Research On Mechanical Performance Of Concrete-Filled Circular Steel Tubular Column With Large Slenderness Ratio

Concrete-filled circular steel tubes(CFCST) are widely applied in buildings for its better carrying capacity, convenient construction, regular shape and excellent earthquake resistant properties. Yet for now, many researchers have studied on the behavior of concrete-filled circular steel tubes subjected to bending, compression or eccentric compression. While the behavior subjected to compression- bending torsion and compression-bending-shearing is rarely to see. Present bearing capacity of the CFCST members subjected to complicated loading conditions is mostly established on a large of calculation results owing to the lack of the experimental results. So more experimental researches on CFCST should be done to ensure its application in engineering. This dissertation studied on its mechanical behavior of the members subjected to complicated loading conditions. The main works are listed as follows:(1) The static behavior of two CFCST columns with different axial pressure are studied. The load-displacement curves and load-strain curves were obtained. The failure characteristic, working mechanism and bearing capacity were analyzed. The experimental results show that the failure mode of two specimens were the buckling of bottom walls which was basically identical. And there were cracks in concrete varied from 400 mm to 1600 mm at the bottom of columns region. Increasing the axial pressure can reduce the bearing capacity, but it had a little influence on its initial stiffness.(2) The formulator for strength of CFCST columns subjected to complicated loading conditions was deduced. And the bearing capacity of the specimens obtainedby calculating formula agreed well with those of experimental results. So the method that was mentioned in the article can be used to calculate the of CFCST columns under compression-bending-shearing.(3) Finite elements analyses on the behavior of the experiment specimens were conducted by using the ABAQUS6.12. The agreement between the calculated results and experimental ones showed that the method of modeling was efficient. The influences of the axial compression ratio, the strength of concrete and steel, steel ratio,moment distribution and boundary conditions were studied. The results showed that the greater the axial compression ratio and moment distribution of column, the smaller the bearing capacity; increasing the the strength of concrete and steel and steel ratio of the specimen can improve the initial stiffness and bearing capacity, but it could reduce the ductility performance of the specimen; Stiffness ratio of stiffener mainly affect the development form of tension field. Design recommendations for CFCST columns have been put forward basis on the comparisons.(4)Three low cyclic loading tests on compressive-flexural members of CFCST columns were carried out. The load-displacement hysteretic curves, skeleton curves,the characteristic loads and displacements and seismic behavior indexes were obtained. The experimental results showed that the columns had higher load carrying capacity, good ductility and seismic behavior. The existence of concrete can improve the initial stiffness and energy dissipation capacity of specimens, But its capacity and rigidity decreases when the axial compression ratio increases.( 5) The load-displacement hysteretic of the specimens calculated by related theoretical model agreed well with experimental results. Based on the famous damage model Palmgren-Miner’s rule, the cumulative damage of the specimen’s seismic behavior were evaluated. The experimental results showed that specimens had a part of damage, but they didn’t reach the limited state.