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Chapter: Civil : Design of Reinforced Concrete Elements : Limit State Design Of Footing

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

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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Civil : Design of Reinforced Concrete Elements : Limit State Design Of Footing : Design of Isolated Column Footing |


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