The advantages of using cold-formed steel are high strength-to-weight ratio, flexibility in fabricating different cross-section shapes, easy for construction and so on. Cold-formed steel sheets are usually manufactured into open sectional profiles by cold-rolling and brake-pressing methods. Doubly symmetric built-up sections, with the advantage of having larger torsional rigidity compared to traditional singly symmetric open sections, have great potential to extend the application of cold-formed steel to structural members with larger scale. The doubly symmetric built-up open and closed sections can be made by connecting two channel sections using self-tapping screws, which are also convenient for industrialized production. In this study, the structural performance of cold-formed steel built-up section beams with intermediate stiffeners and built-up section beams with web perforations was examined through experimental and numerical investigations. The impact of different screw arrangements on the moment capacities of built-up section beams was also studied. A total of 113 beam tests that include simply supported beam tests of 16 pairs of channel sections with stiffened webs, 19 built-up sections with intermediate stiffeners, 43 built-up sections with web perforations, as well as 35 built-up sections with different screw arrangements was conducted. The material properties of the test specimens were also obtained by tensile coupon tests. The main objective of the experimental investigation is to obtain the section moment capacities and understand the structural behaviour of the built-up sections under bending. Finite element (FE) models of the beam specimens were also developed and numerical analysis that includes material nonlinearity, geometric nonlinearity and contact nonlinearity (for built-up section beams only) was performed and the results were verified against the test results. The validated FE models were employed for extensive parametric studies of 382 numerical simulations that include 101 channel sections with stiffened webs, 113 built-up sections with intermediate stiffeners, 80 built-up sections with web perforations, as well as 88 built-up sections having different screw arrangements. A wide range of section slenderness was considered in the built-up sections and different failure modes of beams were investigated. Therefore, a comprehensive study on the influence of stiffener features, different sizes of web holes, and various screw arrangements on the structural performance of cold-formed steel built-up section beams was carried out. The results obtained from the experimental and numerical studies were employed for evaluating the design rules of cold-formed steel built-up section beams. It should be mentioned that the existing direct strength method (DSM) for cold-formed steel members does not cover the design of built-up section beams. Hence, suitable design rules based on DSM have been proposed for cold-formed steel built-up section beams with intermediate stiffeners and built-up section beams with web perforations under bending. The design equations for considering the screw spacings of built-up closed section beams have also proposed. Furthermore, the suitability of the modified design rules, as well as the existing DSM for cold-formed steel built-up section beams was also evaluated through a reliability analysis. ; published_or_final_version ; Civil Engineering ; Doctoral ; Doctor of Philosophy