Structural instabilities : General principles

The excellent strength and stiffness properties that characterize structural steels explain why metal construction makes extensive use of slender structural elements. A corollary is that structural instability becomes a major concern when designing projects.

Structural instability can affect bar, plate, or shell structures alike. There are many different types of instability, each with its own specific characteristics. Rigorous treatment of these phenomena quickly reaches its limits in terms of mathematical analysis, and most of the practical solutions recommended involve experimentation and numerical tools.

In this file:

• The relationship between equilibrium and stability is introduced intuitively, and the basic phenomena of instability are clearly identified for further examination later on.

• We discuss and illustrate the two types of elastic instability –: bifurcation instability and limit point instability –, and describe their characteristics and properties.

• We emphasize what differentiates the actual structural element, as produced by standard manufacturing methods and therefore subject to imperfections and made of real material, from the ideally perfect structural element made of a material with "theoretical" behavior that is infinitely elastic.

• They are associated respectively with the ultimate load, which alone represents the actual load-bearing capacity, and the critical elastic load, which nevertheless plays a decisive role in the assessment of the former.

The following files (including  ) deal successively and separately, but always at a conceptual level, with the specific instabilities of bars, plates, and shells. Regulatory aspects are addressed in the files specifically concerned with the structural elements in question.