Seismic Behavior Of Elliptical Concrete Filled Steel Tube And Design Method Of Elliptical FRP Confined Concrete Filled Steel Tubular Stub Columns

With the development of social economy and urbanization construction,the new architectural form and modeling continuously appears.The application of structural members with elliptical cross-section becomes increasingly widespread.Elliptical concrete-filled steel tube(ECFST)is a new type of composite members,it not only has great aesthetical appeal in external appearance,but also can provide sound structural properties because of the synergistic effect between the two materials,with small friction factor of fluid resistance.Therefore,they are gradually applied into bridges,theaters,airport ternimals and other practical projects.However,research on ECFST mainly focus on static performance at home and abroad at present,few studies on the seismic properties of ECFST have been reported,which limited the application of ECFST in practical projects.In order to investigate the distinct mechanical properties of ECFST,and further promote the practical application,the thesis presents the experimental numerical and analytical studies on the seimic performance of ECFST and the mechanical properties and design method of the fiber reinforced polymer(FRP)confined ECFST(FRP-ECFST).The research work of this thesis is aimed to provide advices for design of elliptical members,so that the application of these members in practice can be further developed.The main research work in this thesis is organized as follows:(1)Six ECFST stub columns were tested under monotonic axial compression and twelve ECFST stub columns were tested under cyclic axial compression.Based on the test results,the effects of different aspect ratio,concrete strength and cyclic number on the mechanical performanc of ECFST stub columns under monotonic and cyclic axial compression were discussed at the same time.(2)Numerical simulation of the ECFST stub columns under monotonic aixal compression was conducted using finite element(FE)program ABAQUS.The FE models of the ECFST stub columns were established applying the modified concrete plastic-damage model to consider the non-uniform confinement.The accuracy of the FE models was validated against the generated test data.Then the effects of different aspect ratio,concrete strength and steel strength on the bearing capacity of ECFST stub columns were studied by parametric analysis.According to the results of parametric analysis,the design formula of axial bearing capacity of ECFST stub columns based on superposition theory is proposed.(3)Thirteen ECFST specimens were tested under constant axial compression and monotonic horizontal loading and the other thirteen specimens were tested under cyclic horizontal loading.Failure modes,horizontal load-displacement hysteresis curves,skeleton curves,bearing capacity,ductility performance,strength degration,stiffness degration and energy dissipation of ECFST specimens were analyzed.The effects of different aspect ratios,concrete strengths,steel strengths,axial compression ratios,slenderness ratios and loading direction on the seisimic behavior of ECFST columns have been obtained.(4)Numerical analysis models of ECFST columns subjected to constant axial compression and cyclic horizontal loading were established based on fiber model.The accuracy of proposed numerical models is verified by comparing the horizontal load-displacement hysteresis curves and skeleton curves of experiments and FE models.Then the effects of different aspect ratios,concrete strengths,steel strengths,axial compression ratios,slenderness ratios and loading direction on the seisimic behavior of ECFST columns were studied by parametric analysis.At the same time,the applicability of the interaction formula of circular CFST beam-columns adopted in the Technical Code for Concrete Filled Steel Tubular Structures was verified.(5)The simplified calculation method of hysteresis model for ECFST was proposed on the basis of experimental research and numerical analysis combining parametric analysis,which can be refered in practical projects(6)FRP-ECFST was proposed to prevent the local buckling and corrosion of steel tube and low ductility of ECFST members.Design model of FRP-ECFST stub columns was established by theoretical analysis.Firstly,a simple and accurate stress-strain model for FRP-steel confined concrete in circular columns was formulated for use in design based on the analysis-oriented model.Secondly,parameter of the elliptical section,volume ratio of FRP and steel tube were introduced to obtain design-oriented stress-strain model for FRP-steel confined concrete in elliptical columns.Lastly,design model of FRP-ECFST stub columns was established.(7)The flexural and axial bearing capacity formulas for FRP-ECFST stub columns were derived through couducting section analysis using the proposed design-oriented stress-strain model for FRP-steel confined concrete.In addition,the simplified design equations for FRP-ECFST stub columns were finally proposed followed by introducing an equivalent rectangular stress block according to the Chinese code for design of concrete structures.