Seismic Performance of Multi-Cavity Concrete-Filled Steel Tubular Branch Columns Under Different Loading Directions

Literature Overview

The study by Wu, Cao, Dong, Yin, and Li (2018), published in Journal of Vibration and Shock (Vol. 37, No. 18, pp. 78–85), investigates the seismic performance of irregular-section multi-cavity CFST branch columns under different loading directions. The research was funded by the National Natural Science Foundation of China (Grant 51578020) and conducted at Beijing University of Technology. The authors conducted low-cycle cyclic loading tests on 5 specimens subjected to two cycles of horizontal force in three directions (long axis, short axis, and 45°), with two joint core configurations (basic and reinforced with additional steel cavity).

Core Technical Findings

The study demonstrates that the loading direction has a significant influence on the seismic performance of irregular-section multi-cavity CFST branch columns. The following table summarizes the key results:

The study also found that the joint core reinforcement can improve the load capacity, stiffness, and energy dissipation of the specimens to a certain extent. The damage and failure of all specimens were initiated at the weld of the lower horizontal diaphragm in the lower column, with crack propagation leading to plate tearing. The deformation and energy dissipation were primarily concentrated in the upper column, accounting for 60% to 70% of the total.

Interpretation of Technical Points

The finding that the long axis direction provides the best seismic performance is consistent with the geometric properties of irregular-section columns. The long axis direction has a larger moment of inertia and greater section modulus, leading to higher flexural stiffness and load capacity. The short axis direction has the smallest moment of inertia, resulting in lower stiffness and earlier failure. The 45° direction provides intermediate performance due to the combined effect of both principal axes.

The observation that the weld at the lower horizontal diaphragm is the critical failure location is of great practical significance. This weld is a primary load path for transferring forces between the upper and lower columns, and any weld defect can initiate crack propagation under cyclic loading. The study highlights the importance of weld quality in the seismic performance of CFST branch columns.

From a welding and fabrication perspective, the joint core reinforcement introduces several technical considerations:

The weld at the lower horizontal diaphragm is the most critical weld in the branch column, and its quality must be ensured through rigorous welding procedures and NDT inspection. The use of SAW or FCAW with full-penetration groove welds is recommended, and 100% UT inspection should be performed to detect any weld defects. The weld metal should be matched to the base metal, and the welding parameters should be optimized to minimize HAZ hardness and residual stress.

Engineering Practice Implications

For engineers designing CFST branch columns in seismic regions, this study provides several important guidelines:

1. The loading direction should be considered in the design, with the short axis direction being the most critical for seismic loading.
2. The joint core reinforcement should be applied to improve the seismic performance, particularly in the short axis direction.
3. The weld at the lower horizontal diaphragm should be designed and fabricated with the highest quality standards, as it is the critical failure location.
4. The multi-cavity configuration should be optimized to provide balanced performance in all loading directions.
5. The deformation and energy dissipation capacity of the upper column should be considered in the design, as it accounts for 60% to 70% of the total.

The study also has implications for the inspection and repair of existing CFST branch columns. When weld defects are detected at the lower horizontal diaphragm, immediate repair should be performed to prevent crack propagation and potential failure. The repair procedure should include:

• Thorough cleaning of the weld area to remove rust and paint.
• Removal of the defective weld by grinding or machining.
• Repair welding with matching weld metal and optimized welding parameters.
• NDT inspection of the repair weld to ensure compliance with applicable standards.

Key Questions and Reflections

One question that arises is whether the findings are applicable to other types of irregular-section CFST columns, such as cruciform, T-shaped, or L-shaped sections. The study focuses on multi-cavity branch columns with a specific irregular section, and the generalization to other section shapes requires further investigation.

Another reflection concerns the effect of loading amplitude and number of cycles on the seismic performance. The study subjects the specimens to only two cycles of horizontal force, which may not capture the full range of seismic demand. Further research with more cycles and varying loading amplitudes is needed to evaluate the fatigue behavior and cumulative damage.

Study Insights and Implications

This paper provides valuable experimental evidence on the seismic performance of irregular-section multi-cavity CFST branch columns under different loading directions. The key takeaway for practicing engineers is that the loading direction has a significant influence on seismic performance, with the short axis direction being the most critical, and that the weld at the lower horizontal diaphragm is the critical failure location requiring the highest quality standards. For welding and fabrication engineers, the study underscores the importance of rigorous weld quality control, particularly for the critical welds in the joint core, and the need for 100% NDT inspection to detect and prevent weld defects. Future research should extend the investigation to other section shapes, evaluate the fatigue behavior under more cycles, and develop simplified design equations for irregular-section CFST branch columns based on the experimental data.