Foundation Uplift
I am planning to build a 2 story standard colonial (28 x 36) in lower Michigan using Sips upper walls and a PWF conditioned crawlspace foundation. I am somewhat puzzled by how uplift is prevented in some of the PWF recommendations I see proposed. I will not be back filling the interior walls so it appears there are 2 options. Pour a 3-1/2″ concrete floor( attach to wall via a screed board with embedded fasteners) or build a treated regular joist floor? I can understand the lateral load being counteracted by these 2 methods, but simply having a joist floor seems rather anemic for uplift on a 4′ tall crawlspace. If I do elect to use the concrete floor method would it be practical to install 4″ of rigid foam underneath against a taller screed board, then pour concrete on top of this? I presume a 3rd option may simply use ICF’s or Form_A-Drain as a footer form on top of gravel base, bolt PWF wall to that, then I can use whatever needed for finished floor inside. Gee and I thought designing / planning a retirement home would be a snap.
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Replies
William,
I had always understood foundation uplift only to be a problem when the soils underneath were subject to freezing. Is there another reason so suspect that your house may be subject to it? Expansive soils? This is all new to me.
It is my understanding uplift can also occur due to the force against the side of a building attempting to push it up and or over. Think of the side of a building as a large piston, a small amount of wind creates a large force over the large area. Maybe I'm wrong, but I would think this would have to be considered.
William,
Ah - I think the confusion comes from calling it "uplift", which has a specific structural meaning.
I quite agree that PWFs can pose a problem dealing with horizontal forces. That horizontal loading above grade is typically dealt with by shear-walls, moment connections and other bracing. I assume wind and seismic loading would be dealt with in a very similar way below grade in a PWF. How they resist soil pressure -which I think is what you are asking about - is something I'm sorry to say I don't know enough about to be of any help. Hopefully someone with more experience with PWFs will chime in.
Williwm,
The PWF design specification has a detail for anchoring, against uplift, the PWF walls to a concrete slab. It doesn't require a screed board but 16d ss spikes "half" driven into each stud and surrounded by concrete. Adding sub slab foam shouldn't matter. The design specification text indicates the anchorage is only needed when the weight of the structure is not sufficient to resist the uplift. It is usually easier to have them in the plans than convince the"authorities" that they aren't needed.
Edit Malcolm is "right on" Shear walls, including the PWF walls themselves, deal with horizontal forces from all sources. The continuous nature of the shear due to soil pressure requires that tabulated diaphragm shear resistance be de- rated by 0.56. The floor above the PWF is part of the PWF and resists the soil forces inward pressure.
Edit 2 There is much heat loss out the edges of a slab, i'd suggest foam blocks between studs to serve as your "screed".
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I'm not sure how this uplift anchor detail works:
In most cases, wind-driven uplift is not a real concern. My house has been sitting on dry-laid stone since 1830 with no apparent movement, and I'm on top of a hill with a design wind speed of 110 mph. You can use the Component Cladding Loads portion of chapter 3 in the IRC to determine how much force is actually acting on your house and then use physics to determine whether there is any chance of the house tipping over. If you need to use hold-downs, helical metal piles are one readily available option.