Research on the Influence of Axial Compression Ratio on the Seismic Performance of Fully Bolted Octagonal Core-Tube Joints in Square Steel Tube Columns

In practical engineering， steel columns are often subjected to different vertical axial compression ratios. To study the mechanical properties of the octagonal core tube full-bolted connection joints of square steel tube columns under different axial compression ratios， eight sets of finite element joint models were established， with axial compression ratios of vertical -0.2， -0.1， 0， 0.1， 0.2， 0.3， 0.4， and 0.5. The influence of different axial compression ratios on the seismic performance of the joints under bidirectional horizontal loading was analyzed. The results showed that when subjected to tension-bending and pure bending， the joints ultimately exhibited tensile failure of the flange high-strength bolts； when subjected to compression-bending with an the axial compression ratio of 0.1， the joints ultimately still failed due to tensile failure of the flange high-strength bolts， whereas when the axial compression ratio ranged from 0.2 to 0.5， the failure occurred at the connection between the lower column and the column base stiffener. Under tension-bending condition， the flange plates， flange high-strength bolts， and the lower section of the core tube bore substantial loads， while the load-transfer capacity of the joints was relatively weak， resulting in less load being transferred to the lower column. During severe earthquakes， when the structural elastic-plastic inter-story drift angle reached the limit value of 0.02 rad， excessive tensile deformation of the flange high-strength bolts led to joint failure. In practical engineering， special attention should be paid to the design of flanges and high-strength bolts for scenarios where tension or excessive seismic intensity may occur. Under pure bending and compression-bending loading， as the axial compression ratio increased， the bearing capacities of the flange plates， flange high-strength bolts， and the lower section of the core tube decreased， while the bearing capacity of the lower column increased. Concurrently， the joints' load-transfer capacity， energy dissipation capacity， and ductility were enhanced， but the overall bearing capacity decreased and stiffness degradation accelerated. When the axial compression ratio was 0， the joints had the highest bearing capacity， but relatively weak energy dissipation capacity， and ultimately underwent joint failure. Therefore， it is recommended to control the axial compression ratio at 0.2 in joint design， to ensure the joint possesses both high bearing capacity and favorable ductility and energy dissipation capacity.