Footing size question
I’m planning an ICF guest house. It will rest on a continuous footer 32″ below grade, poured on virgin clay soil. The ICF walls will start right on top of the footer and extend 8′ above ground. Engineered wood trusses will support a metal roof. A concrete slab on grade will be floated between the walls. The envelope is a rectangle with outside dimensions of 29′ x 40′. There are no interior load bearing walls. There is no official snow load info for my area, but I’m using 30psf because that’s what the truss engineer used, and that is probably overkill.
In looking at the 2015 IRC (code in my area), it looks like my situation would fit under Table R403.1(3), and the applicable cell would call for a 15″ x 6″ footer. But the footnotes explain that the “model” house for the calculations is 32′ wide with an interior center bearing wall. It also says that the bearing wall carries “half of the tributary attic, and floor framing.” I’m not sure how to interpret that, but I’m thinking they mean floor framing where the joists rest on the center wall (which I don’t have), and attic framing if it were framed out as habitable space? I’m assuming they do not mean the roof trusses, or they would say that.
All of this wondering is because I want to know if I can still use the table for my build, even though I don’t have a center bearing wall. Since I don’t have the framing they are assigning to that wall, it seems like in both the model house and my house, the exterior walls carry the load of the exterior walls themselves and the roof, i.e., they’re both loaded the same. To bolster my case, in the diagram beneath the table of the slab on grade house, there is no center footer.
Yes, I know that an engineer can tell me how big my footers should be, and I will have an engineer look at my plans, but it always makes me happy to be able to figure something out myself first, and it makes me happy to learn how things work.
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Replies
Zoe, that table assumes a center wall, which means that only about 1/4 of the roof and ceiling load are reaching the footing. For your design, 1/2 of the load will reach the footing. The footnote explicitly forbids extrapolation, so without a prescriptive solution you will need to engage an engineer. With such a simple design it shouldn't be too expensive to hire an engineer. You can ask them to focus only on this aspect of your design, but I'd recommend a full review if possible.
What your engineer will do is add the live and dead loads of your roof and ceiling and the dead load of your walls. (If you're in a high wind or seismic zone there will be additional calculations.) You can find values for those in Chapter 3 and 6. They divide that load by the bearing capacity of the soil to find a minimum bearing width, then use the formulas in Chapter 4 to determine the footing height.
Thank you, Michael! I had thought about the scenario you described as well, but if the center footing, which would be sized the same as the others, can carry the 1/2 load of the model house, that's what I'm asking my footers to bear as well...1/2 the load of the house. I will be engaging an engineer, but it just seemed to me that the math of it may still work with the IRC table.
Zoe, part of the load on the perimeter footings is from the roof and ceiling, but part is from the wall assembly itself, so it's not a linear adjustment. Also, although the table allows interpolation--adjusting values down if your building is smaller--it expressly forbids extrapolation, or adjusting for a building wider than their example (or without a center-bearing for the roof). I'm going by your original post where you said there would be no interior bearing walls, which means the full roof and ceiling load lands on the exterior walls.
In reality the differences in loading in your case compared to their model home will probably not make much difference in the size of the footing, and your code official may not require an engineer's stamp, but according to the IRC they are supposed to require it. Especially over clay soil, which varies a lot in its bearing capacity.
Thanks for all the input, Michael! I appreciate it!