CFA's Tech Talk
By: Ed Sauter
Do you ever notice how closely wisdom for life is patterned after building wisdom? For instance, how often did you hear the phrase, "Keep your feet firmly planted on the ground," as you were growing up? This piece of wisdom not only applies as a life lesson, but also as a fundamental component of residential concrete construction.
Footings are located below nearly every residential foundation and serve two distinct functions. Footings distribute the building loads to the ground and they provide a platform for constructing the foundation wall. As such a crucial component of a residence, special consideration must be given to footings during the construction process to ensure a solid base for the home.
Typically, homeowners are only made aware of footings if there is a problem with the foundation wall, yet proper information about the role of footings is essential to understanding the basics of residential construction. A footing helps to distribute the weight of the home to prevent settling in the future. By distributing the loads over an expanded area, the footing reduces the risk of settlement that could, in severe cases, cause cracks in the foundation, frame walls and ceilings.
The first step to ensure proper footing performance is an evaluation of soil conditions.
For typical soils, a standard footing can withstand the weight of an ordinary home. However, soft zones or clay soil under a traditional footing can lead to above average settlement. In some areas, soil testing is required and it may also be a necessity when the residence is to be constructed on fill. At least two borings should be taken from the site--on opposite ends of the structure--and the depth should be 5 feet below the anticipated depth of the bottom of the footing to identify any problems that might be just below the excavation.
The test identifies the type of soil encountered at various depths and the bearing capacity of each soil type. This information will reveal any potential problems that could affect the construction process. As such, the test locations should be very carefully chosen and more samples ordered if there is any uncertainty of soil bearing capacity or the presence of expansive soils.
Weathering Mother Nature
As with any project, Mother Nature can wreck havoc on footing construction. Footings should not be cast in standing water, on mud, saturated soils, expansive soils or materials that have been washed into the footing excavation.
Standing water in the excavation must be removed by draining or pumping. Further, any soil deposited in the excavation by water or from the sidewalls of the excavation must be removed before casting the footing. This is crucial, because the soils from a wash or cave-in of the sidewall are typically of a different consistency and type than those at the bottom of the excavation. These soils have been loosened or disturbed; thus, they will exhibit different bearing properties than the undisturbed soils on the bottom of the excavation.
Special consideration must also be given to frost-covered or frozen ground. If the footing substrate ground is frozen, the frost must be removed with heat or by excavating the frozen soil. The void can then be replaced with controlled fill or by placing concrete in the additional depth. Footings may, however, be cast on frost if the frost depth does not exceed 2 inches and the footing is covered after pouring, if permitted by a structural engineer. The heat of hydration from the curing concrete is most often sufficient to pull a shallow frost penetration out of the ground. Remember frozen ground at any location or depth is expanded.
Basics of footings
Footing types differ depending on the host of variables that must be considered, including soil conditions; building load on the footing; environmental conditions the foundation will experience (floods, earthquakes, groundwater and tidewaters); as well as the impact around the buildings--heavy land traffic, machinery and dynamic movement. These factors and other considerations determine whether the footing will be a trench, post and beam, narrow, spread or post tensioned design or perhaps even a slab on grade with thickened edges.
Footings of any type described above should never be placed on poor soils or other inappropriate materials. When these conditions are encountered, the material should be completely removed and replaced with controlled fill or concrete.
There are two basics fundamentals necessary for installing a footing--location and shape. The location is decided by the load that is to be distributed. The shape depends on the design engineer's analysis or variables considered. To form the shape of the footing in southern areas of the country, shallow footings can be installed by trenching or cutting the ground to the desired depth and using the ground as a form. Deeper foundations require extensive excavation with the footing being shaped with the use of side wall forms.
The most common forming material for footings, standard dimensional lumber, can be used several times before it needs to be replaced. If the soil conditions permit, "2-by-4s" or other dimensional wood forms the top of the footing. Then, the ground is excavated from the area between the forms until the proper footing depth is reached. If the soil is not self-supporting, the side forms must be the full height.
Some contractors, however, opt to use steel forms because of their durability and rigidity. Typically 4 inches in depth, steel channels and tubing are the most commonly used materials. Loops or slots for installing steel stakes are cut into or welded to the forms. A variety of manufactured forming systems made of metal, fabric or plastic are also available. Some of these systems are left in place and incorporate the foundation drainage into the form system.
The final option is to place the footings without forms. This is done directly on flat surfaces on rock or high capacity soils when the footing does not need to distribute the load of the structure.
Width and depth
Generally, footings are constructed to extend 2 to 3 inches beyond each side of the foundation wall to provide a sufficient platform for the wall forms. This design also exceeds the normal requirements for load transfer based on the soil bearing capacity. However, if the soil capacity is poor (less than 1500 psf) or loads are high, wider footings may be required that necessitate transverse reinforcing or greater bearing area. Typically, basement footing depths of 8 to 10 inches are adequate and common. In any situation, however, the footing should not be less than 6 inches or one-half the footing width minus the wall thickness, whichever is greater.
Transverse reinforcing may be needed if the footing extension beyond the wall exceeds the depth of the footing. For example, an 8-inch-deep footing with an extension of 10 inches beyond the side of the foundation wall would require transverse reinforcing. A licensed engineer should be consulted to determine the specific requirements for the reinforcing. Longitudinal reinforcing is common to control shrinkage cracking and to increase strength over narrow excavations.
Educating the industry
Jim Baty, technical director for the Concrete Foundations Association, said the footings should not be underestimated.
"It is the base on which the entire structure rests. Our association has recognized for a long time that education in the fundamentals of soil capacity and performance, as well as foundation construction and design, is a necessity. We take pride in having provided this service now for more than 15 years."
The CFA, and other industry trade associations, offer a variety of educational seminars on the basics of foundation construction, which include a detailed analysis of proper footing construction techniques. Dates for these seminars offered by the CFA are available online at www.cfawalls.org.
For more information on the detailed construction recommendations for footings, review the CFA Standard or contact the American Concrete Institute (ACI) for copies of their Guide to Residential Concrete Construction (332R) or Requirements for Residential Concrete Construction, as well as the current International Residential Code (IRC).
Ed Sauter is executive director of the CFA. He can be contacted at 319-895-6940 or at www.cfawalls.org.