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.