Research Of Seismic Performance On CFRP Confined Concrete-filled Steel Tubular Columns

Concrete filled steel tube is widely used in the construction of buildings, bridges, long-span and high-rise structures due to excellent mechanical properties and seismic behavior. It was demonstrated that local plastic buckling often occurred at the ends of the steel tube under earthquake actions. This failure mode is very difficult to repair if not impossible, and also results in unstable hysteretic loading capacity, particularly for columns with higher axial load. The confined concrete filled steel tube is conceived based on the failure mode additional transverse reinforcement is designed for the potential plastic hinge regions, to achieve improved seismic performance. It was shown that the Fiber Reinforced Polymer especially Carbon FRP can postpone or restrict the local buckling of the steel tube, in addition, it can increase the confinement behavior of the core concrete to improve the mechanical behavior. At present, the research on CFRP confined concrete filled steel tube focused on axial compressive behavior and flexural behavior, while the the compressive-bending composition behavior under seismic loading was neglected.In 2012 years, the CFRP confined concrete filled steel tube has been successfully used in the design of the piers of highway bridges. With the application and development of this new kinds of structure, it is very important to improve the research on the working mechanism and design method. Based on the present researches,a series of compressive studies were conducted on mechanical behavior and calculation researches via experiment research,FE model calculation, and theoretical analysissimulated method of finite element and theoretical analysis.The main research contents are listed in the following:(1) 9 large-scale concrete filled steel tube columns were tested subjected to cyclic loading under constant axial load by the experimental parameters included axial compression ratio, loading sequences, and diameter-thickness ratio. Based on the test result, the failure modes, degeneration of strength and stiffness, mechanical property were investigated. The results showed that different loading sequences have effects on the load carrying capacity, ductility and energy dissipation capacity of CFT columns, as well as the failure modes of the CFT columns.(2) Bsaed on the experimental study of concrete filled steel tube, 7 CFRP confined concrete filled steel tube columns were carried out to study the seismic properties. The failure mode, hysteretic performance, load-displacement curve, degeneration behavior and energy dissipating ability were compared and analyzed. It was found that appropriate confined method improves the seismic performance and mechanical property of concrete filled steel tube. It was shown that two-direction confinement method by setting cushion layer could largely increase the confined area and improve deformation of specimens.(3) A finite element analysis(FEA) model of CFRP confined concrete filled steel tube member was developed through the FE software ABAQUS. In this software, the constitutive relations of different material(including steel tube, core concrete, CFRP), material and geometric nonlinearity as well as the interaction of different materials were considered. The predicted results are in good agreement with experimental results, and can be applied to the future mechanics performance analysis.(4) A typical finite element analysis(FEA) model of CFRP confined concrete filled steel tube member subjected to cyclic loading under constant axial load was proposed according to test specimen and size of fact. Full range analysis on CFRP confined concrete filled steel tube member under combined compression was presented by the FEA model. The failure mode, material(including steel tube, core concrete, CFRP) stress distribution, contact relationship of different materials and load transfer mechanism were investigated. The analysis results revealed that the confinement performance of CFRP appeared post-peak, and CFRP can provide confinement 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. Based on the analysis made by the various parameters of the height of confined areas and the thickness of buffering layer, adding a buffering layer between steel tube and the CFRP would change the stress distribution of the tube and the core concrete, and lead to better capacity of energy dissipation and ductility of the column. In order to ensure the optimal confinement effect, the height of the additional confined area are recommended to choose should as twice the tube diameter. Key parameters were selected(including axial compressive ratio, steel diameter to thickness ratio, steel tube and core concrete strength, and the layers of CFRP) for a systematic parametric analysis, which providing a theoretical basis of the design method for the subsequent study.(5) The enhanced coefficients of the capacity and ductility of the concrete filled steel tubes were proposed under different axial compression ratios, and the corresponding model of load-displacement skeleton curve was suggested. A hysteresis curve model was recommended based on the confining coefficient of CFRP and the enhanced coefficient of ductility. Meanwhile, structure measurement of CFRP confined concrete filled steel tubes was given under the design target based on capacity and the ductility.