| Instability mechanism and design theory of the distortional buckling, and the interaction buckling of distortional and local modes of cold-formed thin-walled steel columns under axial compression have been studied in the thesis, including experimental study, theoretical analysis and design theory of the distortional buckling, and the interaction buckling of distortional and local modes of columns. Based on these systematic studies, and a research on the existing design methods for cold-formed thin-walled steel columns, a novel design theory is proposed, namely instability coefficient method.An experimental research was conducted to investigate the distortional buckling behavior and failure modes of cold-formed thin-walled steel columns under concentrically load. Nine web-stiffened lipped channel sections with narrow flanges and nine web-stiffened and flange-stiffened lipped channel sections with wide-flange were tested. The effect of the columns length, plates with intermediate stiffener, web to flange ratio, initial geometrical imperfections were studied. Axial compressive load vs. strains curves, axial compressive load vs. distortional displacement curves and axial compressive load vs. axial shortening curves were experimentally measured and analyzed. A three-dimensional finite element analytical model of cold-formed thin-walled steel columns was developed by using ABAQUS software to analyze those specimens. A parametric analysis employing the FEA model for cold-formed steel columns was performed. Theoretical analysis for the deformation modes and the performance of plate-stiffened of cold-formed thin-walled steel columns undergoing distortional buckling was conducted on the basis of the experimental results and the finite element analysis results. Instability mechanism of distortional buckling mode is revealed by the studies. The main cause that the post-buckling strength of the distortional buckling mode is compared lower with the local buckling mode is explained. The influence of geometries, initial imperfections and boundary conditions to the ultimate load-carrying capacity of steel columns are researched.Numerical methods and simplified analysis models for the elastic distortional buckling analysis were summarized. The source, evolution and performance of the formulas and test data for the two strength design curves developed by Hancock are studied, for predicting the load-carrying capacity in the distortional mode. A proposed strength design curve based on available test data and Hancock’s strength design curves are then compared with the current design methods, the Direct Strength Method and the Effective Width Method, which are incorporated in the "North American specification for the design of cold-formed steel structural members"(AISI-NAS:2007),"cold-formed steel structures"(AS/NAS4600:2005), and the Chinese "Technical specification for low-rise cold-formed thin-walled steel buildings"(JGJ227-2011). The results indicate that the current design standards adopted the two strength design curves for the DSM and EWM, but they have some differences at the partial extent. A novel formula is proposed for dealing with this problem. The range of applicability of the proposed strength equation is extended from that in AS/NZS4600and is shown to be more accurate than AS/NZS4600when compared with that in the NAS S100. Based on the recent test results, the strength design curve of distortional mode in inelastic buckling range in the DSM is modified.An experimental research was conducted to investigate the interaction of distortional and local buckling behavior and failure modes of cold-formed thin-walled steel columns under concentrically load. Nine web-stiffened lipped channel sections with smaller edge stiffeners and nine web-stiffened lipped channel sections with larger edge stiffeners were tested. The effect of the columns length, plates with intermediate stiffener, the size of edge stiffeners, initial geometrical imperfections were studied. Axial compressive load vs. strains curves, axial compressive load vs. distortional displacement curves and axial compressive load vs. axial shortening curves were experimentally measured and analyzed. The local-distortional and distortional-local interaction modes, which are dissimilar structural behavior, were analyzed. A three-dimensional finite element analytical model of cold-formed thin-walled steel columns was developed by using ABAQUS software to analyze those specimens. A parametric analysis employing the FEA model for cold-formed steel columns was performed. Theoretical analysis for the coupled deformation modes and the performance of plate-stiffened of cold-formed thin-walled steel columns experiencing the interaction of distortional and local buckling was conducted on the basis of the experimental results and the finite element analysis results. Instability mechanism of the interaction of distortional and local mode is revealed by the studies. The main cause that the post-buckling strength of the interaction of distortional and local modes is compared lower with the distortional and/or local buckling mode is explained. The influence of geometries, initial imperfections and boundary conditions to the ultimate load-carrying capacity of steel columns undergoing the interaction behavior are researched.The two design formulae NLD and NDL of load-carrying capacity in the direct strength method for the interaction of distortional and local buckling are studied in depth. The two design formulae ELD and EDL of load-carrying capacity for the interaction of distortional and local buckling, which is in harmony with the effective width method, are proposed. The four design formulae are then compared with the experimental results undergoing the interaction of distortional and local buckling in this work and collecting from the literature. Design theory concerning the load-carrying capacity of the distortional and the interaction between distortional and local modes is developed. A criterion for identifying the buckling mode between distortional and distortional-local, and between local and local-distortional is established. According to the criterion, two formulae to check the buckling modes are presented. The two formulae are then compared with the experimental results experiencing the interaction between distortional and local buckling in this work and collecting from the literature. An accuracy of determination concerning the criterion is analyzed.The Effective Width Method, Direct Strength Method and Strength-reduction Method for calculation of the load-carrying capacity of cold-formed thin-walled steel columns under axial compression are studied in depth. A novel design theory, Instability Coefficient Method, is then developed. A conception of instability coefficient for an element and member is put forward. A general formula for calculation the load-carrying capacity of cold-formed steel columns is presented. Several instability coefficients concerning the local, distortional, Euler buckling and interaction buckling between them are derived. Three Tables of the local, distortional and Euler buckling are given according to the corresponding instability coefficients. A theory checking buckling mode and failure mode, which can identify if the pure buckling mode or mixed mode will be occurred for a given column, is presented. A calculation process of the Instability Coefficient Method is also given. The calculated results on the basis of the Instability Coefficient Method are then compared with the experimental results in this work and collecting from the literature. An accuracy of determination concerning the method is analyzed.The following is a summary of the main creative points of this thesis.1. Through the experimental research, finite element parametric analysis and theoretical analysis, the mechanical performance of the distortional buckling is studied systematically and in-depth. The instability mechanism of the distortional buckling mode is revealed. The root causes, which the post-buckling strength of the distortional mode is lower than the local mode, are clarified. A proposed EWM design formula for predicting the load-carrying capacity in the distortional mode is presented.2. Through the experimental research, finite element parametric analysis and theoretical analysis, the mechanical performance of the interaction of distortional and local buckling mode is studied systematically and in-depth. The instability mechanism of the interaction of distortional and local buckling mode is revealed. The root causes, which the post-buckling strength of the interaction of distortional and local buckling mode is lower than the distortional and/or local mode, are clarified. The experimental clearly evidence of the occurrence of the local-distortional and distortional-local interaction modes, which are dissimilar structural behavior, is provided and analyzed.3. The two design formulae ELD and EDL of load-carrying capacity of cold-formed thin-walled steel columns experiencing the interaction of distortional and local modes are developed in the shape of effective width method. A criterion for identifying the buckling mode between distortional and distortional-local, and between local and local-distortional is established for the first time. According to the criterion, two formulae to check the buckling and failure modes are also presented with clear physical meaning.4. A unified design theory, Instability Coefficient Method, is presented for the first time for predicting the load-carrying capacity of cold-formed thin-walled steel columns. The method deals with the local, distortional, Euler buckling and the interaction buckling between them at the same time. The three tables of the local, distortional and Euler buckling are given according to the corresponding instability coefficients. A theory checking buckling mode and failure mode, which can identify if the pure buckling mode or mixed mode will be occurred for a given column, is presented. Based on these available conditions, the load-carrying capacity of a given column will be determined by the instability coefficient multiplying yielding carrying capacity of steel. |
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