Bearing and Retaining

1 · Input data

Footing bearing pressure, the eccentricity of the resultant, and the two stability factors of safety, to ACI 318-25 Chapter 13 as far as the code reaches and statics where it does not, with ASCE/SEI 7-22 as the source of the loads. The clauses this card rests on are §13.2.6.1, §R13.2.6.1, §13.3.1.1, §13.2.6.4, §R13.2.6.3 and §13.3.4.3 of ACI 318-25, with §5.3.8 for the load factor on lateral earth pressure H, and ASCE/SEI 7-22 §2.3 and §2.4 for the load combinations the numbers come from. ACI 318-25 supplies no bearing-capacity equation, no presumptive bearing values, no settlement criterion and no sliding or overturning procedure. The permissible bearing pressure below is yours, from the geotechnical report — the page never invents one, and it refuses the run without it. Every parameter is required; press Solve the bearing and stability check when the card is complete. If anything is missing or malformed the page opens a pop-up naming each parameter and computes nothing.

2 · Solution

The standard behind it

ACI 318-25, Chapter 13 — Foundations, with ASCE/SEI 7-22 Chapter 2 for the load combinations and Chapter 3 for the lateral soil load, and ACI's own Chapter 5 for the load factor on it. Each clause below was read in the filed code text; the printed page is given with it.

The clauses this calculator rests on:

• §13.1.1 (printed page 205) — the chapter's scope: shallow foundations, and by name cantilever retaining walls, counterfort and buttressed cantilever retaining walls and basement walls.
• §13.2.6.1 (p. 208) — foundations are proportioned for bearing effects and for stability against overturning and sliding at the soil-foundation interface in accordance with the general building code. This is the clause that makes the two factors of safety on this page the engineer's, not the code's.
• §R13.2.6.1 (p. 208) — permissible soil pressures are determined by principles of soil mechanics and in accordance with the general building code.
• §13.3.1.1 (p. 211–212) — the minimum base area is proportioned to not exceed the permissible bearing pressure, and permissible bearing pressures are determined through principles of soil or rock mechanics.
• §13.2.6.4 (p. 209) — a foundation system may be designed by any procedure satisfying equilibrium and geometric compatibility. The arithmetic on this page is that procedure.
• §R13.2.6.3 (p. 209) — for a single concentrically loaded spread footing the soil pressure is the factored load divided by the base area; for eccentric loading, applied factored loads may be used, and where the resulting pressures are incompatible with the geotechnical design the design is adjusted in coordination with the geotechnical engineer.
• §13.3.4.3 (p. 213) — the distribution of bearing pressure under a combined footing or a mat shall be consistent with properties of the soil or rock and the structure, and with established principles of soil or rock mechanics.
• §5.3.8 (p. 72) — the load factor on lateral earth pressure H: 1.6 where it acts alone or adds to the primary load effect, 0.9 where it is permanent and counteracting, and omitted where it is not permanent but counteracts.
• ASCE/SEI 7-22 §2.3 (printed page 7) and §2.4 (p. 8) — the load combinations for strength design and for allowable stress design: the source of the loads entered, which this page does not build.
• ASCE/SEI 7-22 §3.2.1 (printed page 12) — lateral soil loads. Where they are not given in a geotechnical report approved by the authority having jurisdiction, the values of Table 3.2-1 (printed page 11) are the minimum design lateral soil loads; foundation walls and walls with horizontal movement restricted at the top take at-rest pressure, while a wall free to move and rotate at the top, such as a retaining wall, is permitted to be designed for active pressure.
• ASCE/SEI 7-22 §3.2.2 and §3.3 (p. 12) — uplift loads on floors and foundations, and the alternative method that separates soil pressure from ground-water pressure.

The clauses the stage page listed for the member checks — this calculator does not perform them:

• §13.3.1.2 (p. 212) — overall depth selected so the effective depth of the bottom reinforcement is at least 6 in.
• §13.2.6.2(a) and (b) (p. 208) — for a member continuously supported by soil and designed on the rigid-behaviour assumption, one-way shear may use V_c = 2λ√f′_c b_wd and the two-way size-effect factor is λ_s = 1.0.
• Table 13.2.7.1 with §13.2.7.2 (p. 210) — where the critical section for flexure sits for each support condition, and where the critical section for shear is measured from.
• §13.2.8.1 through §13.2.8.4 (p. 211) — development of reinforcement in shallow foundations, and the critical sections for it.
• §13.2.9.1 (p. 211) — cover for foundation reinforcement.
• §13.3.6.1 and §13.3.6.3 (p. 213) — the stem of a cantilever retaining wall is designed as a one-way slab, with the critical section at the interface between stem and footing.
• §13.3.7.1(a) through (e) (p. 214) — basement walls resisting out-of-plane lateral earth pressure, including V_c = 2λ√f′_c b_wd and λ_s = 1.0.
• §11.1.4 (p. 179) — design of cantilever retaining walls is in accordance with Chapter 13.
• §16.3.1.1 (p. 240) — the transfer of factored forces and moments at the base of a column, wall or pedestal to the foundation.

What the code does not give

This panel is on the page on purpose. A calculator that hides these gaps is more dangerous than one that shows them:

• No bearing-capacity equation and no presumptive bearing values appear anywhere in ACI 318-25. The permissible bearing pressure is a value from the geotechnical report; the code names no standard that supplies it.
• No settlement or differential-settlement criterion.
• No factor of safety against sliding or overturning, and no rule for where the resultant must fall on the base. Both are deferred to the general building code in a single sentence, §13.2.6.1. The two factors of safety this page computes are therefore labelled as a stability check the code does not provide, and the page applies no minimum of its own.
• No pressure-distribution rule for an eccentrically loaded footing. The middle-third check (e ≤ B/6) and the triangular distribution that follows when it is not satisfied are statics, not code provisions: they follow from a linear pressure over a base that cannot take tension. The page prints them as statics and cites no clause for them. The comparison against the permissible bearing pressure, which the page does make, rests on §13.3.1.1.
• No lateral earth-pressure magnitude and no coefficient: the demand side of a wall is ASCE/SEI 7-22 §3.2.1 and the geotechnical report, not ACI.
• Nothing on mechanically stabilised earth, segmental block, soil-nailed, sheet-pile or soldier-pile walls. The page will not pretend to cover them.
• An asymmetry worth knowing: a basement wall gets a dedicated earth-pressure clause (§13.3.7), while a cantilever retaining wall's stem gets only a member-type assignment (§13.3.6). The page will say so rather than dress §13.3.6 up as a wall design procedure.
• No clause that governs a cantilever retaining wall's own footing. §R13.3.6.1.1 says only that §13.3.6.1.1 does not apply to wall footings. That such a footing is designed as a strip footing under §13.1.1(a) is an inference from §13.1.1(a) together with §R13.3.6.1.1 — the code does not print that sentence, so this page does not cite it.