Research On Dynamic Behavior Of FRP Confined Square Concrete Filled Steel Tube Under Axial Impact Loading

Concrete filled steel tube(CFT or CFST)columns have the advantages of high strength,good ductility and convenient construction.However,when the external load exceeds the bearing capacity of the CFT member,the local buckling problem occurs in the steel tube,resulting in a rapid decline in load carrying capacity.The lack of effective confinement after steel buckling leads to increased brittleness of infilled concrete.FRP-confined concrete filled steel tube(CCFT)effectively prevents or delays local buckling of the steel tube by additional confinement on CFT,while the core concrete is more confined,and the steel tube in the compression zone will be in a tri-axial compression state.Previous studies have proved that FRP confined concrete filled steel tubular structures have good mechanical properties under static loads.During the life cycle of a structure,the components are often subjected to impact loads in addition to receiving normal static loads during their service life.Currently,the research on the dynamic behaviour of CCFT under impact load is still insufficient.Especially most research only concentrates on the circular section.The dynamic performance of CCFT with square section under impact load has not been reported.However,in practical engineering,square section columns are widely used in buildings,so it is very important to study the dynamic performance of FRP-confined square CFT columns under impact loading.In this dissertation,the axial static and dynamic behaviour of FRP confined concrete filled steel stub columns with square section are studied.The main research work and findings are as follows:(1)The axial compression static tests of different FRP materials confined square concrete-filled steel tube stub columns were carried out.The test parameters are FRP type,steel-FRP bonding condition,and their influences on the static axial compression behaviour of FRP-confined square CFST stub columns were studied.The failure modes,ultimate bearing capacity,ultimate strain and ductility of such members under axial load are analyzed.The test results show that the additional FRP confinement can improve the bearing capacity and deformation performance of concrete-filled square steel tubular columns.(2)The axial drop-weight impact tests of square concrete filled steel tube stub columns were carried out.The effects of different impact speeds on the impact resistance of the concrete-filled steel tube stub columns were studied through experiments.The dynamic response of such components under different impact velocities,such as impact process,failure mode,impact time histories,displacement time histories and dynamic increase factor(DIF),are analyzed.The experimental results show that the concrete filled steel tube columns have good impact resistance.(3)The axial drop-weight impact test of different FRP materials confined square concrete-filled steel tube stub columns were carried out.The effects of FRP type and steel-FRP interface bonding condition on the dynamic behaviour of FRP-confined square CFT stub column under impact loading were studied by experiments.The dynamic response of the impact process,failure mode,impact time histories,displacement time histories and dynamic increase factor(DIF)of such components under impact load are analyzed.Compared with CFT,the peak impact force of CCFT increased by 15%and the impact duration shortened by 24%.The experimental results show that the additional FRP constraint can improve the impact resistance of CFST stub columns.(4)The finite element analysis model of the FRP confined square steel tubular short column under impact load is established in LS-DYNA.The model considers the effects of material and geometric nonlinearities,interactions between steel and concrete.The comparison with the test results of current drop-wright tests in this dissertation shows that the model has good reliability.Then the model was used to analyze the energy changes and the deformation behaviour of the specimens during the impact process.Based on the reliable FE model,a detailed parametric analysis is carried out to investigate the effects on the impact behavior of FRP confined square CFT stub columns,such as concrete strength,steel yield strength and FRP layer.(5)A simplified calculation model for the bearing capacity of FRP confined square CFT stub columns under axial compression and impact loads is proposed.The comparison with the test results shows that the calculation model has good reliability.