Combined footing: when to use and how to design ultimate guide
introduction
In civil engineering and structural design, foundations play the most critical role in transferring building loads safely to the ground. Among the many types of foundations, combined footing is a widely used solution when individual isolated footings cannot serve effectively. Understanding when to use combined footing and how to design it is essential for civil engineers, contractors, and construction students.
This article explores the concept of combined footing in depth—from definition, types, and situations where it is required to detailed steps of design and construction practices.
What is combined footing?
A combined footing is a type of shallow foundation that supports two or more columns. Instead of constructing separate isolated footings for each column, a single footing is built to spread the loads evenly to the soil.
The key idea behind combined footing is achieving uniform soil pressure distribution and ensuring stability when space or load conditions make isolated footings impractical.
Why not always use isolated footings?
Normally, isolated footings (single pad foundations) are the most economical choice for supporting columns. However, certain site and load conditions make isolated unsuitable
- Columns located too close to each other, causing overlapping of isolated footings.
- Columns placed near property boundaries where an isolated footing would extend beyond the site limits
- High load differences between adjacent columns, requiring redistribution through a shared base
- Soil bearing capacity limitations that need larger footing areas to prevent excessive settlement
In such cases, combined footing becomes the most efficient solution.
Situations When Combined Footing is Used
Civil engineers generally select combined footing in the following scenarios:
1. Columns Near Property Line
When a column is close to the property line, an isolated footing may extend beyond the boundary. In this case, the footing is combined with an adjacent column to balance the load and stay within site limits
2. Closely Spaced Columns
If two columns are very close, separate isolated footings may overlap. Constructing a single combined footing saves material, space, and cost.
3. Unequal Column Loads
In multi-story buildings or industrial structures, columns often carry different loads. A combined footing ensures that soil pressure remains uniform by adjusting the footing shape and reinforcement.
4. soil of Low Bearing Capacity
On weak soils, the required footing area increases significantly. Instead of building very large isolated footings, engineers prefer combining adjacent columns into a larger but manageable combined footing
5. Structural Continuity Requirement
Sometimes, to improve rigidity and reduce differential between adjacent footings, a combined footing is adopted as a design choice
Types of Combined Footing
Combined footings are classified based on shape and load conditions:
1. Rectangular footing
- Used when both columns carry equal or nearly equal loads.
- The footing is shaped as a rectangle, with loads distributed symmetrically
- Suitable when column spacing is moderate and soil pressure is uniform
2. Trapezoidal Combined Footing
- Adopted when loads on columns are unequal
- The footing is wider on the side of the heavier load to balance soil pressure
- Common in industrial buildings where load variations between columns are significant
3. Strap (or Cantilever) Footing
- Technically a variant of combined footing.
- Two isolated footings are connected with a beam (strap) that transfers load between them.
- Especially useful when one column is near a boundary line.
Design considerations for combined footing
Designing a combined footing requires both structural analysis and geotechnical understanding. The main objective is to keep the soil pressure uniform while ensuring the footing is structurally safe against bending, shear, and settlement.
Key design considerations include:
- load from Columns
- Dead loads, live loads, and additional effects (wind, seismic) must be considered.
- Unequal loads require adjusting the shape of the footing
2. Soil bearing capacity (SBC)
- The ultimate soil bearing capacity, reduced with a factor of safety, determines the maximum allowable pressure.
- Geotechnical investigation reports provide SBC values.
3. Footing Depth
- Must be sufficient to prevent shear failure of soil and resist uplift from groundwater or frost.
- Generally ranges from 1.0 to 3.0 meters depending on conditions.
4. Position of Resultant Load
- The centroid of the footing must coincide with the resultant of column loads to avoid eccentric loading.
5. Shape of Footing
- Rectangular when loads are equal
- Trapezoidal when loads are unequal
6. Structural Checks
- One-way and two-way shear resistance
- Bending moment along footing length.
- Adequate reinforcement in both longitudinal and transverse directions.
Step-by-Step Design of Combined Footing
A simplified stepwise procedure for designing combined footing:
Step 1: Collect Data
- Column load (factored)
- Spacing between columns
- Soil bearing capacity (SBC)
- Cover, allowable stresses, and material properties (concrete grade, steel grade).
Step 2: Calculate Required Area
A=(P1+P2)/qallow
Where
- P1+P2 = Column loads
- qallow = Allowable soil pressure
Step 3: Decide Shape and Dimensions
- If P1≈P2, use rectangular footing.
- If P1≠P2, use trapezoidal footing with dimensions adjusted to maintain uniform soil pressure.
Step 4: Locate Resultant Load
Ensure the centroid of the footing area coincides with the resultant of the column loads. Adjust lengths accordingly.
Step 5: Structural Design
- Longitudinal Bending: Treat as a beam supported by soil reaction.
- Transverse Bending: Provide sufficient reinforcement across width.
- Shear Checks: Verify one-way shear (critical at distance d from column face) and punching shear around columns.
Step 6: reinforcement detailing
- Provide main reinforcement along the length (tension at the bottom).
- Distribute steel along the width.
- Adequate anchorage and lap lengths as per code (IS 456:2000, ACI 318, or Eurocode).
Step 7: Depth of Footing
- Calculate from shear considerations. Usually, depth increases with load intensity.
Practical Example (Simplified)
Suppose:
- Column loads: KN, P2=1200 KN
- Spacing between columns = 4.0 m.
- SBC = 200 kN/m².
Step 1: Required Area
A=(800+1200)/200=10m²
Step 2: Shape
Since loads are unequal, use trapezoidal footing.
Step 3: Proportions
The width on column 1 side is smaller; the width on column 2 side is larger. Adjust until the centroid coincides with the resultant.
Step 4: Structural Design
Bending moments, shear checks, and reinforcement are designed according to standard codes.
Construction Guidelines for Combined Footing
1. Excavation
- Excavate to the required depth based on soil report
- Ensure firm soil strata without loose material
2. Base Preparation
- Provide lean concrete (PCC) bed before reinforcement
- Ensure surface is level
3. Reinforcement Placement
- Place bars as per design
- Maintain cover blocks to ensure durability
4. Formwork
- Proper shuttering to avoid leakage of slurry
5. Concrete pouring
- Use high-quality concrete of required grade
- Vibrate adequately to remove voids
6. Curing
- Keep footing moist for at least 14 days to achieve strength.
7. Backfilling
- After de-shuttering, backfill with selected soil in layers.
Advantages of Combined Footing
- More economical than separate oversized isolated footings
- Provides uniform soil pressure distribution
- Useful in space-constrained sites
- Reduced chances of differential settlement
- Flexible design options (rectangular, trapezoidal, strap).
Limitations of Combined Footing
- Requires careful design and detailing
- More formwork and reinforcement compared to simple isolated footings.
- Not suitable for very poor soils (deep foundations may be needed).
Conclusion
Combined footing is a smart and practical solution when isolated footings are not feasible due to site or load constraints. By spreading the loads of two or more columns over a single base, it ensures structural safety, economic efficiency, and uniform soil pressure
Civil engineers must carefully evaluate conditions such as column loads, soil bearing capacity, spacing, and site boundaries before choosing combined footing. With proper design, detailing and construction practices, combined footing can deliver a strong and reliable foundation system for a wide range of structures
