Experimental and numerical investigations of the octagonal concrete-filled thin-walled tube columns with binding bars (O-CFT-WBB) under compressive pressure and thermal loads

An experimental and theoretical investigation is conducted to evaluate the behavior of CFT columns with a unique geometric cross-section under axial loading and in a thermal environment. In this study, based on previous research on octagonal CFT columns, this geometry was chosen as a favorable option compared to other possible configurations. Additionally, binding bars were employed to enhance these columns' mechanical and thermal behavior. A thermal gradient (temperature changes ranging from 0 to 600 degrees) was applied in the laboratory to replicate extreme environmental conditions. By experimental results, a theoretical investigation was conducted to derive suggested formulas for considering confinement effects in O-CFT-WBB columns, and the results were then compared with experimental results and the closest existing formulas. After examining the experimental results, it was shown that the presented formulas had more accuracy than the existing ones. Additionally, based on the experimental results, the presence of binding bars at different temperatures led to different behaviors in the axial capacity of the columns. According to the results, the binding bars in the O-CFT-WB columns can lead to different behaviors in the axial capacity of the columns. Based on a constitutive model of confined concrete, a method for calculating the maximum strength of L-shaped CFT columns with and without binding bars is proposed. The method is verified with experimental results from this test program and data from other experiments.