Structural Investigation of Shear Performance of Square CFST Column Joints with Extended Hollo-Bolts

The performance of concrete-filled steel tubular (CFST) column joints with a group of extended hollo-bolts has been experimentally investigated under the influence of shear loading. The primary focus was to understand the performance of extended hollo-bolts in enhancing the transfer of shear load to the concrete core of the column by bolt bearing. The joint assembly was fabricated with a rigid end-plate, thereby overcoming the influence of endplates in the joint global behavior. A series of full-scale group hollo-bolted CFST column joint tests was carried out, where eight specimens were fabricated with a group of two hollo-bolts, arranged in one row and two rows in the assembly; another five specimens were fabricated with a group of four hollo-bolts, arranged in two rows. Apart from bolt arrangement, the other parameters studied included the use of standard and extended hollo-bolts, the embedment length of the hollo-bolt, and the bolt pitch distance. From the investigation, it is observed that all the joints failed in pure shear signifying utilization of the full capacity of the hollo-bolt, and no prominent bearing failure of concrete was observed. Enhanced composite behavior was achieved using the extended hollo-bolts as the shear load was transferred to the concrete core effectively. A group of two extended hollo-bolts in a single row transmits equal forces to the concrete core, whereas, when two or four extended hollo-bolts are in two rows, the upper row transmits more forces as compared to the bolts in the lower row. Lastly, with pitch distance of 2.5 times bolt hole diameter and beyond, the total strength of the joint is equal to the sum of strength of individual bolts, which confirms that the group action did not deteriorate the joint capacity. Subsequently, an analytical model for the global force-displacement behavior and joint shear strength is proposed by calibrating the test data obtained through this study.