Effect of Structural Configurations on Interfacial Bond Performance of Square Steel Tube High-Strength Concrete

Literature Overview

This paper by Dong Hongying and colleagues from Beijing University of Technology, published in the Journal of Tianjin University in 2021 (Volume 54, Issue 1, pages 91–100), addresses a fundamental challenge in steel tube concrete (STC) structural design: the insufficient interfacial bond strength between square steel tubes and high-strength concrete (HSC). The study investigates eleven different structural configurations through push-out tests on large-scale specimens, providing comprehensive data on bond strength, energy dissipation capacity, and configuration efficiency.

Core Technical Findings

The research systematically evaluates five types of structural configurations and their combinations to improve the interfacial bond performance of square steel tube HSC members. The key findings are summarized below:

Technical Analysis of Load-Displacement Behavior

The push-out tests reveal distinct load-displacement curve characteristics for different configuration types:

No Configuration (Baseline)

The load-displacement curve exhibits an ascending branch followed by a flat plateau branch. This behavior indicates that once the interfacial friction is fully mobilized, the bond strength remains constant with increasing slip. The plateau branch reflects the pure frictional resistance between the steel tube inner surface and the concrete surface.

Vertical Configuration Only

Specimens with only internal vertical structural measures (such as vertical rib plates) exhibit two ascending branches with different slopes. The first ascending branch corresponds to the initial frictional and mechanical interlock resistance, while the second ascending branch represents the progressive engagement of the vertical ribs as slip increases. The transition between the two branches occurs when the vertical ribs begin to bear the shear load.

Configurations with Shear Elements

Specimens equipped with shear elements (shear studs, transverse rib plates) exhibit a descending branch after the peak load. This post-peak softening behavior is attributed to the progressive fracture of the shear elements as slip increases. The rate of strength degradation depends on the ductility of the shear elements and the concrete cover thickness.

Configuration Efficiency Analysis

The concept of configuration efficiency is defined as the ratio of the actual bond strength (or energy dissipation) improvement to the theoretical superposition of individual configuration contributions. The study finds that:

1. Shear studs alone achieve the highest bond strength improvement per unit of configuration complexity, making them the most efficient for bond strength enhancement.
2. Transverse rib plates alone achieve the highest energy dissipation improvement, making them the most efficient for ductility enhancement.
3. Combined configurations show synergistic effects: the combination of shear studs and transverse rib plates yields a bond strength 7.2% higher than the simple superposition of individual contributions, while the energy dissipation is 3.2% lower than superposition. This suggests that the shear studs contribute more to peak strength while the transverse ribs contribute more to post-peak ductility.
4. Complex combinations such as transverse ribs + vertical ribs + rebar cage achieve an 11.2× improvement in bond strength, demonstrating that multiple mechanisms can be combined to achieve dramatic performance enhancement.

Verification of Shear Strength Calculation Methods

The study verifies existing shear strength calculation methods for shear studs and transverse rib plates used in the configurations. The results show that the existing calculation methods are conservative (biased toward safety) compared to the experimental results. This conservatism is acceptable for engineering design but may lead to over-design if not properly calibrated.

The verification process involves:

1. Calculating the theoretical shear strength of each shear element using the relevant design codes (GB 50010 for concrete structures, GB 50017 for steel structures)
2. Comparing the calculated values with the experimental values obtained from the push-out tests
3. Assessing the degree of conservatism and identifying potential areas for calibration improvement

Engineering Practice Considerations

Selection of Configuration Based on Design Requirements

Construction Practicality

The practical implementation of each configuration has different construction implications:

• Shear studs: Welded to the inner surface of the steel tube before concrete placement; requires access to the tube interior
• Transverse rib plates: Fabricated as integral part of the steel tube; requires precise welding and positioning
• Vertical rib plates: Similar to transverse ribs but oriented vertically; may interfere with reinforcement placement
• Connecting reinforcement bars: Embedded in the concrete and anchored to the steel tube; requires careful coordination with reinforcement layout
• Chambered steel plates: Fabricated as internal partitions within the steel tube; may complicate concrete placement and compaction

Key Questions and Reflections

The study provides comprehensive experimental data on the interfacial bond performance of square steel tube HSC members with various configurations, but several practical questions remain. First, the push-out test configuration may not fully replicate the three-dimensional stress state in actual structural members. The boundary conditions in the push-out test are relatively simple compared to the complex loading conditions in practical structures, where biaxial or triaxial stresses may be present at the steel-concrete interface.

Second, the study focuses on quasi-static loading, but seismic loading involves cyclic loading that may cause progressive degradation of the interfacial bond. The fatigue behavior of the shear elements under cyclic loading is not addressed, which is critical for seismic design.

Third, the long-term performance of the configurations under sustained loads and environmental exposure is not investigated. The corrosion of shear studs and reinforcement bars, as well as the carbonation of the concrete, may affect the long-term bond performance.

Study Insights and Outlook

The research provides a systematic framework for selecting appropriate structural configurations to improve the interfacial bond performance of square steel tube HSC members. The concept of configuration efficiency is particularly useful for practical design, as it allows engineers to optimize the trade-off between performance improvement and construction complexity. The finding that existing shear strength calculation methods are conservative is encouraging for design safety but suggests opportunities for more refined analytical models. The combination of multiple configuration types offers a pathway to achieve dramatic improvements in bond performance, but practical implementation requires careful consideration of construction feasibility, cost, and long-term durability. Engineers should use the configuration efficiency data to make informed decisions about the most appropriate configuration for each specific application, balancing structural performance requirements with economic and practical constraints.