A solid foundation supports the structure to stand marvellously. Spread footing spreads the load evenly, enabling the building to bear the heavy loads. A strong foundation is key for structures, as it acts as a structural element for bearing and distributing the loads in a smooth and equal manner. By spreading the weight across the foundation, it reduces the stress on the soil below. In this article, we are going to explore types of spread footing, applications, advantages and disadvantages, and design considerations.
What are Spread Footings?
A solid foundation system is used to support building columns and bridges with good load-bearing capacity. The potential failure of soil can be prevented by equally distributing the heavy loads of the structures at every edge. The applied load of the structure increases the stability of the soil and load-bearing capacity while integrated with the spread footing foundation system.
A spread footing is a foundation type that supports tall and heavy structures with their unique properties. Spread footings effectively stabilise the building by balancing the upward soil pressure with the downward load of the structure. While designing the spread footings, the linear distribution of soil pressure can be measured to guarantee that the footing is rigid enough to cause only small deformations when a heavy load is applied.
Types of Spread Footings
In the case of building bridges, each column should be supported individually to create a robust foundation system. Here are the types of spread footings that can be widely used in providing tremendous supporting foundations:
- Wall Spread Footing (Strip Footing)
The weight of the building is spread across a larger area in a wall-spread footing. It is created by adding thickness to either reinforced or unreinforced concrete behind load-bearing walls. Wall footings are frequently used in dwellings to give both structural and non-structural walls solid support. Concrete, brick, or stone are used to build wall spread footing.
- Combined Footing
Two or more columns arranged in a straight line can be supported by combined footing. In some situations, such as when building a house, combined footings are utilised even though isolated footings are more economical for each column.
Two primary categories of combined footing exist:
- When the columns are comparable in size, rectangular combined footing is utilised.
- When there are size differences between the columns and restrictions on the amount of space they can occupy, the trapezoidal combined footing is utilised.
- Isolated Spread Footing (Column Spread Footing)
Any individual column can be supported by isolated spread footing. It is popular in areas with stable soil conditions, is cost-effective, and is simple to build.
- Continuous Spread Footing
A continuous spread footing foundation is frequently used for stability in earthquake-prone areas, supports many columns, and comprises a continuous beam linking each column base.
- Strap Footing
Strap footing connects two columns with a strap or cantilever footing to help transmit weights evenly, particularly in situations where column separation is significant.
- Grillage Footing
A grillage footing is a shallow foundation that is utilised in loose or damp soil, is supported by large loads, and is composed of several beam layers over concrete.
- Inverted Arch Footing
Inverted arch footings disperse loads over soft soil and were historically common for multi-story buildings, but they are rarely used today.
- Raft Foundation (Mat Foundation)
This reinforced concrete slab, also known as a mat foundation, distributes loads over a wide area and supports several columns or walls, making it perfect for soft soil.
Applications of Spread Footing
- Spread footings are reasonably priced and appropriate for a range of constructions, including factories, residences, and commercial buildings.
- Strip footings are frequently used to continually support load-bearing walls in warehouses and multi-story buildings.
- Raft footings are perfect for major constructions like bridges and high-rise buildings because they distribute loads evenly and steadily.
- Pile foundation, which are frequently employed in infrastructure projects, oil facilities, and marine construction, are required for difficult soils.
- Because they require less excavation and take up less space, combined footings are ideal for structures with intricate designs or small foundation areas.
Advantages of Spread Footing
As a popular foundation system in the construction field, spread footings have the following benefits:
- Compared to load-bearing foundations, they feature a wider base, which helps to distribute the weight over a greater area for increased stability.
- Spread footings offer constant support throughout the building, in contrast to spot footings, which only make contact with the ground at a single point.
- They support the foundation or pillars of a building.
- Usually constructed of reinforced concrete, spread footings increase strength and lower failure risk.
- Structures with spread footings are more resilient and less likely to sustain damage since they disperse the weight more evenly.
Disadvantages of Spread Footing
While using spread footing as a foundation, one may encounter specific disadvantages as follows:
- Different kinds of soil require different spread footing methods.
- Because a lot of concrete needs to be prepared, it is challenging to fill the entire space at once.
- Concrete pouring is a time-consuming task that demands a team rather than a single person.
Materials Used in Spread Footings
The basic materials that can be used to construct spread footings are as follows:
- Plain concrete or reinforced concrete
- Stone
- Brick
Construction Process of Spread Footings
The initial stage of the construction process: Digging the trench, laying forms, adding reinforcement rods, pouring concrete, and then backfilling the pit with the excavated dirt are the first steps in the spread footing building process.
Excavation Depths: Wall support is usually not necessary for shallow excavations (up to 5 feet), but sloping or wall support is necessary for deeper excavations (particularly for depths over 5 feet) for safety.
Supporting Excavation Walls: Vertical, supported walls are required for excavations close to structures or property lines. Vertical timber boards can be used for excavations under 10 feet deep; more substantial support systems are required for deeper excavations.
Techniques for Deep Excavations: Steel sheet piles or soldier pile systems are used to support vertical cuttings in place of timber sheeting for excavations deeper than 12 to 15 feet.
Special Support Systems: Steel sheet piles may be utilised in place of soldier piles in unstable soils, such as dry sand, and tiebacks or inclined braces may be utilised to support soldier piles in specific soil situations.
Design for Spread Footing
Check for Soil Bearings:
Calculates the size of the footing and confirms that the soil can sustain the weight of the construction without failing. The actual pressure beneath the footing must be less than the soil’s allowable bearing capability.
Check for Shear:
Guarantees that the footing is capable of withstanding both one-way and two-way shear stresses. One-way shear makes sure the shear doesn’t go beyond the permitted shear by checking it across the footing’s width. To avoid failure, two-way shear checks punching shear close to columns.
Flexural Check:
Establishes how much reinforcement is required to withstand the bending (moment) brought on by the load on the structure. Guarantees that the footing’s moment capability exceeds the actual moment.
Inspecting Development Length:
Confirms that the reinforcing bars are embedded into the footing for a sufficient amount of time to reach their full potential strength.
Verification of Stability:
By comparing the resisting and overturning moments, the overturning check makes sure the footing won’t topple over when the structure is loaded.
- Sliding Check: Verifies that under horizontal stresses, the footing won’t slide.
- Uplift Check: This check verifies that the footing is stable and able to withstand upward forces from the structure.
Determining the soil’s bearing capacity, shear resistance, bending strength, development length, and stability under different forces are all important considerations when building a spread footing. Every step guarantees that the foundation can sustain the weight and stay stable.
What is the Difference Between Spread Footings and Mat Foundations?
Here we can see the tabular difference between spread footings and mat foundations as follows:
| S.No | FEATURES | MAT FOUNDATIONS | SPREAD FOOTINGS |
| 1. | Cost | More expensive due to higher concrete, steel, and labour requirements | More affordable as it requires less material and labour. |
| 2. | Performance | More efficient in distributing loads evenly and reducing soil stress | Less efficient, localised load distribution might stress the soil more |
| 3. | Construction | More complex, requiring more excavation, formwork, and curing | Easier and quicker to construct with less excavation and formwork |
| 4. | Durability | More durable, less prone to cracking, settlement, and erosion | Less durable and may experience more cracking and settlement. |
| 5. | Maintenance | Requires less maintenance, especially in stable conditions | May need more frequent repairs depending on soil and load conditions. |
Conclusion
Spread footings are versatile. They can support various construction applications like buildings, bridges, towers, industrial, commercial, and residential structures. It uniformly transmits the heavy load of the structure to the ground and ensures the stability of the building constructed above it.
FAQ’s
A popular foundational system that distributes the weight of the structure or building is termed a spread footing. They effectively stabilise the building by balancing the upward soil pressure with the downward load of the structure.