I know none of us have experienced this condition, but we all know someone who has had the problem of pitching, often after a period of personal discussion with some of George Thorogood’s friends.

In this case, I’m not thinking of the example above, nor of the tasty oatmeal raisin cookies my grandmother made for us as kids. I am referring specifically to the pre-cast concrete chunks, usually four to six inches thick and 12 to 18 inches in diameter, that are sold or provided for pole building bases.

The basic concept is to drop concrete biscuits into the bottom of the drilled holes and place the building columns directly on top of them. The general idea is that the biscuits support the weight of the building, to prevent them from settling.

My recommendation: RUN, DON’T WALK, far from this as a design solution.

Why?

They are a failure looking for a place to spend.

Let’s see what a foundation is supposed to do. The dead weight of the building PLUS all imposed live loads must be distributed to the soils below the building. Sounds pretty simple, huh?

For starters, International Building Codes require that concrete bases be a minimum of six inches thick. This immediately removes any particular cookie that is less than this thick (most of them).

Consider 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 on the materials used. Steel roofing and siding will be lighter than wood shingles and siding. For the sake of this example, we’ll use a fairly light build weight of 10 psf (pounds per square foot). The Code specifies a minimum roof live load of 20 psf. This means that each footing must support the weight of half the width (15 feet) multiplied by the column spacing (8 feet) multiplied by 30 pounds per square foot. Doing the math, 3600 lbs.

In many parts of the country, soil head pressures are as low as 1,500 or even 1,000 psf. Basically, the easier it is to dig, the less the soil is capable of supporting a vertical load.

For every foot of depth below ground level, the capacity of the ground increases by 20%. Aside from 1000 psf soils, for every foot of width over a foot, capacity also gets a 20% increase.

With 1,500 psf of soil and the bottom of the footing four feet below grade, a 12-inch footing will support 2,700 pounds per square foot.

A 12-inch diameter base covers 0.785 square feet, a 16-inch 1.4, an 18-inch 1.77, a 24-inch 3.14.

The 16-inch base would support exactly the 3,600 pounds in the example above. However, many places across the country have snow loads (which foundations must support) and many buildings are more than 30 feet wide or have columns placed more than eight feet apart.

Testing a 40 foot span, with a roof snow load of 40 psf, the same eight foot column spacing, would mean resisting a load of 8000 pounds! With 1500 psf soils, even a two foot diameter footing would be inadequate.

In most cases, the use of concrete biscuits as foundation pads is inappropriate and a waste of money. To ensure that a building will not settle (from inadequate foundations), seek a plan prepared by a registered design professional who is competent in post frame building design. He/She will have the history and training to design her/his building to bear the loads… which start with the foundation.