I know none of us has ever experienced this condition, but we all know of someone who has had the hurling issue, often after a period of personal discussion with some of the friends of George Thorogood.
In this instance, I’m not thinking either of the example above, or the tasty oatmeal raisin cookies my grandma made for us when we were kids. I am making specific reference to the pre-cast chunks of concrete usually four to six inches in thickness and 12 to 18 inches in diameter which are sold or provided for footings in pole buildings.
The basic concept is to throw concrete cookies in the bottom of the augered holes and place the building columns directly upon them. The general idea is for the cookies to support the weight of the building, to prevent settling.
My recommendation – RUN, DO NOT WALK, away from this as a design solution.
They are a failure looking for a place to happen.
Let’s look at what a footing is supposed to do. The dead weight of the building PLUS all imposed live loads must be distributed to the soils beneath the building. Sounds pretty simple, eh?
To begin with, the International Building Codes require concrete footings to be a minimum of six inches in thickness. This eliminates immediately any concrete cookies which are less than this thickness (most of them).
Examine a fairly small example – a 30’ wide building with columns spaced every eight feet. The actual weight of the building (dead load) will vary greatly depending upon the materials used. Steel roofing and siding will be lighter than shingles and wood sidings. For the sake of this example, we will use a fairly light 10 psf (pounds per square foot) building weight. The Code specifies a minimum roof live load of 20 psf. This means each footing must carry the weight of one-half of the width (15 feet) times the column spacing (8 feet) times 30 psf. Doing the math, 3600 pounds.
In many parts of the country soil bearing pressures are as little as 1500 or even 1000 psf. Basically – the easier it is to dig, the lower the capacity of the soil to support a vertical load.
For every foot of depth below grade, the soil capacity is increased by 20%. Other than with 1000 psf soils, for every foot of width over one foot, the capacity also gets a 20% increase.
With 1500 psf soil, and the bottom of the footing four feet below grade, a 12 inch footing will support 2700 pounds per square foot.
A 12 inch diameter footing covers 0.785 square feet, a 16 inch 1.4, 18 inch 1.77, 24 inch 3.14.
The 16 inch footing would support exactly the 3600 pounds from the example above. However – lots of places in the country have snow loads (which the footings must support) and many buildings are wider than 30 feet, or have columns placed over eight feet apart.
Trying a 40 foot span, with a 40 psf roof snow load, same eight foot column spacing, would mean resisting an 8000 pound load! With 1500 psf soils, even a two foot diameter footing would be inadequate.
In most cases, the use of concrete cookies as footing pads proves to be both inadequate and a waste of good money. To insure a building won’t settle, (from inadequate footings), look for a plan produced by a registered design professional who is proficient in post frame building design. He/She will have the history and training to design your building to withstand the loads…which begins with the foundation.