Zhuojin Pipe Fitting Co., LtdShear Performance Analysis of Round-Ended Elliptical CFST Members
Literature Overview and Research Background
This paper by Wang Jingfeng, Sheng Mingyu, Shen Qihan, and Ma Xianfeng from Hefei University of Technology, published in the "Journal of Hefei University of Technology (Natural Science Edition)" (2020, Vol. 43, No. 1, pp. 81–87), investigates the shear behavior of round-ended elliptical concrete-filled steel tube (CFST) members. The research is supported by the National Natural Science Foundation of China (Grants No. 51478158 and 51178156) and the Ministry of Education New Century Excellent Talents Support Program (NCET-12-0838).
Round-ended elliptical CFST members represent a novel cross-sectional geometry that combines the aerodynamic advantages of elliptical shapes with the structural benefits of concrete confinement. The round-ended configuration eliminates sharp corners that would otherwise create stress concentrations, resulting in a smaller drag coefficient and more uniform stress distribution. These members are particularly suitable for applications such as bridge piers, elevated columns, and arch towers where aerodynamic performance and structural efficiency are both critical design considerations.
Equivalent Constitutive Model for Confined Concrete
A fundamental contribution of this research is the development of a novel equivalent constitutive relationship model applicable to the core concrete confined by round-ended elliptical steel tubes. Traditional CFST constitutive models are typically developed for circular or rectangular cross-sections, and their direct application to complex geometries such as round-ended elliptical sections introduces significant inaccuracies.
The proposed equivalent constitutive model accounts for the non-uniform confinement pressure distribution that arises from the varying curvature of the round-ended elliptical cross-section. At the round-ended portions, the confinement pressure is higher due to the larger radius of curvature, while at the flatter mid-sections, the confinement effect is relatively weaker. This non-uniformity must be captured in the constitutive model to accurately predict the stress-strain behavior of the confined concrete under shear loading.
| Model Parameter | Description | Typical Range |
|---|---|---|
| Concrete unconfined compressive strength f'c | Cylinder compressive strength | 30–60 MPa |
| Steel yield strength f_y | Yield stress of steel tube | 235–460 MPa |
| Steel ratio (含钢率) | A_s / A_total | 5%–15% |
| Confinement factor | Ratio of confined to unconfined strength | 1.2–2.5 |
Finite Element Modeling Approach
The finite element analysis model was established using the proposed equivalent constitutive model for the confined concrete, combined with appropriate material models for the steel tube and boundary conditions that simulate the shear loading configuration. The model incorporates material nonlinearity, geometric nonlinearity, and the complex contact interaction between the steel tube and the core concrete.
The accuracy of the finite element model was validated against experimental test results, confirming that the numerical predictions capture the essential features of the shear behavior including the load-displacement relationship, failure mode, and the influence of key geometric and material parameters.
Shear Failure Modes and Parametric Analysis
The parametric study reveals four distinct failure modes for round-ended elliptical CFST members under shear loading, depending primarily on the shear span-to-depth ratio:
| Failure Mode | Dominant Mechanism | Typical Shear Span Ratio |
|---|---|---|
| Shear failure | Diagonal tension and shear yielding | Low (< 1.5) |
| Bending failure | Flexural yielding and concrete crushing | High (> 3.0) |
| Combined shear-bending failure | Interaction of shear and flexural mechanisms | Intermediate (1.5–2.5) |
| Local bulging failure | Local buckling of steel tube wall | Low (< 1.0, thin-walled) |
The parametric analysis demonstrates that increasing steel strength, concrete strength, and steel ratio all contribute to higher shear capacity. The relationship between shear span ratio and shear capacity is non-monotonic: as the shear span ratio increases from a low value, the shear capacity initially increases due to the transition from shear-dominated to bending-dominated behavior, but beyond a critical value, the capacity decreases as the member becomes increasingly susceptible to flexural failure rather than efficient shear resistance.
Engineering Practice Considerations
From a steel pipe manufacturing and welding perspective, the fabrication of round-ended elliptical CFST members presents unique challenges. The elliptical cross-section with round-ended corners requires specialized rolling or forming equipment capable of producing complex profile shapes with consistent wall thickness. The welding of longitudinal seams, if applicable, must be performed with careful control of weld geometry to avoid creating stress concentrations at the round-ended transitions.
For welded connections in structural assemblies using round-ended elliptical CFST members, the non-circular cross-section complicates the design and fabrication of connection details. Welded joints must accommodate the varying curvature of the tube surface, and weld preparation procedures must be adapted to ensure full fusion and adequate weld penetration at all locations along the connection interface.
The research provides valuable design guidance for engineers considering round-ended elliptical CFST members in shear-critical structural applications. The identified failure modes and their dependence on key parameters enable rational design decisions regarding member proportions, material selection, and detailing requirements.
Key Technical Insights
The development of geometry-specific equivalent constitutive models is essential for accurate analysis of non-conventional CFST cross-sections. The round-ended elliptical geometry, while aerodynamically advantageous, introduces complex confinement patterns that cannot be adequately captured by simplified circular or rectangular confinement models. The non-monotonic relationship between shear span ratio and shear capacity highlights the importance of identifying the critical shear span ratio for each specific member configuration during the design process.
The identification of local bulging failure as a distinct failure mode for thin-walled round-ended elliptical CFST members under low shear span ratios has direct implications for wall thickness selection and local buckling resistance requirements. Design codes should incorporate specific provisions for local buckling checks in these members, potentially requiring increased wall thickness ratios or the addition of internal stiffeners at critical locations.
Study Value and Future Directions
This research establishes a methodological framework for the shear design of round-ended elliptical CFST members that can be extended to other complex cross-sectional geometries. The equivalent constitutive model approach demonstrated here provides a template for developing geometry-specific material models that accurately represent the confinement effects unique to each cross-sectional shape. Future research should address cyclic shear loading behavior for seismic applications, investigate the combined effects of axial compression and shear on the performance of these members, and develop simplified design formulas that can be incorporated into structural design codes for practical engineering use.