Steel Structure Design methods

Steel structure may be design three methods.

Elastic design

Plastic design

Limit sate design

Elastic design is the traditional method. Steel is almost perfectly elastic up to the yield point and elastic theory is a very good method on which ton base design. Structures are anayesed by elastic theory and sections are sized so that permissible stresses are not exceed. Design is in accordance with BS 449: plastic theory developed to take account of behavior past the yield point based on finding the load that cases the structure to collapse. Then working load is the collapse load divided by a load factor. This too is permitted under BS449.

Plastic design

In plastic analysis and design of a structure, the ultimate load of the  structure as a whole is regarded as the design criterion. The term  plastic has occurred due to the fact that the ultimate load is found from the strength of steel  in the plastic range. 

This method is rapid and provides a rational approach for the analysis of the structure. It also provides striking economy as regards the weight  of steel since the sections required by this method are smaller in size than those  required by the method of elastic analysis. Plastic analysis and design has its  -main application in the analysis and design of statically indeterminate framed  structures.

Limit sate design

A Civil Engineering Designer has to ensure that the structures and facilities he designs are (i) fit for their purpose (ii) safe and (iii) economical and durable. Thus safety is one of the paramount responsibilities of the designer. However, it is difficult to assess at the design stage how safe a proposed design will actually be. There is, in fact, a great deal of uncertainty about the many factors, which influence both safety and economy. The uncertainties affecting the safety of a structure are due to 

 · Uncertainty about loading

 · Uncertainty about material strength and 

 · Uncertainty about structural dimensions and behaviour.

These uncertainties together make it impossible for a designer to guarantee that a structure will be absolutely safe. All that  the designer can ensure  is that the risk of failure is extremely small, despite the uncertainties.