(i) To decide the size of the member and amount of the steel reinforcement required.
(ii) To check whether the designed structure will perform safely and satisfactorily during its life span.
Various Methods Used For Design Of R.C.C. Structures Are As Follows:
2. Limit State Method
Working Stress Method (WSM)
Working stress method is the oldest one. The salient features of this method are :
(i) This method is based on the linear elastic theory and assumes that both steel and concrete are elastic and obey Hooke's Law. Therefore, this method is also known as 'Elastic method of Design' or 'Modular Ratio Method'.
(ii) The working stress is based on the behaviour of structure at working loads (service loads). It is assumed that the stresses in steel and concrete do not exceed their permissible values at working loads.
The permissible stress in steel and concrete are obtained by dividing the characteristics strength of the material by the factor of safety. Factor of safety allows for uncertainties in the estimation of working loads and variations in properties of materials.
F.O.S. = 1.78 for Steel
F.O.S. = 3 for concrete
(iii) This method is referred to as 'Deterministic' because it is assumed that loads. Permissible stress and factor of safety are known accurately.
(iv) In this method, the permissible stresses are kept much below the ultimate strength of material.
(v) This method gives no indication to the actual margin of safety against collapse, as the ultimate load carrying capacity cannot be predicted accurately.
Drawback Of Working Stress Method
Following are the main drawbacks of working stress method of design :
(i) This method assumes that both concrete and steel are elastic which is not true.
Concrete is not elastic. The mild steel behaves as an elastic material but HYSD bars do not behave elastically.
(ii) This method uses factor of safety for stresses only which does give true margin of safety because we do not known the failure load.
(iii) This method does not account for shrinkage and creep which are time dependent in nature.
(iv) This method does not use any factor of safety with respect to loads. It mean there is no provision for uncertainties associated with the load.
(v) This method is uneconomical because comparatively large sections with higher quantities of steel reinforcement are obtained.
The working stress method is very simple and reliable but as per IS : 456 - 2000, the working stress method may be used only if it not possible to adopt the limit state method of design. Working stress method is basis method and its knowledge is essential for understanding the concepts of design.
Limit State Method (LSM)
This is the most rational which is based on safety at ultimate loads and serviceability at working loads. The acceptable limit of safety and serviceability requirements before failure of a structure takes place is known as a limit state. A limit state is a state at which one condition relating to a structure attains a limiting or critical value.
The important limit states which are to be considered in design are :
(i) Limit state of collapse
(ii) Limit state of serviceability
Limit state method is referred to as 'Non-deterministic' because it is based on predictions which depend upon experience or on field data.
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