Limit State method VS Working Stress Method for rc structures
The design of reinforced concrete structural members involves the knowledge of loads, material properties and factor of safety. The working stress method is refered to as deterministic because it is presumed that loads, permissible stresses and factor of safety are known accurately. In physical situations, however, these parameters are not always known and the analysis must be based on predictions. The prediction of such parameters is not always known and the analysis must be based on predictions. The prediction of such parameters is normally based on experience or on field data. The design of a structural member depends on how closely an actual situation is predicted. Parameters that involve the element of prediction are refered to as non deterministic and there is no guarantee that they will actually occur. This forms the basis of limit state design.
In the limit state design method these parameters are deterministic based on observations taken over a period of time. These parameters will thus be influenced by chance or random effect not fast at a single instant but throughout the entire period of time or the sequence of time that is being considered. Such a process is refered to as a stochastic process. In a rough sense a stochastic process is an phenomenon that varies to some degree unpredictably as time goes on. It is also refered to as a Random process.
In the limit state design procedure stresses in an element are obtained from the design loads and compared with design strengths. In the working stress design method the stresses. The main distinction between the two methods lie in the fact that in the former, a member is considered in its limit state, whereas in the later, in its working state. The required more stringent conditions that loading conditions leading to collapse as well as partial safety factors be determined very carefully and accurately in the limit state design.