Stress distribution in
rectangular beam under
Singly Rectangular Beam
A Beam is any structural member
which resists load mainly by bending.
Therefore it is also called flexural
member. Beam may be singly
reinforced or doubly reinforced. When
steel is provided only in tensile zone
(i.e. below neutral axis) is called
singly reinforced beam.
DesignofMembers by Working
Plane sections before bending remain
plane after bending.
Normally, concrete is not considered for
taking the tensile stresses except
otherwise specifically permitted.
Therefore, all tensile stresses are taken
up by reinforcement only.
The stress-strain relationship of steel
and concrete is a straight line under
If a beam cross section is limited
because of architectural or other
considerations, it may happen that the
concrete can not develop the
compression force required to resist
the given bending moment, in this
case ,reinforcement is added in the
compression zone, resulting in so
called Doubly reinforced beam
.i.e., one with compression as well as
Why Doubly reinforcement
If concrete can not develop the
required compressive force to resist
the maximum bending moment, then
additional reinforcement is provide in
the compression zone.
Reinforcement is provided in both
compression and tension zone.
Development of moment
In stress distibution,the design
moment is more than the balanced
moment of resistance of the section,
M = M1 + M2
The additional moment M2 is resisted
by providing compression
and additional tensile reinforcement
Moment is obtained by,
Reinforcement in Beam
The total tensile reinforcement Ast
has two components As1+As2 for M1
The equation of Ast,
As2 = M2/[Ó (d-d’)]
Clear cover for design
Not less than 1.5 in. when there is
no exposure to weather or contact
with the ground
For exposure to aggressive weather
CLEAR DISTENCE OF
Clear distance between parallel
bars in a layer must not be less
than the bar diameter or 1 in.