DOUBLY REINFORCED SECTION

DOUBLY REINFORCED SECTION

Whenever the size of beam is restricted
and beam has to bear higher value of B.M.
than the moment of Resistance of the
( M.R.balance)balance section of given beam,
then to resist this higher moment doubly
reinforced section is to be provided.
There are two methods to design such
beams:

1) Increase the concrete mix to increase the capacity of the section

2) Reinforcement are provided in compression zone to give additional strength to the concrete in compression. Such beam are called double reinforced beams.

• A concrete structure when loaded
undergoes elastic as well as plastic
deformation.
• The plastic deformation is termed as
creep.
• Elastic deformation is an instantaneous
process while creep is a long process.
• The creep deformation of concrete
produces additional strain in
compression steel and gradually raises
the level of stress.
• To account for this increases in stress,
the modular ratio is increased.
• IS 456 states that the permissible stress
for compression in bars account, shall
be taken as the calculated compressive
stress in the surrounding concrete
multiplied by 1.5 times the modular
ratio or σsc whichever is lower where
σsc is the permissible stress in
compression in column bars.
• Generally the use of compression steel
is considered uneconomical. However,
in some special cases it can be
advantageous.

• Advantage of compression steel

a) It permits smaller size which look
aesthetic
b) It reduces the long term deflection
and increases ductility of the beam
c) It can be used as anchor bar for
positioning the shear reinforcement
d) As the compression reinforcement
increases ductility of beam, they are
provided (even when not required
for strength) in the seismic zone to
with stand repeated reversals
produces.

Note:-

 In tension steel, shrinkage reduces
the tensile stress and creep produces
additional tensile stress but in compressive
steel both shrinkage and creep add
additional stress so we have to use
different value of modular ratio (m).
In case of Doubly reinforced section stress
in steel on tension side = m × stress in
surrounding concrete. But as per IS456, for
steel in compression zone, Stress in steel =
1.5m × stress in surrounding concrete =
1.5m × fcbc