A footing that is provided under an individual column is known as isolated footing. It is used to carrying a load of superstructure through the Reinforced concrete column and transfer to the soil. This is one of the most economical footings as compared to other types of footing. It is due to the reinforcement provides in this footing is in the bottom layer only. It is the most commonly used foundation.
When to provide and where to provide isolated footing?
- It is used when a safe bearing capacity is very high.
- Loading coming from superstructure is low.
- The distance between the columns is large.
- It is located generally within the property line.
- Most economical
- Easy to construct.
- Highly skilled labors are not required.
Types of Isolated footing
There are three types of Isolated footing which consist of varying footing thickness.
A footing which has a uniform thickness and provided below an individual column is known as pad footing. In pad footing, there is uniform upward pressure exerted by the soil. To get a clear idea, please follow the below Figure.
A footing which consists of a series of the step below an individual footing is known as stepped footing. Steps height should not be greater than ¾ quarter of step length. The thickness at the edge of the footing shall not be less than 150 mm. To get a clear idea, please follow the below Figure.
Slope Footing is one of the types of shallow foundation which is the combination of rectangular and trapezoidal shape. The slope of trapezoidal is 45 degrees from all sides. It reduces the amount of concrete, reinforcement, and Formwork. To get a clear idea, please follow the below Figure.
To know the Complete Quantity of Isolated footing from Excavation to Backfilling. Read this article; Isolated Footing – Complete Quantity Calculation of Excavation, Brick Soling, PCC Work, RCC Work, Neck Column, and Back Filling.
See the plan, and section of different types of isolated footing.
The shape of Isolated footing
- Square footing
- Rectangular footing.
- Circular footing.
See the plan view of different shape of isolated footing.
Design of Isolated Footing
The requirement for the Designing of Footing are;
- Safe Bearing Capacity of soil.
- Grade of concrete.
- Grade of steel.
- A load of Column.
- Size of Column.
Why we design an isolated foundation?
The main objective of isolated foundation is to find out
- Size of Footing.
- The thickness of the footing.
- Calculation of Area of steel in x-direction and y-direction.
- Calculation of Development length.
- Checking of bearing stress.
Following are the design steps of footing as per IS 456-2000.
- Calculation of Area of Footing from factored column load
- Calculation of Net soil pressure
- Calculate of Net factored soil pressure.
- Find the effective depth of Footing.
- Find out the critical Section for bending moment and shear forces
- Check for One-way shear (One-way shear)
- Check for punching shear stress (Two-way shear).
- Calculation of Bending moment
- Calculation of main reinforcement and distribution steel.
- Check for development length
- Check for bearing stress.