This paper presents an experimental study of circular ultra-high-performance concrete-filled steel tube (UHPCFT) columns under monotonic axial compression. Some performance indices, termed concrete contribution ratio (CCR), strength enhancement index (SI) and ductility index (DI), are investigated in relation to the diameter-to-thickness ratio (D/t ) of the UHPCFT columns. The axial bearing capacities of the UHPCFT columns obtained from the test results are compared with Eurocode 4, American Concrete Institute, Australian Standards and Architectural Institute of Japan design codes. The test results show that increasing the steel tube thickness from 2 . 5 mm to 3 . 65 mm enhances the DI by 69%, but the SI only by 5%. Although the Eurocode 4 design code overestimates the axial bearing capacities of the UHPCFT columns, the American Concrete Institute, Australian Standards and Architectural Institute of Japan codes are conservative.

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This paper presents an experimental study of square high strength steel fiber reinforced concrete (SFRC) filled steel tube columns under axial load. A series of tests are performed to investigate the effects of D/t ratio, the bond strength between concrete and the steel tube on behavior and axial load capacity of the high strength SFRC filled steel tube columns. The specimens are separated as greased and non-greased to investigate the bond effect on the axial load capacity. A total of 13 specimens are tested and compared to Eurocode 4, ACI, AS and AISC Codes. The results show that the difference of the axial load capacity between the greased and the non-greased square high strength SFRC filled tube columns is notable. The all design codes overestimate the axial load capacity for the specimens with thinner (D/t ratio is 33.3) steel tube thickness.Keywords: Axial load capacity, Steel fiber reinforced concrete, Design codes, Bond effect, D/t ratio INTRODUCTIONIn high-rise buildings, the use of the high-strength concrete at structural members is quite widespread today. The structural members have to resist axial load and bending moment because of vertical and lateral loads. Therefore, the structural members such as columns have to be designed to sustain all kinds of effects which stem from those forces. The main aim of the concrete filled steel tube columns is to withstand against those forces by combining steel and concrete.There have been many studies about the normal and the high strength concrete filled steel tube columns. Furlong [1] investigates 13 specimens with D/t ratios from 29 to 98. The results show that there is no increase in the load carrying capacity if the steel tube and the concrete resist the load individually. Schneider [2] performs a comprehensive study about axially loaded concrete filled steel tubes. The total 14 specimens are tested under the axial load, and experimental results are compared to numerical solutions. As conclusions states, the circular concrete filled steel tube columns show more ductility and strength compared to square and rectangular ones.One of the most important parameters in this study is the bond effect. Due to the mechanical properties of the high strength concrete such as Poisson ratio and the amount of shrinkage differ from the low or medium strength concrete, the bond strength between the steel tube and the concrete core is critical on the axial load capacity for high strength concrete filled steel tube columns. There have been many studies to investigate the bond effect for the concrete filled tube columns by various researchers. Roeder et.al [3] studies the composite action in concrete filled tubes and emphasizes the importance of the bond stress between the concrete core and the steel tube. The results show that the bond capacity becomes smaller as the diameter and D/t ratios of tubes increase. They develop a formulation that estimates the bond stress capacity. Giakoumelis and Lam [4] investigate 15 circular columns with 30, 60 and 85 MPa concrete stre...

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