Wall footings are structural elements that are designed to support walls by transmitting and distributing their loads to the soil in a way that ensures the load-bearing capacity of the soil is not exceeded. They prevent excessive settlement and rotation, and maintain sufficient safety against sliding and overturning.
Wall footings come in two types: pad or spread footings and strip footings. They are designed to run along the direction of the wall, and the size of the footing and the thickness of the foundation wall depend on the type of soil at the construction site. Typically, the width of the wall footing is 2-3 times the width of the wall.
Wall footings can be constructed using various materials, such as stone, brick, plain concrete, or reinforced concrete. The choice of material depends on the specific requirements of the construction project. When the imposed loads to be transmitted are small, and the underlying soil layer is composed of dense sand and gravels, wall footings can be economically constructed.
Overall, wall footing is best suited for small buildings. By providing a stable and secure foundation for walls, wall footings play a crucial role in ensuring the safety and longevity of a building.
Construction of Wall Footings
1. Brick Wall Footing
A brick wall’s footing typically comprises multiple courses of bricks, with the bottom course usually twice as wide as the wall above. The extra width of the base is achieved through the use of 5cm offsets on each side of the wall. The depth of each course can be one brick thickness or more. The base of the footing wall sits on a plain concrete footing that extends 10 to 15 cm beyond the last brick offset, as illustrated in Figure 1. The width of the footing’s base should be no less than the width of the supported wall plus 30 cm.
2. Stone Masonry Wall Footing
Stone masonry walls typically require offsets of 15 cm with course heights of 30 cm, which is slightly more than that required for brick wall footings. In order to ensure adequate stability, the depth of the concrete used for the footings should be at least 15 cm.
When it comes to the mix proportions for the lean concrete used in these footings, the most common ratios are 1:4:8 (1 part cement, 4 parts fine aggregate, and 8 parts coarse aggregate) or 1:5:10 (1 part cement, 5 parts fine aggregate, and 10 parts coarse aggregate).
It is important to consider the angular spread of load from the wall when designing these footings. In the case of masonry walls, the acceptable spread should be no more than 1 vertical to 112 horizontals, while for cement concrete walls, it should be no more than 1 vertical to 1 horizontal. These guidelines help ensure that the load is distributed evenly and the structure remains stable.
3. Reinforced Concrete Wall Footing
When the load on a wall is heavy or the soil has low bearing capacity, a reinforced concrete strip footing can be used. By reducing the thickness of the strip towards the edge, it is possible to achieve greater economy. This type of footing is known as a reinforced concrete wall footing.
Fig. 3: Reinforced Concrete Wall Footing