Zhuojin Pipe Fitting Co., LtdSeismic Performance of Sleeve-Strengthened Circular CFST Column-Steel Beam Joints
Literature Overview
The paper by Bu Yonghong, Zhao Junhai, Zhang Dongfang, Fan Junchao, and Lan Guanchi (2021, China Journal of Highway and Transport, Vol. 34, No. 5, pp. 110-122) investigates the seismic performance of a novel sleeve-strengthened joint connecting circular concrete-filled steel tube (CFST) columns to steel beams. The study was motivated by safety concerns associated with single-sided bolted connections under seismic loading and proposed a sleeve-strengthened joint as an improved solution.
Test Configuration and Key Variables
Ten half-scale specimens were tested under low-cycle reverse loading with column-end loading. The primary influencing factors were joint construction type and beam-to-column stiffness ratio.
| Test Parameter | Description |
|---|---|
| Specimen scale | 1/2 scale |
| Total specimens | 10 |
| Column type | Circular CFST |
| Joint types | Sleeve-strengthened welded, sleeve-strengthened bolted-welded hybrid, conventional bolted |
| Loading method | Column-end loading, low-cycle quasi-static |
| Key variables | Joint construction type, beam-to-column stiffness ratio |
| Design objective | "Strong joint, weak member" seismic design principle |
Joint Construction Analysis
The sleeve-strengthened joint introduces a cylindrical sleeve around the CFST column at the beam connection zone, allowing the sleeve, bolts, and welds to collectively resist bending moments and shear forces. This is a significant departure from conventional bolted connections where the bolts alone carry the shear and the moment is transferred through bolt tension.
Comparison of Joint Types
| Performance Indicator | Sleeve-Strengthened Bolted-Welded | Sleeve-Strengthened Welded | Conventional Bolted |
|---|---|---|---|
| Bolt loosening | Eliminated | N/A | Present |
| Bolt fracture | Eliminated | N/A | Present |
| Weld cracking | Eliminated | Possible | N/A |
| Connection stiffness | Rigid | Rigid | Semi-rigid |
| Bearing capacity | Highest | High | Lowest |
| Safety reserve | High | Medium-High | Low |
Welding Engineering Perspectives
From a welding and fabrication standpoint, this joint design introduces several critical considerations:
- Sleeve-to-column weld: The sleeve must be welded circumferentially to the CFST column. For circular CFST columns with diameters of 400–800 mm, this circumferential weld requires careful control of angular distortion and requires welding procedures qualified per GB/T 985 or ISO 2553.
- Beam-to-sleeve weld: The connection between the steel beam end plate and the sleeve involves fillet welds that must accommodate seismic inelastic deformation. The weld throat thickness and weld length must be designed to ensure that weld rupture does not precede member yielding.
- Residual stress management: The combined welding of the sleeve to the column and the beam to the sleeve creates a complex residual stress field. Post-weld stress relief or controlled welding sequences should be employed to minimize the risk of brittle fracture under seismic loading.
- Material matching: The sleeve material grade should match or exceed the column steel grade. If the column is constructed from Q345 or Q355 steel, the sleeve should be of equivalent or higher grade to ensure compatible yield behavior.
Failure Mode Analysis
The test results indicate that the sleeve-strengthened bolted-welded hybrid joint successfully avoids bolt loosening, bolt fracture, and weld cracking—failure modes commonly observed in conventional bolted connections. This achievement validates the "strong joint, weak member" seismic design philosophy, where the joint remains elastic while the beam member yields and dissipates energy through plastic deformation.
The joint's equivalent viscous damping coefficient, rotation ductility coefficient, and elastic and elasto-plastic story drift angles all meet or exceed code requirements, confirming that the sleeve-strengthened joint provides adequate seismic performance.
Key Questions and Reflections
An important question for fabrication engineers is: how does the welding sequence affect the residual stress distribution in the sleeve-strengthened joint? The circumferential weld of the sleeve to the column creates a hoop stress field, while the beam-to-sleeve weld introduces localized bending stresses. If these welds are performed without proper sequencing, the superimposed residual stresses could reduce the effective yield strength of the joint.
Furthermore, the paper provides a calculation method for the ultimate moment capacity of the joint, but the validity of this method depends on the assumption that the sleeve and column steel yield simultaneously. In practice, weld imperfections or material non-uniformity could cause premature failure at the weld interface before full yielding of the sleeve.
Study Insights and Outlook
The sleeve-strengthened joint represents a practical engineering solution that transforms a semi-rigid bolted connection into a rigid connection with significantly higher bearing capacity. The hybrid bolted-welded approach is particularly attractive because it combines the adjustability of bolted connections during erection with the structural continuity of welded connections during seismic events. For pipe fabrication workshops, this design requires precision machining of the sleeve-to-column interface and rigorous weld quality control, including full radiographic testing of critical welds per GB/T 3323 or ISO 17636. Future research should extend to full-scale testing and investigate the long-term fatigue performance under combined gravity and seismic loading.