Zhuojin Pipe Fitting Co., LtdBond-Slip Performance of PBL-Stiffened Rectangular Concrete-Filled Steel Tubes
Literature Overview
This paper by Liu Yongjian, Li Hui, Zhang Ning, Liu Junping, and Sun Xiaobo, published in 2015 in the Journal of Architecture and Civil Engineering (Vol. 32, No. 5, pp. 1–7), investigates the interface bond-slip performance of rectangular concrete-filled steel tubes (CFST) reinforced with perforated steel plate connectors (PBL stiffeners). The research was conducted at the National Engineering Laboratory for Bridge Structural Safety Technology, Chang'an University, and was supported by multiple funding sources including the National Natural Science Foundation of China (Grants No. 51178051 and 51378068), the Ministry of Transport Construction Science and Technology Project (Grant No. 2013 318 812 410), and the Shaanxi Provincial Department of Transportation Research Project (Grant No. 13-26K).
Core Technical Approach
The study employs push-out tests to evaluate the shear performance of the steel-concrete interface in rectangular CFST members with PBL stiffeners. The PBL stiffener, or perforated bolted plate connector, consists of a steel plate with regularly spaced holes that is welded to the inner surface of the steel tube. When concrete is cast around the stiffener, the concrete fills the holes and creates interlocking "dowel" action that resists relative slip between the steel tube and the concrete core.
The experimental program systematically varies three key parameters: the spacing between PBL stiffeners, the hole diameter in the stiffener plate, and the maximum aggregate size in the concrete. These parameters are studied to establish their influence on the interface bond strength, shear stiffness, and shear capacity of the composite member. The results are compared with those from conventional CFST specimens without PBL stiffeners to quantify the improvement provided by the stiffener system.
Technical Parameters and Test Configuration
| Test Parameter | Variable Range | Purpose |
|---|---|---|
| PBL stiffener spacing | Multiple values | Evaluate spacing effect on bond strength |
| Hole diameter | Multiple values | Study interlocking capacity |
| Concrete aggregate size | Multiple values | Assess aggregate interlock contribution |
| Concrete strength | Standard range | Control material variability |
| Steel tube grade | Q235/Q345 | Ensure structural steel consistency |
| Push-out load | Monotonic | Measure ultimate shear capacity |
The push-out test setup involves applying a shear force to the steel tube relative to the concrete core and measuring the resulting slip at the interface. The load-slip curve provides the shear capacity, the stiffness (slope of the elastic portion), and the post-peak behavior of the connection. By varying the PBL parameters, the study establishes design guidelines for optimizing the stiffener configuration.
Interpretation of Key Findings
The experimental results demonstrate that PBL stiffeners significantly improve both the shear capacity and the shear stiffness of the steel-concrete interface in rectangular CFST members. The improvement is attributed to the mechanical interlock created by the concrete dowels formed in the perforations of the stiffener plate. The interlocking action provides a direct shear resistance mechanism that supplements the frictional resistance at the bare steel-concrete interface.
The study reveals that the bond strength is sensitive to the PBL stiffener hole parameters. Larger hole diameters provide greater interlocking capacity but may reduce the effective contact area between the stiffener and the steel tube. The spacing between stiffeners also plays a critical role, as closer spacing provides more frequent shear resistance points but may complicate fabrication and assembly. The aggregate size in the concrete has a notable influence on the shear capacity, as larger aggregates create stronger interlock with the concrete dowels in the PBL holes.
A modified calculation formula for the PBL shear capacity is proposed that incorporates a correction factor accounting for the aggregate content in the concrete dowels. The correction factor is found to have a linear relationship with the area ratio of coarse aggregate within the concrete dowels. This modification improves the accuracy of the shear capacity prediction compared to conventional formulas that do not account for aggregate effects.
Design Implications and Engineering Practice
| Design Factor | Recommendation | Rationale |
|---|---|---|
| PBL stiffener spacing | Optimize based on shear demand | Closer spacing increases capacity but adds cost |
| Hole diameter | Select for adequate interlock | Balance interlock strength with contact area |
| Aggregate size | Use appropriate grading | Larger aggregates improve dowel interlock |
| Concrete strength | Match with steel grade | Ensure composite action and avoid brittle failure |
| Fabrication tolerance | Control hole position accuracy | Misaligned holes reduce interlock effectiveness |
The proposed modification to the PBL shear capacity formula provides engineers with a more accurate tool for designing composite members with PBL stiffeners. The linear relationship between the correction factor and the aggregate area ratio simplifies the calculation process while maintaining accuracy. This is particularly valuable for bridge engineering applications, where PBL stiffeners are commonly used to enhance the composite action between steel girders and concrete decks.
Study Insights and Reflections
This research contributes valuable experimental data and analytical insights to the understanding of PBL stiffener performance in rectangular CFST members. The systematic variation of test parameters provides a comprehensive basis for design recommendations. The identification of aggregate size as a significant factor in PBL performance is particularly noteworthy, as it highlights the importance of concrete mix design in composite structural applications.
I note that the study focuses on monotonic loading, and the cyclic behavior of PBL stiffened connections under repeated loading, such as that experienced in seismic regions, would be a valuable extension of this work. Additionally, the long-term behavior under sustained loading, including the effects of concrete shrinkage and creep on the PBL connection performance, deserves further investigation. The practical aspects of fabrication, including welding quality control of the PBL stiffeners to the steel tube, should also be addressed to ensure that the theoretical performance is achieved in practice.
Summary
This literature provides a thorough experimental and analytical study of the bond-slip performance of PBL-stiffened rectangular CFST members. The proposed modification to the PBL shear capacity formula, which accounts for aggregate effects, represents a meaningful improvement over conventional design methods. The systematic investigation of PBL parameters offers practical guidance for engineers designing composite members with enhanced interface shear capacity. The work bridges the gap between fundamental research and practical design, providing tools that can be directly applied in bridge and building engineering projects.