Isolated, Spread, and Strip Footings

Isolated, Spread, and Strip Footings

For July 2026, use the current NCEES PE Civil Reference Handbook, ACI 318-14, and IBC 2018 without supplements. A shallow footing has two interacting designs: soil must support the foundation without bearing or settlement failure, and the concrete footing must carry column or wall forces to that soil. The two designs normally use different load bases.

Select the footing type and load basis

An isolated footing supports one column or pedestal. A spread footing widens the load over soil and can be isolated or continuous. A strip footing runs continuously under a wall or line of closely spaced supports and is often analyzed per foot of length. Geometry also must satisfy property, frost, adjacent-foundation, excavation, and competent-bearing-elevation constraints from IBC 2018 and the geotechnical report.

IBC 2018 pairs allowable soil bearing pressures with its allowable-stress load combinations. Thus footing plan area is commonly proportioned with service or ASD reactions, including footing and overburden weight consistently with whether the geotechnical value is gross or net. Do not add footing weight to a service reaction if the stated net allowable pressure already excludes the same overburden effect.

The concrete footing's flexure, shear, bearing, and development are strength checks using ACI 318-14 and factored load effects. Do not compare factored soil pressure directly with an allowable geotechnical pressure, and do not design reinforcing steel from unfactored pressure when a strength check is requested. Keep two labeled pressure diagrams if necessary.

Soil contact and eccentricity

For a rigid footing with a concentric vertical service resultant P, average pressure is

q = P/A.

If a moment M acts about a centroidal axis and full contact remains, linear elastic pressure is

q = P/A ± M/S,

where S is the footing contact-area section modulus about that axis. For eccentricity e = M/P along footing dimension B, full compression for a rectangular contact requires the resultant within the middle third, |e| ≤ B/6. Check both qmax against allowable bearing and qmin ≥ 0 for a no-tension soil interface.

When qmin is negative, soil does not ordinarily pull the footing downward. Use the no-tension contact distribution required by the problem, resize the footing, add stabilizing load, or adopt a soil-structure interaction model. A triangular compression block is not obtained by simply setting a negative corner to zero while leaving the original positive corner unchanged; equilibrium must be restored.

Soil strength is not the only geotechnical criterion. Estimate total settlement and differential settlement using the report's soil profile and method. Two footings can each satisfy allowable pressure yet settle differently because their sizes, loads, layers, or construction histories differ. Rotation can also change superstructure force distribution.

Worked bearing and strength actions

A square column footing supports service dead load D = 300 kip and service live load L = 200 kip. The geotechnical report gives allowable bearing qallow = 4.0 ksf for the stated service basis, and assume footing weight is already treated consistently. Required area is

Areq = (300 + 200)/4.0 = 125 ft².

Choose a 12 ft × 12 ft footing, A = 144 ft². The concentric service pressure is

qservice = 500/144 = 3.47 ksf < 4.0 ksf.

Bearing passes at this stage, subject to settlement. For a strength example, use

Pu = 1.2D + 1.6L = 1.2(300) + 1.6(200) = 680 kip,

so the simplified uniform factored pressure is

qu = 680/144 = 4.72 ksf.

The column is 24 in square and effective footing depth is d = 25 in = 2.083 ft. For two-way shear, the critical perimeter lies d/2 from the column faces. Its enclosed square has side 24 + 25 = 49 in = 4.083 ft, so enclosed area is 16.67 ft². The factored punching demand is the column force minus upward soil reaction inside that perimeter:

Vu,2way = 680 - 4.72(16.67) = 601.3 kip.

Compare this with the ACI 318-14 two-way shear strength using the actual critical perimeter, depth, concrete strength, column location, and applicable factors.

For one-way shear across the full 12-ft footing width, the critical section is d from the column face. The cantilever length beyond that section is

6 - 1 - 2.083 = 2.917 ft.

Therefore

Vu,1way = 4.72(12)(2.917) = 165.3 kip.

At the column face, the footing projection is 6 - 1 = 5 ft; the factored cantilever moment for one side is

Mu = qu B l²/2 = 4.72(12)(5²)/2 = 708 kip-ft.

These are demands, not capacities. Select thickness and reinforcement so ACI flexure and shear strengths exceed them.

Complete the detail

Bottom reinforcement generally resists upward-soil-pressure bending in an isolated footing, but local moments, walls, or combined actions can change required faces and directions. Extend bars far enough to develop beyond the governing section; merely placing the calculated steel area at midspan is insufficient. Check column-to-footing bearing, dowels or developed column bars, pedestal interfaces, concrete cover cast against earth, and any shear-friction, key, or anchorage path for lateral force.

For a strip footing, repeat the equilibrium per unit wall length and check one-way behavior and transverse flexure under the applicable ACI model; concentrated pilasters can introduce local two-way effects. Finish by reconciling service bearing, settlement, factored structural design, and construction geometry. Passing only P/A completes none of the local concrete checks. Along a long strip footing, also examine stiffness changes and soil transitions that can concentrate differential movement beneath the supported wall.

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