3. Structural elements, ideal and real

The concept of critical elastic load is closely associated with an ideal structural element, both in terms of its initial configuration (free of any geometric imperfections, such as lack of straightness or flatness), the material it is made of (indefinitely elastic), and the conditions under which the stress to which it is subjected is applied (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., an element that is geometrically and structurally imperfect, made of a material that responds to an inelastic constitutive law and subjected to small, undesirable but unavoidable eccentricities. The ultimate load is therefore considered to be representative of the effective load-bearing capacity.

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