| As an innovative composite member,FRP-interlayer-steel confined concrete columns are composed of FRP tube,grouting material,steel tube and concrete from external to internal.In FISC columns,the height of steel tube and concrete is larger than that of FRP tube and grouting material,thus FRP and grouting material do not bear the vertical load directly.FISC can effectively solve the corrosion problem of traditional concrete-filled steel tubular columns(CFST)and the early-cracking problem of resin matrix in FRP confined concrete-filled steel tubular columns(CCFT).The core concrete in FISC columns is confined by both FRP and steel tubes,enhancing the bearing capacity and ductility.Thus,FISCs have great potential for applications in bridge piles and other corrosive fields.Currently,the research of mechanical behaviors of FISC columns is limited,and the confinement mechanism and load transferring mode are different from those in CCFT columns and FRP-concrete-steel double-tube concrete(DTCC)columns.Therefore,this thesis systematically presents an experimental and analytical study on FISC short columns,including behaviors under monotonic axial compression,cyclic axial compression,and eccentric compression.And the dual-confinement mechanism and load transferring mode in FISC short columns are revealed.This thesis mainly contains the following parts:(1)25 short columns including 19 FISC specimens were tested under monotonic compression,with key parameters of the FRP type,FRP thickness,concrete strength,FRP-to-steel confinement ratio and loading patterns.The failure mode,bearing capacity,deformation performance and sectional strain distribution of FRP and steel tubes were analyzed by the test.The characteristics of load-displacement curves of FISC short columns were summarized.Based on test results,the steel hoop stress and FRP hoop strain were analyzed and the sectional force balance analysis was conducted.The dual-confinement mechanism of the FRP-interlayer-steel composite tube on core concrete was revealed.(2)The finite element(FE)model of FISC short columns under axial compression was established using the software ABAQUS.The influence of grouting material thickness on the ultimate bearing capacity and deformation was investigated,the uneven confinement mechanism of FRP and steel tube on the grouting material was revealed,and the FRP fracture location of specimens at failure was determined.Based on the test and FE results,the calculation formula of axial bearing capacity was proposed,and the axial stress-strain model of concrete under FRP-steel dual confinement was established.(3)18 short columns including 16 FISC specimens were tested under cyclic compression,with key parameters of loading patterns,FRP type,FRP thickness,concrete strength and FRP-to-steel confinement ratio.The characteristics of axial stress-strain envelope curves,unloading curves and reloading curves in FISC short columns under different cyclic loading patterns were summarized.The relationship between residual strain and unloading strain was obtained,and the cumulative effects of loading history on plastic strain and stress degradation were found.Based on test results,the envelope and internal cyclic stress-strain models of FISC short columns were established.(4)20 short columns including 17 FISC specimens were tested under eccentric compression,with key parameters of eccentricity,FRP-to-steel confinement ratio,FRP type and thickness.The failure mode,load-deflection curves,lateral deformation curves and strain distribution of FRP and steel tubes were analyzed by the test.The non-uniform confinement mechanism of FRPinterlayer-steel composite tube on core concrete was revealed.Considering the influence of eccentricity on the stress-strain relationship of the concrete and grouting material,the sectional numerical analysis model was established.Based on analysis results,a simplified calculation method was proposed to predict the compression-bending strength of FISC sections. |
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