Study On The Effect Of Material Stiffness On The Performance Of Concrete Filled Steel Tube Columns Under Low Cyclic Loading

The research is supported by National Natural Science Foudation Item ’Study on the structure interface damage and energy dissipation mechanism of large scale steel and reinforced concrete composite high pile wharf of inland river’.Through method of the combination of field test and finite element simulation to study the factors affecting the stiffness of the composite steel tube column cross section composite material,that is steel pipe thickness,core reinforcement ratio and concrete strength,which will influence the performance of the RCFST column.The main contents of the research include: P-△hysteretic curves,skeleton curves,strength degradation,stiffness degradation,energy dissipation,deformation and ductility.The main conclusions are as follows:(1)In addition to tests on the damage basis,the P-△hysteresis curves are all full of the field test of RCFST columns and the calculation of the finite element model.It shows that the RCFST column has good ductility and good energy dissipation capacity,and it can absorb the energy from earthquake and horizontal cyclic loading better.(2)For a specimen with no damage to the base,with the increase of steel tube thickness,the stronger the restraining effect of the steel pipe on the core concrete,the weaker the strength degradation and the improvement of the horizontal bearing capacity.The energy dissipation coefficient and the equivalent viscous damping coefficient of the specimen all decrease,and the energy dissipation capacity of the specimen becomes weaker.The increase of steel tube thickness has a weak influence on the stiffness degradation of the RCFST column;the ductility coefficient μ of each test specimen is more than 3,and the thickness of steel pipe increases,the ductility coefficient is also increased.The external steel tube can effectively restrict the transverse deformation of the internal concrete and make the steel pipe concrete structure have good plastic deformation ability.(3)For the test specimens based on the damage,the P-△hysteresis curve is not full,showing a more obvious reverse S shape,and the pinching phenomenon occurs.The damage of the base is disadvantageous to the energy dissipation and ductility of the specimen.And,from the strength degradation curve,it can be concluded that the damage foundation reduces the hoop effect on the RCFST column,and exacerbates the degradation of strength and stiffness of the RCFST column.(4)With the increase of reinforcement ratio,the skeleton curve obviouslysteepened,the energy dissipation coefficient and the equivalent viscous damping coefficient increase obviously.The plastic deformation ability and energy dissipation performance of the RCFST column are enhanced effectively.In the wake of the increase of concrete strength,the overall trend trend of the skeleton curve is slightly steepening,but the amplitude is smaller,and the energy dissipation coefficient and the equivalent viscous damping coefficient increase slightly,indicating the increase of concrete strength has limited influence on the plastic deformation capacity and energy dissipation performance of RCFST columns.(5)The foundation failure of all test models is cracking along the vertical direction of the horizontal force from the bottom of the column,and develops on both sides and the steel tube of RCFST column is prone to buckle deformation or even bend drum at the bottom of the column.In practical engineering,in order to ensure the effective performance of the load bearing performance and energy dissipation performance of RCFST columns,the location of the foundation cracking and the buckling deformation of the steel pipe can be strengthened.