Working Stress Method

  • Working Stress Method is the traditional method of design not only for Reinforced Concrete but also for structural steel and timber design.
  • The conceptual basis of the Working Stress Method assumes that the structural material behaves in a linear elastic manner and that appropriate safety can be ensured by suitably limiting the stresses in the material due to the presumed working loads (service loads) on the structure.

  • Working Stress Method also assumes that both the steel reinforcement and concrete act together and are perfectly elastic at all stages, and hence the modular ratio can be used to determine the stresses in steel and concrete.
  • The stresses under the working loads are obtained by applying the methods of ‘strength of materials’ like the simple bending theory. The limitations due to non-linearity and buckling are neglected.

  • The stresses caused by the ‘characteristic’ or service loads are checked against the permissible (allowable) stress, which is a fraction of the ultimate or yield stress. The permissible stress may be defined in terms of a factor of safety, which takes care of the overload or other unknown factors.

Permissible (allowable) stress = Ultimate or yield stress/Factor of safety

Thus, in Working Stress Method,
Working stress ≤ Permissible stress

Limitations of Working Stress Method

1.The main assumption of a linear elastic behavior and the implied assumption that the stresses under working loads can be kept within the ‘permissible stresses’ are found to be unrealistic. Many factors are responsible for this, such as the long-term effects of creep and shrinkage and other secondary effects.

2.The use of the imaginary concept of modular ratio results in larger percentage of compression steel and generally larger member sizes than the members designed using ultimate load or limit states design. However, as a result of the larger member sizes, they result in better performance during service.

3.The stress–strain curve for concrete is non-linear and is time dependent. Thus, the elastic modulus is a function of the stress level (it may also change with age) and hence the modular ratio is not really constant. This method does not consider the consequences of this material non-linearity.
4. Working Stress Method does not discriminate between the different types of loads that act simultaneously but have different degrees of uncertainty. This may result in unconservative designs, particularly when two different loads (say, dead loads and wind loads) have counteracting effects.