Types and Design of Shallow Foundation :A shallow foundation is a type of foundation that transfers the load from the structure to the earth near the surface, as opposed to deep foundations, which transfer loads to deeper layers.
Shallow foundations are commonly used when the load-bearing capacity of the soil near the surface is sufficient to support the load of the structure. The depth of a shallow foundation is typically less than its width.
There are several types of shallow foundations, each with its specific use based on the type of soil, load requirements, and structural considerations. Below are the main types of shallow foundations explained in detail:
Types of Shallow Foundation
1. Spread Footing or Isolated Footing
A spread footing is one of the most common types of shallow foundations used to support individual columns or piers. This type of foundation spreads the load of the structure over a wide area of the soil, increasing the surface area of contact between the structure and the ground, which helps in distributing the load evenly.
- Construction: It consists of a wide base, typically made of reinforced concrete, which is placed below the column. The size of the footing is determined based on the load carried by the column and the soil’s bearing capacity.
- Application: Spread footings are ideal for lightly loaded structures like small buildings where the soil has sufficient load-bearing capacity near the surface.
- Advantages: Easy to construct, cost-effective, and suitable for low-rise structures.
2. Wall or Strip Footing
A strip footing is similar to a spread footing but is used for supporting walls rather than individual columns. It is a continuous strip of concrete or masonry that runs along the length of the wall, transferring the load evenly to the ground beneath.
- Construction: Strip footings are typically used for load-bearing walls and are generally wider than the wall they support. They are constructed from reinforced concrete and extend along the length of the wall.
- Application: These are used in structures where load-bearing walls are employed, such as in residential houses or low-rise buildings.
- Advantages: Economical and simple to construct, especially for long walls.
3. Combined Footing
A combined footing is used to support two or more columns when the columns are close enough to each other that individual spread footings would overlap. This type of footing is typically rectangular or trapezoidal in shape.
- Construction: The design of a combined footing is influenced by the position of the columns and the load they carry. It is generally constructed from reinforced concrete.
- Application: Combined footings are used where individual footings would not be feasible due to space constraints or when the soil’s bearing capacity is low, and larger areas are required to distribute the load.
- Advantages: It allows efficient load distribution for closely spaced columns and is useful in situations where property lines restrict the width of the foundation.
4. Cantilever or Strap Footing
A strap footing is similar to a combined footing but with a key difference: it uses a strap or a beam to connect an eccentric footing (which is not aligned directly beneath the load) to an interior footing.
- Construction: In a strap footing, a beam connects the footings of two columns. The load is transferred through the beam, preventing any eccentric moments in the foundation. It is used when a column is close to a property boundary, and the footing cannot extend beyond the boundary.
- Application: This type of foundation is used when one column is located near the property line, preventing the use of a conventional spread footing. The strap helps in balancing the load and eliminating any tilting effect.
- Advantages: Effective in using space efficiently and avoiding the extension of footings into restricted areas like property boundaries.
5. Raft or Mat Foundation
A raft foundation, also known as a mat foundation, is a large, continuous slab of reinforced concrete that supports multiple columns and walls. It is called a raft because it spreads the load over a large area, much like a raft floating on water.
- Construction: Raft foundations are typically constructed as a large slab, covering the entire footprint of the building. Reinforcement is added in both directions to handle the load effectively.
- Application: These foundations are used for large structures where individual footings are impractical due to weak or compressible soil. Raft foundations are also used in cases where soil settlement needs to be minimized or where the loads from the structure are significant.
- Advantages: Raft foundations distribute loads over a large area, reducing differential settlement and the need for deep foundations. They are especially useful in areas with weak soil or heavy structures.
6. Grid Foundation
A grid foundation, sometimes called a cellular raft or grillage foundation, is made up of intersecting beams arranged in a grid pattern. This system provides a lighter form of foundation compared to a solid raft, using less concrete while still distributing the load evenly.
- Construction: The grid is made from reinforced concrete beams placed at regular intervals, creating a lattice or grid that supports the structure above. The voids between the beams may be filled with compacted soil or left empty.
- Application: Grid foundations are used in areas with moderate load requirements, such as in industrial structures or warehouses where the loads are spread over a large area.
- Advantages: Grid foundations use less material compared to solid raft foundations while still providing good load distribution. They are more cost-effective in situations where load distribution is important but a full mat foundation is not necessary.
7. Slab-on-Grade Foundation
A slab-on-grade foundation consists of a simple concrete slab poured directly onto the ground. This type of foundation is common in residential construction and areas with stable soil conditions.
- Construction: The slab is usually reinforced with steel and may include a moisture barrier or insulation depending on the climate. The edges of the slab may be thicker to provide additional support where the load is concentrated.
- Application: Slab-on-grade foundations are often used for single-story homes, garages, and light commercial buildings, particularly in areas where frost action is not a concern.
- Advantages: This foundation is quick and easy to construct, cost-effective, and suitable for flat terrains. It also helps in providing a smooth, solid base for the building.
Shallow foundations are a practical and cost-effective option for a wide range of construction projects, especially when the soil near the surface has adequate load-bearing capacity. The choice of a particular type of shallow foundation depends on several factors, including the type of structure, the load it will carry, soil conditions, and space constraints.
Design of Shallow Foundation
1. Finalizing Plan Size depends Safe Bearing Capacity (SBC)
The plan size of the footing is determined based on the Safe Bearing Capacity (SBC) of the soil. SBC is the maximum pressure the soil can safely bear without risk of shear failure or excessive settlement. To decide the plan size, we calculate the load from the structure (serviceability load) and divide it by the SBC. The serviceability load is used because the SBC represents the allowable pressure under normal conditions, not extreme loads. The formula is:
Plan size of footing= Load on footing*1.1/SBC
P=Output of structural Analysis and input for footing
NGL=Natural Ground Level
BP=bearing Pressure ( exerted by a structure (or load)or column on the surface of a foundation or soil.
BC= Bearing Capacity (ability of soil or rock to support the loads applied to the ground by a foundation, structure, or other built object.)
2. Deciding Thickness Based on Shear and Flexure
The thickness of the footing is determined to ensure the footing can resist various types of shear and bending moments:
- One-way shear: This type of shear occurs along a single plane.
- Two-way shear (punching shear): This occurs when the load tries to punch through the footing around the column.
- Flexure: The bending of the footing due to the load from the column.
The thickness must be sufficient to resist these forces safely. The required depth of the footing is typically calculated using shear and flexure analysis, ensuring it has adequate strength to support the structure without cracking or failing.
3. Reinforcement Design Based on Flexure Requirements
Reinforcement bars (steel bars) are provided primarily to resist flexural stresses in the footing. The amount and spacing of reinforcement are calculated based on the bending moments caused by the loads. The reinforcement is typically placed in both directions in isolated footings, which are used for individual columns. In isolated footings, the centerline of the footing aligns with the column, ensuring that the load is evenly distributed.
Deciding the Founding Depth
The depth at which the footing is placed is known as the founding depth. It is usually chosen based on the following considerations:
- Frost line: In colder climates, the footing must be below the frost line to avoid damage from freeze-thaw cycles.
- Soil type: The depth also depends on the type of soil and its bearing capacity at different depths.
- Water table: The footing should be above the water table to avoid problems with water seepage, which could weaken the soil and the foundation.
- Minimum depth requirement: Usually, a minimum depth (e.g., 1.5m) is recommended for stability.
Why Use Serviceability Load to Find Plan Size
The serviceability load is used to find the plan size because this load represents the actual, long-term working loads that the structure will experience during its lifespan. The SBC is designed for these conditions and is a reduced value of the Ultimate Bearing Capacity (UBC), considering a factor of safety. The formula for SBC is:
SBC= Ultimate Bearing Capacity (UBC)/Factor of Safety
Using the serviceability load ensures that the footing can safely support the structure without excessive settlement, which could lead to cracking or other damage.