The thickness of a strip foundation is influenced by various factors such as the type of soil, foundation depth, and soil bearing capacity. The load conditions and the depth of the foundation are two significant factors that determine the thickness of a strip foundation.
When designing a strip foundation, the soil’s load-bearing capacity must be taken into consideration to ensure that the foundation can support the weight of the structure above it. Depending on the soil type and bearing capacity, a thicker foundation may be required to ensure stability.
The depth of the foundation is another crucial factor that affects the thickness of a strip foundation. Deeper foundations may require a thicker strip to distribute the load more evenly and prevent settlement or failure.
Additionally, the strip foundation’s thickness must be determined based on the load conditions that the foundation will be subjected to. The weight of the structure, as well as any additional loads, such as wind or snow, must be considered when calculating the required thickness.
Therefore, it is essential to consider all these factors when designing a strip foundation to ensure that the foundation is strong, stable, and capable of supporting the structure above it for many years.
Fig.1: Strip Foundation
Thickness Requirements of Strip Foundations
Thickness of Strip Foundation Carrying Light Loads
The thickness of a strip foundation is commonly designed to be equal to the projection from the face of the footing or wall, but it should not be less than 150mm. This minimum thickness requirement is in place to ensure that the strip foundation is sufficiently rigid and able to bridge weak pockets in the soil. It also helps the foundation withstand longitudinal forces that may be generated by thermal contraction and expansion, as well as movements caused by changes in moisture content in the soil underneath the footing or wall.
In particular, when the soil underneath the foundation is composed of clay, which is known for its swelling properties, the pressure exerted on the foundation can be significant. To mitigate this, a minimum thickness requirement is necessary to ensure that the strip foundation can withstand the potential swelling of clay and remain stable over time. This minimum thickness also helps prevent damage to the footing or wall due to movements caused by changes in moisture content in the clay soil, which can further ensure the stability and durability of the foundation.
Thickness of Strip Foundation Supporting Heavy Loads
According to the given context, the ability of strip foundations to support large loads is dependent on the strength of the foundation to withstand shear and bending moments that may cause failure of the foundation projection. The thickness of the foundation is therefore determined by this strength, ensuring that it can support the loads it is intended to carry without succumbing to shear or bending failure.
Two types of failure were described in the context – bending and shear failure. Bending failure occurs when the foundation projection is unable to support the bending moments exerted upon it, causing it to fail. Shear failure, on the other hand, occurs when the foundation is unable to withstand the shear forces acting upon it.
The use of reinforcements is an important consideration in strip foundation design. If reinforcements are not embedded into the foundation, the thickness of the foundation may be controlled by bedding failure, which can compromise the integrity of the foundation. Therefore, the inclusion of reinforcements is necessary to ensure that the strip foundation is able to withstand the loads it is designed to carry, and avoid potential failure modes such as bending, shear or bedding failure.
Fig.2: Bending and shear failure of strip foundation
To prevent bending failure, it’s crucial to ensure adequate concrete thickness in construction. One effective approach is to implement stepped or sloped transitions in the concrete thickness from the wall face to the bottom width. In some cases, strip foundations are designed conservatively by selecting a thickness that prevents tension development on the strip’s underside. This thickness is often twice the strip projection. However, when considering a 45-degree load distribution at the strip foundation’s base, a small degree of tension stress at the foundation base may be acceptable. The magnitude of this tension stress, however, is currently unknown.
Thickness of Deep and Wide Strip Foundation
When designing strip foundations, it is important to consider the balance between the depth and width of the foundation in relation to the amount of concrete used. Using excessive amounts of concrete can be wasteful and costly, particularly if it does not contribute to the load-bearing capacity of the foundation. One way to reduce the amount of concrete used is to create stepped projections, but the cost of constructing the necessary formwork must be weighed against the potential savings. Sloping projections can also be an economical choice, as long as the slope ratio does not exceed one vertical to three horizontal, as greater slopes require more expensive formwork. For wide or heavily loaded strip foundations, it may be worth comparing the costs of reinforced and unreinforced construction, particularly if the foundation needs to extend below a layer of weak soil. In some cases, unreinforced foundations may be more cost-effective, as they require less concrete and can be constructed with a lower ratio of 1:9 in non-aggressive soil.