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Andrew: what alan and matt are telling you is that there is no “formula” for your situation, it is completely dependent on your existing foundation and soil conditions. Without a soils report, a competent PE will make conservative assumptions on allowable bearing capacity and strength and condition of existing footer and foundation walls. Find a good PE, bring as much information as possible regarding footing size, depth, wall cross sections, plans, etc. A good engineer can tell you if it will work for a reasonable fee ($200 min would be my guess)
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Andrew
Ditto to what Fred and the other guys said. Foundation sizes are all but irrelevent without a soils engineer. Depending on your area, how old your house is (they were a lot more forgiving about the load bearing capacity of soil at one time, I would guess)and the efficiency of your engineer, there is nothing saying the soil your house is in now is up to standards.
To do a self-test and presuming you can access the soil the footings are sitting on, take a sharpened rod of eight or even ten gauge and push it into the soil. If at best, you can only push it in a few inches, things look pretty good. Five to six and beyond is getting dicey. This is only to give you an idea of where you stand, if you proceed, naturally you'll still need the engineer.
Good luck.
cybergreg
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Andrew,
The penetrometer test CG mentions only works in undisturbed soil and the diameter and bluntness of the rod are critical as is the weight applied to penetrate to specified depths.
Contact your county office of soil conservation. If you can't find then, contact your county agricultural extension service. Every state has these things.
They can get you a coppy of your county's soil survey. It's a free book that includes many, many arial maps with soil types overlayed on the maps. These will tell you what type of soil you have on your property. They are so detailed that I have three different soil types on my acre and a half.
The book also includes a couple of chapters on soil mechanics and tells the bearing capacities of the different types.
The surveys are free, provided by the government.
however, hurry to get one, the last survey was done in the 70's and many places don't have any surveys left. The information will probably still be accurate in a million years but there won't be any books.
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Example: 2 story residence
Footing design load: 1,690 # / '; max. allowable soil pressure: 2000 # / s.f. minus 125# / ' weight of footing = 1875 # / s.f. Required w = 1690 / 1875 = .90 say 1 foot. So 16" or 18" in most cases with light loads is adequate.
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Can anyone tell me the formula by which a foundation cross sectional area is determined (including the footer). I have a one story wood framed house and am trying to determine if the existing foundation will support a second story.
*Hi Andrew.The following information is taken from the Internation One and Two Family Dwelling Code 1998 - Building Officials and Code Administrators International, Inc (BOCA), 4051 West Flossmoor Road, Country Club Hills, Illinois 60478-5795 (708) 799-2300:SECTION 403 FOOTINGS:403.1.1- Minimum size. Minimum sizes for concrete and masonry footings shall be in accordance with Table 403.1.1 and Figure 403.1.1(1). The footing width, W, shall be based on the load-bearing value of the soil in accordance with Table 401.4.1. Spread footings shall be a minimum of 6 inches (152mm) thick. Footing projections, P, shall be a minimum of 2 inches (51mm) and shall not exceed the thickness of the footing. The size of footings supporting piers and columns shall be based on the tributary load and allowable soil pressure in accordance with Table 502.3.4(2). Footings for wood foundations shall be in accordance with the details set forth in Section 403.2, Figure 403.1.1(2) and Figure 403.1.1(3).Table 401.4.1- Presumptive Load-Bearing Values of Foundation Materials Class of Material:Clay, sandy clay, silty clay, and clayey silt...2,000 psf. Class of Materials: Sand, silty sand, clayey sand, silty gravel and clayey gravel...3,000 psf.Table 403.1.1 - Minimum Width of Concrete or Masonry Footings Conventional wood frame construction: Load-Bearing value of soil (psf) 2,000 2-story 15" min. width Load-Bearing value ............ 3,000 2-story 10" min. widthAll this information is found in Chapter 4 - Foundations.Hope this helps.
*In NC, we build by the Cabo building code. Strip footers are min 16" wide and 6" thick for 1 story and, min 16" wide by 8" thick for 2 story. This is assuming a soil that supports at least 2000 PSF which is about what clay gives you. Generally footers are dug 18" wide as that is a common width for a backhoe bucket. Also the footer must project beyond the foundation by min 2". Pier footer sizes (and pier sizes) are dependent on tributary area. I'd have to look those up, although this is normally determined by an engineer - which, BTW - if you proceed with this you really do need a PE. As far as the foundation cross section thickness, as with piers sizes, it is partially determined by the height of the foundation, but generally 8" is standard for 1 & 2 story houses. In the case of a brick veneer, foundation walls are sometimes built thicker to provide adequate bearing surface for the floor system.
*Andrew: what alan and matt are telling you is that there is no "formula" for your situation, it is completely dependent on your existing foundation and soil conditions. Without a soils report, a competent PE will make conservative assumptions on allowable bearing capacity and strength and condition of existing footer and foundation walls. Find a good PE, bring as much information as possible regarding footing size, depth, wall cross sections, plans, etc. A good engineer can tell you if it will work for a reasonable fee ($200 min would be my guess)