Behavior Of Hybrid Columns Of Concrete-Filled Square Steel Tube With FRP-Confined Concrete Core Under Axial Compression And Seismic Loading

A novel column consisting of fiber-reinforced polymer(FRP),steel and concrete is presented.The sectional form of steel-concrete-FRP-concrete(SCFC)columns has a square steel tube as the outer layer and one or more circular filament-wound FRP tubes as the inner layer,with concrete placed between these two layers and within the FRP tubes.Thus the column can be regarded as a concrete-filled square steel tube(CFST)with one or more FRP-confined concrete cores(FCCC).Experimental investigations,theoretical studies,and finite element analysis were all developed.1.The experimental investigation of 62 SCFC stub columns under axial compression was carried out.The specimens with a single FCCC were loaded under two schemes,monotonic axial compression and cyclic axial compression.The influences of four design parameters on the mechanical behavior of stub columns were examined.The specimens with multiple FCCC’s were loaded under monotonic compression only.Meanwhile,the effects of sectional configuration,thickness,and material strength on the mechanical behavior were considered.Additionally,a three dimensional scanning technique was adopted during experiments to detect the deformation of specimens at several significant loading steps.The dilation process of the steel tubes and corresponding stress states were both determined using finite element analysis.From the axial compression experiments,the failure process and failure mode were both obtained,which reveal that the SCFC has several significant advantages which differ from traditional elements,including significant post-yield stiffness and post-peak residual load-bearing capacity.2.A load-strain model for SCFC section was proposed which is established by considering the interaction mechanisms among steel,FRP,core concrete,and sandwiched concrete and attempts to model the mechanical responses of four key stages including the elastic stage,yielding transition stage,strengthening stage and residual load-bearing stage of SCFC sections under axial compression.Most importantly,a stress-strain relationship of concrete simultaneously confined by both steel and FRP tubes was proposed and the stress-strain relationship of steel tubes with large axial and lateral deformations was developed.Further,a parameter analysis was conducted using the solid element model of finite element analysis.Last,the load-strain model was determined based on the discussions above.3.In the seismic experimental investigation,14 SCFC specimens were fabricated with four differing parameters: axial compression ratio,concrete strength,FRP tube thickness,and steel tube thickness.Their energy dissipation capacity,the length of plastic region,and the type of envelope curve were discussed.Additionally,the seismic performances of SCFC were evaluated by using the fiber model in OpenSees.A similar hybrid section of double-skin steel tubular column was chosen to compare with the mechanical behavior and seismic performance of SCFC.Results show the high level of load-bearing and energy dissipation capacity of SCFC,which fully took the advantages of material strength and their interaction mechanisms.Also,the calculation method for moment bearing capacity was proposed.Above,a design method was proposed for the novel SCFC,which demonstrates the distinctive and superior behavior and broader prospects of applications in heavy structures.