Limit state of serviceability for flexural
The check for deflection is done
through the following two methods specified by IS 456:2000 (Refer clause 42.1)
In this method, the deflection criteria of the
member is said t o be satisfied when the actual val ue of span to depth ratio
of the member is less than the permissible values. T he IS code procedure for
calculating the permissible values are as given below
Choosing the basic values of span to effective
depth ratios (l/d) from the following, depending
on the type of beam
Cantilever = 8
Simply supported = 20
Continuous = 26
the value of basic span to depth ratio to get the allowable span to depth
Allowable l/d = Bas ic
l/d x Mt x Mc x Mf
Where, Mt = Mo ification factor
obtained from fig 4 IS 456:200 0. It depends on the area of tension reinforcement
provided and the type of steel.
Mc = Modificatio n factor obtained from
fig 5 IS 456:2000. Thi s depends on the area of compress ion steel used.
Mf = Reduction f actor got from
fig 6 of IS 456:2000
Note: The basic values of l/d
mentioned above is valid upto spans of 10m. The basic values are multiplied by
10 / span in meters except for cantilever. For cantilevers whose span exceeds
10 m the theoretical method shall be used.
method of checking deflection
The actual deflections of the members are calculated as per
procedure given in annexure 'C' of IS 456:2000. This deflection value shall be
limited to the following
i. The final
deflection due to all loads including the effects of temperature, creep and
shrinkage shall not exceed span / 250.
deflection including the effects of temperature, creep and shrinkage occurring
after erection of partitions and the application of finishes shall not exceed
span/350 or 20 mm whichever is less.