WORKING STRESS METHOD

WORKING STRESS METHOD

In this article we discuss about working stress method for the design of beams 

Then we will discuss modular ratio,uses of modular ratio, assumption in Working stress method also moment of resistance offered

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• This method is a traditional design method in which concrete & steel are 

assumed to behave in elastic limit (linear behavior).

• In this method permissible stresses are considered for design which have linear 

value as compared to design stresses.

• Factor of safety for concrete = 3.

• 𝜎CbC = 𝑓𝑐𝑟/3= 0.33fck

• factor of safety for steel = 1.78

𝜎st = 𝑓 𝑦/1.78= 0.55 fy.

where,

 𝜎Cbc = permissible bending compressive stress in concrete

𝜎st = permissible tensile stress in steel

Modular Ratio ( m ) 

m = 𝐸s/Ec =𝐸𝑠/5000√𝑓𝑐𝑘

m = 280/3𝜎𝑐𝑏𝑐

• Effect of creep & shrinkage is considered the modulus of elasticity of concrete 

decreases & modular ratio increases.

However the effect of creep shrinkage are not considered for working stress 

method.

• As per IS456:2000

m = 280/3𝜎𝑐𝑏𝑐

Assumptions

1. Plane section remains plane before & after bending. It represent the variation of 

strain is const. along the given span.

2. All the tensile forces are carried by steel only & compressive forces are carried 

by concrete only.

3. There is no initial stress in steel when steel is embedded in coreete.

4. The bond b/w concrete & concrete & steel is perfect within elastic limit.

5. The modulus of elasticity of concrete & steel are remains constant & does not 

change with time.

• Critical depth of N.A. represents the depth of axis when concrete & steel attains its max. permissible values.

• The depth of critical axis depends upon grade of steel only.