Working stress Method OR Modular ratio method

1. This was the first theoritical method developed in 1900's for the design of rcc structure.
2. In this assumed that both concrete and steel act togather and prefectly elastic at all stages so that modular ratio (Modili of elastic of steel and concrete) can be used to determine the stresses in steel and concrete.
3. This method adopts permissible stresses which are obtained by applying factor of safety on material for design.
4. factor safety used for cubic strength of concrete = 3  and 1.8 for yeild stregth steel
5. this method has three defects :

• This method neighter show real strenght nor gives true factor of safety againt the structure failure.
• it gives uneconomic section.section are consrtucted big than limited state design
• concrete does not have definite modular of elasticity as in steel.

1. Design moment and shear in structure are calculated by elastic analysis with respect to service load applied on structure.
2. stresses in the concrete and steel are calculated on the basis elastic behaviour of the section.
3. Modular ration is taken may be either a conatant value for all strenght of concrete or may be vary according to strenght of concrete and steel.
4. IS 456 recommands modular ratio of elasticity of concrete which varies with rescepct to the strenght of concrete.