A combined footing is a spread foundation that carries two columns on a single base. It is built when an isolated footing cannot take the applied load on its own or when two columns stand so close together that their pads would overlap. The structural logic is simple: two columns share one rigid slab, the soil pressure spreads over the combined area, and the footing acts as an inverted beam between them. A worked combined footing design example is the fastest way to see how the sizing and reinforcement checks fit together. The sections below explain when a combined footing is justified, the shapes it takes, and the design and construction points that control the outcome.
When to Use a Combined Footing
Three situations push the design from isolated pads toward a combined base: heavier column loads that require larger pads, weaker soil that inflates the required area, and property lines that stop footings from extending past the boundary.
Pressure Bulb Overlap
Every footing pushes a bulb of stress into the soil beneath it. When the pressure bulbs of two adjacent footings overlap, the soil in the overlap zone carries double stress, which can trigger a shallow foundation failure. Combining the two footings into one slab spreads the same total load over a continuous area and removes the high-stress zone.
Boundary Columns and Property Lines
When a column must sit at the edge of the site, the footing cannot project beyond the boundary. A combined footing ties that edge column to the first interior column so the two pads resist the moment together. The interior column acts as a counterweight and keeps the resultant pressure within the footing plan. For lightly loaded walls the same site constraint is handled with simpler construction such as a stone masonry footing, but engineered columns need the combined base.
Types of Combined Footings
Combined footings come in a few standard shapes, and the shape follows the site and the loads. The main types are rectangular, trapezoidal, irregular, strap, and raft footings. A combined footing excel sheet is a practical starting point for the sizing runs, because the iterative area and pressure checks are easy to automate.
Rectangular Combined Footings
The rectangular combined footing carries two columns on a slab of constant width, and it is the most commonly constructed type. Its length and reinforcement stay uniform, which simplifies the bar schedule and the formwork. The columns may sit anywhere along the slab, including at the edge, which happens when a boundary wall stops the foundation from continuing into the adjacent land. Two reinforcement nets are typical: a top net and a bottom net, each sized for its zone of the bending field.
Edge Column Arrangements
When one column lands at the edge of the rectangular slab, that side of the footing behaves like a cantilever and the soil pressure under the edge must stay compressive. The interior column provides the balancing load, and the design check verifies that the resultant of the column loads falls close to the centroid of the footing plan.
Trapezoidal and Irregular Shapes
A trapezoidal combined footing tapers in width so its centroid matches the resultant of the two column loads, which keeps the soil pressure uniform. The varying width makes construction harder: every short-direction bar has a different length and the formwork cannot be reused, so trapezoidal shapes are not popular in construction. The taper also reduces the plan area compared with a rectangle of the same scale, yet that rarely cuts cost because labor and formwork dominate. Irregular shapes are reserved for special conditions where neither shape fits the site.
| Shape | Column positions | Construction effort | Typical use |
|---|---|---|---|
| Rectangular | Anywhere along the slab | Low | Most common; boundary and interior columns |
| Trapezoidal | Centroid matched to loads | High | Varying loads or site limits |
| Irregular | Site-specific | Very high | Special constraints only |
| Strap | Two separate pads | Medium | Eccentric boundary columns |
| Raft | Whole building | High | Weak soil and heavy loads |
Reinforcement Detailing for Combined Footings
A combined footing bends in two directions: longitudinally between the columns and transversely across the width, so the reinforcement layout must match that two-way field. The reinforcement detailing of footing elements finalizes the design, because bar cutoffs and laps have to line up with the moment envelope.
Top and Bottom Nets
The bottom net resists the upward soil pressure that bends the slab like an inverted beam between the two columns. The top net picks up the negative moment over each column. In a rectangular combined footing both nets run the full length of the slab, which keeps bar lengths constant and the schedule simple.
Bar Lengths in Tapered Slabs
In a trapezoidal footing every short-direction bar has a different length because the width changes continuously. Fabricators cut each bar to its own length and the field crew places them in order from the wide end to the narrow end. This labor cost is the main reason designers avoid the shape unless the loads demand it.
Laps, Cutoffs, and Cover
Longitudinal bars are lapped away from the peak moment zones, and cutoff points follow the bending moment envelope so no bar ends where tension is highest. Clear cover to the bottom of the slab follows the exposure class, typically 50 mm for concrete cast against soil, and the top bars get cover per the environmental conditions.
Design Considerations and Checks
The design of a combined footing follows the same limit state framework as any spread foundation, with extra attention to the longitudinal bending between the columns. The footing is idealized as a rigid beam on an elastic soil, which means the soil pressure is assumed to vary linearly across the plan. The design proceeds through a short sequence of checks, summarized below.
- Size the plan area from the service loads and the allowable bearing capacity.
- Locate the resultant of the column loads and align it with the footing centroid.
- Calculate the soil pressure diagram at the ultimate limit state.
- Check one-way shear and punching shear at the critical sections.
- Design the longitudinal reinforcement for the moment envelope between and over the columns.
- Verify the transverse reinforcement and minimum steel requirements.
Centroid Alignment and Pressure Distribution
The resultant of the two column loads should fall at the centroid of the footing plan so the soil pressure stays uniform. If the resultant drifts off-center, the pressure diagram becomes trapezoidal and the edge pressure on one side rises. The design limits that edge pressure to the allowable bearing capacity and keeps the resultant inside the middle third so no part of the base lifts off the soil.
Load Combinations and Strength Factors
Service loads size the plan area, while factored loads drive the strength checks. The load factors and strength reduction factors come from the governing code, and the isolated footing design guidelines based on ACI 318-14 set out the same framework that applies to combined bases, with longitudinal beam action added for the two-column case.
Shear, Bending, and Deflection Checks
One-way shear is checked across the full width of the slab at the critical sections near each column, and punching shear around each column perimeter. Longitudinal bending between the columns sizes the main bars, while transverse bending sizes the distribution steel. Minimum reinforcement and crack width limits keep serviceability under working loads.
Construction and Cost Considerations
Construction effort, not material volume, decides which combined footing shape is economic. A rectangular footing needs simple formwork, straight reinforcement, and repeatable bar spacing, so site labor stays low. A trapezoidal footing needs tapered formwork and custom-length bars, and the added labor cancels the small saving in concrete volume.
Formwork, Excavation, and Concreting
Excavation follows the footing outline, and the base is blinded with lean concrete before the reinforcement goes in. Formwork for a tapered slab is cut to the slope and the top surface screeded to the fall on the drawings. Concreting runs in one continuous pour to avoid cold joints under the columns, with compaction concentrated around the starter bars.
When the Base Becomes a Wall Footing
When a combined base is long and narrow and supports a continuous wall line rather than discrete columns, the design converges with strip behavior. The analysis and design of RC wall footing based on ACI 318-19 covers that geometry directly, including the transverse distribution of the line load and the longitudinal temperature steel.
| Shape | Formwork effort | Reinforcement labor | Overall cost |
|---|---|---|---|
| Rectangular | Low | Low | Lowest |
| Trapezoidal | High | High | Higher despite less concrete |
| Irregular | Very high | Very high | Highest |
| Strap | Medium | Medium | Medium |
| Raft | High | High | High for large footprints |
Selecting the Right Combined Footing
Selection starts with the site geometry and ends with the construction cost. Rectangular combined footings win when the column loads are similar and the site allows a constant width. Trapezoidal footings earn their extra labor only when the load resultant sits far off-center. Strap footings are the economical middle ground when pads can stay separate but need to share load.
A Quick Decision Framework
- Similar column loads and an open site: rectangular combined footing
- Resultant far from the center and a restricted site: trapezoidal
- Pads can stay separate with an eccentric edge column: strap footing
- Weak soil across the whole footprint: raft foundation
Confirming the Final Scheme
Once the shape is chosen, the designer confirms the plan dimensions against the allowable bearing pressure, re-checks the centroid position, and runs the shear and moment checks with the final geometry. Small adjustments to the slab length shift the centroid, so the sizing loop repeats until the resultant and the centroid agree.
Combining two functions in one element is common across building design, and it shows up beyond foundations in spaces such as a multi-functional entryway that merges pantry storage with circulation. The payoff is the same everywhere: fewer elements, less material, and a simpler structure. A combined footing applies that logic to the ground, and a clear picture of the load path, the shape options, and the construction effort keeps the choice practical.