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Civil - Design of Reinforced Concrete Elements - Limit State Design For Flexure

Limit state of serviceability for flexural members

   Posted On :  16.07.2016 09:43 pm

The check for deflection is done through the following two methods specified by IS 456:2000 (Refer clause 42.1)


Limit state of serviceability for flexural members:

 

Deflection

 

The check for deflection is done through the following two methods specified by IS 456:2000 (Refer clause 42.1)

1      Empirical Method

 

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

 

a.                 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

b.    Modify the value of basic span to depth ratio to get the allowable span to depth

 

ratio.

 

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.

 

2      Theoretical 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.

 

ii.    The 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.

 

 

Tags : Civil - Design of Reinforced Concrete Elements - Limit State Design For Flexure
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