Torsional Performance and Capacity Calculation of Elliptical Steel Tube Concrete Members

Literature Overview

This paper by Song Shunlong et al. (2017), published in the Journal of Hefei University of Technology (Natural Science Edition) (Vol. 40, No. 7, pp. 952-959), investigates the torsional behavior and torsional capacity of elliptical steel tube concrete (ESTC) members through finite element analysis using ABAQUS. The study conducts parametric analysis examining the influence of steel strength, concrete strength, section steel ratio, section area, and major-to-minor axis ratio on torsional performance. The research reveals the mechanical behavior of ESTC members under pure torsion and proposes a simplified torsional capacity calculation formula. The work is supported by the National Natural Science Foundation of China (51478158) and the Ministry of Education New Century Excellent Talent Support Program (NCET-12-0838).

Core Technical Content

Finite Element Modeling and Parametric Analysis

The ABAQUS finite element model was developed to simulate the torsional behavior of elliptical steel tube concrete members. The model incorporated appropriate material models for both steel and concrete, including non-linear constitutive relationships and interaction definitions between the steel tube and concrete core. The parametric study examined five key variables:

Torsional Behavior and Stages

The torque (T) versus rotation angle (θ) curve for ESTC members was identified to have three distinct stages:

1. Elastic stage: Linear relationship between torque and rotation angle, with both steel and concrete behaving elastically.
2. Elasto-plastic stage: Non-linear behavior begins as either steel or concrete yields, with progressive plastic deformation developing.
3. Plastic strengthening stage: Continued increase in torque with rotation, indicating strain hardening and effective composite action between steel and concrete.

The presence of all three stages indicates good plastic performance and ductility in ESTC members under torsional loading, which is favorable for structural design in seismic regions.

Torsional Capacity Calculation Formula

The authors proposed a simplified calculation formula for the torsional strength capacity of elliptical steel tube concrete members. The formula integrates the contributions of the steel tube and concrete core to the overall torsional resistance, accounting for the elliptical geometry and composite action.

Technical Analysis of Torsional Mechanics

Mechanism of Torsional Resistance

The torsional resistance of an ESTC member develops through several mechanisms:

• Steel tube contribution: The elliptical steel tube resists torsion through shear flow around its cross-section, with the elliptical geometry providing different resistance characteristics in the major and minor axis directions.
• Concrete core contribution: The concrete core contributes to torsional resistance through shear stress distribution, with the confined concrete experiencing complex stress states under torsional loading.
• Composite interaction: The interface between steel and concrete transfers shear stresses, enabling effective composite action that enhances overall torsional capacity beyond the sum of individual components.

Influence of Geometric Parameters

The major-to-minor axis ratio of the elliptical cross-section significantly affects torsional behavior. An elliptical section has different torsional rigidity in different directions, and the axis ratio determines the degree of anisotropy. Members with higher axis ratios exhibit more pronounced directional differences in torsional performance.

Parametric Study Results

The parametric analysis revealed several important trends:

• Torsional strength capacity increases with increasing steel strength, as higher-strength steel provides greater shear resistance.
• Torsional strength capacity increases with increasing concrete strength, as stronger concrete contributes more to the composite torsional resistance.
• Torsional strength capacity increases with increasing section steel ratio, as a thicker steel tube provides more material to resist torsion.
• Torsional strength capacity increases with increasing section area, as larger sections have greater polar moment of inertia.
• The major-to-minor axis ratio has a complex influence, with both geometric and material interaction effects contributing to the overall behavior.

Engineering Practice Implications

Applications of Elliptical Steel Tube Concrete

Elliptical steel tube concrete members find applications in several structural contexts:

• Bridge structures: Elliptical tubes can be used in bridge girders and columns where aerodynamic performance and torsional resistance are important.
• Building structures: Elliptical columns can provide improved torsional stiffness in structures subjected to torsional loads.
• Special structures: Elliptical sections may be used in structures where specific geometric constraints require non-circular cross-sections.

Design Considerations

The proposed torsional capacity formula provides a practical tool for designing ESTC members, but several design considerations must be addressed:

1. Material selection: The steel grade and concrete strength should be selected to achieve the required torsional capacity while considering economic factors.
2. Section optimization: The major-to-minor axis ratio should be optimized based on the specific loading conditions and geometric constraints.
3. Connection design: Torsional loads must be effectively transferred at member connections, requiring careful connection design.
4. Serviceability: Torsional deformation must be checked against serviceability limits, particularly rotation angles at service loads.

Comparison with Circular Steel Tube Concrete

Elliptical steel tube concrete members offer certain advantages over circular sections:

• Better fit to architectural and geometric constraints.
• Potentially improved aerodynamic performance in certain applications.
• Different torsional resistance characteristics that may be advantageous in specific loading scenarios.

However, elliptical sections also present challenges:

• More complex fabrication and welding requirements.
• Potentially lower torsional efficiency compared to circular sections of equivalent area.
• More complex finite element modeling and analysis.

Study Insights and Recommendations

This research provides valuable insights into the torsional behavior of elliptical steel tube concrete members, filling a gap in the existing knowledge base for non-circular steel tube concrete cross-sections. The proposed simplified calculation formula offers a practical design tool that balances accuracy with computational efficiency.

For steel pipe manufacturing and welding engineers, the key consideration is that elliptical steel tubes require more sophisticated fabrication techniques than circular tubes. The forming process must achieve precise elliptical geometry with consistent wall thickness, and any welding defects or geometric deviations can significantly affect torsional performance. The elliptical shape introduces additional complexity in terms of forming tolerances, weld quality, and dimensional accuracy, all of which must be carefully controlled during manufacturing.

The research also highlights the importance of comprehensive parametric studies in understanding structural behavior. The identification of key parameters and their influence on torsional capacity enables more informed design decisions and helps optimize structural performance for specific applications.