1. 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.
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