WSM (Working Stress Method ) is a traditional method. WSM was introduced in 1900's. This is based on linear elastic theory.This method was not only for structural steel but also for reinforced concrete and timber design.
Assumption:-
a) Structural member behave in linear elastic manner.
b) Permissible (Allowable) stress are kept well below the material strength.
c) In this design method, members are designed to never beyond their elastic range.
d) The allowable stress is determined by suitably restricting the materials yield or ultimate strength.
| Allowable stress= | yield stress or ultimate stress |
| factor of safety |
Yield Stress - Ductile materials (i.e, Steel)
Ultimate stress - Brittle materials (i.e, Concrete)
Factor of safety (FOS):- As per IS (456-2000)
Concrete = 3
Steel = 1.78
** When we apply loads, stress induced in materials and we can calculate them using the principles of strength of materials (SOM).
**Reinforced cement concrete is a composite material.It has two different material concrete and steel.
Draw back of WSM :-
a) The actual stress distribution of concrete is not described. It is assume to be elastic but in reality, concrete is not elastic.
b) Same factor of safety is used for different type of loads.
c) The failure mode cannot be observed.
d) The warning before failure cannot be studied precisely.
e) We cannot account shrinkage and creep effects by using Working Stress Method.
Note:- As per IS-456 clause 18.22 , working stress method may be used only if it is not possible to adopt the limit state design.
