In this method of design, the is designed to withstand the safety of for all loads liable to act on it throughout its life, it also satisfied the serviceability requirements such as limitation on deflection and cracking.
The aim of design through this method is to achieve acceptable probabilities that structure will become unfit for use to which it is built. It means that it will not reach a limit state or critical state.
All relevant limit states should be considered in design to ensure an adequate degree of safety and serviceability. In general, the structure should be designed for the most critical limit state and should be checked for the other limit states.
The limit state design philosophy uses a multiply safety factor format, which attempts to provide adequate safety at ultimate loads as well as adequate serviceability at service loads, by considering all possible limit states.
For ensuring the above objectives, the design should be based on characteristics values for material strengths and applied loads, which takes into account, the variations in the material strength and in the loads to be supported. The characteristics values should be based on statistical data if available, where such data are not available, they should be based on experience. The “design values” are derived from the characteristic values through the use of partial safety factor, one of material strength and other for loads.
The principle limit states are;
(1) Limit state of collapse
(2) Limit state of serviceability
