This paper describes a study of the S650 high strength steel material properties including the effect of cold-formed angle. Coupon specimens with different cold-formed angles (90°, 100°, 120°, 140°, 160° and 180°) and different thicknesses (4 mm and 6 mm) were examined. Relationships between cold-formed angle and yield stress as well as tensile stress of the material were determined, based on the tensile coupon test results. Yield and tensile stresses assessed by considering the influence of the cold-formed angles were compared with those without considering this influence. Analyses revealed that both yield and tensile stresses decreased with increasing cold-formed angle. Ductile-damage material models available in the finite element analysis software ABAQUS were used to simulate tensile coupon tests. The experimental and numerical results showed good agreements.

This paper describes a study of the behaviour of cold-formed high strength steel angles. Thirty-six specimens with different cold-formed angles (90°, 100°, 120°, 140°, 160°, and 170°) and different thicknesses (4 mm and 6 mm) were considered. The initial geometric imperfections of the specimens were determined using the 3D laser scanning method. The magnitudes of these geometric imperfections for torsional and torsional-flexural buckling and flexural buckling analyses were proposed. The commercial finite element analysis (FEA) programme ABAQUS with shell elements S4R was used for finite element analyses. Different material strengths in corner and flat parts along with different proof stresses (0.2%, 0.01%, and 0.006%) were considered in the numerical models. The experimental and FEA results showed good agreement. Influence of cold-formed angle on non-dimensional slenderness and reduction factor curves of the 4 mm thick columns with 90° and 120° cold-formed angles was analysed.

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.

Hollow sections have been widely used for steel structures. Applications of High Strength Steel (HSS) on cold-formed hollow sections lead to thinner and lighter structures. This paper presents numerical investigations on the lateral-torsional buckling resistance of the cold-formed high strength steel rectangular hollow beam. Results from finite element analysis using Abaqus software are compared to calculations accroding to Eurocode 3. First, buckling analyses are carried out to investigate buckling behaviors of the rectangular hollow beams. In this step, elastic critical moments have been investigated and compared to calculations according to SN003. Buckling shapes have been also obtained in this step. Secondly, nonlinear analyses using Riks method have been performed. Buckling resistances of the beams with various thicknesses, various high strength materials S460, S650 and S960 have been compared to calculations according to EC 3. Further investigations on geometric imperfection sensitive have been carried out with different amplitudes L/1000, L/750, L/500 and L/250.

This paper presents results from experiments performed on cold-formed high strength steel circular and polygonal sections. The test specimens were conceived to provide necessary information for development of tubular towers for wind turbines. A total of 32 high strength steel specimens were tested under uniaxial compression. Various thicknesses, openings and geometric imperfections were studied. Sixteen specimens without opening and sixteen specimens with opening were tested under axial compression in order to investigate influences of opening on the resistances. Tensile tests of coupon specimens taken from flat part, circular part and corner part were used to investigate basic properties of high strength steel material S650 and to evaluate influence of corner on the material strength and ductility. Initial geometric imperfections of the specimens were determined by using a 3D laser scanning method. The coupon test results were used to calibrate the ductile damage material models for modelling of fracture. Results from the experiments are fully described in this paper

This paper is the second part of the study on the cold-formed high strength steel circular and polygonal sections intended to be used in tubular wind towers. Results from 32 numerical finite element analysis (FEA) models were compared with and calibrated against results of the tests on 32 corresponding specimens. The FEA results agreed well with the experimental results in terms of resistances and load-displacement curves. Further investigations on the numerical models were performed. Yield stress used in the FEA significantly affected the resistances of the numerical models. Using 0.2% proof stress leaded to higher resistance than the experimental results. Corners significantly influenced buckling behaviour in the polygonal section models. Analyses of an oval opening in the tubular specimens showed that peak stresses around the opening were considerably higher in the polygonal section models than in the circular section models. Finally, investigation of sensitivity to geometrical imperfections indicated that failure modes of numerical models with geometrical imperfections according to EC3 significantly differed from those of tested specimens and numerical models with geometrical imperfections obtained from the 3D scans.

Extensive research is conducted on the improvement of renewable energies. One field is the use of wind energy, where the tower construction is one of the main issues. This paper deals with new ideas and ongoing research in this area. To raise the height of steel tubular towers, fatigue as the design limit and constraints due to transportation issues have to be overcome. Changes in the cross-section are considered as one of possible solutions. This work presents an extensive finite element study dealing with different ways to improve shell stability, which become the limiting criteria if a friction connection substitutes the common flange connection between two tower segments. The use of circular and polygonal cross-sections is briefly described and will be investigated in an experimental program.

Towers for wind turbines are very sensitive to geometrical imperfections. Pattern and amplitude of imperfections significantly influence the strength of the towers. Rather limited number of experiments exists on a tubular tower like structure and no experiments are available considering door opening in towers with cylindrical or polygonal cross-section. One of the objectives of the RFCS research project “HIGH STEEL TUBULAR TOWERS FOR WIND TURBINES, HISTIWIN2” was to investigate current practice for design of door openings and propose a competitive solution, possibly using higher strength steel grades. A comprehensive experimental program was conceived to gain experimental evidence for the design of stiffening of the door openings.The following compression tests were evaluated in the paper:•4 static down-scaled compression tests of components of circular tubes •4 static down-scaled compression tests of components of polygonal tubes•4 static down-scaled compression tests of components of circular tubes with opening•4 static down-scaled compression tests of components of polygonal tubes with openingPlate thickness is down-scaled to 4 mm thick plate made of S650 steel grade.Most of FE nonlinear analyses of thin walled shell structures were performed using geometrical imperfections according to Eurocode 3. In this approach, geometrical imperfections are based on the pattern that coincides with the critical buckling mode. A comprehensive numerical investigation was made to conclude whether there is need to use a real geometrical imperfection pattern. The real geometry of the imperfections was obtained by laser scan measurements. Both, the measured geometrical imperfections and the geometrical imperfections according to Eurocode 3 were considered. Furthermore, results obtained from polygonal specimens were compared with specimens with circular cross sections.

With increasing demand of sustainable energy production, the harvesting of wind power by using wind turbines have become increasingly important over the last few decades. Most of the wind turbine towers are made from steel. Towers are an important part of the investment in wind parks and contribute about 20% of costs for the onshore towers. It supports the nacelle and is designed to sustain wind load and load caused by the operation of the wind turbine. This thesis presents results of experimental and numerical investigations on behavior of the steel tubular tower components made of circular and polygonal cross sections focusing on resistance of the door opening. An experimental program was planned to investigate the material and initial geometrical imperfections of the down scale specimens. Investigation of the basic material properties of the down scale specimens was carried out by using coupon specimens cut off from the down scale specimens. Influences of cold forming on the properties of the material were also studied by using coupon specimens with different cold formed angles. 3D scanning laser was used to determine initial geometrical imperfection of the down scale specimens. The measured data was analyzed by Matlab and Gom Inspect software. Results from these experiments were used as input data for the numerical models. Experiments on down scale specimens of the tubular tower with polygonal and circular cross sections were performed in order to investigate structural resistance. Finite element program Abaqus was used to simulate the tested specimens in order to compare their results to the experiments. Elastic, nonlinear geometry and nonlinear materials analyses were performed in order to investigate the ultimate resistance of the down scale specimens. Comparison of resistances of the circular and polygonal cross sections with Eurocode 3 was carried out.