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This doctoral thesis addresses an increasing demand from the cold-formed steel production industry for more stable load-bearing elements. As buildings constructed entirely from cold-formed steel profiles and sheeting increase their spans, heights and bays, light steel frames not only become more impressive, but the applications of light-steel framing widen.The limited stability of standard cold-formed steel elements is resolved in the thesis by using primary load-bearing elements composed of multiple profiles. Built-up (composed) members:• are assembled from multiple, standard cold-formed steel profiles,• offer a number of benefits in terms of production, transportation, storage, flexibility in shape and arrangement, and overall cost,• can carry higher loads than the sum of what the constituents can take up when used separately.The thesis concerns built-up columns of intermediate and high slenderness; namely, these are elements that undergo all of the common types of buckling that occur in thin-walled structures — local, distortional and overall (flexural, torsional, andtorsional-flexural) buckling.Built-up sections are investigated due to their potential to achieve notably higher load-bearing capacity and avoid overall and distortional buckling occurrences that may compromise a structure’s integrity. Currently, no reliable design methods existfor such elements in structural design codes worldwide.The research aims to (1) understand the response of such members in terms of load-bearing capacity, stiffness, failure modes, scatter in capacity, and (2) recognise the design methods that are required. The focus of the doctoral studies is onmembers loaded in axial compression; however, the design under combined forces is also addressed.Experimental and numerical investigations are performed in order to study the behaviour of built-up columns and beams in weak-axis bending. Results are compared to analytical predictions based on existing design methods. Investigations concern a variety of cross-sections, some of which have been taken from design practice, while others are contrived and optimised within the doctoral programmewith the goal to reach considerably higher load-bearing capacity and better stability.Design partially or fully based on numerical analysis is considered and its advantages and disadvantages are presented.The manuscript documents the rather complex instability of built-up thin-walled systems, which was observed both experimentally and in numerical analysis. On many occasions, the presented failures comprise multiple, interacting or mixedbuckling modes, often combined with plastic yielding in the higher load stages.Based on the results from the various investigations presented in the manuscript, a design method is proposed which is an extension based on currently existing design techniques.A special emphasis is put on simplifying the design process. The goal is to eliminate ambiguities, avoid a lack of clarity and structure, and thus prevent mistakes in everyday design practice. This is in line with a contemporary tendency in coldformed steel design to simplify design methods and step away from overcomplicated, unclear methods.An indirect benefit of a simple and concise design method is the promotion of the structural system itself — a simple, clear and structured design technique will stimulate engineers to use light steel as a solution to their everyday design tasks.