Working Stress method
This method of design was the oldest one. It is based on the elastic theory and assumes that both steel and concrete are elastic and obey Hook’s law. It means that the stress is directly proportional to strain up to the point of collapse. Based on the elastic theory, and assuming that the bond between steel and concrete is perfect, permissible stresses of the materials are obtained. The basis of this method is that the permissible stresses are not exceeded any where in the structure when it is subjected to worst combination of working loads.
In this method, the ultimate strength of concrete and yield strength or 0.2% proof stress of steel are divided by factors of safety to obtain permissible stresses. These factors of safety take into account the uncertainties in manufacturing of these materials. As per IS456, a factor of safety of 3 is to be used for bending compressive stresses in concrete and 1.78 for yield/proof strength of steel.
The main drawbacks of the working stress method of design are as follows :
(i) It assumes that concrete is elastic which is not true as the concrete behaves in-elastically even on low level of stresses.
(ii) It uses factors of safety for stresses only and not for loads. Hence, this method does not give true margin of safety with respect to loads because we do not know the failure load.
(iii) It does not use any factor of safety with respect to loads. It means, there is no provision for the uncertainties associated with the estimation of loads.
(iv) It does not account for shrinkage and creep which are time dependent and plastic in nature.
(v) This method gives uneconomical sections.
(vi) It pays no attention to the conditions that arise at the time of collapse.
The working stress method is very simple and reliable but as per IS 456:2000 the working stress method is to be used only if it is not possible to use limit state method of design. Working stress method is the basic method and its knowledge is essential for understanding the concepts of design.
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