Research on post-fire cold-formed steel back-to-back built-up columns with local buckling

Cold-formed steel back-to-back built-up columns (CFS-BBC) are extensively used in lightweight steel structures because of their high load-bearing capacity and torsional stiffness. When the CFS structure suffers from a fire, some columns may still be elastic due to low-level internal force, so reusing them after the fire is very economical. However, research on evaluating the residual load-bearing capacity of CFS-BBC after fire exposure is still lacking. This study conducted experimental and finite element analyzes on the post-fire local buckling behavior of CFS-BBC. Experimental research included shear tests of self-tapping screw fasteners after fire exposure and axial compression tests of post-fire CFS-BBC. Subsequently, a finite element model was developed and calibrated, and parametric analyzes were carried out to extensively investigate the local buckling behavior of the CFS-BBC. Test results indicated that the ultimate load-bearing capacity of local buckling CFS-BBC was initially increased and then decreased with the heating-treatment temperatures. After exposure to 600 degrees C, the load-bearing capacity was enhanced by within 10 %. However, there was a 20 % decrease in load-bearing capacity after exposure to 800 degrees C. Based on the test and parametric analysis results, a predictive formula for the reduction factor of the post-fire local buckling load-bearing capacity of CFS-BBC was proposed. Furthermore, the coefficients in the Direct Strength Method (DSM) were modified to have a better prediction when the slenderness factor was relatively small.