ABSTRACT
The early codes of practice for reinforced concrete and steelwork design were based on the concept of working stresses. That is, the maximum elastic stresses in the materials under the design, or working, loads were compared to allowable values, based on the stresses at failure of the material divided by a suitable factor of safety. It was soon realized that concrete is an inelastic material, although elastic methods are still suitable for design load conditions for prestressed concrete members. This was reflected in the first code of practice for prestressed concrete, British Standard CP115, which adopted separate approaches for the design load and ultimate load behaviour of members. This was an early example of limit state design – identifying all the possible loading conditions for a member and choosing the most critical as the basis of design, while checking the other load conditions afterwards. A general approach based on limit state principles, which also identifies factors affecting the performance of structures other than loading, was incorporated into British Standards CP11O and BS8110, covering both reinforced and prestressed concrete, and has been retained in EC2.
