Zhuojin Pipe Fitting Co., LtdEffect of Combined Connectors on Steel Tube-Concrete Bonding Performance
Literature Overview
This paper, published in the Journal of Zhejiang University (Engineering Science) in 2018 by Wang Hailong, Zhu Yuqi, Xia Jin, and Sun Xiaoyan from Zhejiang University, investigates the influence of combined connectors on the bonding and slip behavior between steel tubes and surrounding concrete. The research was funded by the National Natural Science Foundation of China (51579220, 51408537) and the Zhejiang Provincial Natural Science Foundation (LY16E080004, LZ16E080002). The combined connector system consists of ring plates, stiffener plates, and headed studs, and the study examines their collective effect on interface bond-slip performance.
Core Technical Findings
The experimental and numerical investigation reveals several important findings regarding the interaction between steel tubes and concrete through combined connectors. The characteristic slip values for steel tubes with combined connectors exceed 6 mm, satisfying the ductile connector requirements specified in Eurocode 4. The combination of ring plates and stiffener plates more fully utilizes the compressive capacity of concrete, while the inclusion of headed studs at appropriate ratios effectively enhances the elastic anti-slip stiffness of the interface.
| Connector Configuration | Characteristic Slip | Elastic Stiffness | Ultimate Strength |
|---|---|---|---|
| Without connectors | < 3 mm | Low | Low |
| Ring plates only | Moderate | Moderate | Moderate |
| Stiffener plates only | Moderate | Moderate | Moderate |
| Combined connectors (ring + stiffener + studs) | > 6 mm | High | High |
| High stud ratio, multiple rows | > 6 mm | Very high | Very high |
The study confirms that increasing the stud ratio improves both the plastic anti-slip stiffness and the ultimate anti-slip strength of the interface. Importantly, even at high stud ratios with multiple rows of studs, the studs retain their ductile connector characteristics, meaning they do not become brittle failure elements. The actual ultimate load of individual studs is found to be consistent with the calculated values from design codes.
Welding Quality and Connector Integrity
From a welding engineering perspective, the installation of combined connectors involves critical welding operations that must be executed to the highest quality standards. The headed studs are typically welded to the steel tube surface using stud welding (flash butt welding or arc stud welding), while ring plates and stiffener plates are attached through fillet welds or full-penetration butt welds. The quality of these welds directly determines the effectiveness of the connector system.
Key welding quality considerations include:
- Stud welding must achieve full fusion between the stud base and the steel tube surface, with no slag inclusions or incomplete penetration
- Ring plate welds must provide full load transfer capability, requiring adequate weld throat thickness and uniform weld geometry
- Welding-induced residual stresses in the steel tube wall may affect the local buckling behavior under combined axial and interface shear loading
- Heat-affected zone (HAZ) softening around connector welds could reduce the local yield strength of the steel tube, particularly for high-strength steel grades
Engineering Practice Implications
The findings of this study have direct implications for the design and fabrication of composite steel-concrete structures, including steel tube concrete (STC) columns, bridge girders, and composite floor systems. The confirmation that combined connectors can achieve ductile behavior with characteristic slips exceeding 6 mm provides confidence in the design of composite structures that rely on interface bond for load transfer.
For pipe fabrication engineers, the practical recommendation is to ensure that connector welding operations are performed by qualified welders using approved welding procedures. Non-destructive testing (NDT) of connector welds should be performed according to relevant standards, with acceptance criteria tailored to the criticality of the connector function. The study's finding that individual stud ultimate loads match code predictions suggests that current design codes are reasonably accurate for stud connectors, but the combined effect of multiple connectors with ring and stiffener plates may not be adequately captured by simple superposition methods.
This research contributes significantly to the understanding of steel-concrete interface behavior and provides validated design parameters that can be directly applied in engineering practice, enhancing the reliability and performance of composite structural systems.