Load carrying capacity of systems within a global safety perspective. Part I. Robustness of stable equilibria under imperfections

The problem of the practical stability of structures is addressed in a modern way by considering the effects of both static and dynamic perturbations. The major historical contributions, due to Euler, Koiter and Thompson, are reviewed and illustrated by an archetypal model permitting to highlight the main mechanical and dynamical points. It is found that a global approach is necessary for a reliable safety estimation, especially in the neighborhood of (static) critical loads. Considering that the admissible load threshold has to account for robustness to finite perturbations, the Koiter critical load must be lowered, obtaining the so called Thompson critical load. It is shown how these two thresholds share some properties (e.g., both depend in a sensitive way on imperfections, which must be known for practical calculations), while having a deep different meaning: the former is related to static imperfections, and requires only a local analysis, while the latter is related to dynamical imperfections, and requires a global analysis. It is shown that P-Euler(Crit) >= P-Koiter(Crit) >= P-Thompson(Crit), i.e., that the advancement of knowledge leads to a lower estimation of the actual critical load. (C) 2011 Elsevier Ltd. All rights reserved.