This paper investigates complex instability phenomena of cold-formed steel members subjected to fire, using a modal identification approach based on the constrained finite strip method. During fire, elevated temperatures produce degradation of mechanical properties and thermal deformations on steel members, altering their expected performance at ambient conditions. The non-uniform temperature distribution through the cross-section of thin-walled members changes over time of fire exposure, potentially impacting the interaction of buckling modes. For instance, members normally controlled by symmetric local buckling at ambient temperature may develop a significant participation of non-symmetric distortional buckling, and a more pronounced interaction of modes at elevated temperatures. A set of thin-walled cold-formed steel members is investigated under thermal gradient. Modal identification based on the deformation field obtained through shell finite element nonlinear analysis is used to quantitatively assess the evolution of modal participations in thin-walled members as the temperature changes. The results provide the ability to track how the modes vary under evaluated temperature, and identify how the failure modes are coupled. In addition, the results can potential reveal the impact of thermal gradients on the load-carrying capacity of structural members under fire.
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