Study On Seismic Behavior Of CFST Members With Initial Concrete Imperfection Under Compression-bending-torsion Combined Load

The cooperative work of steel tube and core concrete in concrete-filled steel tube(CFST)makes CFST members have a series of combined advantages,such as high bearing capacity,great seismic behavior and constructed conveniently.So it can adapt to the requirements in the modern structures development,especially to prefabricated buildings,so it has been widely used in engineering field.However,the advantage of CFST members is that they are based on ideal conditions of construction,maintenance and service.Due to the fact that concrete in CFST mostly adopts cast-in-place construction modem,and it can lead to the core concrete shrinks radial in the later stage,settlement and residual air in the tube,and then form a gap between core concrete and steel tube.It may affect the mechanical properties of the members.In this paper,based on the experimental study and theoretical analysis,the seismic behavior of CFST members with initial concrete imperfection under axial-flexure-torsion combined load is carried out deeply.The main works can be summarized as follows:1.The present situation of research on CFST members with initial concrete imperfection and the seismic behavior of CFST members under compound loading are systematically described.2.Including 18 CFST with initial concrete imperfection specimens and 8 counterparts,26 CFST members are tested under compression-bending-torsion combined load in all.The experimental parameters include gap ratio,the gap type,bending-torsion ratio,axial compression ratio.The effects of the above parameters on the seismic performance of CFST members with initial concrete imperfection under compression-bending-torsion combined load are clarified by analyzing the test results.3.A nonlinear finite element model is developed,where the nonlinear material behaviour and the effect of gap on the interface behaviour of the concrete and steel tube are included.And,the reliability and accuracy of finite element model of are verified by using the existing test results and the above experimental results.4.On the basis of verifying the reliability and accuracy of the finite element model,and firstly the finite element model is used to analyze the stress and strain distribution of the steel tube and the core concrete,and the contact stress between the steel tube and the concrete and so on.The effect mechanism of initial concrete imperfection on CFST members under compression-bending-torsion is clarified from those mechanical properties.Secondly,the effect of different parameters on the bearing capacity coefficient of CFST members under axial-flexure-torsion combined load is analyzed.At last,the maximum limit of the gap ratio is proposed for CFST stub columns with circumferential gaps,and a simplified formula is proposed to estimate the effect of circular-segment gap on the ultimate strength of CFST stub columns as well.Finally,the maximum limit of the gap ratio is proposed for CFST stub columns with circumferential gaps,and a simplified formula is proposed,which can predict the ultimate moment of CFST beams with circular-segment gaps.