Finite Element Modelling and Parametric Studies of Semi-Closed Thin-Walled Steel Polygonal Columns - Application on Steel Lattice Towers for Wind Turbines

Abstract: The trend of structural engineering in the recent years is toward the use of lighter and moreeconomical structural elements. In steel construction, peculiarly, thin-walled structural elements arebecoming more popular and have a growing importance. Improved techniques in a manufacturing ofthin-walled elements have led to increased competitiveness of such products in the buildingapplications. Some examples of such structural element can be found in everyday life in form ofcolumns, studs, roofing trusses, and light-weight frames. However, the use of slender profiles and acomplex cross sections shape lead to requirements to study instability phenomenon in a form of local,distortional, flexural, torsional and coupled instability. Such complex structural behavior is inevitablyaccompanied by demand to improve calculation methods and design provisions. In this thesis, aninnovative solution of structural element composed of thin-walled plates is proposed for theapplication on lattice support structure of wind turbine.Thin-walled cold-formed profiles are steel products usually made from cold rolled coils and folded inthe second step. In this way, only open profiles can be produced. The predominant problem of theopen cross-section is the excessive torsional effect caused by the non-coincidence between the shearcenter and mass center, and a poor torsional resistance. A better response is possible with closed crosssections, but such profiles could not be produced by the folding. The solution is to make semi-closedsection by assembling them into polygonal profiles with mechanical fasteners, as presented in thisthesis.Objective of this work is to study the proposed structural sections in design situation and toinvestigate possible design models. The expected structural behavior of the column is a mixturebetween the open and closed cross-section. These cases will be investigated through numerical study.In this thesis presented a comprehensive parametric study on the ultimate strength of proposed coldformedsteel columns using the Finite Element package ABAQUS. FE models were first developedfor columns by using automation that was made through MATLAB and Python script. The bucklingand non-linear FE study was done for the investigation of local (L), distortional (D) and global (G)possible buckling failures and ultimate resistance, respectively. Modelling issues such as boundaryconditions, meshing, initial imperfections, material models, and non-linear solution controls in FEAwere also addressed.The parametric study involved series of profiles of built-up polygonal cross-section types with variedthickness (t), number of corners (n), diameter (d), slenderness (slend), yield strength (fy), number ofpoints along corner radii (np), extension lip length (lext), gusset plate thickness (tg), member length (l),and density of fasteners (s/d ratio), loaded in compression and bending moment. The bending momentoccurs as the effect of forces acting on the connection. The purpose of this analysis is to study thecritical load, cross-sectional behavior, influence of the amplitude of initial imperfections on theultimate load, and influence of each parameter used in the analysis through Full Factorial Design.