Finite element analysis of the mechanical behavior of stiffened cold-formed built-up CFST flat columns

To address insufficient lateral restraint and the susceptibility of the flat walls of traditional concrete-filled steel tubular (CFST) rectangular flat columns, a built-up rectangular CFST flat column with integrated longitudinal cold-formed stiffening ribs is proposed. The FE model was validated against the global response of a welded-plate CFST flat-column specimen. The model was then used to study the axial-compression and quasi-static cyclic behavior of the proposed columns. The coupled effects of aspect ratio, tube thickness, and rib dimensions were evaluated. The individual effects of tube thickness and rib dimensions under axial compression were further examined through the supplementary decoupled analysis. The results indicate that, compared with the non-stiffened control specimen, the stiffened configuration provides higher axial and cyclic load-bearing capacity, and more effective suppression of local outward bulging. Within the investigated parameter range, the coupled increase in tube thickness and rib dimensions improves axial and cyclic performance, while the decoupled analysis indicates that tube thickness is the dominant parameter governing axial load-bearing capacity and rib dimensions mainly contribute to local outward deformation control and post-peak load retention. These findings provide a reference for the parametric design and engineering application of stiffened cold-formed built-up rectangular flat CFST columns.