Mechanical Properties of PBL-Strengthened Rectangular Concrete-Filled Steel Tubes

Literature Overview

The paper by Liu Yongjian, Zhang Ning, and Zhang Junguang, published in the Journal of Architecture and Civil Engineering (2012, Vol. 29, No. 4, pp. 13–17), proposes and investigates a novel PBL (Perforated Bearing Length) shear connector-strengthened rectangular concrete-filled steel tube (CFST) member. The concept involves installing perforated steel plate longitudinal ribs inside the rectangular steel tube, which serve a dual function as PBL shear connectors and stiffening ribs. Axial compression tests were conducted on short columns with and without PBL stiffening ribs, and the shear transfer mechanisms at the steel-concrete interface were analyzed. The research was supported by the National Natural Science Foundation of China (51178051).

Core Technical Findings

PBL Shear Connector Configuration

The PBL shear connector is a well-established component in composite bridge engineering, consisting of a steel plate with a series of perforations through which longitudinal rebars pass. In the context of CFST members, the perforated steel plate is installed longitudinally inside the steel tube, creating a mechanical interlock between the steel tube and the infill concrete. This configuration enhances the composite action between the steel tube and concrete by providing a direct shear transfer path.

Load Capacity and Ductility Improvements

The experimental results demonstrated that the PBL-strengthened rectangular CFST members exhibited a 14% to 28% increase in axial compression load capacity compared to conventional CFST members without ribs. The ductility of the specimens was also significantly improved, indicating that the PBL configuration not only increases strength but also enhances the deformation capacity of the member.

The improvement in composite action is attributed to the enhanced confining effect of the steel tube on the concrete, facilitated by the PBL ribs. The perforated steel plate provides additional bearing surfaces that resist the lateral expansion of the concrete, thereby increasing the confining pressure and the triaxial compressive strength of the concrete core.

Shear Transfer Mechanism

The study analyzed the shear transfer mechanism at the steel-concrete interface in both configurations. In conventional CFST members, shear is transferred through friction and adhesion at the interface, which can be unreliable under cyclic loading or when the interface is contaminated. In PBL-strengthened members, the PBL connectors provide a mechanical shear transfer path that is independent of interface friction, making the composite action more reliable and predictable.

The PBL connectors effectively shorten the load transfer path from the steel tube wall to the core concrete, improving the stress distribution in the member and reducing localized stress concentrations. This results in a more uniform stress state and better overall structural performance.

Interpretation of Technical Points

Dual Function of PBL Ribs

The dual function of the PBL ribs as both shear connectors and stiffening ribs is a key innovation of this research. As shear connectors, the PBL ribs provide mechanical interlock between the steel tube and concrete, enhancing composite action. As stiffening ribs, they provide local reinforcement to the steel tube walls, delaying local buckling and improving the post-buckling behavior of the member.

This dual function makes the PBL-strengthened configuration particularly attractive for practical applications, as it achieves multiple performance improvements with a single structural element. The simplicity of the configuration also reduces fabrication complexity compared to alternative strengthening methods that require separate shear connectors and stiffeners.

Interface Behavior and Composite Action

The steel-concrete interface in CFST members is a critical factor in determining the composite action between the two materials. In conventional CFST members, the interface behavior is governed by friction, adhesion, and mechanical interlock from surface roughness. However, this interface can be unreliable under certain conditions, such as when the concrete is placed in a contaminated or wet steel tube, or when the member is subjected to cyclic loading that can cause interface separation.

The PBL connectors bypass the interface by providing a direct mechanical connection between the steel tube and concrete. This ensures reliable composite action regardless of interface conditions, which is particularly important for seismic applications where interface separation can occur under cyclic loading.

Connection with Engineering Practice

Fabrication and Welding Requirements

The fabrication of PBL-strengthened rectangular CFST members requires precise installation of the perforated steel plate ribs inside the steel tube before concrete placement. The ribs must be welded to the inner surface of the steel tube with high-quality welds that can transfer the shear forces between the ribs and the tube. The welding of internal ribs inside a confined space presents challenges related to weld accessibility, positioning, and NDT verification.

Welding procedures must comply with relevant standards such as GB/T 19804, SY/T 4103, or ISO 3834. For internal welds, narrow-gap welding techniques such as GTAW (gas tungsten arc welding) or FCAW (flux-cored arc welding) with appropriate filler metals should be used. NDT verification of internal welds is challenging but can be achieved using phased array ultrasonic testing (PAUT) or digital radiographic testing (DR).

The perforations in the PBL steel plate must be precisely machined to ensure proper engagement with the longitudinal rebars or other connecting elements. The plate thickness and perforation geometry must be designed to provide adequate shear capacity without excessive material usage.

Concrete Placement Challenges

The presence of internal PBL ribs creates additional challenges for concrete placement. The concrete must be pumped or poured around the ribs, ensuring full compaction without voids or segregation. The ribs divide the concrete flow path, creating potential zones of incomplete filling. Post-placement inspection using UPV testing or electromagnetic methods is recommended to verify concrete quality and detect voids.

For large-scale applications, the concrete placement sequence should be carefully planned to ensure uniform compaction around all ribs. Vibrating equipment must be positioned to effectively compact the concrete in the confined spaces between the ribs and the tube walls.

Key Questions and Reflections

The long-term durability of PBL-strengthened CFST members in corrosive environments is a concern. The internal steel ribs are susceptible to corrosion, which can reduce the shear capacity of the PBL connectors over time. Corrosion protection measures such as epoxy coatings, galvanizing, or cathodic protection should be applied to the internal ribs before concrete placement.

The scalability of the PBL-strengthened configuration to larger cross-sections and longer members is another question. The experimental results are based on short columns, and the behavior of longer members may be influenced by additional factors such as slenderness effects, imperfections, and second-order moments. Parametric studies with longer members are recommended to validate the applicability of the PBL-strengthened configuration for practical structural applications.

The cost-effectiveness of the PBL-strengthened configuration must be evaluated against alternative strengthening methods. While the 14–28% improvement in load capacity is significant, the additional cost of fabricating and installing the PBL ribs must be justified by the performance gains. Life-cycle cost analysis should be conducted to evaluate the economic viability of this approach.

Study Insights and Implications

This research presents an innovative and practical approach to enhancing the mechanical performance of rectangular CFST members through the use of PBL shear connector-strengthened ribs. The dual function of the PBL ribs as both shear connectors and stiffening ribs provides a simple and effective solution to improve composite action, load capacity, and ductility. The experimental results demonstrate significant improvements in axial compression performance, with load capacity increases of 14–28% and improved ductility. For practitioners in bridge engineering and building structures, the PBL-strengthened CFST configuration offers a promising approach to achieving higher performance with minimal fabrication complexity. The key challenges for practical implementation include ensuring weld quality for internal ribs, achieving proper concrete compaction around the ribs, and providing adequate corrosion protection for the internal steel components. Further research and validation through full-scale testing are recommended before widespread adoption in critical structural applications.