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The multimodal buckling of thin-walled isotropic columns with open cross-sections
under uniform compression is discussed. Column lengths were selected to enable strong interactions
between selected eigenmodes. In the case of short columns or very long ones subjected to compression,
single-mode buckling can be observed only and the effect under discussion does not occur. In the
present study, the influence of higher global modes on the load-carrying capacity and behavior in the
post-buckling state of thin-walled structures with open cross-sections is analyzed in detail. In the
literature known to the authors, higher global modes are always neglected practically in the analysis
due to their very high values of bifurcation loads. However, the phenomenon of an unexpected loss
in the load-carrying capacity of opened columns can be observed in the experimental investigations.
It might be explained using multimode buckling when the higher global distortional-flexural buckling
modes are taken into account. In the conducted numerical simulations, a significant influence of
higher global distortional-flexural buckling modes on the post-buckling equilibrium path of uniformly
compressed columns with C- and TH-shaped (the so-called “top-hat”) cross-sections was observed.
The columns of two lengths, for which strong interactions between selected eigenmodes were seen,
were subject to consideration. Two numerical methods were applied, namely, the semi-analytical
method (SAM) using Koiter’s perturbation approach and the finite element method (FEM), to solve the
problem. The SAM results showed that the third mode had a considerable impact on the load-carrying
capacity, whereas the FEM results confirmed a catastrophic effect of the modes on the behavior of the
structures under analysis, which led to a lack of convergence of numerical calculations despite an
application of the Riks algorithm. All elastic-plastic effects were neglected.
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