General specification for flexure design
Beams are designed on
the basis of limit state of collapse in flexure and checked for other limit
states of shear, torsion and serviceability. To ensure safety the resistance to
bending, shear, torsion and axial loads at every section should be greater than
the appropriate values at that produced by the probable most unfavourable
combination of loads on the structure using the appropriate safety factors. The
following general specifications and practical requirements are necessary for
designing the reinforced cement concrete beams.
a. Selection of grade of concrete
Apart from strength and deflection,
durability shall also be considered to select the grade of concrete to be used.
Table 5 of IS 456:2000 shall be referred for the grade of concrete to be used.
In this table the grade of concrete to be used is recommended based on the
different environmental exposure conditions.
b. Selection of grade of steel
Normally Fe 250, Fe 415 and Fe 500 are
used. In earthquake zones and other places where there are possibilities of
vibration, impact, blast etc, Fe 250 (mild steel) is preferred as it is more
c. Size of the beam
The size of the beam shall be fixed
based on the architectural requirements, placing of reinforcement, economy of
the formwork, deflection, design moments and shear. In addition, the depth of
the beam depends on the clear height below the beam and the width depends on
the thickness of the wall to be constructed below the beam. The width of the
beam is usually equal to the width of the wall so that there is no projection
or offset at the common surface of contact between the beam and the wall.
The commonly used widths of the beam are
115 mm, 150 mm, 200 mm, 230 mm, 250 mm, 300 mm.
d. Cover to the reinforcement
Cover is the certain thickness of
concrete provided all round the steel bars to give adequate protection to steel
against fire, corrosion and other harmful elements present in the atmosphere.
It is measured as distance from the outer concrete surface to the nearest
surface of steel. The amount of cover to be provided depends on the condition
of exposure and shall be as given in the Table 16 of IS 456:2000. The cover
shall not be less than the diameter of the bar.
e. Spacing of the bars
The details of spacing of bars to be
provided in beams are given in clause 26.3.2 of IS 456. As per this clause the
following shall be considered for spacing of bars.
The horizontal distance between two
parallel main bars shall usually be not less than the greatest of the following
Diameter of the bar if the diameters are
The diameter of the larger bar if the
diameters are unequal
5mm more than the nominal maximum size
of coarse aggregate
Greater horizontal spacing than the
minimum specified above should be provided wherever possible. However when
needle vibrators are used, the horizontal distance between bars of a group may
be reduced to two thirds the nominal maximum size of the coarse aggregate,
provided that sufficient space is left between groups of bars to enable the
vibrator to be immersed.
Where there are 2 or more rows of bars,
the bars shall be vertically in line and the minimum vertical distance between
the bars shall be of the greatest of the following
Maximum size of aggregate
Maximum size of bars
Maximum distance between bars in tension
maximum distance between parallel reinforcement bars shall not be greater than
the values given in table 15 of IS 456:2000.