Recent research on built-up cold-formed steel structures

• Comprehensive summary of recent research on built-up cold-formed steel members • Research on built-up members with three or four composite sections • Comprehensive account of full-scale test programs • Advanced numerical techniques for designing built-up CFS members • Proposed design rules The paper describes the main outcomes of a recently completed project on built-up cold-formed steel structures, including experimental, analytical and numerical advances. The project covers columns failing by distortional buckling or interactive global-local or global-distortional buckling, and laterally restrained and unrestrained beams. The cross-sections studied feature two, three or four component sections, arranged to produce singly or doubly cross-sections, including open sections and sections with closed loops. The paper first summarises the main experimental observations and results, then presents analytical solutions for determining the flexural, torsional and warping rigidities of built-up sections, followed by recent finite strip analyses to determine the local and distortional buckling loads of built-up sections accounting for discrete fasteners, including the Compound Strip Method and the modal Finite Strip Method, the latter for determining the pure modes of built-up sections. The development of fully nonlinear shell finite element models is outlined next, as specific for built-up sections. Lastly, the paper summarises proposed provisions for the design of built-up sections with two or more component cross-sections, covering columns and beams failing by local, distortional and/or global modes as well as combinations of these modes.