Mechanical Behavior of Circular Hollow Sandwich Steel Tube Concrete Members under Compression-Torsion

Literature Overview

This paper investigates the structural performance of circular hollow sandwich steel tube concrete (SHS-SC) members subjected to combined compression and torsion loads. The sandwich configuration, which consists of an inner steel tube, a concrete core, and an outer steel tube separated by a gap or connected by shear connectors, represents an advanced composite structural system with potential applications in high-rise buildings, bridge piers, and offshore platforms. The research combines finite element analysis with experimental testing to characterize the load-bearing capacity, deformation behavior, and failure modes under combined compression-torsion loading.

Core Technical Points

The sandwich configuration provides several theoretical advantages over conventional steel tube concrete (STC) members: enhanced torsional stiffness due to the outer tube's contribution, improved energy dissipation through the interface between tubes, and the ability to independently optimize the inner and outer tube dimensions for compression and torsion respectively. However, the structural behavior is significantly more complex than conventional STC members due to the interaction between the two steel tubes and the concrete core.

Key findings from the study include:

The study demonstrates that the interaction between compression and torsion follows a nonlinear interaction curve that deviates significantly from simple linear interaction models. The proposed interaction equation takes the form: (P/P_0)^α + (T/T_0)^β ≤ 1.0, where α and β are exponents that depend on the member geometry and material properties, with typical values of α = 1.5-2.0 and β = 1.8-2.5 for the studied configurations.

Interpretation of Technical Points

The failure mechanism under combined compression-torsion loading is highly dependent on the load ratio. At low torsion ratios (T/T_0 < 0.2), the failure is dominated by concrete crushing in the compression zone with the steel tubes yielding in tension on the opposite side. As the torsion ratio increases, the failure mode transitions to a diagonal tension failure pattern where the concrete core develops spiral cracks and the steel tubes exhibit torsional buckling of their walls.

The sandwich configuration provides a unique advantage in torsional resistance because the outer tube contributes directly to the torsional moment resistance through shear flow, while the inner tube and concrete core contribute through warping torsion. The study finds that for members with outer tube thickness ratios above 0.03, the outer tube can carry 60-70% of the torsional moment, significantly reducing the demand on the concrete core.

The gap between the inner and outer tubes plays a crucial role in the deformation behavior. A small gap (0-10 mm) allows independent deformation of the two tubes while maintaining composite action through shear connectors, resulting in enhanced ductility. However, if the gap is too large (>20 mm), the shear connectors may not effectively transfer shear between tubes, leading to a loss of composite action and reduced capacity.

Process and Standards Analysis

Current design standards for steel tube concrete members (GB 50017, CECS 230, AS 4100) do not address sandwich configurations, creating a significant gap in design guidance. The study proposes a design methodology based on the following principles:

1. The compression capacity is calculated as the sum of the inner tube capacity, concrete core capacity (with confinement enhancement), and outer tube capacity, with a reduction factor of 0.9 for the composite action.
2. The torsion capacity is calculated using the thin-walled tube theory for each steel tube plus a contribution from the concrete core, with the outer tube contribution weighted by its torsional stiffness ratio.
3. The interaction between compression and torsion is evaluated using the proposed nonlinear interaction equation with exponents calibrated from the experimental and numerical results.

The study recommends specific manufacturing tolerances for the sandwich configuration:

Integration with Engineering Practice

The sandwich steel tube concrete configuration finds practical application in structures requiring high torsional resistance, such as bridge piers subjected to seismic loading, offshore platform legs exposed to wave-induced torsional moments, and high-rise building cores that must resist torsional effects from asymmetric loading. The study provides design guidance that enables engineers to exploit the advantages of the sandwich configuration while ensuring adequate safety margins.

A practical implementation case from a bridge pier project demonstrated that the sandwich configuration with an outer tube diameter of 1200 mm, inner tube diameter of 900 mm, and concrete core of C50 achieved a torsional capacity 45% higher than a conventional single-tube STC member of equivalent cross-sectional area, while maintaining comparable compression capacity. The additional cost of the sandwich configuration was approximately 20% higher than the conventional STC member, representing a favorable cost-benefit ratio for applications where torsional resistance is critical.

The construction sequence for sandwich members requires careful planning: the inner tube is fabricated and erected first, followed by placement of shear connectors, then the concrete core is poured and cured, and finally the outer tube is installed and welded to the shear connectors. This sequence ensures proper composite action while minimizing construction complexity.

Key Questions and Reflections

Several important questions arise from this research. First, the long-term behavior of the sandwich configuration under sustained combined compression-torsion loads, including creep and relaxation effects in both the steel tubes and concrete core, requires further investigation. Second, the effect of cyclic loading on the interface between the inner and outer tubes, particularly the potential for fatigue damage at shear connector welds, is not fully addressed. Third, the fire performance of the sandwich configuration, where the gap between tubes may create a thermal barrier but also a potential weak point, needs comprehensive evaluation.

The study also raises questions about the optimal design of shear connectors. While the study examines welded studs as shear connectors, alternative configurations such as perforated plates, friction stir welding, or mechanical interlocks may offer different performance characteristics that warrant investigation.

Study Insights and Implications

This research provides essential technical data for the design of sandwich steel tube concrete members under combined compression-torsion loading, filling a significant gap in the structural engineering literature. The proposed interaction equation and design methodology offer a practical framework for engineers to design these advanced composite members with confidence.

The study demonstrates that the sandwich configuration offers significant advantages in torsional resistance while maintaining comparable compression capacity to conventional STC members. This makes it an attractive option for applications where torsional loads are significant, provided that the additional construction complexity and cost are justified by the performance benefits. Future research should focus on simplifying the design methodology into code-ready formats, investigating alternative shear connector designs, and developing construction guidelines that ensure reliable composite action in practice.