ACI 318-19 Footing Design Methodology

The footing must safely transfer the column load to the soil without exceeding its allowable bearing pressure. calcnote computes the factored net soil pressure from the column loads divided by footing area, minus the footing self-weight.

For eccentric or lateral loading, the resultant must land within the kern (middle third of the base) to keep the entire base in compression. Overturning and sliding stability are checked as service-level ratios.

Footing proportioning (sizing) uses service-level loads, while strength design of the footing section uses factored loads. calcnote separates these two stages: the engineer sizes the footing for allowable soil pressure, then the calculator checks the section for factored-load strength.

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The critical section for flexure is at the face of the column (isolated footing) or the face of the wall (wall footing). The factored upward soil pressure on the cantilevered overhang produces the design moment.

Minimum reinforcement per Section 13.3.3.2 referencing Table 24.4.3.2: A_s,min = 0.0018 · b · h for Grade 60 deformed bars, or 0.0020 · b · h for Grade 40/50 bars. The strength-reduction factor is φ = 0.90 (tension-controlled, Table 21.2.2).

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The critical section for one-way shear is at a distance d from the face of the column or wall. The concrete shear strength V_c is computed from Table 22.5.5.1 using the detailed ρ_w form (no shear reinforcement in typical footings).

The strength-reduction factor is φ = 0.75 (Table 21.2.1). The check compares the factored shear V_u at the critical section against φ · V_c.

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Punching shear is evaluated on a closed critical perimeter at d/2 from the face of the column or loaded area (Section 22.6.4.1), where d is the average effective depth of both reinforcement layers (Section 22.6.2.1).

Three v_c equations from Table 22.6.5.2 govern: (a) the general two-way shear equation, (b) the column aspect-ratio (β) equation, and (c) the α_s equation (interior, edge, or corner column). The smallest of the three controls.

This check applies to isolated footings only. Wall footings are exempt because the loaded area extends the full wall width (no closed perimeter forms).

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The bottom reinforcement must be developed within the available length: footing overhang minus cover. The required development length l_d is computed per the governing clause 25.4.2.3 via the equation in 25.4.2.4a, with ψ factors from Table 25.4.2.5.

Minimum l_d is 12 in. per Section 25.4.2.4. When A_s,provided exceeds A_s,required, the excess-reinforcement reduction applies. calcnote reports both the required and available development lengths and flags a warning when the bar cannot be fully developed.

Note: the engine's user-facing warnings cite "25.4.2.4a" (the specific equation); this hub cites both 25.4.2.3 and 25.4.2.4 (governing clause + equation section). Both are correct references at different granularity levels.

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