Zhuojin Pipe Fitting Co., LtdShear Capacity of Double-Skin CFST External Ribbed Ring-Plate Joints
Literature Overview
Zhang Yufen, Wang Fu, and Zhu Ge (2018), from North China University of Technology, published a study in World Information on Earthquake engineering on the shear behaviour of joints in double-skin CFST columns. The joint under investigation features an external ribbed ring-plate connection, which is a relatively novel configuration designed to enhance the shear capacity of the joint panel zone. The work was supported by the National Natural Science Foundation of China (Grant No. 51478004).
This research is particularly relevant to engineers working on heavy industrial structures, nuclear facilities, and high-rise buildings where double-skin CFST columns are employed for their superior fire resistance, corrosion protection, and post-yield ductility.
Shear Mechanism and Analytical Model
The authors decomposed the shear resistance of the joint panel zone into three distinct contributions:
- Shear contribution of the inner and outer steel tube webs within the joint region.
- Shear contribution of the vertical ribs and anchored web plates of the main connecting components.
- Shear contribution of the concrete core within the joint panel.
By superimposing these three components, the authors derived closed-form expressions for both the yield shear capacity and the ultimate shear capacity of the joint. The analytical results were then compared with experimental data, showing good agreement.
Shear Contribution Decomposition
| Component | Yield Stage Contribution | Ultimate Stage Contribution | Key Geometric Parameters |
|---|---|---|---|
| Inner steel tube web | Elastic shear resistance | Plastic shear resistance with local buckling | Wall thickness, height, material yield stress |
| Outer steel tube web | Elastic shear resistance | Plastic shear resistance with local buckling | Wall thickness, height, material yield stress |
| Vertical ribs and anchored plates | Elastic shear resistance | Plastic shear resistance | Rib thickness, plate dimensions, weld strength |
| Concrete core | Limited contribution at yield | Contributes via diagonal compression strut | Concrete strength, core dimensions |
The Shear Capacity Reserve Coefficient
A notable contribution of this paper is the introduction of the shear capacity reserve coefficient (抗剪能力储备系数). This dimensionless index quantifies the safety margin between the shear capacity of the joint and the shear demand imposed after the beam has yielded. The rationale is straightforward: in seismic design, the principle of "strong shear, weak bending" (强剪弱弯) requires that the joint panel zone remains elastic while the beams form plastic hinges. The reserve coefficient provides a quantitative measure of whether this design principle is satisfied.
The authors found that the novel external ribbed ring-plate joint provides sufficient shear reserve even after beam yielding, thereby ensuring a ductile beam-hinge failure mode. This is a significant advantage over conventional single-skin CFST joints, where the joint panel zone can become a weak link.
Fabrication and Welding Considerations
From a fabrication standpoint, the external ribbed ring-plate joint introduces several quality control challenges:
- Rib-to-tube welding: The vertical ribs are typically welded to the outer surface of the CFST column. These fillet or full-penetration welds must be inspected via ultrasonic testing (UT) or magnetic particle testing (MT) to detect lack of fusion, porosity, or undercut. The weld profile should conform to the applicable standard, such as GB/T 12467 or ISO 2553.
- Ring plate attachment: The ring plate itself is welded to the column and provides a stiffening effect. The weld quality at the ring plate-to-column junction is critical, as this is where the shear force is transferred.
- Concrete placement: In a double-skin CFST column, the concrete core must be placed between the inner and outer tubes. Ensuring full compaction and avoiding voids is essential, particularly in the joint region where the geometry is complex.
Weld Quality Control Checklist
| Inspection Item | Method | Acceptance Criteria |
|---|---|---|
| Rib-to-tube fillet welds | MT or PT | No cracks, lack of fusion > 2 mm |
| Ring plate butt welds | UT (PAUT preferred) | No planar defects > 3 mm |
| HAZ hardness | Microhardness test | ≤ 350 HV (for Q345 steel) |
| Weld dilution | Spectroscopic analysis | Carbon equivalent within limits |
| Concrete fill density | Radiographic test or density measurement | ≥ 95% of specified density |
Engineering Implications
The introduction of the shear capacity reserve coefficient is a welcome addition to the design methodology for CFST joints. It provides a clear, quantitative criterion that can be incorporated into design checklists and peer reviews. Engineers should aim for a reserve coefficient greater than 1.0, with a target value of 1.2 to 1.5 depending on the seismic intensity and the importance category of the structure.
The paper also reinforces the importance of the joint panel zone in seismic design. In conventional steel frames, the joint panel zone is often overlooked, but in CFST systems, it can be the governing failure mode. The external ribbed ring-plate configuration is a promising solution that deserves further investigation, particularly regarding its fatigue performance under cyclic loading and its behaviour at elevated temperatures.
Summary
This paper presents a rigorous analytical framework for the shear design of double-skin CFST joints with external ribbed ring plates. The decomposition of shear resistance into three components and the introduction of the shear capacity reserve coefficient are both valuable contributions to the field. Engineers should pay close attention to the fabrication quality of the rib and ring plate welds, as these details are critical to achieving the predicted shear performance. The findings support the adoption of this joint type in seismic design, provided that the analytical model is validated against full-scale tests for each specific geometry.