Bond-Slip Behavior of PBL-Stiffened Rectangular CFST Interfaces

Literature Overview

This paper, published in Journal of Architecture and Civil Engineering in 2015 by Liu Yongjian, Li Hui, Zhang Ning, Liu Junping, and Sun Xiaobo from Chang'an University, Northwest A&F University, and Fuzhou University, investigates the interface bond-slip behavior of rectangular steel tube concrete (CFST) members reinforced with perforated angle steel (PBL) stiffeners. The research, supported by multiple funding sources including the National Natural Science Foundation (Grants 51178051 and 51378068) and the Ministry of Transport Construction Science and Technology Program (Grant 2013318812410), presents pullout test results and proposes a modified calculation method for PBL stiffener shear capacity that accounts for the influence of coarse aggregate content in concrete.

Technical Background

The interface between the steel tube and the concrete core in CFST members is a critical zone for load transfer and composite action. In rectangular CFST members, the interface area is larger than in circular sections, and the bond-slip behavior is more complex due to the non-uniform stress distribution. PBL stiffeners — perforated angle steels welded to the interior of the steel tube — are commonly used to enhance the shear transfer capacity at the interface, particularly at critical locations such as beam-column joints, splice connections, and regions of high shear stress.

PBL Stiffener Configuration

Experimental Program

The pullout tests were conducted on rectangular CFST specimens with PBL stiffeners, with the following test parameters varied:

1. Hole spacing of PBL stiffeners: Different spacing configurations were tested to evaluate the effect of hole density on shear transfer capacity.
2. Hole diameter of PBL stiffeners: Varying hole sizes to study the relationship between the concrete dowel action through the holes and the overall shear capacity.
3. Coarse aggregate size of concrete: Different aggregate sizes were used to investigate the influence of aggregate interlock on the bond-slip behavior.

Comparison tests were also conducted on plain CFST specimens without PBL stiffeners to quantify the enhancement provided by the stiffeners.

Key Experimental Findings

The results demonstrate that PBL stiffeners significantly enhance both the shear strength and stiffness of the steel tube-concrete interface. The bond-slip curves of PBL-stiffened specimens show a more gradual post-peak degradation compared to plain CFST specimens, indicating improved ductility and energy dissipation capacity.

Modified Calculation Method

A key contribution of this paper is the proposed modification to the PBL stiffener shear capacity formula that accounts for the influence of coarse aggregate content in the concrete. The traditional PBL shear capacity formula, derived from Eurocode 4 (EN 1994-1-1), is based on the concrete dowel action through the holes and does not explicitly consider the aggregate size effect.

Modified Formula Approach

The proposed modification introduces a correction factor that is linearly related to the area ratio of coarse aggregate within the concrete dowels formed in the PBL holes. The logic is as follows:

• The concrete dowels that pass through the PBL holes are the primary mechanism for shear transfer.
• The presence of coarse aggregate within these dowels affects their tensile strength and failure mode.
• A higher aggregate area ratio in the dowels leads to a higher tensile capacity of the dowels, but also introduces stress concentrations around the aggregate particles.
• The correction factor captures this nonlinear relationship between aggregate content and dowel capacity.

Comparison with Eurocode 4

Engineering Practice Implications

Design Recommendations

Based on the findings of this study, the following design recommendations are provided for PBL-stiffened rectangular CFST members:

1. PBL hole spacing: Optimal hole spacing should be selected based on the concrete aggregate size to ensure that each hole contains at least one aggregate particle while maintaining sufficient concrete dowel cross-section. A spacing of 2 to 3 times the maximum aggregate size is generally recommended.
2. Hole diameter: The hole diameter should be at least 3 to 4 times the maximum aggregate size to ensure proper concrete flow and aggregate distribution around the holes.
3. Plate thickness: The perforated plate thickness should be sufficient to resist the bearing forces from the concrete dowels and to prevent local buckling under shear loading.
4. Concrete placement: Special attention should be paid to concrete compaction around the PBL stiffeners to avoid voids or honeycombing that would reduce the effective dowel action.

Quality Control Considerations

From a quality control perspective, the following checks are important during fabrication and construction:

• Weld inspection: The welds connecting the PBL stiffeners to the steel tube interior must be fully inspected using ultrasonic testing (UT) or magnetic particle testing (MT) to ensure complete fusion and absence of cracks.
• Concrete placement quality: The concrete around the PBL stiffeners should be carefully placed and vibrated to ensure full penetration into the holes and elimination of voids.
• Dimensional accuracy: The hole spacing and diameter should be verified against design specifications during fabrication, as deviations can significantly affect the shear capacity.

Study Insights and Reflections

This research makes a valuable contribution to the understanding of interface behavior in PBL-stiffened rectangular CFST members. The proposed modification to the PBL shear capacity formula addresses a gap in the existing design codes by explicitly considering the influence of coarse aggregate content, which is a practical parameter that varies significantly in construction practice.

One important observation from the experimental results is the strong correlation between the PBL shear capacity and the aggregate area ratio in the concrete dowels. This finding has practical implications for concrete mix design — the aggregate grading and maximum size should be considered not only for the bulk concrete properties but also for the interface shear transfer capacity. In engineering practice, this means that the concrete mix design for CFST members should be optimized for both strength and interface performance, which may require a different aggregate grading than for conventional reinforced concrete.

Another consideration is the effect of the PBL stiffeners on the overall structural behavior of the CFST member. While the PBL stiffeners enhance the interface shear capacity, they also introduce stress concentrations at the weld toes where the perforated plates are welded to the steel tube. Under cyclic loading, such as seismic events, these weld toes are susceptible to fatigue cracking. Future research should investigate the fatigue performance of PBL-stiffened CFST connections under repeated loading.

Furthermore, the interaction between the PBL stiffeners and the concrete core under high axial compression deserves further attention. The concrete dowels in the PBL holes are subjected to combined shear and compressive stresses, and the failure mode may shift from dowel shear to dowel crushing under high axial loads. The proposed correction factor should ideally be validated under such combined loading conditions.

In summary, this study provides practical guidance for the design of PBL-stiffened rectangular CFST members, with the modified shear capacity formula offering a more accurate prediction that accounts for the influence of concrete aggregate content. The findings are directly applicable to engineering practice in the design of composite bridge structures, building frames, and other CFST applications where interface shear transfer is critical.