Torsional Performance of Hexagonal Steel Tube Concrete Members

Literature Overview

This study by Wang Sihao, Yu Xin, and Wang Zhibin from Fuzhou University investigates the pure torsional mechanical behavior of hexagonal steel tube concrete (STC) members using finite element analysis. Published in Progress in Steel Building Structures (Volume 23, Issue 1, 2021, pages 18-24), the research was supported by the National Natural Science Foundation of China (Grant 51708118), the Fujian Provincial Natural Science Foundation (Grant 2019J01234), and Fuzhou University equipment testing fund (Grant 2019T031).

Research Motivation and Geometric Considerations

Hexagonal steel tube concrete members represent a relatively novel structural cross-section that combines the advantages of circular tubes (uniform confinement of concrete) and square tubes (ease of connection fabrication). The hexagonal geometry provides a higher torsional constant compared to square sections of equivalent perimeter while offering better connection compatibility than circular sections. The torsional performance of such members is particularly relevant for applications involving eccentric loading, seismic action, and wind-induced torsion.

The research methodology followed a systematic approach: first validating the finite element model against experimental data for conventional square and circular STC members, then applying the validated model to hexagonal sections for parametric analysis.

Finite Element Model Development and Validation

The ABAQUS finite element model was developed and validated through comparison with experimental results for both square and circular STC torsion specimens. The model incorporated:

The validation process confirmed that the model accurately predicted the torque-shear strain (T-γ) relationship curves, torsional failure modes, and ultimate torsional capacity for both square and circular sections. This validation step is critical for ensuring the reliability of subsequent parametric analysis results.

Torsional Performance Analysis Results

The parametric analysis examined the T-γ relationship curves, shear stress distribution patterns, and steel tube confinement force distribution for hexagonal STC members. Key findings include:

1. The T-γ curve exhibits a linear elastic phase followed by a nonlinear hardening phase and eventual softening upon steel tube yielding and concrete crushing.
2. Shear stress distribution is non-uniform across the hexagonal section, with higher stresses at the corners and lower stresses at the mid-span of each face.
3. The steel tube confinement force is not uniformly distributed; it is higher near the flat faces where concrete dilation is more constrained and lower near the corners.
4. The torsional capacity of hexagonal STC members falls between that of equivalent circular and square STC members, with values closer to the circular section due to the hexagonal geometry's higher torsional constant.

Simplified Calculation Formula Development

Based on the parametric analysis results, the authors proposed a simplified calculation formula for the torsional capacity of hexagonal STC members. The formula incorporates the following parameters:

• Section geometric properties (torsional constant, section modulus)
• Material properties (steel yield strength, concrete compressive strength)
• Confinement effect factor
• Interaction factor between steel tube and concrete

The simplified formula was validated against the finite element results, demonstrating good agreement across the parametric range studied. This formula provides engineers with a practical design tool for preliminary sizing of hexagonal STC members subjected to torsional loading.

Comparison with Conventional Cross-Sections

The torsional performance of hexagonal STC members was implicitly compared with circular and square sections through the validated modeling approach. The hexagonal section offers:

• Approximately 5-10% higher torsional constant than a square section of equivalent perimeter
• Approximately 3-8% lower torsional constant than a circular section of equivalent perimeter
• Better connection compatibility than circular sections for standard steel tube connection details
• More uniform stress distribution than square sections, reducing stress concentration at corners

Engineering Practice Considerations

For practical applications, hexagonal STC members offer a balanced solution between the torsional efficiency of circular sections and the fabrication convenience of square sections. The hexagonal geometry can be produced through cold forming or hot rolling processes, with the six faces providing multiple planes for connection welding. The torsional performance data from this study can inform the design of hexagonal STC columns in seismic regions where torsional loading is significant, as well as in applications such as transmission towers, crane rails, and marine structures.

Study Insights and Practical Implications

This research contributes to the growing body of knowledge on non-circular steel tube concrete members, which are increasingly being considered for applications requiring both structural efficiency and fabrication practicality. The finite element modeling approach—validated against experimental data before parametric extension—sets a methodological standard for computational studies of novel structural systems. The simplified design formula provides a practical tool that can be integrated into structural design software or used for preliminary design calculations. Engineers considering hexagonal STC members for torsional applications should note that the non-uniform stress distribution requires careful attention to the corner regions, where stress concentrations may initiate local buckling or concrete crushing. The work demonstrates that computational methods, when properly validated, can efficiently explore the design space of novel structural cross-sections, accelerating the transition from research concepts to practical engineering solutions.