Plastic analysis - PLASTIC ANALYSIS 35 1.0 INTRODUCTION...

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PLASTIC ANALYSIS Version II 35 - {PAGE } PLASTIC ANALYSIS 1.0 INTRODUCTION The elastic design method, also termed as allowable stress method (or Working stress method) , is a conventional method of design based on the elastic properties of steel. This method of design limits the structural usefulness of the material upto a certain allowable stress, which is well below the elastic limit. The stresses due to working loads do not exceed the specified allowable stresses, which are obtained by applying an adequate factor of safety to the yield stress of steel. The elastic design does not take into account the strength of the material beyond the elastic stress. Therefore the structure designed according to this method will be heavier than that designed by plastic methods, but in many cases, elastic design will also require less stability bracing. In the method of plastic design of a structure, the ultimate load rather than the yield stress 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 also known as method of load factor design or ultimate load design . The strength of steel beyond the yield stress is fully utilised in this method. This method is rapid and provides a rational approach for the analysis of the structure. This method also provides striking economy as regards the weight of steel since the sections designed by this method are smaller in size than those designed by the method of elastic design. Plastic design method has its main application in the analysis and design of statically indeterminate framed structures. 2.0 BASIS OF PLASTIC THEORY 2.1 Ductility of Steel Structural steel is characterised by its capacity to withstand considerable deformation beyond first yield, without fracture. During the process of 'yielding' the steel deforms under a constant and uniform stress known as 'yield stress'. This property of steel, known as ductility , is utilised in plastic design methods. Fig. 1 shows the idealised stress-strain relationship for structural mild steel when it is subjected to direct tension. Elastic straining of the material is represented by line OA . AB represents yielding of the material when the stress remains constant, and is equal to the yield stress, f y . The strain occurring in the material during yielding remains after the load has been removed and is called the plastic strain and this strain is at least ten times as large as the elastic strain, e y at yield point. © Copyright reserved 35
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PLASTIC ANALYSIS Version II 35 - {PAGE } When subjected to compression, the stress-strain characteristics of various grades of structural steel are largely similar to Fig. 1 and display the same property of yield. The major difference is in the strain hardening range where there is no drop in stress after a peak value. This characteristic is known as ductility of steel.
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Plastic analysis - PLASTIC ANALYSIS 35 1.0 INTRODUCTION...

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