Concrete Footing Techniques

Footings and Foundations

• Houses in areas where the ground freezes are typically constructed on a concrete footing and foundation system. The system is a T-shaped structure, with the foundation wall tied to the footing. The footing, or spread footing, distributes the weight of the structure onto the soil. Footings are placed under all the load-bearing parts of the house, including piers and columns as well as the foundation wall. The most important factor in footing design is the bearing capacity of the soil.

Footings and Soil Bearing Capacity

• The load of the house spreads out within the footing and then spreads out in the soil at an angle of about 60 degrees. The most pressure is directly under the footing. Weak soil can lead to the footing settling, and cause uneven settling of the house, eventually causing damage. Fine clays and silt soils are lowest in bearing capacity; sands and gravels are higher. Weak soils can be addressed by importing granular material to increase bearing capacity.

Footing Design

• The dimensions of the footing can change according to the conditions, but usually the footing is 16 inches square by 8 inches thick and the foundation wall is 8 inches wide. The bottom of the footing should be at least 12 inches below the grade, or below the frost line to avoid pressure from freezing water. Concrete footings are reinforced with steel and the foundation wall is doweled into the footing with the rebar.

More Footing Techniques

• For most conditions, the typical footing design is sufficient for the performance of the footing, but for active soil conditions variations on the footing structure may be recommended. Installation of drainage pipes to keep moisture away from the foundation is required in most building codes. In locations where freeze and thaw activity isn't an issue, a slab-on-grade structure needs only a thickened edge on the slab to serve the purpose of the footing.