Cross-sectional Shape Optimization of Cold-Formed Steel Columns for Better Structural and Thermal Performances

Abstract

Cold-formed steel (CFS) sections can be rolled into different cross-sectional shapes, and optimizing these shapes can further improve their load-bearing capacities, resulting in a more economical and efficient building solution. However, the high thermal conductivity of steel can lead to thermal bridges, which can significantly reduce the building's thermal performance and energy efficiency. Although many studies can be found regarding the optimization of CFS sections for better structural behaviours, or thermal performance improvement, there is no existing research found in the literature on the optimization of CFS sections from both perspectives at the same time. Therefore, this research aims to find an optimal cross-sectional shape for a CFS column that maximizes its thermal resistance and axial strength with fixed coil width and given thickness. An extensive parametric study was undertaken to numerically investigate the effect of CFS columns' cross-sectional shape and size on the thermal and structural performances. Three cross-sectional shapes were considered, unlipped channel sections, simple lipped channel sections, and simple lipped channel sections with flange indentations. The parametric study also varied the steel thickness, web depth, flange length, lip size, flange indent size, indent width, and indent angle. In total, 324 FE models were computed and analyzed regarding thermal and structural performances, respectively. According to the obtained results, the thermal resistance of the CFS framed wall decreases with increased steel thickness, while the strength of the CFS section increases with the thickness. The thermal resistance of the CFS framed wall decreases when the flange length increases due to the increased steel flange contact surface. Furthermore, the results enhance the need for a small indentation in the flange to increase the thermal resistance. The average increase in the thermal resistance was found to be 1.41%, 2.61%, and 3.74% for an indent size of 2, 5, and 8 mm, respectively, considering a steel thickness of 2.5 mm. However, this flange indentation adversely affected the axial capacity of the CFS section. The average percentage of decrease in the strength is 0.87%, 2.05%, and 4.72% for a flange indent size of 2, 5, and 8 mm, respectively, considering a steel thickness of 2.5 mm. The optimal design of the CFS columns found among the 324 FE models with the greatest thermal resistance and axial capacity for each cross-sectional shape was present in this research. The maximization of thermal resistance always leads to decreased axial strength, and, oppositely, the maximization of axial strength always leads to a drop in thermal resistance. A set of Pareto-optimal solutions was also presented.