Research On Axial Compressive Behavior Of Round-ended Concrete-filled Steel Tubular Long Columns

The round-ended concrete-filled steel tubular can balance both good restraint performance and high load-carrying performance preferably,and have been widely used in bridge engineering in recent years.Domestic and foreign scholars have researched and discussed the mechanical properties of round-ended concrete-filled steel tube under axial compression,eccentric compression,pure bending,and complex forces.But now the lack of the studies on the properties of RCFST long columns under axial load,This article makes a deep and systematic study of the axial compressive performance of RCFST long columns under axial load through experimental research and finite element analysis.The specific research contents are shown as below:⑴The axial compression tests were performed on one RCFST short column and five RCFST intermediate long columns.In the test,three test parameters of changes were considered which were slenderness ratio,height to width aspect ratio and stiffness ratio.This paper analyzed the above parameters for the influences of the mechanical properties such as failure form and bearing capacity.The result shows that the short column was strength failure and the intermediate long column experienced elastic stage,yield stage and destruction stage,which failure modes are elastic-plastic instability;the ultimate bearing capacity of specimens is proportional to the stiffness ratio and has an inverse relationship with the slenderness radio,and for specimens with larger height to width aspect ratio,the stiffness ratio has a greater impact on the bearing capacity;By analyzing the load-longitudinal strain curve of each test piece,it can be known that the limit slenderness ratio of the short column and the intermediate long column is between 10.76 and 21.53.⑵ This paper carried out the finite element analysis of the axial compressive performance of RCFST long columns under axial load,and studied the mechanical properties such as the stress distribution and interaction force of the whole loading progress of specimens.By analyzing a large number of parameters of slenderness ratio,aspect ratio,steel strength,concrete strength,steel content,etc.,the different failure modes of short,intermediate long columns and long columns is obtained.To investigate the effects of various parameters on the axial compressive properties of RCFST intermediate long columns,we can conclude that short columns occurred buckling failures,intermediate long columns occurred elastoplastic instability failures,and long columns occurred elastic instability failures;With the increase of steel strength,steel content,and concrete strength,the ultimate bearing capacity of members under axial compressive increases.The strength of concrete has a greater impact on intermediate long columns and a smaller effect on long columns.When H /B is increased,the weak axis stability and ultimate bearing capacity of the component are improved,but has little effect on the initial stiffness.⑶This paper determined the boundary slenderness ratio formula between RCFST short columns,intermediate long columns,and long columns,and proposed a simplified formula for the axial compressive bearing capacity of RCFST long columns.This article summarized the calculation methods of the bearing capacity of concrete filled steel tube long columns at home and abroad,and used the test data to compare and analyze the accuracy of each calculation method.And the paper proposed the calculation method for the bearing capacity of RCFST long columns.⑷From the failure mode,carrying capacity and interaction,etc,this article used finite element analysis method to contrast and analyze the axial compression performance of circular,rectangular and RCFST columns,and discussed the effect of parameters such as slenderness ratio,material properties,and steel content on components.The research shows,all three members are caused bending deformation,the middle of the rectangular member is accompanied by buckling deformation,and the round-end and circular cross-section members have not suffered local buckling failure;the ultimate bearing capacity of RCFST members is between circular and rectangular,which is closer to the rectangular cross-section;RCFST members have better restraint performance and larger moment of inertia,and have better ductility in the descending section;the steel content ratio and steel strength have the greatest influence on rectangular section members,and have the smallest impact on circular section members,round end section members are in between,and concrete has little effect on the bearing performance of the three sections.The paper researched the axial compressive performance of RCFST long columns,and proposed the boundary slenderness ratio formula between RCFST short columns,intermediate long columns,and long columns,and revealed the different failure mechanism between RCFST short columns,intermediate long columns,and long columns,and analyzd the influence of axial compression performance of RCFST long columns of the parameters such as slenderness ratio,steel content,etc.and proposed the calculation method for the bearing capacity of RCFST long columns.The research results provided a theoretical basis for the research on eccentric of RCFST,improved the system of RCFST,and provided a theoretical basis for the practical engineering application of RCFST.