Section 13.2.7/Minimum Footing Depth
ACI 318-19 Section 13.2.7 establishes minimum depth requirements for footings based on support condition and the need to develop reinforcement and resist shear.
The overall depth of footings must be sufficient to develop the required reinforcement and to resist both one-way and two-way (punching) shear without shear reinforcement if practical. For footings supported on soil, the depth of concrete above the bottom reinforcement must be at least 6 inches. For footings on driven piles, the depth above the bottom reinforcement must be at least 12 inches. The effective depth d used in shear and flexure calculations equals the overall depth minus the concrete cover (3 inches for concrete cast against earth per Section 20.6.1) minus one-half the bar diameter. The footing depth is typically governed by two-way (punching) shear at the column perimeter rather than by flexure, especially for square footings with concentrated column loads.
Why this section exists
Footings transfer column and wall loads to the soil. An undersized footing depth creates two problems: insufficient effective depth for shear resistance (risking a brittle punching failure at the column), and insufficient concrete below the reinforcement for proper bond and anchorage. The 6-inch minimum for soil-supported footings ensures enough concrete mass for durability and shear transfer. The 12-inch minimum for pile-supported footings accounts for the concentrated uplift and lateral forces at pile heads and the need to develop pile reinforcement into the cap.
What plan reviewers look for
Plan reviewers check the foundation plan for footing dimensions and verify that the specified depth satisfies the minimum requirements. They check the structural calculations for both one-way shear (at d from the column face) and two-way shear (at d/2 from the column face) to confirm the footing depth is adequate without shear reinforcement. They verify the concrete cover is 3 inches for footings cast against earth. They check that the reinforcement development length can be achieved within the footing depth.