Influence of Local Imperfections on the Global Instability of Plane Trusses and Built-Up Columns

To illustrate the mode interaction in stability analysis of structures, trusses and built-up columns offer a simple context where global flexural modes of the structure can interact with local flexural buckling modes of the members. Truss and built-up column structures are often modelled under the assumption of zero-moment at connections. However, to capture the effect of local buckling in a 2D truss or built-up column member, FE models need to employ beam-type elements with 3Degrees-Of-Freedom-Per-Node to consider nodal rotations and capture bending behaviour of elements in local buckling modes. In this paper, we model trusses and built-up columns of zero-moment at connections using beam-type elements, by changing the standard FE assemblage procedure. Effect of local imperfections on the global buckling behaviour is examined in case studies, which show that the assumptions on connection type might influence the shape and critical loads for local buckling modes as well as the imperfection sensitivity of the structure. The case studies include snap-through as well as bifurcation-type instabilities. The effect of susceptibility to localized buckling in predicting the buckling and post-buckling behaviour is also examined.