Isolated footing is a type of shallow foundation used to support individual columns or pillars. It is typically used when the loads from the structure are relatively light and the soil is strong enough to support the weight of the structure.
Isolated footings are designed to transfer the load from the column or pillar to the soil below. They are usually square or rectangular and are constructed of reinforced concrete. The size and depth of the footing are determined by the load of the column and the properties of the soil.
Different types of isolated footing
Isolated footing types based on the shape
- Square footing
- Rectangular footing
- Circular footing
- Trapezoidal footing
- Triangular footing
Square footing
This is the most common type of isolated footing. It is a square-shaped footing that supports a single column or pier.
Advantages of square footing
- Easy to construct
- Suitable for square or rectangular columns
- Provides uniform distribution of load over the soil
- Less chance of soil settlement or uneven settlement.
Disadvantages of square footing
- Not suitable for circular columns
- Not suitable for irregular column loads
- Requires larger area compared to circular footing for the same load-bearing capacity.
Rectangular footing
Similar to a square footing, but with a rectangular shape. It is used when the column load is not symmetrical.
Advantages of rectangular footing
- Suitable for rectangular or long columns
- Provides uniform distribution of load over the soil
- Less chance of soil settlement or uneven settlement
- Can be designed for irregular column loads.
Disadvantages of rectangular footing
- Not suitable for circular columns
- Requires larger area compared to circular footing for the same load-bearing capacity.
Circular footing
This type of footing is circular and is used when the column load is circular.
Advantages of circular column
- Suitable for circular columns or loads
- Requires smaller area compared to rectangular or square footing for the same load-bearing capacity
- Provides uniform distribution of load over the soil.
Disadvantages of circular column
- Difficult to construct
- Not suitable for irregular column loads.
Trapezoidal footing
This type of footing is used when the column load is not symmetrical. It is a trapezoidal-shaped footing that is wider at the bottom and narrower at the top.
Advantages of Trapezoidal footing
- Suitable for irregular column loads
- Provides uniform distribution of load over the soil
- Requires less area compared to rectangular footing for the same load-bearing capacity.
Disadvantages of Trapezoidal footing
- Difficult to construct compared to the square or rectangular footing.
Triangular footing
This type of footing is used when the column load is not symmetrical. It is a triangular-shaped footing that supports the load of a single column or pier.
Advantages of triangular footing
- Suitable for irregular column loads
- Requires less area compared to rectangular or square footing for the same load-bearing capacity.
Disadvantages triangular footing
- Difficult to construct compared to the square or rectangular footing.
Isolated footing types based on loads
- Strap footing
- Stepped footing
- Sloped footing
Strap footing
This type of footing is used when the column load is eccentric or when two columns are very close to each other. It consists of a rectangular or trapezoidal shape footing connected by a narrow strip of concrete to another rectangular or trapezoidal footing.
Advantages of Strap footing
- Suitable for cases where the column loads are eccentric or unequally distributed
- Provides greater stability against lateral loads
- Reduces the width of the foundation.
Disadvantages of Strap footing
- Difficult to construct
- Requires special formwork and reinforcement.
Stepped footing
This type of footing is used when the soil-bearing capacity is not uniform. It consists of two or more steps, with each step having a smaller area than the one above it.
Advantages of Stepped footing
- Suitable for cases where the soil-bearing capacity is not uniform
- Provides stability against lateral loads
- Reduces the width of the foundation.
Disadvantages of Stepped footing
- Difficult to construct
- Requires special formwork and reinforcement.
Sloped footing
This type of footing is used when the soil is sloped. The footing is sloped to match the slope of the soil.
Advantages of Sloped footing
- Suitable for cases where the soil slope is significant
- Provides stability against lateral loads.
Disadvantages of Sloped footing
- Difficult to construct
- Requires careful consideration of the slope and its effect on the foundation.
Advantages of Isolated Footing
- Cost-effective: Isolated footings are cost-effective to construct compared to other types of foundations, such as raft foundations or pile foundations.
- Easy to construct: These footings are relatively easy to construct, requiring less labor and equipment compared to other types of foundations.
- Requires less excavation: These footings require less excavation compared to other types of foundations, reducing the amount of soil that needs to be removed from the construction site.
- Customizable: Isolated footings can be customized to suit different soil and load conditions, making them a flexible option for a variety of construction projects.
- Suitable for light loads: These are suitable for supporting light loads, such as residential and light commercial structures.
- Less disruptive to neighboring structures: These do not require deep excavation or significant soil disturbance, making them less disruptive to neighboring structures and the environment.
- Provides stability: These footings provide stability to the column or pillar they support, ensuring the safety of the structure.
- Reduces settling: These footings help to distribute the load of the structure evenly over the soil, reducing the likelihood of settling or sinking over time.
- Allows for easy maintenance: These footings are easy to access and maintain, allowing for easy inspection and repair if necessary.
- Fast construction: These footings can be constructed quickly, reducing the overall construction time of the project.
Disadvantages of Isolated Footing
- Limited load capacity: These footings have a limited load-bearing capacity and may not be suitable for heavy structures or loads.
- Vulnerable to differential settlement: These footings can be vulnerable to differential settlement, which occurs when the soil under one part of the foundation settles more than another. This can cause the foundation to become uneven and lead to structural damage.
- Depth limitations: These footings may be limited in depth by the presence of underground utilities, rock formations, or other obstacles.
- Vulnerable to overturning: Isolated footings may be vulnerable to overturning forces in areas with high winds or seismic activity.
- Unsuitable for poor soil conditions: These footings may not be suitable for construction sites with poor soil conditions, such as soft or compressible soils.
- Prone to moisture: These footings may be prone to moisture damage if not properly designed and constructed, which can lead to structural damage and decay.
- Requires careful planning: Isolated footings require careful planning and design to ensure they are properly sized and located to support the intended load.
- Limited use in complex structures: These footings may not be suitable for complex or irregularly shaped structures that require more specialized foundation systems.
A quick comparison between strap stepped, and sloped footing
| Feature | Strap Footing | Stepped Footing | Sloped Footing |
| Load Capacity | High | Moderate | Moderate |
| Stability | High | High | High |
| Lateral Support | Excellent | Excellent | Good |
| Constructability | Medium | Medium | Medium |
| Cost | High | High | High |
| Application | Uneven loads, closed columns | Uneven soil, unbalanced loads | Sloped terrain |
