Foundations (Footings)
2011-09-16
Foundations, or “footings”, transfer the loads of a structure into the ground and anchor the structure in place. Perhaps the most obvious load that foundations supports is weight due to gravity. But foundations are also the end of the load path for wind, earth, water, and all other loading conditions which may act on a structure. A foundation should be capable of properly supporting the weight of the structure and its contents, in addition to all other loads the structure may be expected to experience in its lifetime.
There are numerous types of foundations used in residential construction. Some of the more common types are continuous foundations, which are also called ‘spread’ or ‘strip’ footings. These are continuous, rectangular-shaped strips of concrete upon which load bearing walls are erected. Isolated footings are square or rectangular shaped pads of concrete placed on the ground often used to support point loads, such as columns or posts. Monolithic footings are called such because they are placed monolithically with a concrete slab on grade. They can be simplistically thought of as “thickened edges” around the perimeter of a slab. Piles and grade beams are sometimes employed where surface soil conditions are poor.
Foundations transfer their loads to the ground by several different mechanisms. Continuous, monolithic, and isolated footings transfer loads primarily through pressure, which is a force applied over an area. The amount of weight supported, including the weight of the foundation itself, is divided by the area of ground covered by the footing. This is the applied pressure which should not exceed the bearing capacity of the soil in order to avoid the potential for significant settlement.
Foundations must also provide uplift and lateral restraint if the structure will be subjected to high winds, seismic activity, buoyancy, retained earth, or water. The terms ‘horizontal’ and ‘lateral’ are used interchangeably here. Uplift and lateral loads are also ultimately transferred to the ground by the foundations. These loads (as opposed to weight over bearing area) may determine the overall size and scope of the foundations.
Piles are another type of foundation which consist of relatively long and slender wood, concrete, or steel members that are vibrated, pounded, pushed or drilled into the ground. Concrete or grouted piles may be cast in a bored hole, with or without steel reinforcement. Piles transfer their loads to the soil by friction along their sides and also through end (bottom) bearing on a stronger soil or rock layer. They may be thought of as ‘underground columns’. The soil surrounding the pile may act as a lateral restraint which prevents pile buckling. In areas of poor surface soil conditions, piles may be used to support grade beams.
Grade beams are concrete beams generally placed at or below grade level, similar to a continuous or monolithic foundation. However, the manner in which they transfer their load to the ground is entirely different. Grade beams usually span between and are supported by piles, similar to a typical elevated building beam spanning between columns. Although they are generally in contact with the soil at their bottom and sides, grade beams do not rely directly upon this contact for their support.
Wood piles are used in numerous residential applications. Many wood piles are embedded in the ground but continuous above the ground. In this manner, they may serve simultaneously as foundations, columns, and or lateral restraint elements. This is often seen in elevated beach homes, docks, boathouses, and seawalls.
As with any structural component or system, problems may arise from improper construction, damage, or deterioration of foundations. Some common problems with foundations and their causes are discussed in Common Problems and Diagnosis.