----------------------- | ------------------ | ----------------- | -------------- | | 50 | 2.0 | 6 × 6 | 14 | | 100 | 2.0 | 8 × 8 | 16 | | 100 | 3.0 | 6 × 6 | 14 | | 200 | 2.0 | 11 × 11 | 20 | | 200 | 3.0 | 9 × 9 | 18 | | 300 | 2.0 | 13 × 13 | 22 | | 300 | 3.0 | 11 × 11 | 20 | | 400 | 3.0 | 12 × 12 | 24 | | 500 | 3.0 | 14 × 14 | 26 | | 500 | 4.0 | 12 × 12 | 24 |

Thickness is governed by punching shear. Values assume f’c = 4,000 psi and 3-inch cover.

Minimum reinforcement and detailing

Common pitfalls and how to avoid confusion

• Self-weight exclusion: When calculating net soil pressure for strength checks, exclude the footing self-weight (it was present before any load was applied). Include self-weight only in the bearing pressure check under service loads.
• Cover at bottom: ACI requires 3 inches of cover on bottom bars cast against earth. Using 1.5 inches (typical for elevated slabs) is non-conservative and risks corrosion.
• Punching perimeter for edge columns: Edge and corner columns have reduced punching perimeters. The full bo = 2×(c+d) formula only applies to interior columns.
• Factored vs service loads: Bearing pressure is checked with service (unfactored) loads. Shear and flexure are checked with factored loads. Mixing these up produces dangerously wrong results.
• Soil bearing is not a structural limit: The structural engineer sizes the footing for soil bearing, then checks concrete strength separately. Both checks must pass independently.

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Worked Example

Problem: Determine the required size of a square isolated footing supporting a W12x65 column with axial dead load of 80 kips and live load of 60 kips. Allowable soil bearing pressure qa = 3.0 ksf. Column base plate is 12 in. x 12 in.

Given:

• PD = 80 kips (dead load)
• PL = 60 kips (live load)
• qa = 3.0 ksf (allowable soil bearing)
• fc' = 3,000 psi (concrete strength)
• Column: W12x65 (d = 12.1 in., bf = 12.0 in.)

Solution:

Step 1 -- Service load for footing sizing (ASCE 7-22 load combination D + L):

Pservice = PD + PL = 80 + 60 = 140 kips

Step 2 -- Required footing area:

A_required = Pservice / qa = 140 / 3.0 = 46.7 ft^2
Side length = sqrt(46.7) = 6.83 ft -> use 7 ft x 7 ft

Step 3 -- Factored load for strength design (ACI 318-19, LC1: 1.2D + 1.6L):

Pu = 1.2 * 80 + 1.6 * 60 = 96 + 96 = 192 kips

Step 4 -- Factored soil pressure:

qu = Pu / (7 * 7) = 192 / 49 = 3.92 ksf

Step 5 -- Check one-way shear at distance d from column face. For a 22 in. thick footing with 3 in. cover, effective depth d = 18.5 in.:

Critical section: bo/2 + d = 12/2 + 18.5 = 24.5 in. = 2.04 ft from column center
Distance from footing edge = 3.5 - 2.04 = 1.46 ft
Vu = qu * 1.46 * 7 = 3.92 * 10.22 = 40.0 kips

phi*Vc = 0.75 * 2 * sqrt(3000) * 84 * 18.5 / 1000
       = 0.75 * 2 * 54.8 * 84 * 18.5 / 1000
       = 127.7 kips > 40.0 kips  OK

Result: A 7 ft x 7 ft square footing, 22 in. thick, provides adequate bearing and shear capacity. Reinforcement: 6-#6 bars each way (temperature and shrinkage minimum per ACI 318-19 Section 7.6).

Frequently Asked Questions

What is the difference between two-way (punching) shear and one-way shear in a footing? Two-way shear, commonly called punching shear, checks whether the column punches through the footing slab along a failure perimeter measured at d/2 from each face of the column, where d is the effective depth of the footing. One-way shear (beam-style shear) checks a vertical plane across the full footing width at a distance d from the column face — it governs on long, narrow footings or strip footings. For typical square spread footings under concentric column loads, punching shear almost always controls and should be checked first; if the footing is thick enough for punching, one-way shear usually passes automatically.

What footing size is needed for a 200-kip service column load on 2,500 psf allowable soil bearing? Required plan area = P / q_allow = 200,000 lb / 2,500 psf = 80 ft². A square footing needs √80 = 8.9 ft per side — use 9 ft × 9 ft (81 ft²). The net soil pressure (excluding footing self-weight of roughly 4.5 ft × 150 pcf = 675 psf) is approximately (200,000 − 9 × 9 × 675) / 81 = 128 ksf net. For strength checks using factored load Pu ≈ 1.4 × 200 = 280 kips with f’c = 3,000 psi, punching shear on a d ≈ 17-inch-deep footing then determines whether the 9-ft size works or a deeper slab is needed.

How do I choose the footing size based on soil bearing pressure? Footing size is governed by soil bearing capacity under service (unfactored) loads, not factored loads, because allowable bearing pressure is a serviceability limit. You divide the total service axial load (column load plus footing self-weight) by the allowable soil bearing pressure to get the required plan area, then round up to a practical size. Once the plan dimensions are set, the footing thickness (h) is then governed by the strength checks — primarily punching shear and one-way shear — using factored loads with ACI phi factors.

What is the ACI 318 one-way shear capacity formula for a footing without shear reinforcement? For normal-weight concrete without shear reinforcement, the simplified one-way shear capacity is Vc = 2λ√f’c × bw × d (US customary, psi units), where λ is the lightweight concrete factor (1.0 for normal weight), f’c is the specified compressive strength in psi, bw is the footing width in inches, and d is the effective depth in inches. This gives Vc in pounds. ACI 318-19 introduced a more detailed table-based approach that accounts for reinforcement ratio and axial load; the simplified 2λ√f’c formula remains conservative and is widely used for preliminary footing sizing.

What is the minimum concrete cover for footing reinforcement, and why? ACI 318 requires a minimum of 3 inches (75 mm) of concrete cover for reinforcement cast against and permanently exposed to earth — which applies to the bottom of footings in direct contact with the ground. This cover protects steel from corrosion and provides fire resistance. If the footing is cast on a concrete mud mat, the cover requirement for the bottom bars may be reduced to 1.5 inches per some interpretations, but the conservative 3-inch minimum is standard practice. Top and side cover is typically 2 inches (50 mm) for footings not exposed to weather.

Why does punching shear capacity decrease when the column is near the footing edge? When a column is located close to the edge of the footing, part of the theoretical punching perimeter (at d/2 from the column face) falls outside the footing boundary and cannot develop shear resistance. The effective critical perimeter is reduced to only the portions within the footing, which directly reduces the punching shear capacity. Edge and corner column footings often require a larger plan size or increased thickness compared to interior columns carrying the same load, specifically to compensate for this reduced perimeter.

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