Research on Non-Bifurcation Buckling Associated with Abrupt Deformation Mode Change in Strong-Axis Eccentrically Compressed Cold-Formed C-Section Steel Members Under Local Buckling

Classical structural stability theory generally categorizes structural （member） buckling into two types： bifurcation buckling and limit-point buckling. In bifurcation buckling， multiple equilibrium paths coexist at the critical point， whereas in limit-point buckling， the deformation mode remains unchanged before and after buckling. These two types correspond to ideal conditions and cases with initial geometric imperfections， respectively. However， studies on ideal cold-formed C-section steel members under strong-axis eccentric compression have shown that， after local buckling occurs， the member undergoes an abrupt deformation mode change and follows a unique deformation path. This behavior is distinctly different from both bifurcation buckling and limit-point buckling and represents a typical case of non-bifurcation buckling associated with abrupt deformation mode change. To accurately investigate this buckling behavior in cold-formed C-section steel members under strong-axis eccentric compression， this study adopted a combined approach using three-dimensional scanning technology and finite element simulation. The full-range mechanical responses of both ideal members and members with initial geometric imperfections were analyzed， and the influence of initial geometric imperfections on this buckling mode was thoroughly examined. The results indicated that， compared with ideal members， the neutral axis of imperfect members underwent a slight rotation during the early stages of loading. Regardless of whether imperfections were present， once the neutral axis began to rotate， its rotation angle continued to increase with increasing load. The triggering mechanism for the non-bifurcation buckling associated with abrupt deformation mode change in cold-formed C-section steel members under strong-axis eccentric compression was local buckling， which induced stress redistribution across the cross-section， leading to rotation of the neutral axis and overall torsional deformation of the member.