A cantilever footing is a combined footing type that uses cantilever action to balance the eccentricity induced when a column load sits off the center of its footing. The eccentrically loaded footing, the strap beam, and the inner column foundation act together, and the behavior of this assembly is explained in our combined footing design article. Because the strap beam bends like a cantilever, these foundations are named cantilever footings.
This article covers when cantilever footings become necessary, how load eccentricity changes soil pressure, the strap beam design procedure, and how the system compares with other options.
What Is a Cantilever Footing Foundation?
Columns cannot always be centered on an isolated footing. When a building extends to the boundary of the plot, the footing cannot be built on the neighboring land, so the column sits at the edge of its own footing. The eccentric load increases soil stress directly under the column while the pressure on the far side drops. If the pressure exceeds the allowable bearing capacity, the footing settles unevenly or the soil fails in bearing.
The standard solution ties the eccentrically loaded footing to an interior foundation with a stiff beam. The beam spans between the boundary footing and the inner column footing, and rotation of the outer footing lifts the beam at its inner support, like the free end of a cantilever. This upward reaction balances the overturning effect, which is why the system counts as a spread footing, not a deep foundation. In low-rise masonry buildings, a stone masonry footing is used where the wall can be centered on the strip, but framed structures with a boundary column do not.
When a Cantilever Footing Is Needed
- The column sits on a property line or very close to it, so the footing cannot project into the adjacent plot.
- The eccentricity is large enough that a simple rectangular footing would exceed the allowable soil pressure or lose soil contact on one side.
- An interior column stands close enough to carry the balancing reaction from the strap beam.
- Soil conditions still allow a shallow foundation, so piles or a raft are not required.
None of these conditions makes the cantilever footing mandatory. A footing can be designed with eccentricity, and codes allow pressure to redistribute within limits. The arrangement becomes attractive when the eccentric footing alone would need an oversized base or when settlement control is strict.
Why Load Eccentricity Matters
Soil pressure under a concentric footing is roughly uniform. Once the load moves off center, the pressure becomes trapezoidal, and at large eccentricities triangular, with zero pressure on the far edge. The maximum edge pressure can be estimated with the expression p = P/(B L) (1 + 6e/B), where P is the column load, B is the footing width along the eccentricity, L is the perpendicular length, and e is the eccentricity. When e exceeds B/6, part of the footing loses contact with the soil and the design uses a reduced effective base.
Pressure Distribution Under an Edge Load
The factor 6e/B controls how uneven the pressure becomes. At e = B/6 the far-edge pressure is exactly zero. At e = B/12 the far edge carries half the average pressure, which most codes accept. Designers compare the maximum edge pressure with the allowable bearing capacity before the footing is sized. For strip foundations, the wall footing design steps follow a simpler uniform-pressure check because the load stays centered on the wall.
Keeping the Resultant Inside the Kern
The kern, or middle third, is the central zone where the resultant of all loads must land if the full base is to stay in contact with the soil. Keeping the resultant inside the middle third is a common design rule for eccentrically loaded footings. When it cannot be met, the engineer enlarges the footing, adds the strap beam, or moves to a deep foundation.
Exceeding the allowable pressure does not fail the footing immediately. The first symptom is differential settlement, which tilts the column and cracks partitions. If the edge pressure keeps climbing, the soil beneath the loaded edge yields and bearing failure follows. These shallow foundation failure modes are why boundary columns get the cantilever treatment.
Cantilever Footing Design Procedure
The design follows the same logic as strap footing design, with one addition: besides bending from vertical loads at the ultimate limit state, the designer must include the bending and shear forces induced by the load eccentricity. The procedure below models the strap beam as a simply supported beam on hinges at the two footing centers.
Step-by-Step Design Sequence
- Collect the column loads and the distance between the boundary column and the inner footing center. The eccentricity is the offset between the boundary column and the footing centroid.
- Assume provisional footing sizes and compute the net soil pressure after deducting self-weight and overburden.
- Treat the strap beam as a simply supported beam. The boundary footing reaction equals the column load plus the eccentric moment effect, and the inner footing reaction must balance it.
- Size the inner footing for its column load plus the strap reaction, and check the soil pressure stays below the allowable value.
- Design the strap beam for the maximum bending moment and shear, including the eccentric moment, with top bars over the supports and bottom bars in the span.
- Check stability: the upward reaction at the inner footing must not exceed the footing weight and overburden, or it will lift.
- Detail the reinforcement with proper anchorage, laps, and cover, following the reinforcement detailing of footing requirements.
Most designers make the strap beam much stiffer than the footings so it behaves as a rigid link. A flexible beam changes the assumed hinge model and shifts the pressure distribution. The beam is kept off the soil with a gap or compressible layer so it does not pick up bearing pressure that would alter the reactions.
Proportioning and Sizing the Footings
Proportioning starts from the allowable bearing capacity and the load eccentricity, then works outward to practical sizes. The isolated footing design guidelines from ACI 318-14 give the load combinations, the shear checks, and the flexural design used for each pad, and the strap beam is then added between them.
Typical Proportions for the Strap Beam
The strap beam dominates the proportions because it carries the balancing moment. Its depth is commonly one-sixth to one-eighth of the column center-to-center spacing, and its width about one-third of the depth or a practical minimum of 300 mm. The boundary footing is sized so the resultant of the column load and the strap reaction stays within the middle third, while the inner footing is only slightly larger than a normal isolated footing because it receives the downward balancing reaction.
| Item | Typical value |
|---|---|
| Strap beam depth | 1/6 to 1/8 of the column spacing |
| Strap beam width | 300 mm minimum, or one-third of the depth |
| Boundary footing | Resultant kept inside the middle third |
| Inner footing | Column load plus strap reaction, checked against allowable pressure |
| Clear cover at the soil face | 75 mm for concrete cast against earth |
| Strap beam clearance | 75 to 150 mm gap below the beam |
The gap under the strap beam is easy to get wrong. If the beam bears on soil, it picks up pressure the analysis never included and can crack in service. A void former or a layer of soft fill keeps the beam free to work as a structural link.
Construction Details and Site Practice
Construction of a cantilever footing demands care because the two footings and the beam must behave as one assembly. Excavation proceeds to the depth set in the geotechnical report, and the bearing stratum is inspected before concrete is placed. Worked examples of cantilever footing design cover the same geometry and can be checked against the procedure below.
Excavation, Formwork, and Concrete Placement
- Trim the excavation sides and place a lean concrete blinding layer so the reinforcement stays clean and cover is easy to control.
- Set the boundary footing forms, then the inner footing forms, then the strap beam forms, keeping the beam soffit at the design clearance above the soil.
- Place reinforcement in the sequence shown on the shop drawings, threading the strap beam bars through the footing cages before pouring.
- Compact the concrete in layers, watching the beam haunches and the junctions where the beam meets each footing.
- Cure the concrete for at least seven days and keep the surface moist, because early shrinkage cracks the beam more easily than the pads.
Working in Weak Soils
When the bearing stratum is weak, over-excavate and replace the top layer with compacted granular fill, or place a lean concrete raft below the footings. Dewatering may be needed when the water table sits above the footing base, and the excavation sides must be battered or supported in loose soil. Soft spots found at formation level should be dug out and backfilled before the blinding layer goes down.
Quality control concentrates on geometry: the column position, the eccentricity, and the beam clearance all affect the pressure distribution, so a survey check of the column starter bars against the setting-out plan takes only minutes.
Cantilever Footing vs. Other Foundation Options
The cantilever footing is one of several ways to handle a boundary column, and the choice depends on column spacing, soil strength, and cost. The cantilever action here differs from a cantilever retaining wall, which resists lateral earth pressure rather than balancing a vertical eccentric load, but both use a stiff stem working against a stabilizing mass.
How the Options Compare
| Foundation option | Eccentric load handling | Typical application | Relative cost |
|---|---|---|---|
| Isolated footing | Poor at a property line | Interior columns | Low |
| Rectangular combined footing | Good when columns are close | Two columns sharing one base | Medium |
| Cantilever (strap) footing | Good with a stiff beam | Boundary columns with interior support | Medium |
| Raft foundation | Good, spreads all loads | Weak soil, many columns | High |
An isolated footing is cheapest when the column can be centered, which is why interior columns keep it. A rectangular combined footing suits two closely spaced columns. The strap footing wins when the boundary column is far from the next column or when the interior load is small enough to take the balancing reaction. A raft spreads every column load over the whole footprint and suits weak soils, at higher cost.
For a side-by-side comparison of the two systems, the article on balanced footings and cantilever footings walks through when each works best and where the load paths differ. In practice the decision reduces to three numbers: the allowable soil pressure, the eccentricity, and the distance to the nearest interior column.