Study On Mechanical Behaviour Of Concrete Filled CFRP-Steel Tubular Beam-Columns

Concrete filled steel tubular (CFST) structures have been presented more than100years. Because of significant advantages in aspect of mechanical properties and construction technology, concrete filled steel tubular structures have been widely used in bridges, factories, high-rises, underground structures, and so on. With the development of the modern structural technology, large-scale concrete filled steel tubular structures appeared. Only thick-walled steel tube and high-strength steel tube can provide sufficient constraining force to concrete. And therefore the cost of project will be increased. Carbon fiber reinforcement polymer (CFRP) which is composited of carbon fiber material and substrate material at a certain ratio through certain technique is a high-performance novel material. CFRP gets attention with its advantages such as lightness, high strength, corrosion stability and fatigue resistance in engineering. Concrete filled CFRP-steel tubular (CFRP-CFST) structures were proposed in the early21st century. CFRP can provide constraining force with steel tube together to core concrete and can substantially delay and even completely suppress the development of local buckling deformation in the steel tube. Make structures have higher bearing capacity and better durability. Hot spot of research on concrete filled CFRP-steel tubular structures were formed at home and abroad. With theoretical foundation established this new structural form was pioneered the application in road engineering in our country in2012. Connection between theoretical research and engineering application was established. Meanwhile, the potential application of concrete filled CFRP-steel tubular structures was proved.Domestic and international researches focused on axial compressive behaviour and flexural behaviour. But the load-bearing form of structural members is various. With application of concrete filled CFRP-steel tubular structures popularized, existing structural theory could not meet the demands of guidance for practical engineering. Based on researches in being, study on mechanical behaviour of concrete filled CFRP-steel tubular beam-columns was expanded through ways of experimental research, simulated method of finite element and theoretical analysis. The main aspects of research work in this thesis are listed as follows:(1) Experimental study on static property of concrete filled CFRP-steel tubular beam-columns was carried out. Laws of bearing capacity and mechanical property affected by slenderness ratio and load eccentricity ratio were investigated. The axial load versus displacement of mid-span curves of concrete filled CFRP-steel tubular beam-columns can be divided into three stages:elastic stages, elastic-plastic stage and decline stage. Failure mode of specimens belongs to ductility damage. Because undergoing of larger deformation specimens can sustain a certain bearing capacity. Experimental results reveal that steel tube and CFRP can cooperate in both longitudinal direction and transverse direction from initial of the test to the end. The redistribution of internal force of steel tube was analyzed based on the test results. Longitudinal strain of steel tube accords basically with plane section assumption along the height of cross section; distribution of deflection along member height of specimens approximates half sine wave.(2) Theoretical analysis on static behaviour of concrete filled CFRP-steel tubular beam-columns was conducted and the formula of ultimate bearing capacity was proposed. Axial load versus deflection of mid-span curves and failure mode of concrete filled CFRP-steel tubular structures were simulated quite well through large finite element software ABAQUS. The results of simulation agree well with test data. The change regularities of stress, strain and interaction of specimens and combined materials such as CFRP, steel tube and concrete, were analyzed. The analysis results agree well with experimental results. It is found that adhesive strength between steel tube and core concrete dose almost not influence the bearing capacity, stiffness in elastic stage and interaction between steel tube and core concrete of members slightly. And loading path affects static performance of specimens slightly. The analysis results also reveal that transverse CFRP in compressive areas constrains specimens obviously and delays outward buckling of steel tube, longitudinal CFRP in tensile areas improves bending capacity and delays bending deformation of specimens. Based on analysis of systematic parameters, the bearing capacity calculating expressions for circular section and square section respectively were proposed herein for concrete filled CFRP-steel tubular beam-columns. The predicted results are in good agreement with experimental results.(3) A series of concrete filled CFRP-steel tubular beam-columns under cyclic load tests were carried out. Laws of lateral load versus displacement of mid-span hysteretic curves, lateral load versus displacement of mid-span envelope curves, moment versus curvature hysteretic curves, moment versus curvature envelope curves, stiffness, stiffness degradation, strength degradation, ductility index, accumulated energy dissipation and equivalent viscous damping coefficient affected by axial compression ratio and longitudinal reinforced coefficient of CFRP were investigated. Experimental results reveal that CFRP shows the mechanical property of transverse confinement and longitudinal strengthening for specimens, steel tube and CFRP can identically cooperate in longitudinal direction and transverse direction under cyclic load, the lateral load versus displacement of mid-span hysteretic curves and the moment versus curvature hysteretic curves of all the specimens display perfect hysteretic behaviour without pinch, Axial compressive load affects load bearing capacity obviously during the later loading period. Analysis results of hysteretic performance index show that the strength degradation which is not obvious for circular specimens happens in all square specimens, as axial compression ratio and longitudinal reinforced coefficient increase, the bending capacity and stiffness are enhanced and the stiffness degradation is delayed, but it can decrease accumulated energy dissipation and ductility, generally speaking, axial compression ratio is beneficial to seismic behaviors in certain range.