Study On The Hysteretic Behavior And Design Method Of Thin-walled Concrete-filled Square Steel Tube Columns

Concrete-filled steel tube(CFST)have been extensively used in high-rise structures,industrial buildings,underground and bridge structures due to their high bearing capacity,good plasticity,fire resistance and easy construction.However,it is less economical to apply CFST columns to multi-layer structures and small and medium span bridges,because of the strict restrictions on the width-thickness ratio(diameter-thickness ratio)in domestic and foreign codes.The thin-walled CFST columns inherit the advantages of traditional CFST columns,and significantly decrease the amount of steel used and reduce the welding and fabricating workload of steel tubes,thereby cutting down the cost and improving the economic benefits.In this paper,experimental tests,finite element simulations,and theoretical analyses were carried out to study the hysteretic behavior of the thin-walled square CFST columns.The main contents are as follows:(1)Twelve thin-walled square CFST columns were tested under constant axial force and lateral cyclic loads.The specimens’ failure modes were attained,and the influences of key parameters,such as the axial compression ratio,stiffener type,connection mode between the steel tube and the stiffener,and the steel strength,on the bearing capacity,ductility,and energy consumption of the specimens were investigated.The failure modes of the specimens were generally characterized by visible bulging of the steel tube within1/2 times side length at the column end,where the concrete was crushed accordingly.The stiffened thin-walled square CFST columns performed a good energy-dissipation ability with relatively full hysteretic curves.Nearly all the stiffened specimens had a displacement ductility coefficient larger than 4,showing excellent ductility and deformability.The octagonal and circular linings are effective in improving the hysteretic performance of the thin-walled CFST specimens.The bearing capacities of specimens with slot welding connections are slightly higher than those with self-tapping screw connections,while the ductility and energy dissipation of the former are decreased compared to the later.It is suggested that slot welding connections be used to connect the circular lining thin-walled CFST columns in the project,which can meet the requirements of bearing capacity and rigidity,as well as strong ductility and energy consumption.(2)Finite element(FE)analysis of the thin-walled square CFST columns was conducted based on ABAQUS and Open Sees software.The simulation results by the FE models agreed well with the test results in terms of load-carrying capacities and failure modes of the specimens.On the basis of the push-over analysis,and the stress development and resistance mechanism of the specimens were revealed.The FE parameter analysis on thirteen circular lining thin-walled square CFST columns was carried out,considering the influence of axial compression ratio,width-thickness ratio,length-to-width ratio,steel strength,and concrete strength.Within the parameter scopes,as the axial compression ratio increases,the initial stiffness increases and the lateral bearing capacity of the specimens increases first and decreases,but the ductility coefficient and energy dissipation decrease.The initial stiffness,lateral bearing capacity,ductility,and the energy dissipation reduced significantly with the increasing of the widthto-thickness ratio of the steel tube and the length-to-width ratio of the specimens.It is effective to improve the stiffness and bearing capacity of the specimens by increasing the strength of the steel and concrete materials.However,using concrete with higher compressive strength could also reduce the ductility and loop areas in the hysteretic curves.In engineering application,various indicators should be considered comprehensively,and reasonable axial compression ratio,width-thickness ratio,lengthto-width ratio,concrete strength,and steel strength matching the concrete strength should be selected.It is recommended that the axial pressure ratio does not exceed 0.4 and the width-thickness ratio does not exceed 120.(3)The design methods of the load-carrying capacity of thin-walled square CFST columns under combined compression and bending were summarized,and their predictions were compared with the test results in this paper.The Eurocode 4 provides a relatively precise prediction on the bending capacity of unstiffened specimens.The calculation results of the Japanese specification(AIJ)are close to the test results of the circular lining specimens,and the results of Chinese specification(GB50936-2014)accord well with the test results of the octagonal lining specimens and the slot weld specimens.Based on the load-carrying mechanism,the restoring force model of the thinwalled square CFST columns was proposed,and the predicted results agree well with the tested load-displacement curves.