House Foundations & Footings
Footing & House Foundation Systems
Foundations are one of the most important aspects of construction. This is the part of the structure which interacts with the earth. At first blush, this "interaction" may not appear to be significant. However, one must realize that the surface of the earth is covered by many different materials (soils). These materials (soils) have vastly different properties and react quite differently when loads are placed upon them. The problem is further complicated when we disturb these soils and move them from one location to another. This practice is generally referred to as "cutting and filling." Read this article for more information on cutting and filling.
Footing and foundation systems, when properly designed and constructed, allow us to construct buildings which will withstand the powerful forces of nature. Some of these forces are quite impressive. Gravity, soil swelling, frost heaving, hydrostatic pressure, etc. are just a few of the subtle hidden forces that can affect your house or building. These forces are powerful and should not be underestimated. For example, I have witnessed firsthand the ability of a 2' x 2' area of soil to raise 1,200 lbs of steel and concrete 1/2". This occurred after the soil underneath a column footing had been saturated with water after a moderate rainfall. I was quite impressed. You must respect these forces.
Let's start with the basics. Footings are the structural members which transmit the concentrated loads of the structure to the soil. These members come in various shapes and sizes and are generally constructed of steel-reinforced concrete. The footings are generally a minimum of two to three times wider than the width of the foundation wall. The thickness of the footer is a function of the weight of the structure above and the strength of the soil below the footer. A thicker footer (10 -12") will be stronger than a thinner (6 - 8") footer. The footer is usually installed immediately after excavation. The foundation is then constructed on top of the footer. Generally speaking, the footer is constructed independently of the foundation.
Foundations are also structural members. They basically are nothing more than giant beams. They carry loads, similar to structural steel I-beams. Foundations can be constructed from a variety of materials. They can be made from concrete, stone, concrete block, wood, steel, etc. Since they are difficult to replace, it is a wise idea to construct foundations using strong, durable, water resistant materials. The design criteria concerning foundations is very technical. You should consider consulting a structural engineer prior to building a foundation. This person will properly size the foundation and specify the materials from which it should be constructed.
The following books offer a wealth of information concerning footings and foundations. In the event you cannot locate these books, your local library will probably have many similar books on the same topic. Good Luck!
- Residential Foundations
Home Builder Press, 2000
- Simplified Design of Building Foundations, 2nd ed.
John Wiley & Sons, Inc. 1988
- Foundations and Concrete Work
Fine Homebuilding Magazine
Taunton Press, 2002