Design of Isolated Column Footing
The objective of design is to determine
• Area of footing
• Thickness of footing
• Reinforcement details of footing (satisfying moment and shear considerations)
• Check for bearing stresses and development length
This is carried out considering the loads of footing, SBC of soil, Grade of concrete and Grade of steel. The method of design is similar to the design of beams and slabs. Since footings are buried, deflection control is not important. However, crack widths should be less than 0.3 mm.
The steps followed in the design of footings are generally iterative. The important steps in the design of footings are;
• Find the area of footing (due to service loads)
• Assume a suitable thickness of footing
• Identify critical sections for flexure and shear
• Find the bending moment and shear forces at these critical sections (due to factored loads)
• Check the adequacy of the assumed thickness
• Find the reinforcement details
• Check for development length
• Check for bearing stresses
Limit state of collapse is adopted in the design pf isolated column footings. The various design steps considered are;
• Design for flexure
• Design for shear (one way shear and two way shear)
• Design for bearing
• Design for development length
The materials used in RC footings are concrete and steel. The minimum grade of concrete to be used for footings is M20, which can be increased when the footings are placed in aggressive environment, or to resist higher stresses.
Cover: The minimum thickness of cover to main reinforcement shall not be less than 50 mm for surfaces in contact with earth face and not less than 40 mm for external exposed face. However, where the concrete is in direct contact with the soil the cover should be 75 mm. In case of raft foundation the cover for reinforcement shall not be less than 75 mm.
Minimum reinforcement and bar diameter: The minimum reinforcement according to slab and beam elements as appropriate should be followed, unless otherwise specified. The diameter of main reinforcing bars shall not be less 10 mm. The grade of steel used is either Fe 415 or Fe 500.
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