Bending Performance and Bearing Capacity Calculation of Elliptical CFST Members

Literature Overview

This study by Jiang Han, Wang Jingfeng, and Shen Qihan from Hefei University of Technology investigates the bending performance and bearing capacity of elliptical cross-section CFST members. Published in the Journal of Building Steel Structure Progress in 2018, the research was supported by the National Natural Science Foundation of China (grant 51478158) and the Ministry of Education New Century Excellent Talent Support Program (NCET-12-0838). The authors established numerical analysis models using ABAQUS software, validated the models against six pure bending tests, conducted parametric studies, and proposed simplified calculation formulas for the flexural bearing capacity of elliptical CFST members.

Research Background and Significance

Elliptical cross-section CFST members offer potential advantages over circular cross-section members in certain structural applications. The elliptical shape provides greater section modulus in one direction while maintaining a more compact profile in the other direction, which can be beneficial in space-constrained structural applications such as building columns, bridge piers, and industrial facilities. However, the complex geometry of elliptical cross-sections introduces challenges in terms of concrete filling, stress distribution, and bearing capacity calculation that are not addressed in existing design codes.

Numerical Analysis Methodology

Model Development

The authors developed ABAQUS finite element models of elliptical CFST members under pure bending. The models accounted for three critical aspects:

Complex contact problems between the steel tube and the concrete core, including frictional contact and potential separation under bending.
Elliptical cross-section characteristics, including the varying curvature along the cross-section perimeter.
Material nonlinearity for both steel and concrete, including plastic deformation of steel and nonlinear stress-strain behavior of concrete.

Model Validation

The numerical models were validated against six pure bending test specimens. The validation process involved comparing the numerically predicted load-displacement curves, stress distributions, and failure modes with the experimental results. The close agreement between numerical and experimental results confirmed the accuracy and reliability of the models for parametric studies.

Parametric Study Results

Influence of Key Parameters

The parametric study investigated the influence of steel strength, concrete strength, diameter-to-thickness ratio, and axis ratio (ratio of major axis to minor axis) on the ultimate bending moment of elliptical CFST members. The results are summarized as follows:

Parameter	Direction of Influence	Magnitude of Effect	Design Implication
Steel strength	Higher strength increases ultimate moment	Moderate	Higher grade steel tubes improve flexural capacity
Concrete strength	Higher strength increases ultimate moment	Moderate	Higher grade concrete improves flexural capacity
Diameter-to-thickness ratio (D/t)	Smaller ratio increases ultimate moment	Significant	Thicker walls improve flexural capacity and delay local buckling
Axis ratio (a/b)	Higher ratio increases ultimate moment in major axis direction	Significant	More elongated ellipses provide greater section modulus

Failure Modes

The parametric study revealed the failure modes of elliptical CFST members under pure bending. The failure mode depends on the D/t ratio and the axis ratio:

For members with low D/t ratios, failure is characterized by concrete crushing in the compression zone and steel yielding in the tension zone, with minimal local buckling of the steel tube.
For members with high D/t ratios, local buckling of the steel tube in the compression zone occurs before concrete crushing, leading to a ductile failure mode with significant deformation capacity.
The axis ratio affects the location and pattern of local buckling, with buckling typically initiating at the region of highest curvature (the minor axis ends) under bending about the major axis.

Simplified Calculation Formula

Based on the parametric study results, the authors proposed simplified calculation formulas for the flexural bearing capacity of elliptical CFST members. These formulas provide a practical tool for structural designers to estimate the ultimate bending moment of elliptical CFST members without resorting to complex numerical analysis. The formulas incorporate the key parameters identified in the parametric study, including steel strength, concrete strength, D/t ratio, and axis ratio.

Engineering Practice Implications

From a steel pipe fabrication and welding perspective, the study of elliptical CFST members has several important implications.

Steel Tube Fabrication Challenges

Fabricating elliptical cross-section steel tubes presents greater challenges than fabricating circular cross-section tubes. The bending process must produce a uniform elliptical shape without introducing excessive residual stresses or geometric imperfections. The welding of elliptical tube segments must also be carefully controlled to maintain the elliptical cross-sectional geometry along the length of the tube. Any deviation from the ideal elliptical shape can affect the stress distribution and bearing capacity of the CFST member.

Concrete Filling Process

The concrete filling process for elliptical CFST members is more challenging than for circular CFST members. The varying cross-sectional dimensions along the perimeter of the ellipse can lead to uneven concrete flow and potential voids or honeycombing, particularly at the minor axis ends where the cross-section is most compact. Proper concrete mix design and filling procedures are essential to ensure complete and uniform concrete filling of the elliptical tube.

Weld Quality Considerations

For longitudinally welded elliptical steel tubes, the weld quality is particularly important because the weld zone is subjected to complex stress states under bending. The varying curvature of the elliptical cross-section means that the weld zone experiences different stress states depending on the location along the tube perimeter. Welds located at the minor axis ends, where the curvature is highest, may experience higher stress concentrations under bending. The weld procedure must be designed and executed to ensure adequate weld quality at all locations along the tube perimeter.

Study Insights and Reflections

This study provides valuable insights into the bending performance of elliptical CFST members and offers simplified calculation formulas for practical design. The parametric study results clearly demonstrate the influence of key parameters on flexural capacity, providing guidance for optimizing the design of elliptical CFST members. For steel pipe engineers, the study highlights the unique challenges associated with fabricating and welding elliptical steel tubes, as well as the importance of concrete filling quality in achieving the expected structural performance. The proposed simplified calculation formulas represent a significant step toward enabling the practical application of elliptical CFST members in structural engineering. Future research should focus on developing comprehensive design provisions for elliptical CFST members that address not only pure bending but also combined loading conditions, including axial compression with bending and shear with bending, as well as fatigue and fracture behavior under cyclic loading.