Footings are the primary structural elements that transmit the load of the entire superstructure to the ground (underlying soil below the structure). Footings are designed to transmit these loads to the soil without exceeding its safe bearing capacity. Therefore, laying out the foundation requires  learning about the property, soil conditions, temperatures and amounts of precipitation. A footing is placed below the frost line and then the walls are added on top, the footing is wider than the wall which will provide extra support at the base of the foundation.

Soil conditions impact your choices around using concrete piers, poured-concrete footings, and slab foundations for spreading the weight of a structure. Footings are an important part of foundation construction. They are typically made of concrete with rebar reinforcement that has been poured into an excavated trench. The purpose of footings is to support the foundation and prevent settling. Footings are especially important in areas with troublesome soils.
 

By far, the most common footing in residential construction is a continuous concrete spread footing, however, conditions give rise to some engineering concerns that need to be addressed, these conditions include:  high-wind conditions; inland or coastal flooding conditions; high-hazard seismic conditions; and poor soil conditions.

 

Simple Concrete Footing

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Building codes for residential construction contain tables that prescribe minimum footing widths for plain concrete footings. Alternatively, the footing widths may be determined based on a site’s particular loading condition and presumptive soil-bearing capacity.

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The following are general rules of thumb for determining the thickness of plain concrete footings for residential structures, once the required bearing width is calculated:

 

• The minimum footing thickness should not be less than the distance the footing extends outward from the edge of the foundation wall, or 6 inches, whichever is greater.

• The footing width should project a minimum of 2 inches from both faces of the wall (to allow for a minimum construction tolerance), but not greater than the footing thickness.

• It should also be understood that footing widths generally follow the width increments of standard excavation equipment (a backhoe bucket size of 12, 16 or 24 inches).

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NOTE: Some designers and builders may specify one or two longitudinal No. 4 bars for wall footings, steel reinforcement is not required for residential-scale structures in typical soil conditions. For situations where the rules of thumb or prescriptive code tables do not apply or where a more economical solution is possible, a more detailed footing analysis may be considered.

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Concrete Footing Design

 

For the vast majority of residential footing designs, it quickly becomes evident that conventional residential footing requirements found in residential building codes are adequate, if not conservative. However, to improve performance and economy or to address peculiar conditions, a footing may need to be specially designed.

 

A footing is designed to resist the upward-acting pressure created by the soil beneath the footing; that pressure tends to make the footing bend upward at its edges. According to the American Concrete Institute ACI-318, the three modes of failure considered in reinforced concrete footing design are one-way shear, two-way shear, and flexure. Bearing (crushing) is also a possible failure mode, but is rarely applicable to residential loading conditions. To simplify calculations for the three failure modes, the following discussion explains the relation of the failure modes to the design of plain and reinforced concrete footings. The designer should refer to ACI-318 for additional commentary and guidance. The design equations used later in this section are based on ACI-318 and principles of engineering mechanics as described below. Moreover, the approach is based on the assumption of uniform soil-bearing pressure on the bottom of the footing; therefore, walls and columns should be supported as close as possible to the center of the footings.

 

Reinforced Concrete Footing Design

 

For infrequent situations in residential construction where a plain concrete footing may not be practical, or where it is more economical to reduce the footing thickness, steel reinforcement may be considered. A reinforced concrete footing is designed similar to a plain concrete footing; however, the concrete depth to the reinforcing bar is used to check shear instead of the entire footing thickness. In addition, the moment capacity is determined differently due to the presence of the reinforcement, which resists the tension stresses induced by the bending moment. Finally, a higher resistance factor is used to reflect the more consistent bending strength of reinforced concrete relative to unreinforced concrete.

 

As specified by the American Concrete Institute ACI-318, a minimum of 3 inches of concrete cover over steel reinforcement is required when concrete is in contact with soil. In addition, ACI-318 does not permit a depth less than 6 inches for reinforced footings supported by soil. These limits may be relaxed by the designer, provided that adequate capacity is demonstrated in the strength analysis; however, a reinforced footing thickness of significantly less than 6 inches may be considered impractical even though it may calculate acceptably. One exception may be found where a nominal 4-inch-thick slab is reinforced to serve as an integral footing for an interior load-bearing wall (that is not intended to transmit uplift forces from a shear wall overturning restraint anchorage in high-hazard wind or seismic regions). Further, the concrete cover should not be less than 2 inches for residential applications, although this recommendation may be somewhat conservative for interior footings that are generally less exposed to ground moisture and other corrosive agents.

 

Minimum Reinforcement

 

Owing to concerns with shrinkage and temperature cracking, American Concrete Institute ACI-318 requires a minimum amount of steel reinforcement. The following equations determine minimum reinforcement, although many plain concrete residential footings have performed successfully and are commonly used. Thus, the ACI minimums may be considered arbitrary, and the designer may use discretion in applying the ACI minimums in residential footing design. The minimums certainly should not be considered a strict “pass/fail” criterion. Designers often specify one or two longitudinal No. 4 bars for wall footings as nominal reinforcement in the case of questionable soils, or when required to maintain continuity of stepped footings on sloped sites, or under conditions resulting in a changed footing depth. However, for most residential foundations, the primary resistance against differential settlement is provided by the deep beam action of the foundation wall; footing reinforcement may provide limited benefit. In such cases, the footing simply acts as a platform for the wall construction and distributes loads to a larger soil-bearing area.

 

Lap Splices

 

Where reinforcement cannot be installed in one length to meet reinforcement requirements (as in continuous wall footings), reinforcement bars must be lapped to develop the bars’ full tensile capacity across the splice. In accordance with American Concrete Institute ACI-318, a minimum lap length of 40 times the diameter of the reinforcement bar is required for splices in the reinforcement. In addition, the separation between spliced or lapped bars is not to exceed eight times the diameter of the reinforcement bar, or 6 inches, whichever is less.