Reinforcement Detail of Spread or Isolated Footing
Among various types of foundation, the isolated footing is considered as one of the most recognized and easiest types of foundation. Foundations play vital role as the complete load of the building is transmitted to ground via the foundation.
Isolated footing is very effective for reinforced cement concrete column since it is simple and very inexpensive. The purpose of an isolated is to provide support for a single column. Isolated footings stand for independent footings which are arranged for each column.
Isolated footing is provided in the following conditions :-
• Columns are not placed narrowly.
• Loads on footings are fewer.
• The Safe bearing strength of the soil is normally greater.
The Isolated footings are made of a bottom slab. There are three basic types of bottom slabs :
• Pad footing having identical thickness
• Stepped footing having inconsistent thickness
• Sloped footing (trapezoidal section)
Due to upward soil pressure, footing twits in a dish-shaped form. So, it is necessary to arrange reinforced steel mesh to combat the two major internal forces like shear force and bending moment. When there are boundary of land or a road where it is not possible to rest the footing in neighbors’ property, footings should be designed as combined footings or even eccentric footing is utilized as a substitute of isolated footings.
The Design of Isolated Footing - The Isolated Footings is designed for the following purposes :-
1. Area of footing
2. Thickness of footing
3. Reinforcement Details of footing with a satisfying moment and shear force consideration.
4. Check for development length and shearing stresses.
All these things are performed on the basis of the loads on footings, safe bearing capacity (SBC) of the soil, grade of concrete and steel. Minimum cover to main reinforcement should not be lower than 50mm on the surface of the contact with the earth face. The diameter of main reinforcement should not be lower than 10mm.
The Steps involved in footing design :
• Determine an area of footings depending on factored loads.
• Guess suitable thickness of footing
• Determine the critical section for flexure and shear
• Determine the bending moment and shear forces at critical section
• Verify the suitability of the assumed thickness
• Determine the reinforcement detail
• Examine the bearing stresses
• Examine development length