In civil Engineering, What is the difference between working stress approach and limit state approach?

For working stress approach, service loads are used in the whole design and the strength of material is not utilized in the full extent. In this method of design, stresses acting on structural members are calculated based on elastic method and they are designed not to exceed certain allowable values. In fact, the whole structure during the lifespan may only experience loading stresses far below the ultimate state and that is the reason why this method is called working stress approach. Under such scenario, the most economical design can hardly be obtained by using working stress approach which is now commonly used in the design of temporary works.

For limit state approach, for each material and load, a partial safety factor is assigned individually depending on the material properties and load properties. Therefore, each element of load and material properties is accurately assessed resulting in a more refined and accurate analysis of the structure. In this connection, the material strength can be utilized to its maximum value during its lifespan and loads can be assessed with reasonable probability of occurrence. Limit state approach is commonly used for the majority of reinforced concrete design because it ensures the utilization of material strength with the lowest construction cost input.

Working stress method uses service loads only for the design of structures, while limit state method used both service loads and factored loads for design. WSM is based on simple bending theory (i.e Stress-Strain graph is a straight line) and therefore no consideration of stress concentrations, creep etc. are taken into account whereas LSM takes into account the actual stress strain curve of steel which helps in more accurate analysis.  WSM assumes concrete and steel to be a homogeneous material and therefore uses a factor called modular ratio (m) which is ratio of Youngs modulus of steel to that of concrete for all the calculations where as no such factor is required in LSM since concrete and steel are considered as two different materials. There are many more such differences.