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Elastic vs. Plastic Analysis

Elastic or first-plastic-hinge design has been used for century and plastic analysis is mainly limited to portal frame design. While steel accepted for use in building structures is reasonably ductile and has a minimum elongation at fracture of 15%, the ignorance of favorable effect in redundant structures by using elastic analysis is unjustifiable and un-sustainable for several reasons as follows.

  •  The elastic analysis discourages engineers to design robust and redundant structure against local failure since the design stops at the first plastic hinge. The approach does not allow engineers to consider in an analysis the strength reserve after first yield of structures while redundant and robust structures do not fail at the first plastic hinge.
  • Engineers quite often like to make use of ductility of steel in their design and, for extreme loads during rare events, elastic design is uneconomical and puts a consultant using the elastic design into a non-competitive position.
  • From past record, one can hardly find frame failure initiated by formation of plastic hinges in beams. Inspection of steel structures after earthquake showed that member buckling and cracking at connections were more common but plastic hinge in beams was unusual. This indicates buckling and connection are two important aspects in structural steel design against collapse and design allowing for plastic behaviour and stress re-distribution is both a safe and a sensible direction for design.
  • For design of structures under static loads, the authors suggest the use of P-Δ-δ plastic analysis in ultimate load design and P-Δ-δ elastic analysis for design under working loads, with both analyses allowing for mandatory frame and member imperfections. This ensures that the structure will not collapse under ultimate loads or yield to store energy under working load condition.

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