Zhuojin Pipe Fitting Co., LtdComparison of Seismic Performance Between Externally Reinforced Ring and Internal Diaphragm Square CFST Column-Steel Beam Frames
Literature Overview
This paper by Ni Tiekuan, Yuan Wenqi, and Xu Chengxiang (2016), published in the Journal of Wuhan University of Technology, presents a comparative study of the seismic performance of square concrete-filled steel tube (CFST) column-steel beam frames with two different connection types: externally reinforced ring connections and internal diaphragm connections. The authors designed and tested a three-story, two-bay frame with externally reinforced ring connections under low-cycle reverse loading and compared the results with previously published test data for internal diaphragm connections. The comparison covers hysteresis curves, skeleton curves, ductility, energy dissipation capacity, strength degradation, and stiffness degradation.
Connection Design Comparison
The two connection types represent fundamentally different approaches to achieving moment continuity at the column-beam joint in CFST frames.
Externally Reinforced Ring Connection
The externally reinforced ring connection uses a welded steel ring (or collar) placed around the exterior of the CFST column at the beam connection level. The beam is welded to this external ring, transferring moments through the ring to the column. Key design features include:
- The ring is welded to the column tube at the top and bottom edges, creating a rigid connection.
- The beam-to-ring weld provides the primary moment transfer mechanism.
- The ring thickness and height are designed to prevent local yielding and buckling under cyclic loading.
- The connection detail avoids cutting or modifying the CFST column itself.
Internal Diaphragm Connection
The internal diaphragm connection uses a steel plate inserted inside the CFST column at the beam connection level. The beam is welded to this internal plate, with the diaphragm transferring forces to the column through shear connection with the concrete core. Key design features include:
- The diaphragm plate is inserted during column fabrication, before concrete filling.
- The plate is welded to the column tube at the insertion point.
- Shear transfer to the concrete core relies on the bond between the plate and the concrete.
- The connection requires access to the column interior during fabrication.
Experimental Results Comparison
| Performance Indicator | Externally Reinforced Ring | Internal Diaphragm | Comparison |
|---|---|---|---|
| Hysteresis curve shape | Full and stable | Full but with earlier degradation | Ring superior |
| Ductility coefficient | >4 | >4 | Both meet code requirements |
| Ultimate bearing capacity | Significantly higher | Lower | Ring advantage 15-25% |
| Energy dissipation capacity | Higher cumulative energy | Lower cumulative energy | Ring superior |
| Strength degradation rate | Slower degradation | Faster degradation | Ring more stable |
| Stiffness degradation rate | Moderate degradation | Faster degradation | Ring maintains stiffness better |
| Damage pattern | Ring weld yielding | Diaphragm plate yielding | Both acceptable |
Seismic Performance Analysis
Hysteresis and Skeleton Curves
The hysteresis curves for both connection types exhibit full and well-defined loops, indicating good energy dissipation capacity and stable cyclic behavior. However, the externally reinforced ring connection demonstrates more consistent loop shapes throughout the loading history, while the internal diaphragm connection shows more pronounced loop pinching at higher displacement levels. This pinching behavior indicates progressive damage accumulation at the diaphragm-to-concrete interface.
Ductility and Energy Dissipation
Both connection types achieve ductility coefficients exceeding 4.0, which satisfies the seismic design requirements for special moment frames. The externally reinforced ring connection achieves higher ductility values, typically in the range of 4.5-5.5, compared to 4.0-4.8 for the internal diaphragm connection. The energy dissipation capacity, quantified as the cumulative area enclosed by the hysteresis loops, is consistently higher for the externally reinforced ring connection across all displacement levels.
Strength and Stiffness Degradation
The externally reinforced ring connection exhibits slower strength degradation under cyclic loading, maintaining over 85% of peak strength at the ultimate displacement limit. The internal diaphragm connection shows faster degradation, with strength retention dropping to approximately 75-80% at comparable displacement levels. Similarly, the stiffness degradation is more gradual for the ring connection, indicating better post-yield behavior and more predictable deformation characteristics under seismic loading.
Engineering Practice Implications
The superior seismic performance of the externally reinforced ring connection has several practical implications for structural engineers:
- The ring connection is more suitable for high-seismicity regions where enhanced ductility and energy dissipation are critical.
- The ring connection requires no modification to the CFST column fabrication process, making it compatible with existing production methods.
- The internal diaphragm connection, while adequate for moderate seismicity, may require additional reinforcement or larger plate dimensions to achieve equivalent performance to the ring connection.
- The ring connection's superior strength retention under cyclic loading provides better margin against collapse under prolonged earthquake shaking.
Design Recommendations
For seismic design of CFST column-steel beam frames, the following recommendations emerge from this study:
- Prefer externally reinforced ring connections for special moment frames in high-seismicity zones.
- Ensure ring thickness is at least 1.5 times the column tube thickness to prevent local buckling.
- Use full-penetration groove welds for the ring-to-column and beam-to-ring connections.
- Provide adequate clear space between the ring and the beam flange to accommodate plastic hinge formation.
- Consider the interaction between the ring connection and adjacent column segments when designing the overall frame stiffness.
Study Insights and Reflections
This comparative study provides valuable experimental evidence for the selection of connection details in CFST moment frames. The results clearly demonstrate that the externally reinforced ring connection offers superior seismic performance across all measured parameters, including ultimate capacity, ductility, energy dissipation, and damage tolerance. The practical advantage of the ring connection—its compatibility with standard CFST fabrication processes without requiring internal modifications—makes it particularly attractive for large-scale construction projects.
However, the study also highlights the importance of considering construction feasibility and cost when selecting connection types. The internal diaphragm connection, while showing somewhat inferior seismic performance, may be preferred in applications where column fabrication access is limited or where the diaphragm can serve a dual purpose as a construction platform. Engineers should weigh the performance benefits of the ring connection against the additional fabrication and welding costs when making final design decisions. The overall conclusion is that for seismic-critical applications, the externally reinforced ring connection is the preferred choice for square CFST column-steel beam frames, providing a robust and reliable connection detail that maintains structural integrity throughout the full range of seismic demands.