STUDY ON BEHAVIOR OF ULTRA HIGH STRENGTH CIRCULAR CFT STUB COLUMNS IN UNIAXIAL COMPRESSION

Concrete filled steel tube (CFT) is a member consisting of a steel tube filled with concrete. It is well known that the strength and ductility of CFT columns increase due to the effect of mutual confinement between the concrete and the steel tube. However, the prescriptive design rules of ultra-high strength CFT is less established due to lack of the laboratory database. In this study, experiments on circular CFT stub columns consisting of 780N/mm2 or 550N/mm2 class steel with 23 to 45 diameter thickness ratio and 100N/mm2 class concrete (ultra-high strength CFT) were carried out. Steel tube columns and plain concrete columns were also individually tested to characterize the behavior of CFT stub columns. The biaxial strain is measured to calculate the axial and circumferential stress in the steel tubes by the incremental method. Then, the relationship between axial and confinement stress in infilled concrete is quantitatively evaluated to understand the details of mutual confinement effect in ultra-high strength CFT stub columns. Results are summarized as follows: 1) The strength ratio of the CFT column calculated by concrete cylinder strength was almost proportional to a steel tube strength ratio. 2) Clear relations were not observed between the strength ratio calculated by concrete material strength and the diameters of CFT specimens. 3) The load of CFT stub column with 550N/mm2 class steel and 100N/mm2 class concrete becomes larger than the simple cumulative load of steel tube and plain concrete when strain increased by 0.03 to 0.06% after the fracture strain of plain concrete. During this strain level, the circumferential strain of steel tube in CFT stub column becomes larger than that of individual steel tube. 4) The axial stress of infilled concrete almost agrees with the stress of plain concrete until the fracture strain of plain concrete is reached. The circumferential stress in the steel tube of CFT stub column was generated between the fracture strain of plain concrete and that of cylinder concrete. After fracture strain of cylinder concrete, the axial stress in infilled concrete has climb gradient. 5) If the fracture strain of concrete is lower than the yield strain of steel tube in CFT stub column, the gradient of confining stress – axial stress curve changes smoothly. On the other hand, if the yield strain of steel tube in CFT stub column is lower than the fracture strain of concrete, only the confining stress was observed and the axial stress