Experimental Study on Square Hollow Sandwich Steel Tube Concrete Compression-Bending-Torsion Members

Literature Overview

This paper by Huang Hong, Zhu Qi, Chen Mengcheng, and Li Ting, published in "China Civil Engineering Journal" (Vol. 49, No. 3, 2016, pp. 91-97), presents an experimental and numerical study on square hollow sandwich steel tube concrete (SHS-STC) members subjected to combined compression, bending, and torsion. The research was conducted at East China Jiaotong University and the Jiangxi Provincial Key Laboratory of Building Process Simulation and Control, and was supported by the National Natural Science Foundation of China (Grants 51378206, 51008122), the Jiangxi Provincial Young Scientists Fund (Grant 20143ACB21020), and the Jiangxi Provincial Young Scientist Cultivation Program (Grant 20133BCB23015).

The study investigates seven specimens: five square hollow sandwich CSTC members and two square solid CSTC members, with slenderness ratio, hollow ratio, and axial compression ratio as the primary variation parameters. The work addresses an important gap in structural engineering knowledge, as the behavior of CSTC members under combined compression-bending-torsion loading is less well understood than under pure axial compression or pure bending.

Core Technical Points

Specimen Configuration and Test Parameters

The specimens were square hollow sandwich CSTC members, meaning they consisted of an outer square steel tube with an inner square steel tube, and concrete was cast in the annular space between the two tubes. The hollow ratio, defined as the ratio of the inner tube cross-sectional area to the outer tube cross-sectional area, was varied to investigate its effect on torsional behavior. The slenderness ratio and axial compression ratio were also varied to capture the influence of member geometry and loading conditions.

Numerical Analysis

The torsion-angle relationship for each specimen was calculated using ABAQUS finite element analysis. The calculated torsion-angle curves showed good agreement with experimental results before reaching the ultimate load-carrying capacity. This validation confirms the reliability of the numerical model for predicting the torsional behavior of SHS-STC members.

Key Findings

The study identified several important relationships governing the torsional behavior of SHS-STC members:

1. As the axial compression ratio increases (n >= 0.2), the torsional load-carrying capacity decreases. This negative interaction between axial compression and torsion is attributed to the progressive crushing of the concrete core under combined loading, which reduces the concrete's contribution to torsional resistance.
2. When the eccentricity is small, increasing the hollow ratio leads to greater torsional load-carrying capacity and improved ductility. This finding is somewhat counterintuitive, as a higher hollow ratio means less concrete, but the explanation lies in the increased contribution of the steel tubes to torsional resistance when the concrete contribution is reduced.
3. The working mechanism analysis, based on typical calculation examples, examined three aspects: the load shared by the steel tubes versus the concrete, the influence of axial compression ratio on torsion-angle curves, and the effect of hollow ratio on torsion-angle curves.

Working Mechanism Analysis

The load sharing between steel tubes and concrete was analyzed to understand the relative contributions of each material to the overall torsional resistance. The steel tubes, particularly the outer tube, provide significant torsional stiffness through their thin-walled section geometry. The concrete in the annular space contributes through its shear resistance and by providing confinement to the steel tubes, preventing local buckling.

The axial compression ratio influence analysis revealed that higher axial loads reduce the effective torsional capacity by pre-damaging the concrete and reducing its ability to resist additional shear stresses from torsion. The hollow ratio analysis demonstrated that, under low eccentricity conditions, the increased steel tube contribution (due to the larger inner tube) can compensate for the reduced concrete contribution, leading to improved torsional performance.

Technical Analysis of Torsional Behavior

From a structural engineering perspective, the torsional behavior of hollow sandwich CSTC members is governed by the interaction between the steel tubes and the concrete infill. The Vlasov theory of thin-walled section torsion provides a useful framework for understanding the behavior, but the presence of concrete infill modifies the classical thin-walled behavior by providing additional shear resistance and by preventing local buckling of the steel tubes.

The finding that higher hollow ratios improve torsional capacity under low eccentricity conditions has important implications for design optimization. It suggests that, for torsion-dominated applications, using a larger inner tube (and thus less concrete) may be beneficial, provided that the axial compression ratio is not too high. This insight can lead to material savings without compromising structural performance.

Comparison with Solid CSTC Members

The two solid CSTC specimens served as reference cases for comparison. The comparison between hollow sandwich and solid CSTC members under compression-bending-torsion loading provides insight into the relative advantages and disadvantages of the hollow sandwich configuration. The hollow sandwich configuration offers the benefit of reduced self-weight and the ability to route utilities through the hollow core, but the trade-offs in terms of torsional capacity and ductility must be carefully evaluated.

Engineering Practice Integration

For steel pipe fabrication, the manufacturing of square hollow sandwich CSTC members requires precise dimensional control of both the outer and inner square tubes. The inner tube must be accurately positioned within the outer tube to ensure uniform concrete thickness, which is critical for consistent structural performance. Any eccentricity in the inner tube placement can lead to uneven concrete distribution, creating weak zones that may initiate failure under torsional loading.

The welding of stiffeners, diaphragms, and connection plates to the square steel tubes must be performed with attention to the torsional load paths. Welded connections that interrupt the continuity of the steel tube walls can significantly reduce torsional capacity. Therefore, the placement and design of welded connections should be optimized to minimize the disruption of the torsional load flow.

From a quality control standpoint, non-destructive testing of all welds is essential, as any weld defect—such as a lack of fusion, porosity, or undercut—can serve as a stress concentration site under torsional loading. Ultrasonic testing (UT) and magnetic particle testing (MT) should be performed on all critical welds, with particular attention to the welds at the ends of the inner tube where stress concentrations are likely to occur.

Key Questions and Reflections

The study is limited to relatively few specimens (seven total), which may not provide a comprehensive understanding of all the complex interactions between slenderness ratio, hollow ratio, and axial compression ratio. A more extensive parametric study, either experimental or numerical, would be beneficial to establish more robust design guidelines.

The numerical model validation was limited to the pre-ultimate loading range, with good agreement before reaching the ultimate load-carrying capacity. The post-ultimate behavior, which is important for ductility assessment and energy dissipation capacity, was not as thoroughly validated. Future work should focus on improving the model's accuracy in the post-ultimate range.

The study does not address the effect of material nonlinearity due to cyclic loading, which is relevant for seismic applications. The torsional behavior of SHS-STC members under cyclic compression-bending-torsion loading may differ significantly from the monotonic loading behavior studied here, particularly in terms of energy dissipation and stiffness degradation.

Study Insights and Implications

This research contributes valuable experimental data and analytical insights into the torsional behavior of square hollow sandwich CSTC members. The finding that higher hollow ratios can improve torsional capacity under low eccentricity conditions offers a counterintuitive but valuable design optimization opportunity. The identification of the negative interaction between axial compression and torsion highlights the importance of considering combined loading effects in design.

For engineers and pipe manufacturers, the study underscores the importance of precise fabrication and quality control in hollow sandwich CSTC members. The uniformity of concrete distribution, the accuracy of inner tube positioning, and the quality of welded connections are all critical factors that influence the torsional performance of these members.

Conclusion

The study by Huang et al. provides meaningful insights into the compression-bending-torsion behavior of square hollow sandwich CSTC members. The experimental and numerical results demonstrate that the hollow sandwich configuration can offer improved torsional performance under certain conditions, particularly when the eccentricity is low and the hollow ratio is optimized. The identification of key parameters and their influence on torsional behavior offers practical guidance for the design of CSTC members in applications where torsion is a significant design consideration. The research highlights the need for careful fabrication and quality control to ensure the predicted structural performance is achieved in practice.