Rational Width-to-Thickness Ratios for Square Concrete-Filled Steel Tubular Columns: A Comparative Study on Cold-Formed and Tailor-Welded Tubes

The width-to-thickness ratio (B/t) is a pivotal design parameter for square concrete-filled steel tubular (CFST) columns, governing the suppression of local buckling and directly influencing the load-bearing capacity and ductility of members. To establish differentiated B/t limits for cold-formed (CFCFST) and tailor-welded (CFTWST) square CFST columns, this study critically reviews and compares five representative design formulas from international codes and the literature. Their applicability, theoretical rationality, and inherent limitations are systematically assessed. Particular attention is paid to the distinct mechanical characteristics induced by cold forming (strain hardening) and welding (residual stresses), based on which the suitability of each formula for CFCFST and CFTWST columns is separately evaluated. Through parametric analyses using a rigorously validated finite element (FE) model—verified against 14 in-house experiments and 210 literature specimens—and grounded in the steel–concrete compatibility mechanism, targeted B/t limits are proposed for each tube type.