The interaction of local and overall buckling of cold-formed stainless steel columns.

Abstract: The objective of this research is to investigate the interaction of local and overall flexural buckling in cold-formed stainless steel columns. Literature study exposes a lack of understanding of this subject and a need for experimental data, particularly on the local-overall interaction buckling of stainless steel open sections. Two separate experimental programs were therefore carried out. The first program included 36 tests on pin-ended lipped channel columns. Three alloys were considered: AISI 304, AISI 430 and 3Cr12. The specimens were designed to fail by local-overall interaction buckling in the inelastic stress range, thus highlighting the non-linear behaviour of stainless steel. Half of the specimens were tested under a concentric load. The other half had the load applied with a nominal eccentricity of Le/1500. The test results demonstrate the imperfection sensitivity of local-overall interaction buckling and illustrate the shift in effective centroid in pin-ended columns with singly symmetric cross-section. The second experimental program studied local-overall interaction buckling in 24 pin-ended stainless steel I-section columns. The specimens consisted of plain channels connected back-to-back using sheet metal screws. Two alloys were considered: AISI 304 and AISI 404. Local and overall imperfections were carefully measured in both experimental programs. Extensive material testing was carried out on the alloys employed in the experimental program, in order to determine tensile and compressive material properties, anisotropic parameters and enhanced corner properties. A detailed finite element model is presented, which includes non-linear material behaviour, anisotropy, increased material properties of the corner areas and local and overall imperfections. The model was verified against the two aforementioned experimental programs and against additional data available in literature on stainless steel SHS columns. The model yielded excellent predictions of the specimen failure mode, ultimate strength and load-deformation behaviour. The finite element model was used to generate additional data for stainless steel columns with lipped channel, plain channel, SHS and I-shaped cross-section, failing by local-overall interaction buckling. The parametric studies covered the practical ranges of overall and cross-sectional slenderness values. The Australian/New Zealand, European and North American standards for stainless steel were evaluated using the available data. The comparison reveals an inability of the design codes to properly account for the interaction effect as the cross-sectional slenderness increases. Predictions are unsafe for I-section columns with intermediate or high cross-sectional slenderness. A direct strength method is proposed for stainless steel columns, accounting for the local-overall interaction effect. The method offers a simple design solution which fits within the framework of the current Australian and North-American standards

Cross-sectional optimization of cold-formed steel channels to Eurocode 3

Cold-formed steel structural systems are widely used in modern construction. However, identifying optimal cross section geometries for cold-formed steel elements is a complex problem, since the strength of these members is controlled by combinations of local, distortional, and global buckling. This paper presents a procedure to obtain optimized steel channel cross-sections for use in compression or bending. A simple lipped C-shape is taken as a starting point, but the optimization process allows for the addition of double-fold (return) lips, inclined lips and triangular web stiffeners. The cross-sections are optimized with respect to their structural capacity, determined according to the relevant Eurocode (EN1993-1-3), using genetic algorithms. All plate slenderness limit values and all limits on the relative dimensions of the cross-sectional components, set by the Eurocode, are thereby taken into account as constraints on the optimization problem. The optimization for compression is carried out for different column lengths and includes the effects of the shift of the effective centroid induced by local buckling. Detailed finite element models are used to confirm the relative gains in capacity obtained through the optimization process

The application of plastic flow theory to inelastic column buckling

The paper presents a theory of inelastic column buckling which is consistent with the principles of plastic flow theory. The theory accounts for flexural, torsional and flexural-torsional modes. While the use of the tangent modulus to describe inelastic flexural buckling has been common place for a long time, efforts to comprehensively unite the torsional and flexural-torsional modes with the principles of plastic flow theory have so far been hampered by the ‘plastic buckling paradox’. New theoretical developments presented in this paper provide a way to achieve this goal. The solution hinges on the derivation of the inelastic shear stiffness while considering an infinitesimal solid element embedded within the column at a stage immediately past the point of buckling. The proposed inelastic column theory is verified against selected experimental data pertaining to aluminium and stainless steel columns of various cross-sections. Particular attention is paid to the torsional buckling problem of the inelastic cruciform section column

Experimental investigation of local-flexural interactive buckling of cold-formed steel channel columns

This paper presents the results of a comprehensive experimental programme aimed at studying the interaction of local and overall flexural buckling in cold-formed steel (CFS) plain and lipped channels under axial compression. The results were further used to verify the accuracy of the current design procedures in Eurocode 3, as well as to evaluate the effectiveness of a previously proposed optimisation methodology. A total of 36 axial compression tests on CFS channels with three different lengths (1 m, 1.5 m and 2 m) and four different cross-sections were conducted under a concentrically applied load and pin-ended boundary conditions. The initial geometric imperfections of the specimens were measured using a specially designed set-up with laser displacement transducers. Material tests were also carried out to determine the tensile properties of the flat parts of the cross-sections, as well as the cold-worked corner regions. A comparison between the experimental results and the Eurocode 3 predictions showed that the effective width approach combined with the P–M interaction equation proposed in Eurocode 3 to take into account the shift of the effective centroid consistently provided safe results. However, the Eurocode 3 procedures were also quite conservative in predicting the capacity pertaining to local-global interaction buckling, especially for plain channels. Furthermore, the experimental data confirmed the results of an optimisation study and demonstrated that the optimised CFS columns exhibited a capacity which was up to 26% higher than the standard channel with the same amount of material taken as a starting point

Special Article: Physical Activity, Physical Fitness, and Cardiovascular Risk Factors in Childhood

In adults, physical activity and exercise training are associated with reduced cardiovascular morbidity and mortality, a reduced likelihood of developing adverse cardiovascular risk factors, and improved insulin sensitivity. In childhood, participation in appropriate physical activity may prevent the development of cardiovascular risk factors in the future and complement treatment of existing cardiovascular risk factors, including hypertension, dyslipidemia, and overweight. Exercise in children can also significantly improve insulin sensitivity independent of weight loss. These e fects are mediated in overweight children by increases in lean body mass relative to fat mass and associated improvements in inflammatory mediators, endothelial function, and the associated adverse hormonal milieu