Behavior of eccentrically loaded square spiral-confined high-strength concrete-filled steel tube columns

Concrete-filled steel tube (CFST) columns have been widely used in super tall buildings. Since these columns are subjected to very high axial loads, the use of high-strength concrete (HSC) is preferred in order to reduce their cross-sectional size. For applications in seismic regions, additional confinement to the HSC may be needed to ensure ductility. Among various approaches, spiral confinement is an effective and practical way for square CFST columns. Six spiral-confined concrete-filled steel tube (SCCFST) columns and three CFST columns with the concrete compressive strength of 111 MPa were tested under eccentric axial loading. The behavior of the columns under different levels of eccentricity ratio was investigated, and the use of high-strength spirals to further enhance confinement was also examined. Concrete crushing was found to be the main cause for the loss of the load-carrying capacity. Spiral reinforcement is not effective in increasing the load-carrying capacity; however, it can considerably improve ductility, which is improved with an increase of the yield strength of spiral reinforcement. Based on the test results, an equivalent uniaxial stress-strain model was developed for the HSC in CFST and SCCFST columns. A fiber beam-column element using the proposed stress-strain models was found to provide good predictions of the measured load-displacement responses. Such fiber element can be used for modeling of SCCCFST columns with HSC in nonlinear structural analysis of tall buildings.