House in a pole building shell, foundation to wall detail
Hi,
I’m in northeast Missouri, zone 5, and my day job is drawing plans for some west coast architects, and the occasional house plan locally. Two very different markets!
The local folks are almost always outside city limits, and building to a budget, and rarely interested in green for green’s sake, unless it can be shown to save money/pay for itself within a pretty near horizon.
I’m beginning to put together a plan that is a little different for me. The shell will be a 80’x120′ pole structure with 18′ side walls, and 30′ of one of the ends of the building will be their residence (so 30×80, with 80′ being the gable end, and facing W), and the remaining 80×90 will be a shop for their seed business and farm equipment.
Since pole buildings are generally fully supported through their poles, and the drilled footings that support those poles, I’m at the least interested in reducing the concrete that would go into a conventional frost footing (typically 48″ deep here). If the main building loads go into the poles, the lengths of walls between poles don’t need any significant foundation support, so I’m wanting to do a frost-protected shallow foundation/thickened slab between the drilled pole footings. That’s the detail I’ve begun with here.
I think I can get them to go for it if it is easily buildable, saves them money in concrete and excavation, offers comfort advantages (they’re talking in-floor heat, and I’d like this insulated slab to meet that need instead), and of course integrates to the pole building’s structure.
Bonus: I think the 2-pour version of this detail would lend itself well to a concrete free slab, but that may be a bridge too far for them…
Thanks for any feedback- I’ve enjoyed learning a lot through this site recently, and finally had a project with a relevant question to ask myself!
-Chad
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The poles through the insulation certainly violates the integrity of the frost protective foundation but it would probably work. You've looked into required fire separation between house and storage occupancy? Since house doesn't need 18' height I think it would be simpler to conventionally build the residence part. They're not planning to deck over the residence and store stuff up there, are they?
Chad, on quick review, that looks like a good detail to me. I've designed similar for a timber-framed structure, attached. On my project the structural posts sat on top of the perimeter foundation wall, which we did as a separate pour for a couple of reasons--one, the owner/builder formed his own frost walls and reused the materials for the framing, and because when you get into taller mono-pours you increase the risk of the top surface plane being lower at the perimeter as the concrete shrinks during the curing process.
You would have some thermal bridging it you have helical piers penetrating the floor system; why not just set the posts on a frost protected shallow foundation?
One item. With floor heat you might need a bit more insulation than shown.
My local winter time soil tempearture is around 50F, so 2" of insulation for slab at 80F, you would loose around 9000btu to the soil. Your ground temperature might be different so losses could different but closer to 3" of foam is better.
Generally for well sealed new construction, you don't need a fully heated slab. The losses in the house are generally low enough that somewhere between 1/2 to 1/3 of the slab needs to be heated. Best to locate these heated areas in high traffic places and add in extra insulation underneath. This would save a fair bit on plumbing and make the house more comfortable as the floor will be "warm" instead of "not cold" with a fully heated slab.
Thanks all.
Joel, the house will occupy the full height of the end of the building. They want scissor trusses there, with a tall living room and also a few bedrooms tucked into a partial second floor. I'll specify the code required fire separation, but I doubt it will be fully adhered to -the "shouse" in these parts is a pretty common building, and is obviously done for people not quite so interested in things like beautiful architecture, or perhaps even codes.
I'm pushing for metal buckets for the posts (from Midwest Perma Column...), rather than burying the posts in concrete. But their footings will most likely be deeper than the shallow one I've shown for the area between posts. The concrete footings will indeed break the lower insulation envelope, but not the exterior insulation. I had thought of this from the perspective of keeping everything stable by either reaching frost depth with the post footings, or protecting against frost with the slab edge between the posts. You've given me something to think about!
Michael, as in my response to Joel, this detail was for the areas between posts. Since each post is supporting an end of a 80' truss, it's going to need more than a typical residential 2-story perimeter foundation. But functionally, yes, I intend to set the posts on top of the foundation rather than embed them.
I've always been leery of installing anchors through a cold joint- I've been told by some engineers not to do it, but that's also for the codes in CA. Using Titens rather than wet-set J-bolts might be ok here.
What purpose does a perimeter drain serve on a shallow footing? I've not seen that done on typical slab-on-grade before.
Akos, I'm trying to talk them out of floor heat, and provide a reasonably well insulated, warmish floor instead. They're outdoors people, and the durability of a concrete floor against mud and grit on boots is something they want. If there's a finished, durable floor that could compete with concrete for cost and be installed on just foam and plywood, I'd be all for it --but I don't know of any?
Thanks again all - I may have more issues as I try to think through the 2nd floor framing inside the shell!
-Chad
Chad, a licensed engineer reviewed and slightly modified my proposed design, with the result being the drawing I posted, including the Titen anchors at the framed wall and Timberlinx connectors at the timber posts. We are in a seismic "B" zone (not very active); working in California is completely different. I had designed a rebar hook to connect the grade beam and the slab, which I've done on other projects, but this engineer thought the hook could lead to cracking. They were not concerned with mechanically connecting the slab and footing, other than the code-required 7" embedment for the anchor bolt.
Water is the enemy of any foundation so I would never pour a footing without a perimeter drain. (Or at least I wouldn't do it again...) We get over 40 inches of rain a year, much of it in relatively large events, and with the frost cycle here in Maine shallow foundations are particularly vulnerable--saturated soil can freeze (and expand); dry soil cannot. The interior "drain" is a for passive radon mitigation, also an issue here.
I should probably add a disclaimer--the drawing I posted was for a specific project; the details may not follow the prescriptive code and should not be used by others without review by a licensed engineer.