Spread footing

A square or rectangular isolated footing under one column, designed to ACI 318-19 (Chapters 13, 22 and 25) with ASCE 7-22 load combinations. The soil checks use the service (ASD) combinations of ASCE 7-22 2.4.1 against the allowable pressure you enter; the concrete is designed for the factored combinations of 2.3.1.

Inputs of the spread footing #

• Footing: length L (x), width B (y), thickness h, the depth of soil over the footing and the soil and concrete unit weights. The footing and soil weights are dead load.
• Column: a concrete column or pedestal (c1 along L, c2 along B, its f'c for bearing on the column base) or a steel column on a base plate (section depth and flange width, plate N × B). The column sits at the footing centre. A round column is entered as the square of equal area (13.2.7.3).
• Loads at the top of the footing, per load type, unfactored: P (compression positive), Mx (moves the resultant toward +x, along L), My (toward +y), and shears Vx, Vy at the top of the footing (they add V × h at the base).
• Soil: allowable bearing pressure qa, its basis (gross pressure, or net pressure above the original overburden γs Df), the base friction coefficient μ, the allowable passive pressure on the footing face (equivalent fluid, 0 to ignore it), and the factors of safety for sliding, overturning and uplift (1.5 prefilled; with the 0.6D combinations some engineers use 1.0, which is your choice).
• Concrete and reinforcement: f'c, fy, λ, clear cover (3 in cast against earth, Table 20.5.1.3.1), bar size and number in each direction, which layer is at the bottom, straight or hooked bar ends, epoxy coating.
• Code options: the size effect factor λs, which 13.2.6.2 lets you neglect for isolated footings (prefilled), or apply.

Auto-design (editor toolbar) varies the plan L × B (1/2-ft steps, square when the input is square), thickness (2-in. steps) and the bars each way (size and count). Objectives: lowest cost index (concrete, formwork, reinforcement); least concrete, then cost; smallest plan, then cost. Every other input is kept; see Auto-design.

Checks performed #

Limitations #

• The column is at the footing centre; footings with offset columns, sloped or stepped footings and pedestals are not covered (enter a pedestal's loads at the top of the footing yourself).
• Plain concrete footings (Chapter 14) and seismic foundation provisions (18.13) are not included.
• The punching shear moment is the column moment (the soil pressure gradient inside the section is not credited).
• No reduction of ℓd for excess reinforcement (25.4.10) is taken.
• Not checked against published worked examples yet (the CRSI Design Handbook, PCA Notes and ACI SP-17 are not on file); the verification below is by closed-form hand calculation.

Verification examples #

Published worked examples (backend/tests/test_verify_spread_footing.py): the free StructurePoint design examples that reproduce Wight, Reinforced Concrete Mechanics and Design, 7th Ed., Examples 15-2 (square footing) and 15-1 (wall footing, checked as a 10 ft length with the wall as the "column"), both to ACI 318-14. Values that ACI 318-19 changed, or that the example computes by another permitted method, are deviations, explained on the Verification page.

Under ACI 318-19 both example footings need a few inches more thickness for one-way shear: the 318-19 strength of members without stirrups depends on the flexural reinforcement ratio and is much lower for lightly reinforced footings.

Hand calculations in closed form: