Concrete walls most often require a foundation to help distribute their load over a larger surface and to make them more stable. Foundations also minimise the effects of frost on the wall. Many municipalities and building code authorities specify the foundation requirements for concrete walls to limit property damage from poorly built walls and to take into account seismic events.
You have to consider a number of variables to determine the depth of a footer for concrete wall foundations. If the location gets frost that goes into the ground, you have to know how far that frost extends. The footer must be set below that level, or be insulated to prevent heaving as the ground expands and contracts. Where there is no frost, the footer depth will be decided by the use of the wall and the soil type.
If the wall will serve as a retainer that holds back soil, then the footer depth will depend upon how much soil it will hold. If the wall will retain 4 feet or less of soil, then the footer placed to the required frost depth will usually be adequate. In frostless zones the base of the footer may range from 4 inches below ground level for a foot-high wall to 16 inches below ground level for a 4-foot wall. When a wall is used for a building, or a portion of a building, then the load of that building will be a factor.
Soil type is another major consideration when deciding the depth of the footer. Soils with a high water table will require a deeper footer, or a footer with drainage installed next to it. Soils with a high clay content may require footers dug to the depth of bedrock.
The loads that the wall footer will carry determine the thickness of the footer. Walls that are primarily used for privacy, and that do not retain soil, will require a footer thickness that takes into account its own weight and the type of soil it bears on. When a wall holds back soil, the footer thickness will increase with the height of the wall.
The footer width varies with the wall's use. For walls that will be loaded from the top, like those supporting buildings or portions of buildings, the wall will be poured to the centre of the footer and the width will most likely be specified by local building codes and the design loads. Retaining walls will often use footers extended beneath the soil that will be retained with the wall being poured to the outside edge of the footer. The footer width in these cases may be five to eight times the width of the wall.
Footer reinforcement is usually rebar or wire mesh. It is determined by the size of the footer and the loads. A footer that is 12 inches wide and 10 inches thick would typically use two to three rebar 5 inches above the bottom of the footer, and running its length.
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