Comparative Study On Axial Compression Behavior Of Recycled Aggregate Concrete Filled Square Steel Tube Long Columns And Recycled Aggregate Concrete Filled Circular Steel Tube Long Columns

In this paper,a comparative test on the axial compression performance of 7recycled aggregate concrete filled square steel tube long columns and 7 recycled aggregate concrete filled circular steel tube long columns is carried out.The ultimate bearing capacity,axial load-displacement curve,axial load-mid-span lateral deflection curve,axial load-mid-span lateral deflection longitudinal strain curve and axial load-mid-span lateral deflection lateral strain curve were obtained.By changing the main influencing factors such as wall thickness,slenderness ratio and adding blocks,the difference of the axial compressive properties between square and circular steel tube specimens was studied.On the basis of experimental research,the finite element simulation and the extended analysis of multi-factor changes are carried out,and the following conclusions are drawn.(1)Recycled aggregate concrete filled square and circular steel tube long columns have the same loading process,and they all go through three stages: elastic stage,elastic-plastic stage and plastic failure stage.(2)The failure characteristics of square steel tube specimens can be divided into two categories,i.e.the buckling failure at the end of columns and the overall bending failure of specimens.In this test,all recycled concrete filled circular steel tube specimens were subjected to bending failure in the middle of the column.(3)The wall thickness of square steel tube will have a significant impact on the failure mode of the specimens,and the specimens with smaller wall thickness will suffer from compressive buckling failure due to end buckling.The square steel tube specimens with the wall thickness of 6 mm show the overall instability failure of the bending in the middle of the column.The failure modes of circular steel tube specimens are all integral bending instability failure.The failure modes of recycled aggregate concrete filled square and circular steel tube specimens are basically not affected by adding blocks.With the increase of slenderness ratio,the failure modes of circular steel tube specimens are all integral bending failure.However,when the slenderness ratio of square steel tube increases to a certain limit value,the failure mode will change from end buckling failure to overall bending failure.(4)With the increase of steel tube wall thickness,the bearing capacity of square and circular steel tube specimens increases,but the bearing capacity of square steel tube specimens increases faster.With the increase of slenderness ratio,the bearing capacity of square and circular steel tube specimens decreases consistently.Adding blocks can reduce the bearing capacity of square and circular steel tube specimens,but it has a greater impact on the bearing capacity of square steel tube specimens.(5)Under the same wall thickness,the initial axial stiffness of circular steel tube specimens is larger than that of square steel tube specimens,and the overall deformation ability is stronger,and the ductility is better.(6)With the increase of steel tube wall thickness,the initial axial stiffness of square and circular steel tube specimens increases slightly.The initial axial stiffness of square and circular steel tube specimens are almost not affected by adding blocks.With the increase of slenderness ratio,the initial stiffness of square and circular steel tube specimens decreases obviously.(7)The lateral deflection deformation curve of circular steel tube specimen approximately satisfies the sinusoidal half-wave hypothesis.The lateral deflection deformation curve of square steel tube specimens does not satisfy the sinusoidal half-wave curve.(8)The longitudinal and transverse strain curves of square and circular steel tube specimens have obvious flow amplitudes,and the plastic properties of steel can be fully developed.The steel tube plays a very good restraint role on the expansion of core recycled concrete.The results show that all the specimens have the cooperative working ability of steel tube and recycled concrete and strong overall deformation ability.(9)Findings from finite element simulation analysis:Under the same parameters,the bearing capacity of round steel tube specimens is higher than that of square steel tube specimens.With the increase of the strength of steel and concrete in square and circular steel tube specimens,the peak load-carrying capacity has been obviously increased.With the increase of slenderness ratio of square and circular steel tube specimens,the displacement corresponding to peak load increases,the initial stiffness of square and circular steel tube specimens decreases,and the bearing capacity of square and circular steel tube specimens decreases.