Seismic Performance of Multi-Chamber Steel Tube Confined Concrete Branching Columns with Different Cross-Sectional Configurations

Literature Overview and Research Context

This paper by Wu Haipeng, Qiao Qiyun, Cao Wanlin, Yin Fei, and Li Xiangyu from the Key Laboratory of Urban and Engineering Safety for the Ministry of Education at Beijing University of Technology investigates the seismic performance of multi-chamber steel tube confined concrete branching columns with different cross-sectional configurations. Published in 2018 in the China Civil Engineering Journal (Vol. 51, No. 6, pp. 23-31), the study was supported by the National Natural Science Foundation of China (51578020) and the China Postdoctoral Science Foundation (2017M520560). Branching columns are critical structural elements in super-high-rise mega-frame structures, where they serve as transition members between columns of different sizes or orientations. The seismic performance of these members is of paramount importance for the safety of tall buildings in earthquake-prone regions.

Experimental Program and Test Configuration

The researchers conducted low-cycle reversed loading tests on five specimens subjected to constant axial compression with horizontal loads applied cyclically twice. The specimens represent different cross-sectional configurations, including one basic configuration and four strengthened configurations. The strengthened configurations include: thickened longitudinal steel plates at the branching face of the upper and lower columns, additional chambers added below the branching face of the lower column, and configurations combining multi-chamber strengthening with angle steel or circular steel tube inserts within the corner chambers.

Specimen Configuration Comparison

Core Findings and Technical Interpretation

The test results reveal several important findings regarding the seismic behavior of multi-chamber steel tube confined concrete branching columns. First, the failure of most specimens occurs at the lower horizontal diaphragm of the lower column, characterized by cracking and tearing of the steel plate in the heat-affected zone (HAZ) near the weld edges. This failure mode indicates that the weld details at the branching connection are the critical weak links in the structural system. The HAZ cracking is particularly concerning because it represents a brittle failure mechanism that can lead to sudden loss of load-carrying capacity.

Displacement Capacity and Ductility

The displacement capacity of the specimens is characterized by three key parameters: the yield displacement angle (approximately 1/101 on average), the peak load displacement angle (approximately 1/42), and the maximum elastic-plastic displacement angle (approximately 1/29). These values indicate that the specimens possess good ductility and displacement capacity, which is essential for the seismic performance of super-high-rise mega-frame structures. The displacement capacity is sufficient to accommodate the large inter-story drifts that may occur during severe earthquakes.

Effect of Cross-Sectional Configuration

The comparative analysis of the different configurations reveals that the arrangement of welds is the primary factor influencing the failure mode of the specimens. The configuration with angle steel or circular steel tube inserts within the corner chambers provides the best seismic performance, followed by the thickened plate configuration. The multi-chamber strengthening configuration provides the least improvement in seismic performance. The upper column exhibits greater deformation and energy dissipation capacity compared to the lower column, and the load-carrying capacity degradation is not significant across all specimens.

Failure Mode Analysis

The failure at the lower horizontal diaphragm of the lower column is consistent with the stress concentration effects at the branching connection. The complex geometry of the branching column creates stress concentrations at the weld junctions, particularly in the HAZ where the material properties are altered by the welding thermal cycle. The cracking and tearing of the steel plate in the HAZ indicates that the weld details require careful design and fabrication to ensure adequate ductility and fatigue resistance.

Engineering Practice Implications

For structural engineers designing super-high-rise mega-frame structures, the findings of this study provide critical guidance on the design of branching columns. The emphasis on weld arrangement as the primary factor influencing failure mode underscores the importance of detailed weld design and quality control. Engineers should adopt weld details that minimize stress concentrations, such as fillet welds with adequate throat thickness, full-penetration groove welds at critical junctions, and proper weld sequencing to minimize residual stresses. The superior performance of the corner insert configurations suggests that local reinforcement at the corners of the branching column can significantly improve seismic performance without substantially increasing the overall weight or cost of the member.

From a welding engineering perspective, the HAZ cracking observed in the tests highlights the need for careful control of the welding process parameters, including preheat temperature, interpass temperature, and post-weld heat treatment. The use of low-hydrogen welding consumables and proper heat input control is essential to prevent HAZ cracking in thick steel plates. Engineers should also consider the use of weldable steel grades with adequate toughness, such as those meeting the Charpy V-notch impact energy requirements at the expected service temperature.

Study Insights and Professional Reflection

This research makes a significant contribution to the understanding of the seismic behavior of branching columns in super-high-rise structures, which are increasingly used in modern architectural design. The systematic comparison of different cross-sectional configurations provides actionable guidance for engineers seeking to optimize the seismic performance of these critical structural elements. The finding that the corner insert configurations provide the best performance is particularly noteworthy, as it suggests a practical and cost-effective strategy for improving the seismic resilience of branching columns. The emphasis on weld quality and arrangement as the primary factor influencing failure mode reinforces the importance of welding engineering in the design and fabrication of complex structural members. Engineers should adopt a holistic approach that integrates structural design, material selection, welding process optimization, and quality control to ensure the reliable seismic performance of branching columns in super-high-rise mega-frame structures.