Structural instabilities - General principles

The very good strength and stiffness properties that characterize structural steels explain why slender structural elements are widely used in steel construction. As a corollary, structural instability is becoming a major concern in project design.

Structural instability can affect bar, plate or shell structures. There are many different instability phenomena, each with its own specific characteristics. Their rigorous treatment quickly reaches its limits, strictly in terms of mathematical analysis, and most practical solutions are based on experimentation and numerical tools.

In this file :

• we intuitively introduce the relationship between equilibrium and stability, and clearly identify the elementary instability phenomena that will be examined in greater detail later;

• the two types of elastic instability – by bifurcation and by limit point – are commented on and illustrated, and their characteristics and properties are given;

• we emphasize the difference between the real structural element as produced by conventional manufacturing methods, i.e. with imperfections and made of a real material, and the ideally perfect structural element made of a material with indefinitely elastic "theoretical" behavior;

• They are associated respectively with the ultimate load, which alone is representative of real load-bearing capacity, and the elastic critical load, which is nevertheless one of the determining parameters in the evaluation of the former.

The following files (including ) deal successively and separately, but always conceptually, with instabilities specific to bars, plates and shells. As for regulatory aspects, these are dealt with in the dossiers specifically addressing the structural elements concerned.