3. Structural elements, ideal and real

The concept of elastic critical load is closely associated with an ideal structural element, both in terms of its initial configuration (free of any geometric imperfections, such as straightness or flatness defects), the material of which it is made (indefinitely elastic), and the conditions of application of the load to which it is subjected (axial resultant or acting strictly in the mean plane).

The concept of ultimate load, on the other hand, characterizes a real structural element, i.e. one that is geometrically and structurally imperfect, made of a material that responds to an inelastic constitutive law and is solicited with slight eccentricities that are undesirable, but unavoidable. The ultimate load is therefore deemed to be representative of the effective load-bearing capacity.

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