Steel reinforcement in concrete beams consists of longitudinal bars and stirrups. Longitudinal bars resist tension forces from bending while stirrups prevent the longitudinal bars from buckling outwards. There are two types of potential beam failures: flexure failure and shear failure. Flexure failure, where vertical cracks develop on the bottom face in tension, is preferred as it provides warning through crack development. Shear failure can occur suddenly without warning signs. Proper design of beams involves selecting rebar grades, sizes, and spacing to ensure flexure failure occurs before unpredictable shear failure.

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40. Figure 1: Steel reinforcement in beams - stirrups
prevent longitudinal bars from bending outwards.
Figure 2: Two types of damage in a beam:
flexure damage is preferred. Longitudinal bars
resist the tension forces due to bending while
vertical stirrups resist shear forces.
Inclined crack
Flexure Failure
Shear Failure(b)
(a)
Bottom face stretches in tension
and vertical cracks develop
Longitudinal Bar
Larger diameter steel bars that
go through the full length of the
beam
Vertical Stirrup
Smaller diameter steel
bars that are made into
closed loops and are
placed at regular
intervals along the full
length of the beam
45
Beam
ColumnColumn
Column
Beam
Beam

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87. Figure 3: Location and amount of longitudinal
steel bars in beams – these resist tension due to
flexure.
At least 2 bars should go
full length of beam
Figure 6: Details of lapping steel reinforcement
in seismic beams – as per IS13920-1993.
Lapping prohibited in
regions where
longitudinal bars can
yield in tension
Lapping of longitudinal bars
Spacing of stirrups
as per calculations
(but not more than
d/2)
d
Spacing of stirrups
as calculated
(but not more than d/4
and 8 times beam bar
diameter)
2d 2d
2d 2d
Figure 5: Location and amount of vertical stirrups
in beams – IS:13920-1993 limit on maximum
spacing ensures good earthquake behaviour.
Spacing of stirrups
not more than 150mm
Figure 4: Steel reinforcement in seismic beams
- stirrups with 135 hooks at ends required as per
IS:13920-1993.
The ends of stirrups
are bent at 135.
Such stirrups do not
open during strong
earthquake shaking.
10 times
diameter of
stirrup
135º
135
Horizontal
Spacing
Total amount of steel
from calculation
Bottom steel at supports
at least half of that at top
Beam
Column Column
Column Column
Beam
Spacing of stirrups
as calculated
(but not more than d/4
and 8 times beam bar
diameter)
Column
Beam
Column
Preferred:
135 hooks in
adjacent
stirrups on
alternate sides