Difference between ultimate strength design and working stress design?

Working stress method of design based on the elastic theory in which materials, concrete and steel are assumed to be stressed well below their elastic limit under the design loads. Working stressess (permissible stress) for the materials are determined by dividing the ultimate or yield stressess by suitable factor of safety values.

Limit state method of design based on limit state concept, te structure shall be designed to withstand safely all loads liable to act on it throughout its life;It shall also satisfy the serviceability requirements, such as limitations on deflection and cracking. The acceptable limit for the safety and serviceability requirements before failure occurs is called a "limit state". The aim of design is to achieve acceptable probabilities that the structure will not become unfit for the use for which is intended, that is that it will not reach a limit state...