Zhuojin Pipe Fitting Co., LtdSeismic Performance of Separated Internal Diaphragm Joint Between Square Steel Tube Concrete Column and U-Shaped Steel Composite Beam
Literature Overview
This paper, authored by Cheng Rui, Zhang Jidong, Xu Xing, Liu Jichun, and Peng Shu from Chongqing University, was published in the Journal of Civil and Environmental Engineering (Chinese and English), Volume 45, Issue 6, 2023, pages 47–58. The research was supported by the National Key R&D Program of China (Grant 2016YFC0701506). The study investigates the seismic performance of a separated internal diaphragm joint connecting square steel tube concrete (CFT) columns and U-shaped steel composite beams through low-cycle reversed loading tests on four specimens.
Core Technical Content
The research designed and tested four joint specimens to evaluate the seismic performance of the separated internal diaphragm joint. The test parameters included the form of the internal diaphragm and the presence or absence of strengthening connections at the beam-column interface.
Specimen Design and Test Parameters
| Specimen | Internal Diaphragm Type | Interface Strengthening | Key Variation |
|---|---|---|---|
| Specimen 1 | Traditional internal diaphragm | No strengthening | Baseline traditional joint |
| Specimen 2 | Separated internal diaphragm (strong axis) | No strengthening | Separated diaphragm, strong axis direction |
| Specimen 3 | Separated internal diaphragm (weak axis) | No strengthening | Separated diaphragm, weak axis direction |
| Specimen 4 | Separated internal diaphragm | With strengthening connection | Separated diaphragm with interface reinforcement |
The low-cycle reversed loading test applied cyclic displacement to the beam end while monitoring the joint's response, including load-displacement hysteresis, stiffness degradation, energy dissipation, and ductility.
Test Results and Seismic Performance Indicators
| Performance Indicator | Range | Interpretation |
|---|---|---|
| Displacement ductility coefficient (μ) | 2.3–3.1 | Moderate ductility; adequate deformation capacity |
| Elastic inter-story drift angle (θy) | 1/68–1/53 | Elastic range; initial stiffness |
| Elastic-plastic inter-story drift angle (θu) | 1/28–1/19 | Ultimate deformation capacity |
| Equivalent viscous damping coefficient (ζeq) | 0.12–0.16 | Moderate energy dissipation capacity |
Failure Mode Analysis
All four specimens exhibited beam-end flexural failure, which is the desired failure mode for ductile seismic design. The hysteresis curves exhibited a reverse S-shape with noticeable pinching, indicating energy dissipation through plastic deformation. The pinching phenomenon is attributed to the slip between the U-shaped steel beam and the CFT column interface, as well as the yielding of the internal diaphragm.
Key Findings
- Internal diaphragm form effect: Changing the internal diaphragm form had a relatively small effect on the joint's load-bearing capacity. However, compared to the traditional internal diaphragm joint, the separated internal diaphragm weak axis specimen exhibited reduced energy dissipation capacity. This suggests that the separated diaphragm design may compromise the joint's ability to dissipate seismic energy, particularly in the weak axis direction.
- Interface strengthening effect: Strengthening the connection at the beam-column interface significantly improved the joint's seismic performance. Specifically, it reduced the rate of stiffness degradation, increased the joint's load-bearing capacity, and enhanced energy dissipation. This finding is particularly important for practical design, as it suggests that additional reinforcement at the interface can compensate for the reduced energy dissipation of the separated diaphragm.
- Deformation capacity: The displacement ductility coefficients of 2.3–3.1 indicate that the joint has moderate ductility, which is generally acceptable for seismic design in moderate seismic zones. However, for high-seismic-intensity regions, further improvement of ductility may be necessary through additional design measures.
Engineering Design Recommendations
Based on the test results, the paper provides design recommendations for the separated internal diaphragm joint:
- Diaphragm design: The separated internal diaphragm should be designed with adequate thickness and connection details to ensure sufficient load transfer between the beam and column. The diaphragm should be detailed to prevent premature buckling or yielding under cyclic loading.
- Interface strengthening: Strengthening connections at the beam-column interface are recommended to improve the joint's seismic performance. This can be achieved through additional stiffening plates, bolted connections, or welded reinforcement at the interface.
- Weak axis consideration: The weak axis direction requires special attention in the design of separated internal diaphragm joints, as the energy dissipation capacity is reduced compared to the strong axis direction. Additional reinforcement or design modifications may be necessary to ensure adequate seismic performance in the weak axis direction.
- Ductility requirements: For structures in high-seismic-intensity regions, the ductility of the joint should be enhanced through design measures such as increased diaphragm thickness, additional interface reinforcement, or the use of high-ductility steel materials.
Key Questions and Reflections
The study raises several important questions for further research and practical application. First, the test was conducted under quasi-static low-cycle reversed loading, which does not fully replicate the dynamic characteristics of earthquake loading. The joint's performance under true dynamic loading, including inertia effects and strain rate effects, may differ from the quasi-static test results. Second, the study does not address the long-term durability of the joint, including the effects of corrosion, fatigue, and cyclic loading over the structure's service life. Third, the paper does not discuss the constructability of the separated internal diaphragm joint, which is a critical consideration for practical implementation. The joint's complexity may increase construction costs and require specialized fabrication and installation procedures.
Study Insights and Implications
This paper provides valuable experimental data on the seismic performance of the separated internal diaphragm joint between square steel tube concrete columns and U-shaped steel composite beams. The key finding that interface strengthening significantly improves seismic performance is particularly important for practical design, as it offers a straightforward method to enhance joint performance without major design modifications. The test results demonstrate that the joint can achieve acceptable seismic performance with proper design, but the reduced energy dissipation in the weak axis direction requires careful attention. For engineers working on composite steel-concrete structures, the study reinforces the importance of detailed joint design and the need for experimental validation of seismic performance assumptions. The findings also highlight the trade-offs between different design options, such as the balance between load-bearing capacity and energy dissipation, and the importance of considering both strong and weak axis directions in seismic design. The paper represents a significant contribution to the understanding of composite joint behavior under seismic loading, and its findings should be considered in the design of future steel-concrete composite structures in seismic regions.