Compression Behavior And Bearing-capacity Calculation Method Of FRP-concrete-steel Tube Composite Square Column

Concrete erosion and rebar corrosion are easy to happen to the engineering structures such as pier columns located in zones where the water level often changes, which have become dangerous risks affecting the safety and durability of structures. Fiber reinforced polymer(FRP) composites have found increasingly wide application in both the retrofit of existing structures and new construction because they possess high strength-to-weight ratio, good corrosion resistance and so on. FRP-concrete-steel tube composite column consists of an outer tube made of FRP and an inner tube made of steel, with the space between filled with concrete. This new form of composite members can utilize the advantages of FRP, concrete and steel tube three constituent materials and possess excellent structural and durability performance, that may be applied in hydraulic, bridge, industrial and civil building engineering and so on. In order to provide the basic reference for design and construction of FRP-concrete-steel tube composite square column(FTCSC) and promote its application in the engineering structures under bad condition including corrosive environment, test study and theory analysis on compression behavior of FTCSC were carried out in this thesis. The main research contents are as follows:(1)The effects of the void ratio, the diameter-to-thickness ratio of the steel tube, and the confinement characteristic value of FRP on the axial compression behavior of FTCSC were investigated through the experiments of twenty-five square columns and six circular columns. The results show that the strength and ductility of concrete in FTCSC are significantly improved because the concrete in the composite square columns is effectively confined by the steel tube and FRP tube. The bearing-capacity of FTCSC increases 9%~41% compared to the bearing-capacity superposition value of concrete and steel tube. The confinement characteristic value of FRP tube has sufficient influence on axial compression behavior of FTCSC and FRP-concrete-steel tube composite solid square column; the bigger of confinement characteristic value, the higher increasing range of bearing-capacity of FTCSC; when the diameter-tothickness ratio of steel tube is below 20, the axial stress and ductility of concrete in FTCSC have great improvement comparison with that of the FRP-confined concrete solid square column(FCSSC), and the bearing-capacity of FTCSC increases 7%~26.5%. Through the approach that square column was equivalent to circular column, and respectively considering the ultimate condition of outer FRP tube, sandwich concrete and inner steel tube, a theory calculation model for ultimate bearing-capacity of FTCSC under axial compression was established.(2)The influences of void ratio, diameter-to-thickness ratio of steel tube and FRP layers upon the bearing capacity of FTCSC were analyzed based on the axial compression tests results from this paper and other relevant references. The results show that when the void ratio is equal, the ultimate bearing-capacity of FTCSC under axial compression significantly decreases with the increase of diameter-to-thickness ratio and increases with the increase of FRP layers; when the diameter-to-thickness ratio of steel tube is similar, the bearing-capacity of FTCSC increases with the increase of void ratio of steel tube. The sandwich concrete of FTCSC was divided into efficient confinement zone and inefficient confinement zone, and then, the relevant simplified calculation model of bearing-capacity of FTCSC under axial compression was put forward based on the statistic analysis on test results of FRP-concrete-steel tube composite circular column from this paper and other relevant references. The calculation formulas of bearing-capacity of FTCSC under axial compression was established based on the comprehensive analysis on test results of FCSSC, FRP-confined concrete hollow square column(FCHSC) and FTCSC from this paper and other relevant references. The impacts of FRP layer, void ratio and diameter-to-thickness ratio of steel tube on the bearing-capacity of FTCSC under axial compression were also considered.(3)The comparative analysis on axial compression behavior of FCSSC, FCHSC and FTCSC was carried out. The results show that the confinement characteristic value of FRP tube has a relatively smaller impact on axial compression behavior of FCHSC than which impact on axial compression behavior of FCHSC and FTCSC. When the void ratio is equal to 0.72, the bearing-capacity of FCHSC with four plies FRP is close to that of FCHSC with two plies, and the maximum growth rate is only 9.4%; the constrain effect of FRP and overall behavior of column will decrease obviously with the increase of inner void ratio.(4)Cyclic axial compression loading has no significant effect on the ultimate bearing-capacity of FTCSC and FTCSC still possesses excellent ductility. Plastic deformation of FTCSC subjected to cyclic axial compression increases with the increase of unloading/reloading cycles. The ultimate bearing-capacity of FTCSC under cyclic compression increases with the decrease of diameter-to-thickness ratio, when diameter-to-thickness ratio reduces from 25.1 to 16.1, the ultimate bearing-capacity of FTCSC grows about 9.7%.(5)The effects of eccentricity and layers of axial FRP pasted on the side farther to the applied load outside the columns on the compression behavior of FTCSC were investigated through the experiments of nineteen FTCSCs subjected to eccentric compression. The results show that the ultimate bearing-capacity of FTCSC under eccentric compression apparently decreases with the increase of eccentricity. When the layers of axial FRP increases from 1 to 2, axial load-lateral deflection curves of FTCSC exhibit more rigid characteristic, and the ultimate bearing-capacity increases 3.6%~13%. Axial load-lateral deflection curves of FTCSC exhibit similar development trend with the eccentricity of 15 mm, 30 mm, 45 mm, only the area between each curve and horizontal axis gradually decreases with the increase of eccentricity. A theory model applicable to calculate the bearing-capacity of FTCSC under eccentric compression was put forward based on the section analysis and strip-partitioning method. By means of the irregular stress diagram of concrete and steel tube being equivalent to rectangle, the simplified bearing-capacity calculation formulas of FTCSC under different compression of steel pipe section were established.