The construction industry uses cold-formed steel (CFS) sheets in the form of galvanised thin-walled profiles and corrugated sheets. In the past decade, CFS profiles have been competing with their hot-rolled counterparts as primary structural members of industrial halls, office buildings and residential housing of up to 3-4 storeys. The spans and column heights achieved with CFS profiles are ever larger. Due to the large slenderness of these members, adequate strength and stability are necessary, as well as reliability in design. Thin-walled members go through buckling during all stages of their working life. Local buckling appears at loads sometimes much lower than the design load. Distortional buckling seriously reduces the member resistance. It interacts with warping and lateral-torsional buckling, being significant for these asymmetric open sections. To restrict these effects, builders employ double sections - usually two standard cold-formed shapes bolted together to form a built-up section. These sections have the advantages of symmetry, higher stability and strength. The design of built-up members involves many uncertainties - although the European standard includes guidelines on the prediction of local, distortional and global buckling, the partial integrity and interaction between the parts of the composed members is still not studied. To study the actual behaviour, built-up members are tested in bending. An optical device for 3D motion analysis measures the displacement of points of interest on the specimen. Two interacting cameras use parallax to obtain the position of an arbitrary number of reflective markers glued to the specimen. The device tracks the movement of the markers in a 3D coordinate system without any contact with the specimen. Standard displacement transducers measure vertical displacements to validate the results. The paper gives an appraisal of the applicability of the method, a summary of the difficulties faced and the outcome of the test campaign.