In construction, the foundation is critical in ensuring the structural stability of a building. Among various types of foundations, a combined footing is commonly used when column loads are heavy and space constraints limit the use of isolated footings. This blog delves into the details of combined footings, their applications, design considerations, and relevant IS codes to ensure compliance and safety.

What is a Combined Footing?

A combined footing is a type of foundation that supports two or more columns. Unlike isolated footings, which serve individual columns, combined footings are employed when the spacing between columns is limited, or when the load from a column is too great for an isolated footing.

Types of Combined Footings

1. Rectangular Combined Footing
   Used when two columns are close to each other and have equal or nearly equal loads.
2. Trapezoidal Combined Footing
   Preferred when one column exerts a significantly larger load than the other. The trapezoidal shape helps distribute the load more evenly.
3. Strap or Cantilever Footing
   A type where an additional beam, known as a strap, connects two isolated footings to act as a single unit.
   

When to Use a Combined Footing?

Combined footings are used in the following situations:

• Space Constraints: When columns are close to property boundaries, making isolated footings infeasible.
• Unequal Loads: When one column carries a heavier load than another, requiring an integrated foundation to balance the load.
• High Soil Pressure: In areas with low soil bearing capacity, combined footings help distribute loads more effectively.
• Overlapping Footings: When isolated footings of adjacent columns overlap, a combined footing can resolve this issue.

Design Considerations for Combined Footings

Designing a combined footing involves analysing the load distribution, soil conditions, and structural requirements. Key considerations include:

1. Load Analysis

• The total load from the columns, including dead load, live load, and any additional forces, must be considered.
• Ensure the load is evenly distributed to prevent differential settlement.

2. Soil Bearing Capacity

• Determine the safe bearing capacity of the soil using standard tests. This value is crucial for calculating the size and thickness of the footing.

3. Geometry of Footing

• Choose between rectangular or trapezoidal shapes based on load conditions and space availability.
• Ensure the centroid of the footing aligns with the resultant force from the columns to avoid tilting.

4. Reinforcement Design

• Use reinforcement bars to strengthen the footing against bending and shear stresses.
• Follow IS 456:2000 for guidance on concrete reinforcement design.

5. Construction Materials

• Use high-quality materials, including cement, aggregates, and steel, to ensure durability and strength.

IS Codes for Combined Footing

For designing and constructing combined footings, the following Indian Standards (IS) codes are applicable:

1. IS 456:2000: Code of practice for plain and reinforced concrete.
2. IS 6403:1981: Code of practice for determining the safe bearing capacity of soil.
3. IS 3370: For reinforced concrete structures exposed to water (if the footing may encounter water or wet conditions).
4. IS 1904:1986: Code of practice for the design and construction of foundations in soils.

These codes provide detailed guidance on material specifications, load calculations, and construction methods.

Steps to Construct a Combined Footing

1. Site Investigation
   Conduct soil tests to determine the bearing capacity and suitability of the site.
2. Excavation
   Excavate the site according to the dimensions and depth of the footing.
3. Base Preparation
   Level the ground and lay a lean concrete layer to provide a smooth surface for footing construction.
4. Formwork Installation
   Install formwork to define the shape of the footing. Ensure it is sturdy to withstand concrete pressure.
5. Reinforcement Placement
   Place reinforcement bars as per the design. Use spacers to maintain the required cover thickness.
6. Concrete Pouring
   Pour concrete uniformly and compact it using vibrators to eliminate air pockets.
7. Curing
   Cure the concrete for the specified duration to achieve maximum strength.

Advantages of Combined Footing

• Cost-Effective: Reduces material usage by combining the foundation for multiple columns.
• Space Efficiency: Ideal for areas with restricted space or proximity to boundaries.
• Load Distribution: Ensures even distribution of loads across the foundation.
• Flexibility: Suitable for varied soil conditions and column loads.

Challenges and Limitations

• Complex Design: Requires precise calculations and alignment to avoid structural failures.
• Higher Initial Costs: The design and construction might be more expensive compared to isolated footings.
• Skilled Labour: Requires experienced engineers and workers for proper implementation.

Conclusion

Combined footings are a practical and efficient solution for supporting multiple columns, especially in situations with space or load constraints. By adhering to design principles and IS codes, engineers can ensure that combined footings provide a safe and durable foundation for any structure. Proper planning, quality materials, and skilled labour are essential to the successful execution of this foundation type.