Seismic Performance of Multi-Chamber CFST Bifurcation Columns Under Different Loading Directions

Literature Overview

This 2018 paper by Wu Haipeng et al. from Beijing University of Technology presents low-cycle reversed loading tests on five specimens of irregular-section multi-chamber CFST bifurcation columns. The study examines three loading directions (long axis, short axis, and 45-degree diagonal) and two joint core constructions (basic and reinforced with additional steel chambers). This research addresses a critical gap in seismic design for super-tall mega-frame structures where branching columns with complex cross-sections are commonly employed.

Core Technical Findings

The experimental results reveal several important patterns. First, the weld arrangement position governs the failure mode — all specimens failed due to weld cracking at the lower horizontal diaphragm of the lower column, with crack propagation leading to steel plate tearing. This finding is particularly significant from a welding engineering perspective, as it identifies the weld at the lower column diaphragm interface as the critical stress concentration zone.

The directional dependence of seismic performance is intuitive — the long axis direction provides the greatest cross-sectional depth and thus the largest section modulus for flexural resistance. However, the 45-degree direction result is noteworthy because it indicates that the multi-chamber configuration provides reasonably balanced performance even under oblique loading, which is significant for real seismic events where ground motion direction is unpredictable.

Welding and Fabrication Analysis

The failure mechanism identified — weld cracking at the lower horizontal diaphragm — demands careful attention to welding quality control in fabrication. From a welding metallurgy perspective, this location represents a T-joint with a thick plate (diaphragm) welding to a thinner wall (column tube), creating a geometry prone to stress concentration and residual stress accumulation. The recommended welding practices include:

• Use of full-penetration welds with adequate root preparation (V-groove or U-groove)
• Preheating temperatures of 100–150°C for steel grades above Q345 to reduce hydrogen-induced cracking susceptibility
• Post-weld heat treatment (PWHT) at 550–620°C for thick sections exceeding 40 mm to relieve residual stresses
• Application of low-hydrogen welding consumables (E7018 or equivalent) with strict moisture control
• Ultrasonic testing (UT) per GB/T 11345 or ASME V Section for 100% inspection of critical welds

The finding that deformation and energy dissipation occur primarily in the upper column portion (60–70%) indicates that the plastic hinge forms above the branch junction, which is favorable from a ductile design philosophy standpoint. This means the lower column and joint region remain largely elastic during seismic events, preserving structural integrity.

Strengthening Construction Effectiveness

The comparison between basic and reinforced joint core constructions demonstrates that adding steel chambers to the joint core region provides measurable improvements in bearing capacity, stiffness, and energy dissipation. However, the paper notes that these improvements are "to a certain extent" — suggesting diminishing returns beyond a certain level of reinforcement. From a cost-benefit engineering perspective, this finding supports selective strengthening rather than uniform over-design of all joint regions.

The observation that bearing capacity degradation under the same reversed loading cycle is not significant is encouraging for seismic performance. It indicates that the multi-chamber CFST bifurcation column maintains stable cyclic behavior, which is essential for withstanding multiple seismic pulses during major earthquakes.

Engineering Practice Integration

For engineers designing super-tall building mega-frames, this research provides actionable guidance. The directional performance variation suggests that seismic isolation or supplemental damping systems should be oriented to minimize loading in the weak short-axis direction. The weld failure identification directly informs quality control procedures — the lower diaphragm welds must receive priority inspection and may require additional reinforcement (weld overlay or stiffener plates) in high-seismicity regions. The 60–70% deformation concentration in the upper column also has implications for detailing — this region should receive enhanced confinement through additional internal chambers or external strengthening to ensure ductile behavior throughout the design displacement range.