Ho's theorem in global–local mode interaction of pin-jointed bar structures

This paper is focused on the interaction phenomena among a global critical mode and some local Eulerian critical modes in pin-jointed structures. These phenomena are framed within Koiter's theory of elastic instability, by an asymptotic reduction into cubic systems. The aim is to present an algorithm for the appraisal of the lowest critical load characterizing the structure under the effect of small imperfections. First of all, the Ho's theorem, concerning the definition of the most dangerous imperfection, is presented and discussed. Then, a FEM code aimed at the determination of the most dangerous shape for the imperfection, and at performing the related sensitivity analysis, is implemented, by superimposing a proper FE beam model (able to model Eulerian instability) to a non-linear FE model for spatial pin-jointed structures. Some numerical results having a practical interest are presented and discussed.