If I remember correctly, the idea of a footing is to spread the load of a wall over a wider base. Typically, though, local codes take national codes to amend for local conditions, like frost lines, soil types, etc. But what I cannot remember is if a frost like is, say, 36″ does this mean the top of or the bottom of said footing is at 36″ (in this example)?
Also, is there a national standard for footing width/height for a given loading? This is all just my Wednesday morning “don’t think about work” curiosity.
Replies
Bottom of footing must be below the frost line.
The IRC will not allow less than a 12" wide footing. I'm sure an engineer could spec much less with the right soil conditions but 12" is the smalles typical.
You typicall don't see a 12" footing anyway, bucket width, room to lay block/set forms, etc. all conspire to have 16-24" footings.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
God, a 12" thick footing would scare me. I am seeing pretty much 24" thick footings here around Gwinnett county, GA, but I have no idea how deep those footings are nor the frost-line depth.
Nuke,When I was talking about 12" I was referring to the width. 8" is pretty standard for the thickness.24" is probably a function of using monolitich slab/footings rather than pouring the footer and then the stemwall. 24" is VERY thick for residential work, at least as far as I have seen.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
the purpose of the minimum 12 inch wide footing is a CMU is 8 inches wide and code say 1 1/2 time the width of block. and that would reduce soil bearing by 25 %.
For years it seems that footing width ended up being a matter of local custom as much as anything when it comes to residential construction. Wherever you are in the country, you tend to find a local standard based upon experience. If it always worked before and nothing has changed, why mess with success?
But as home designs become more complicated and we are now building on more challenging sites, it's worth understanding the mechanics of foundations.
What are actually the determining factors of footing width are the weight of the building, the bearing competence of the soil, and the lateral load on the wall (an engineer will look at the result of this as "moment."
So if you have weak soil, you might put in a wider footing, or you might overexcavate the soil and replace it to increase the bearing competence to support a narrower footing.
As for lateral load, an engineer will calculate the equivalent hydraulic pressure on the wall and specify a footing width for a given wall height. I have built 5' wide footings for an 11' high wall even though the soil was good because the engineer was concerned about tipping the wall.
On the other hand, I've seen plenty of 2' wide (and wider) footings just because the architect was covering his butt and was not paying for the concrete.