In North America, cold-formed square and rectangular hollow sections (collectively referred to as RHS hereinafter) of commonly specified cross-sectional dimensions are produced using either the indirect-forming approach or the direct-forming approach. The indirect-forming approach, as the conventional approach of the two, consists of three steps: (i) roll-forming the coil material progressively into a circular hollow section; (ii) closing the section using electric resistance welding (ERW); and (iii) reshaping the circular section into the final square or rectangular shape. On the other hand, the direct-forming approach, as the new approach of the two, roll-forms the coil material directly into the final square or rectangular shape. RHS with similar cross-sectional dimensions but different production histories (i.e., different cold-forming approaches and post-production treatments) are expected to have significantly different material and residual stress properties. However, RHS design provisions in the existing North American steel design standards (AISC 360-16 and CSA S16-19) are in general developed based on research on indirect-formed RHS and currently do not differentiate RHS cold-formed by different approaches. Based on the research presented in Chapter 1 of this thesis, comparing to indirect-formed RHS, direct-formed RHS in general contain lower levels of residual stresses around cross sections, since the flat faces are not severely cold worked during production. This in turn affects member behaviours under compressive and flexural loadings. The test results presented in Chapters 2 and 4 show that direct-formed RHS have superior stub column and beam behaviours, comparing to their indirect-formed counterparts. In particular, the stub column and beam testing programs, covering a wide range of cross-section dimensions and two strength grades (nominal yield stresses of 350 and 690 MPa), show that the slenderness limits in the existing North American steel design standards are excessively conservative for direct-formed RHS, resulting in unnecessary penalty and member strength underestimation. As a result, the existing design formulae are not suitable for direct-formed RHS. In response to this, subsequent finite element (FE) parametric investigations are performed and presented in Chapters 3 and 5. Modified stub column and beam design recommendations for direct-formed regular- and high-strength RHS are proposed. The effects of post-cold-forming hot-dip galvanizing on material properties, residual stresses, stub column behaviours and beam behaviours of direct-formed regular- and high-strength RHS are also studied in Chapters 1-5 of this thesis. Similar to the application of the heat treatment per ASTM A1085 Supplement S1 or the Class H finish per CSA G40.20/G40.21, post-cold-forming galvanizing improves the stub column (Chapter 2) and beam (Chapter 4) behaviours of direct-formed RHS via effective reduction of residual stresses (Chapter 1). Based on subsequent FE parametric investigations, modified stub column and beam design recommendations catering to galvanized direct-formed RHS are proposed in Chapters 3 and 5. / Graduate