Study On Axial Compressive And Seismic Performance Of Square FRP Tube Confined Steel Reinforced Concrete Columns

To overcome the shortcomings of traditional steel bars reinforced concrete structures,such as relatively weak energy dissipation ability,and insufficient reserves of shear capacity,composite structures,composed by steel tube,section steel and concrete,have been widely used in buildings and bridge structures.However,steel-conrete composite structures are still unable to overcome the corrosion problem of steel.Rencently,Fiber reinforced polymer(FRP)has gradually replaced steel in new concrete structures,due to its advantages of lightweight,high strength,easy construction,good corrosion resistance and fatigue resistance.A novel composite column was proposed by combining FRP tubes with section steel and concrete.The external FRP tubes could not only protect internal steel and concrete from a harsh environment,but also provide confinement to internal concrete.The confinement put concrete under three-direction compression and enhanced the load capacity and deformation ability of the member.The internal concrete could restrain the buckling of the steel and avoid the premature compression wrinkle of the FRP tube.The concrete protection cover outside of steel could be relatively thin because of the protection from the FRP tube and concrete.This could exert the bending and shear resistance of the steel better and improve the energy dissipation ability and shear reserves of the member.Besides,the steel and FRP tube could provide double confinement for internal concrete.This novel composite column could take the advantages of all the three materials and realize the full use of material properties.Current studies have proven the excellent mechanical performance of circular FRP tube confined concrete columns.However,in circular FRP confined steel reinforced concrete columns,the bending and shear resistance abilities of steel are constrained.So,this paper took square FRP-confined Steel Reinforced concrete(FSRC)columns as the research object,and conducted experimental and theoretical analysis on its compressive performance and seismic performance,and investigated the feasibility of this kind of novel composite columns in practical application.The main contents of this paper were:(1)Three kinds of 16 FSRC composite columns(without concrete cover for steel)with different combinations of H steel: cross-shaped-steel(FCRC),welded box steel,and separate box steel,were subjected to monotonic and cyclic axial compression test.The test results showed that FCRCs have better mechanical performance,but the lack of concrete cover for steel would lead to the premature failure of FRP tube.So,square FCRC columns with thin concrete cover for steel were taken as the main research object;(2)38 square FCRC columns with different FRP tube thickness,flange thickness,flange width,and concrete cover thickness for steel were subjected to monotonic and cyclic axial compression tests.Based on the results of the monotonic axial compression test,the effective confinement coefficient and effective confined area of concrete in the square FCRC column were proposed.The relationship between axial strain and hoop strain of square FCRC column was derivated from experiment strain results.On this basis,an analysis-oriented concrete stress-strain constitutive model of square FCRC column was proposed.The axial compression test was simulated by ABAQUS.The simulation results verified the accuracy of the effective confinement coefficient and effective confined area.Based on the test results of the monotonic and cyclic axial compression test,a skeleton curve model of cyclic axial compression,which was also a design-oriented model of monotonic axial compression,and a loading and unloading criterion suitable for OpenSees finite element simulation were proposed.(3)10 square FCRC columns with different axial compression ratios,FRP tube thickness,flange thickness,flange width,and section angle were subjected to a low speed cyclic pseudo-static test.The seismic performance of square FCRC columns,including skeleton curve,residual displacement,and energy dissipation capacity,were deeply analyzed and discussed with test results.OpenSees finite element simulation was carried out and compared with test results.Finally,1920 pushover finite element analyses for square FCRC columns with different specimen design were carried out.The influence of axial compression ratio,FRP tube confinement,steel confinement,shear span ratio,and concrete strength on the lateral load-lateral displacement of square FCRC columns were discussed.The hysteretic lateral load-lateral displacement model for square FCRC columns was proposed,which provided a theoretical basis for the seismic performance design of square FCRC columns.