Zhuojin Pipe Fitting Co., LtdComparison of Seismic Performance of Externally Strengthened Ring and Internal Diaphragm Square CFST Column-Steel Beam Frames
Literature Overview
The paper by Ni Tiekuan, Yuan Wenqi, and Xu Chengxiang (2016), published in the Journal of Wuhan University of Technology (Vol. 38, No. 1, pp. 44–48), compares the seismic performance of square concrete-filled steel tube (CFST) column-steel beam frames with two different connection types: externally strengthened ring connections and internal diaphragm connections. Supported by the National Natural Science Foundation of China (Grant No. 51178057) and the Hubei Provincial Outstanding Young and Middle-aged Scientific and Technological Innovation Team Program (Grant No. T201303), the study designed and fabricated a three-story, two-bay externally strengthened ring connected square CFST column-steel beam frame and subjected it to low-cycle reversed loading tests. The results were compared with previously published test data for internally diaphragm connected frames.
Core Technical Points
Connection Type Characteristics
The two connection types differ fundamentally in their structural behavior and construction methodology:
| Feature | Externally Strengthened Ring | Internal Diaphragm |
|---|---|---|
| Connection location | External to the column | Internal to the column |
| Construction method | Ring plate welded externally to column with beam connections | Diaphragm plate welded internally between column walls |
| Field constructability | Easier field assembly and inspection | Difficult internal welding and inspection |
| Load transfer mechanism | External ring distributes beam forces to column walls | Diaphragm directly transfers beam shear to column walls |
| Access for inspection | Fully accessible | Limited internal access |
Low-Cycle Reversed Loading Test Results
The experimental program employed low-cycle reversed loading to simulate seismic loading conditions. Key performance indicators evaluated include:
- Hysteresis curves: Both connection types exhibited full hysteresis loops, indicating good energy dissipation capability and stable load-bearing performance under cyclic loading.
- Skeleton curves: The externally strengthened ring connection specimens demonstrated significantly higher ultimate bearing capacity compared to internally diaphragm connected specimens.
- Ductility coefficient: Both connection types achieved ductility coefficients greater than 4, satisfying seismic design requirements for ductile structural systems.
- Strength degradation: The rate of strength degradation under cyclic loading was evaluated for both connection types, with the externally strengthened ring showing superior resistance to progressive damage accumulation.
- Stiffness degradation: Stiffness degradation patterns were compared, revealing differences in the post-yield behavior between the two connection types.
Energy Dissipation Capacity
The externally strengthened ring connection demonstrated superior energy dissipation capacity due to:
- Greater plastic hinge formation in the connection region
- More uniform strain distribution in the column walls
- Enhanced confinement effect from the external ring on the concrete core
- Better compatibility between the connection deformation and the column behavior
Engineering Practice Integration
From a practical construction standpoint, the externally strengthened ring connection offers several advantages:
- Fabrication and welding quality: External welding of the ring plate to the column is more accessible for welders and inspectors compared to internal diaphragm welding. This accessibility translates to better weld quality control, easier non-destructive testing (NDT), and reduced risk of weld defects.
- Erection sequence: The external ring can be pre-fabricated in the workshop with high quality control and then bolted or field-welded to the column, facilitating modular construction methods.
- Inspection and maintenance: The fully accessible connection allows for periodic inspection of weld integrity, bolt condition, and any signs of fatigue cracking throughout the structure's service life.
However, the externally strengthened ring connection also presents challenges:
- The external ring increases the overall column cross-section, potentially affecting architectural aesthetics
- Additional material cost for the ring plate and associated connection hardware
- The connection may be more susceptible to local buckling of the ring plate under compressive loading
Key Questions and Reflections
The study's comparison is valuable, but several aspects warrant deeper consideration:
The comparison relies on previously published test data for the internally diaphragm connected specimens. For a rigorous comparison, both connection types should ideally be tested under identical conditions (same loading protocol, same specimen dimensions, same material properties) within the same laboratory environment. Differences in test setup, loading rate, or specimen preparation could introduce systematic biases into the comparison.
Furthermore, the study focuses on the seismic performance of the connection itself but does not extensively discuss the overall frame behavior. In a complete seismic assessment, the interaction between connection behavior and frame global response (including P-Δ effects, story drift demands, and inter-story drift ratio distribution) should be considered.
Study Insights and Implications
This research provides compelling evidence that the externally strengthened ring connection for square CFST column-steel beam frames offers superior seismic performance compared to the internal diaphragm connection. The findings are particularly significant for seismic design of CFST structures in high-seismic zones, where connection ductility and energy dissipation are critical design considerations. For practitioners, the externally strengthened ring connection represents a practical solution that combines excellent seismic performance with improved constructability and inspectability. The results support the adoption of external connection details in CFST frame design, provided that appropriate attention is paid to local buckling resistance of the ring plate and proper detailing of the beam-to-ring connections. The study contributes to the growing body of knowledge on CFST structural systems and provides a basis for code provisions governing CFST frame connections in seismic design standards.