Limit sate designElastic design is the traditional method. Steel is almostperfectly elastic up to the yield point and elastic theory is a very goodmethod on which ton base design. Structures are anayesed by elastic theory andsections are sized so that permissible stresses are not exceed. Design is inaccordance with BS 449: plastic theory developed to take account of behaviorpast the yield point based on finding the load that cases the structure tocollapse. Then working load is the collapse load divided by a load factor. Thistoo is permitted under BS449.
In plastic analysis and design of a structure, the ultimateload of the structure as a whole isregarded as the design criterion. The term plastic has occurred due to the fact that the ultimate load is foundfrom the strength of steel in theplastic range.
This method is rapid and provides a rational approach for the analysisof the structure. It also provides striking economy as regards the weight of steel since the sections required by thismethod are smaller in size than those requiredby the method of elastic analysis. Plastic analysis and design has its -main application in the analysis and designof statically indeterminate framed structures.
Limit sate design
A Civil Engineering Designer has to ensure that thestructures and facilities he designs are (i) fit for their purpose (ii) safeand (iii) economical and durable. Thus safety is one of the paramountresponsibilities of the designer. However, it is difficult to assess at thedesign stage how safe a proposed design will actually be. There is, in fact, a greatdeal of uncertainty about the many factors, which influence both safety and economy.The uncertainties affecting the safety of a structure are due to
· Uncertaintyabout loading
· Uncertainty aboutmaterial strength and
· Uncertainty about structural dimensions andbehaviour.
These uncertainties together make it impossible for adesigner to guarantee that a structure will be absolutely safe. All that the designer can ensure is that the risk of failure is extremelysmall, despite the uncertainties.