A combined footing is a type of foundation that supports two or more columns or walls by distributing the load to a wider area of soil.
Understanding Combined Footing Foundations
At its core, a combined footing foundation is a structural element designed to support more than one vertical load-bearing element, such as columns or walls, using a single, larger footing pad. Instead of providing individual footings under each column or wall, a combined footing ties them together, spreading their collective weight over a larger, continuous area of soil.
This approach is particularly valuable in construction under specific conditions, as highlighted by the definition which notes its common use when the columns or walls are close to each other and the soil's bearing capacity is limited.
Why Use a Combined Footing?
Several factors make combined footings a preferred choice in certain scenarios:
- Limited Space: When columns or walls are spaced too closely, individual isolated footings might overlap. A combined footing neatly encompasses both, saving space and simplifying construction.
- Low Soil Bearing Capacity: If the soil cannot support the concentrated load from individual footings, a combined footing distributes the load over a larger area, reducing the pressure on the soil.
- Eccentric Loading: When a column is located near a property line or obstruction, an isolated footing might experience eccentric loading (where the load is not centered), potentially causing it to tilt. Combining it with an interior column on a single footing can help balance the loads and prevent tilting.
- Structural Stability: Tying multiple columns together with a rigid footing can enhance the overall stability of the structure.
Common Shapes
Combined footings are typically designed in shapes that efficiently distribute the load. The most common shapes include:
- Rectangular Combined Footing: Used when both columns carry approximately equal loads and the footing does not extend significantly beyond either column.
- Trapezoidal Combined Footing: Often used when one column carries a significantly heavier load than the other, or when there's a constraint on one side. The wider end of the trapezoid is placed under the column with the heavier load or the constraint.
By integrating the loads from multiple points onto a single base, combined footings offer a practical and efficient foundation solution for specific structural and soil conditions.
