conditions within. If enclosed and unheated, with an access OH door, then I would not use frost depth. The air within will not likely be freezing. If it is, it will be greatly moderated and 'geothermal' heat below the SOG will prevent freezing. The only exception would be really sub-arctic conditions.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

If enclosed, it is likely that temperatures within the space will be freezing unless you have really cold outside temperatures. The insulation on the U/S of the structure will allow some heat from the structure to 'warm' the space.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TigerGuy]

I would also expect the frost to last longer in this situation, due to a lack of direct sunshine on the slab.

Did the GC forget to read the plans, specs, and geotechnical report before preparing his/her bid? I'm so tired of hearing, "That's not how I bid it." and responding, "Did you read the report? Did you call with questions on the report to make sure you had things covered?"

 

[dik]

I've been involved with several projects where the geotech report was not part of the tender docs... also a couple where the client has decided on what parts of the geotech report are important...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[le99]

".....a drive-in parking garage on the ground floor, surrounded on 3 sides by retaining walls. The grade slopes up to the first floor with a steep slope.

This is essentially similar to the typical underground parking garages with one side open to the environment and interior non-heated. What is the frost depth then?