Initial postbuckling behavior of thin-walled frames under mode interaction

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• A nonlinear 1D model accounting for warping is used for studying thin walled L frames.
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• Different joint configurations and relative warping transmission are considered.
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• Buckling and initial postbuckling are studied, through the asymptotic approach.
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• Mode interaction and imperfection sensitivity analysis are performed.
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• Mode interaction greatly affects the imperfection sensitivity.

An L frame made up by beam and column having channel cross sections, has been analyzed in a previous work by two of the authors [14]. Depending on the aspect ratio and the joint configuration, it has been proved that the structure can exhibit two simultaneous buckling modes. Here using the asymptotic theory of elastic bifurcation that takes into account mode interaction, the initial slope of the bifurcated paths has been determined. Three cases of joint configurations, which are the more common used in welded connections, have been considered. For each case three admissible bifurcated paths have been found. Two of them show a slope having the same order of magnitude of the ones found in the absence of mode interaction while the remaining exhibits a slope largely steepest. Selecting, for each joint case, the bifurcated path with the higher slope and between them the smallest one, it is found that it is associated to the path which bifurcates at the higher critical load. This means that the stiffer structure is also the less imperfection sensitive. Finally for each one of the cases studied, the effect of initial imperfection has been considered and the real load carrying capacity of the frames has been determined. Finally some results have been compared with those obtained using the FE code ABAQUS.