Combined and Strap Footings

Combined and Strap Footings

Use the current NCEES PE Civil Reference Handbook and ACI 318-14 for July 2026. Combined and strap footings commonly solve a geometry problem: an exterior column near a property line cannot sit over the centroid of a symmetric isolated footing. The foundation must restore equilibrium without projecting into the restricted area.

Locate the service-load resultant first

For vertical service column loads Pi at coordinates xi,

Ptotal = ΣPi

and

xR = Σ(Pi xi)/ΣPi.

If a rigid combined footing is intended to have uniform service soil pressure, its plan-area centroid must lie under xR. Then q = Ptotal/A, subject to consistent inclusion of footing and overburden weight. If the load resultant misses the area centroid by eccentricity e, the footing carries a moment M = Ptotal e and pressure becomes linear, P/A ± M/S, while full contact remains valid.

A rectangular footing has its area centroid at midlength, so its edges must be positioned accordingly. A trapezoidal footing shifts the area centroid toward its wider end and is useful when a fixed boundary or column arrangement prevents a suitable rectangle. The trapezoid dimensions must satisfy three conditions: required area, centroid alignment, and constructible widths. Do not select its wide end by intuition alone.

Equal column loads do not guarantee equal isolated footing areas or a symmetric combined footing. One column can have a moment, a different allowable-pressure zone, an edge constraint, or a larger share of footing self-weight. Even with equal vertical loads, unequal distances to the available footing edges can require an asymmetric plan. Use the actual resultant.

Worked rectangular combined footing

An exterior column carries service load P1 = 400 kip at x1 = 2 ft from a property line. An interior column carries P2 = 600 kip at x2 = 14 ft. The footing begins at the property line, and allowable service bearing pressure is 5.0 ksf. Ignore footing weight only because the example states that basis.

The total and resultant location are

Ptotal = 400 + 600 = 1,000 kip,

xR = [400(2) + 600(14)]/1,000 = 9.20 ft.

For a rectangular footing beginning at x = 0, its centroid is at half its length. Select

L = 2(9.20) = 18.40 ft.

Required area is 1,000/5.0 = 200 ft², so required width is

Breq = 200/18.40 = 10.87 ft.

Choose B = 11.0 ft. The service pressure is

q = 1,000/[18.40(11.0)] = 4.941 ksf,

which is below 5.0 ksf. Because the resultant and area centroid both lie at 9.20 ft, this ideal rigid-body pressure is uniform. Settlement still requires evaluation.

The upward line load along the footing is

w = qB = 4.941(11.0) = 54.35 kip/ft.

Starting from the free left edge with shear zero, just before column 1 the upward reaction accumulated over 2 ft is +108.7 kip. The 400-kip downward column load jumps shear to -291.3 kip. From x = 2 to x = 14, 12 ft of soil reaction adds 652.2 kip, so shear just before column 2 is +360.9 kip. The 600-kip column load jumps it to -239.1 kip; the final 4.4 ft of reaction adds 239.1 kip, returning shear to approximately zero at the right edge.

That zero-end-shear check confirms vertical equilibrium. Integrate shear to obtain longitudinal footing moment, with extrema where shear crosses zero or jumps through zero at a column. Use factored column loads and a compatible factored pressure diagram for ACI strength design; the service line-load diagram above is not the reinforcement design diagram.

Strap-footing mechanics

A strap footing uses two separated pads connected by a stiff strap beam. The exterior pad is eccentric to its column because of the property line. The strap transfers moment and shear to the interior pad so the combined system's soil reactions balance the column loads and moments. Analyze the entire foundation first, then isolate each pad and the strap.

A common model assumes the strap is clear of soil or that soil support beneath it is intentionally neglected. Under that assumption, only pad areas produce bearing reactions, and the strap carries internal shear and moment. If the strap bears on soil, it becomes part of the contact system and changes the reactions; do not alternate assumptions midway. The strap-column and strap-pad connections must develop the calculated forces, and the strap needs longitudinal and shear reinforcement with appropriate development.

For preliminary pad sizing, let soil resultants be R1 and R2 at pad centroids. Whole-system equilibrium supplies R1 + R2 = P1 + P2 and a moment equation. Then A1 = R1/qtarget and A2 = R2/qtarget if a common target pressure is chosen. These areas need not match the column-load ratio because the strap moment redistributes soil reactions. Negative pad reaction signals an inconsistent geometry or assumed target pressure.

Structural and service checks

Treat a combined footing longitudinally as an inverted beam and transversely as footing cantilevers from the column line. Check ACI flexure at applicable column faces, one-way shear, punching around each column, bearing, dowels, and bar development. Column forces and upward pressure can cause top reinforcement between columns and bottom reinforcement in other regions; do not assume one tension face everywhere.

Finally compare total and differential settlement at both columns. A mathematically uniform pressure does not guarantee uniform settlement when soil layers vary. Coordinate footing depth, adjacent excavation, groundwater, frost, and construction sequence. The successful design aligns resultants, satisfies pressure and settlement, and then develops every factored internal force through reinforced concrete.

Geometry Acceptance Checklist

1. The selected area carries service demand within the stated bearing basis.
2. The contact-area centroid aligns with the service-load resultant when uniform pressure is assumed.
3. The plan respects property limits and provides constructible column, edge, strap, and reinforcement geometry.