Attached deck column foundations for a building with a PT SOG foundation.

[MJC6125]

Hi. I have a new building that will utilize a PT SOG foundation. I haven't designed a lot of PT SOG foundations, so this issue is something I haven't come across yet. This building will have a wood framed deck attached to it (ledger along the wall of the building supporting the deck joists). The other end of the deck will be supported by exterior columns that are 10' off the building. I'm wondering what is the best type of foundation to use to support the exterior columns.

Option 1: Extend an exterior patio slab at grade that has PT tendons running through it like an extension of the building PT SOG. This is not what the architect wants to do. In particular it would create a discontinuity in their insulation envelope where the exterior slab would have to be tight to the interior slab/foundation.

Option 2: Use a standard shallow foundation (Pad footing or short drilled pier). This seems like it can be designed. However, I'm worried about how much differential movement this foundation element will have compared to the other end of the deck that is attached to the building founded on the PT SOG.

Has anyone ever dealt with this issue? What type of foundation did you use for the exterior columns? Any pointers you have for this condition?

 

[jayrod12]

What's the sub-grade like in the area? I assume if you're doing PT slab you've got expansive clays but with generally good bearing capacities? How deep does the clay go?

 

[MJC6125]

Based on the swell-consolidation tests in the soils report, I actually don't know if the site soils are very expansive. They are clay however, and they are "soft" according to the soils report. They gave a 2,000 psf bearing capacity. The upper 10 ft of the site appears to be the clay material. below that from 10 to 20 ft is a silty sand that is stiffer. Blow counts were 2 to 5 in the upper 10ft, and 9 to 17 in the 10ft-20ft range.

 

[jayrod12]

If you have a geotech on the project, they would likely be the first place I'd start asking. That being said, if frost is not a concern, I'd be bearing the exterior posts on a foundation that had a similar bearing pressure that will be exerted on the loaded parts of the slab on grade to try and keep and load related settlement/movement in the same range.

 

[MJC6125]

Sounds good, thanks for your help. I will ask the geotech for their opinion.

Have you done any PT SOG foundations in the past for expansive soils where you have had to do separate isolated footings away from the building for decks, patio roofs, etc.? I want to make sure it isn't a totally bad idea before I pursue it.

The reason I'm caught up on it is because when we do deep foundations for sites with expansive soils, all exterior deck posts will also be founded on a deep foundation. We always avoid mixing foundation types because of the differential movement issues. I think it would be similar to building exterior deck footings not to frost when the building foundation is to frost. In my experience, those are things that are always avoided. I want to make sure the PT SOG foundation type isn't the same issue.

 

[jayrod12]

I haven't personally done a PT SOG foundation. I don't know for sure, but I imagine that the PT part of it really doesn't affect performance substantially in terms of vertical settlement/movement potential. Therefore I don't see why the exterior columns couldn't be put on a similar end-bearing foundation.

 

[KootK]

I'd be inclined to go with option #2, following the architect's logic. And yeah, if you can get your hands on a geotechnical recommendation, so much the better.

In this situation, both your PT SOG and your isolated deck foundations will be shallow foundations so you're not mixing and matching in that sense. It's just that your building proper will be supported by a version of a raft foundation that will help iron out differential settlement due to localize moisture changes and your isolated deck foundations will not. A prudent strategy to help deal with that might be to create a connection between the deck and the main building that can accommodate a bit of rotation. You can also isolate a large deck, supporting it entirely on isolated foundations. But then you've got potential for threshold problems etc.