Concrete foundation without footings?
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I’m wondering under what conditions footings are not required under a concrete wall? Robert Riversong, at these two links —
http://www.flickr.com/photos/riversong-housewright/4742535309/
http://www.flickr.com/photos/riversong-housewright/4742535287/
— shows concrete foundations without footers. The walls are 10″ thick, rather than 8″ thick, and he shows only two re-bars in the lower section of the wall. Is the use of footers strictly dependent on the weight of the house and the loading characteristics of the soil? It seems much more cost effective to do a single wall pour rather than two separate footer and wall pours.
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Jack,
As long as your foundation design has been approved by an engineer and your local building inspector, you're set to call up the Redi-mix truck. Yes, it's possible to design a foundation wall without a footing, although it may not make economic sense. Talk to a local engineer to see if the soil on your site is suitable for your proposed design to work.
Jack: The point Martin is making is that the footer is nothing magic. I just spreads the load out over a larger area than the foundation wall normally would. If your soil is very solid, you may only need an 8" wide foot, ie, the wall. Some people pour a shallow, frost protected foundation wall 16" wide and about 24" high when they pour the slab; a monolithic pour. Have an engineer guide you or design it. If you diy it, I'd put in 2 pairs of #5 rebar; one in the top, one in the bottom.
Do these figures look correct for a single-story 24'-wide house with 2' eaves?
Roof (light roofing) @ 50psf x 14' = 700#
Attic (limited storage) @ 30psf x 12' = 360#
Main Floor @ 50psf x 6' = 300#
Wall @ 20psf x 8' = 160#
Foundation @ 150#/cu ft x 10"wide x 20"tall = 210#
Total Weight (eave side) = 1730 lbs/running ft, or 2076 lbs/sq ft
According to Charlie Wing's "The Visual Handbook of Building and Remodeling" virtually all soils which have sand or gravel in them and which have very little clay or silt or peat, will have an allowable bearing load greater than 4000 lbs/sq ft, and even most of the fine clayey / silty soils have a allowable bearing value of 2000 lbs/sq ft. Only organic silts and clays, medium and highly plastic clays, and highly organic soils are problematic for a no-footer foundation.
So it appears to me that most small single-story houses houses don't need a footing, and may not even need a 10"-wide concrete wall. Can anyone corroborate this?
FWIW, the US Government has a web site which may help one decide what type of soil there is on their building site. Unfortunately, it doesn't seem to be working at this time.
http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm
Jack,
I'm not going to comment on your calculations, but you are not the first person to note that most soils have enough bearing capacity to support a residential foundation without a footing.
The usual response to your observation is, "The main reason to pour footings is to give the foundation contractor a place to erect his forms." And since it's easy to get the footing forms a little bit out-of-square in the bottom of an excavation, wide footings give the foundation contractor some latitude to correct errors when snapping the lines for the wall forms.
Jack: You may be right, but what if you are off a bit? Is it worth the small savings? That, of course, is your call. Have the soils at your exact location been confirmed as "suitable for a 10 inch footer"? What per cent of the cost of the house is the footer? Pretty tiny? You could spend more on counter tops.
The Southern Pine website gives information for making a Permanent Wood Foundation (using pressure treated wood) in a document downloadable at http://www.southernpine.com/applications_permanent-wood-foundations.asp . Table 1 gives soil loading characteristics similar to those I mentioned above. Table 3 shows that a single-story house on a PWF can typically get away with a 2x6 or 2x8 wood footer, while a two-story house typically needs at least a 2x8 or 2x10 footer plate. It seems to me that the primary difference between a concrete foundation house and a PWF house is the weight of the concrete vs the weight of the wood -- maybe a few hundred pounds per lineal foot for a 2' to 4' foundation wall (with no concrete footer). This suggests to me that a short, no-footer concrete wall is quite feasible. Since concrete has high embodied energy, I would think that proponents of green building techniques might welcome any opportunity to use less of the stuff.