Plastic analysis collapse load

A structure must be designed to resist gross plastic deformation and collapse under all the conditions of loading. The loads to which a process vessel will be subject in service are listed below. They can be classified as major loads, that must always be considered in vessel design, and subsidiary loads. Formal stress analysis to determine the effect of the subsidiary loads is only required in the codes and standards where it is not possible to demonstrate the adequacy of the proposed design by other means such as by comparison with the known behaviour of existing vessels. 

Euler buckling theory predicts collapse at a constant force. However, finite element analysis (FEA) shows that the onset of buckling causes the load bearing capacity to decrease (Fig. 8.8). At high axial deflections, plastic hinges develop at mid-length and the ends of these slender struts. 

Stress-strain (S-S) analysis of the materials provides important information. The tensile S-S curve for steel-pipe material identifies its yield point that is used as the basis in their design. Beyond this static loaded yield point (Chapter 2) the steel will enter into the range of plastic deformation that would lead to a total collapse of the pipe. The allowable design strain used is about two thirds of the yield point. 

Potential failure modes and the various stress limits categories are related. Limits on primary stresses are set to prevent deformation and ductile burst. The primary plus secondary limits are set to prevent plastic deformation leading to incremental collapse and to validate using an elastic analysis to make a fatigue analysis. Finally, peak stress limits are set to prevent fatigue failure due to cyclic loadings. 

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