| Concrete filled FRP (Fiber Reinforced Polymer)-steel tube is a composite system by pouring concrete into a steel tubular confined by FRP sheet in preset ways. FRP provides a better reinforcement for both steel tube and core concrete. It can also be treated as an anti-corrosion jacket for steel tube. Inner steel tube assure the stiffness and ductility for the whole member. Thus concrete filled FRP-steel tube is an ideal combining column. Further study on static and dynamic properties of this new column are performed based on previous research achievements, as followed.1. Establishing theoretical model of whole axial loading process of concrete filled FRP-steel tube. Expressions of stress for the whole structure and every component in it in different loading stages are deducted based on Mises yield criterion, incremental theory of plasticity and Mander model for confining concrete. Equations of whole axial loading-displacement curve are also built. Theoretical models show good agreements with experimental results. Parametric studies are then carried out to determine the effect of FRP confining ratio, across sectional steel ratio, concrete strength and yield stress of steel on the axial loading performance of composite member.2. Proposing compression-bending interaction curve model for concrete filled FRP-steel tube. The performance of concrete filled FRP-steel tube beam-column member is studied based on principle of fiber model. A cross sectional analysis model of concrete filled FRP-steel tube is proposed by referring to the design method of ordinary concrete filled steel tube. Modeling process considers the influence of FRP-steel composite confining level and loading eccentricity on constitutive relation of concrete. It also considers the effect of slenderness ratio on load carrying capacity of the column. Modeling reasonability is verified by existing beam-column theories of concrete filled steel tube and experimental results of concrete filled CFRP-steel tube beam-column member.3. Quasi-static test on concrete filled FRP-steel tube. Quasi-static test is performed on seven concrete filled GFRP/CFRP-steel tube specimens to study seismic performance of this new composite member. Combining loads include axial load and vertical cyclic loading. Mechanical property of concrete filled FRP-steel tube under cyclic loading and the effect of GFRP or CFRP strengthening are investigated. Experimental results reveal that GFRP strengthening has lower damage to ductility and energy dissipation compared with CFRP strengthening, even though it produces little improvement on loading capacity. Finite element models are built and supplementary studies on the effect of FRP confining ratio, axial loading level and cross sectional ratio are performed.4. Quasi-static test and theoretical study on concrete filled FRP thin wall steel tube. Quasi-static test is performed on eleven concrete filled FRP thin wall steel tube to investigate the FRP strengthening effect. Testing results reveal that buckling of steel tube occur on both sides of the loading position, which is the main reason of strength degradation for the composite column. Hoop CFRP strengthening produces prominent improvement for the seismic performance of the composite column. Bi-directional GFRP strengthening can improve loading capacity, ductility and energy dissipation comprehensively. Therefore it is an ideal strengthening method for concrete filled steel tube. Constitutive relation of FRP-steel composite tube considering buckling effect and corresponding hysteretic model are built. Then hysteretic model for concrete filled FRP-steel tube is established and the contributions of core concrete, steel tube and FRP tube to overall accumulated energy dissipation are investigated.5. Experimental study on GFRP strengthening corrosion damaged concrete filled steel tube. Electric accelerated corrosion test on FRP-steel composite plate is performed to value anti-corrosion performance of FRP. Then electric accelerating corrosion test on concrete filled-steel tube is performed to value GFRP strengthening effect and study the seimic performance of concrete filled GFRP strengthening steel tube structure. Experimental results reveal that GFRP can well prevent steel in marine environment from corrosion. The degradation level of hysteretic curve is directly related to the corrosion damage mode. After GFRP strengthening in proper ways, the loading capacity, ductility and energy dissipation of corrosion damaged concrete filled steel tube are recovered effectively or even surpass those of uncorroded members. |
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