Material characterisation and structural behaviour of cold-formed polygonal hollow section steel columns

Abstract

Cold–formed structural steel members are widely adopted in structural engineering owing to their merits of ease of fabrication, high–customised cross-section, and high strength–to–weight ratios. Two commonly used manufacturing methods for cold-formed steel are cold–rolling and press–braking, in which a steel sheet is continuously fed into a successive set of rollers, or predetermined bends are punched along the length of the steel sheet, to produce required cold–formed steel sections. Both of the two methods introduce different levels of plastic deformation into the cold-formed regions of the cross–section. As a result, material properties of steel in the deformed regions vary from those of the parent materials due to the pronounced strain–hardening. A more rounded stress–strain response with an enhanced yield strength, an enhanced ultimate tensile strength and reduced ductility can be observed among those metallic materials that experienced cold–forming. This thesis provides a consistent and thorough investigation into effects of cold–forming on normal strength and high strength steel. Based on the generated test results and collected test data from the global literature, a series of semi–empirical models are proposed to predict the material properties of structural steel after cold–forming, and a material constitutive model which incorporates the proposed predictive expressions is developed to accurately represent the stress–strain response of cold–formed steel. As for structural behaviour of cold–formed steel structures, extensive researches reveal that their structural behaviour was manufacturing method–dependant, which means differences in structural behaviour not only exist among different types of cross–sections, but also among identical cross–sections fabricated from different manufacturing methods. Hence, this thesis conducts a comprehensive investigation into the structural behaviour of cold–formed polygonal hollow section steel columns, including columns made from rectangular hollow sections and octagonal hollow sections. Material tests, residual stress measurements, stub column tests, and long column tests are conducted, and test results are presented in this thesis. Finite element models are developed and validated against experimental results. Effects of material properties, residual stresses, global and local initial geometric imperfections and other parameters are studied. Following the validation of the developed finite element models, extensive parametric studies are carried out to supplement and broaden the test database. Then current design methods for cold–formed steel structures from Australia, China, Europe, and the United States are reviewed. Assessments and modifications to the codified design rules are made based on the test and numerical results. Subsequently, reliability analyses are performed to verify the applicability of the proposed design recommendations.