Mechanics of cold-formed steel built-up box section columns in local buckling modes

The local buckling mechanism of the cold-formed steel built-up box section (CFSBBS) columns is investigated in this paper. The CFSBBS column is assembled using one lipped channel section and one channel section connected by self-drilling screws at the flanges. The buckling behaviors, deformation characteristics, and bearing capacity of columns were experimentally investigated to reveal the true instability mechanism of the CFSBBS columns under the influence of the "plate group effect". The mechanical analysis was conducted on the deformation of each plate during buckling in the built-up columns. A theoretical model of the critical local buckling stress is established. An analytical formula is derived based on the energy method and equilibrium conditions. Subsequently, a design method for predicting the bearing capacity of the CFSBBS column is proposed by incorporating the derived analytical formula into the direct strength method outlined in the AISI S100. Extensive parametric analysis is performed using the finite strip software CUFSM. Finally, the applicability and accuracy of the proposed design method are validated by experimental results. The proposed design method for determining the bearing capacity of the CFSBBS column, considering the "plate group effect," aligns well with the experimental results.