What is a Combined Footing and Its Types for Construction

A combined footing is a specific type of foundation design used in construction to support two or more columns. This innovative design is particularly useful when columns are closely spaced or when the soil conditions are subpar. Combined footings distribute the load more evenly, enhancing stability and reducing the risk of differential settlement.

Key Features of Combined Footing

Combined footings are designed with several key features that make them a reliable choice. They include:

Load Distribution: Combined footings are designed to spread the load from multiple columns over a larger soil area, reducing the pressure on the soil.

Shape and Size: These footings often take the shape of a rectangular slab. The dimensions vary based on the load requirements and the spacing of columns.

Construction: They are typically constructed using reinforced concrete, providing the necessary strength and durability for long-term stability.

Applications: Combined footings are commonly used in situations where columns are closely spaced or when the soil has a low bearing capacity. They are also employed to connect the footings of adjacent columns to prevent differential settlement.

Design Considerations: Factors such as the column loads, soil characteristics, and overall structure stability are crucial when designing combined footings.

Advantages of Combined Footings

Combined footings offer several advantages, including:

Reduction in Risk of Differential Settlement: This ensures the overall stability of the structure. Stable Base for Close Spaced Columns: Combined footings provide a uniform foundation for columns that are placed closely together. Economic Benefits: In some cases, combined footings can be more cost-effective than individual footings.

However, the design and construction of combined footings may require more excavation and concrete compared to individual footings. Additionally, the design can be more complex due to the need to account for interactions between columns.

Types of Combined Footings

Rectangular Combined Footings

Rectangular combined footings are the most commonly used type in construction. They are made in a rectangular shape to easily support multiple columns. The primary advantage of rectangular combined footings is their ability to provide uniform load distribution.

Trapezoidal Combined Footings

These combined footings are used when the load distribution is unequal. A trapezoidal shape is used to accommodate the varying distribution of loads from the superstructure to the subsoil. This type of footing is particularly useful for columns near property lines, where the footing may not extend beyond the property boundary.

Strap Combined Footings

Strap combined footings are used when an external column is very close to the property line. They consist of a connecting beam (strap) that links two spread footings, allowing the heavily loaded column to transfer its load to a less loaded column. This ensures that the entire structure is supported efficiently.

Raft Combined Footings

Raft combined footings are a type of spread footing that provides a large area of support for multiple columns. They are particularly useful in situations with complex load distributions or poor soil conditions. The raft footing distributes the load over a wider area, enhancing stability and reducing the risk of settlement.

Selecting the Ideal Type of Footing

The choice of the ideal type of footing depends on several factors, including the depth at which the bearing strata lies, the soil conditions, and the type of superstructure. When deciding between isolated and combined footings, engineers must consider the economic benefits and structural requirements of the project.

Combined footings are a versatile and effective solution for supporting multiple columns while ensuring stability and load distribution in various soil conditions. Whether you are faced with closely spaced columns or challenging soil conditions, combined footings can provide a reliable foundation for your building project.