Cold-formed steel members are increasingly used in industrial and civil construction. Their use allows optimised member cross sections and shapes, thereby reducing the amount of steel used as well as the weight of the structures and, consequently, reducing harmful effects on the environment. Cold-formed members are manufactured using various methods such as cold rolling or press braking. In the cold rolling method, the cold-formed member is made by passing a flat steel sheet through a series of deformation stages. In the press braking method, the cold-formed member is produced by bending a flat steel sheet along its length. Cold-formed steel circular and polygonal sections have been used for wind turbine tubular towers. In order to harness the maximum amount of the wind's kinetic energy, the height of wind turbine tubular towers has significantly increased over the last few decades. Using high strength steel material has proven to be a feasible solution to the problem of the increasing height of wind turbine towers. The use of high strength steel material for cold-formed members significantly improves their properties. It enables thinner, longer and stronger structures. Moreover, the quantity of steel material required for building cold-formed steel structures is considerably reduced, producing a beneficial effect on the environment.

This thesis describes experimental and numerical investigations of cold-formed high strength steel sections and the effects of cold-formed angles on their properties. The effect of cold-formed angles on the properties of high strength steel was studied using tensile coupon tests. Coupon specimens with different cold-formed angles and different thicknesses were considered. Experimental results revealed that the cold-formed angle has a significant effect on the material properties. Furthermore, the cold-formed angle dependencies of the yield and tensile strengths were determined, and the strengths obtained with/without considering the influence of the cold-formed angle were compared. The yield and tensile strengths both decreased with increasing coldformed angle. The behaviour of cold-formed high strength steel angles was also investigated. Thirty-six specimens with different cold-formed angles (90°, 100°, 120°, 140°, 160°, and 170°) and different thicknesses (4 mm and 6 mm) were used for the investigation. Test results indicated that the resistance of the cold-formed angles significantly decreases, by 84%, with increasing coldformed angles from 90° to 170° (reducing influence of cold-forming). Moreover, a cold-formed angle significantly affects the failure modes of the angles. Experimental and numerical studies of cold-formed high strength steel circular and polygonal sections were carried out. A total of 32 cold-formed high strength steel circular and polygonal specimens, with and without openings, were tested under uniaxial compression. Initial geometric imperfections of the specimens were detected by using a 3D laser scanning method. Finite element analysis (FEA) models were compared with, and calibrated against, test results. The FEA results agreed well with the experimental results.