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The use of cold-formed steel elements in residential and industrial buildings is widely gaining popularity due to their ability to provide cost-effective and sustainable solutions. A high degree of flexibility in the manufacturing of various cross-sectional shapes provides a unique opportunity to further improve the load-carrying capacity of these elements through an optimisation process, leading to more efficient and economical structural systems. This paper aims to offer a practical methodology for the optimum design of CFS beam-column members with different lengths and thicknesses, subject to various combinations of axial compression and bending moment, but with constant material use.The optimisation process is carried out using a Genetic Algorithm and aims to maximise the resistances of CFS members, determined according to the Eurocode 3 design guidelines. Six initial prototype crosssections, including both single and built-up channel sections, are selected and their relative dimensions and edge stiffener configurations are allowed to vary during the optimisation process. To ensure practically relevant solutions EC3 slenderness constraints, as well as a range of practical manufacturing and construction limitations, are imposed on the cross-sections. Standard commercially available single and back-to-back lipped channel sections are taken as the starting points of the optimisation and used to benchmark the efficiency of the optimised sections. Significant gains in capacity (of up to 156 % in the present study) can be obtained compared to the initial cross-sections, while the optimisation results also offer further insights on the material efficiency achievable with various cross-sectional shapes in combined loading scenarios ranging from pure bending to pure compression.

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This study aims to investigate the seismic performance of an innovative cold-formed steel (CFS) moment-resisting frame experimentally and analytically. A half-scale CFS momentresisting portal frame was tested under static monotonic loading until failure. The frame consisted of two box-shaped columns (face-to-face channels connected with inside plates), a back-to-back lipped channel beam section and fully moment-resisting CFS bolted connections. During experimental tests, damage mostly concentrated at the top and bottom of the CFS columns due to the web crippling of the channels close to the connections, while no fracture or obvious slippage was observed at the connection zones. A detailed Finite Element (FE) model was developed using ABAQUS by taking into account the material non-linearity and geometrical imperfections. The lateral load-displacement behaviour, ultimate strength and failure modes predicted by the model were in very good agreement with the experimental results. The validated FE model was then used to assess the effects of key design parameters on the lateral load capacity, ultimate displacement, energy dissipation, ductility, and ductility reduction factor of the frame. It is shown that the proposed system can provide good seismic performance subjected to the appropriate design of the main structural elements. Increasing the axial load ratio of the columns by 50% resulted in 26%, 62%, and 50% decrease in the ultimate lateral load, energy dissipation capacity, and ductility ratio of the CFS frame, 2 respectively. However, the energy dissipation capacity and the ductility ratio of the proposed system increased significantly by increasing the width-to-thickness ratio of the columns.

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