Zhuojin Pipe Fitting Co., LtdUltimate Bearing Capacity of Axially Compressed Cold-Formed Thin-Walled Irregular Octagonal High-Strength Steel Pipe Short Columns
Literature Overview
This paper, published in Journal of Tongji University (Natural Science) (2025, Vol. 53, No. 12), authored by Zhu Jiongyi, Huang Haichao, Liu Haixin, and Yang Qin from Shanghai University, Changsha University of Science and Technology, and China Construction Eighth Engineering Division, investigates the axial compression behavior and ultimate bearing capacity of cold-formed thin-walled irregular octagonal high-strength steel pipe short columns. The study is supported by the National Natural Science Foundation of China (Young Scientists Fund). The work addresses a relatively novel structural cross-section that combines the geometric advantages of octagonal shapes with the material advantages of high-strength steel, offering potential applications in building frames, bridge columns, and industrial structures.
Experimental Program and Results
The study tested eight hollow steel pipe short columns under axial compression, with material tensile tests conducted on different sections of the octagonal steel pipes. The experimental results reveal that the failure mode of irregular octagonal cold-formed thin-walled hollow steel pipe short columns is local buckling instability, and the load-carrying capacity does not reach the yield strength of the material.
Key Experimental Findings
| Parameter | Finding | Engineering Significance |
|---|---|---|
| Failure mode | Local buckling instability | Capacity governed by stability, not material strength |
| Load capacity | Below yield strength | Significant reduction due to local buckling |
| Material tests | Conducted on different sections | Accounts for cold-forming effects on material properties |
The fact that the load-carrying capacity does not reach the yield strength is a critical finding. In conventional structural design, the ultimate capacity of a compression member is often estimated based on the material's yield strength, with adjustments for overall buckling. However, for thin-walled cold-formed members, local buckling can occur at stresses well below the yield strength, significantly reducing the effective cross-sectional area and thus the load-carrying capacity.
Finite Element Analysis and Validation
The authors conducted finite element analysis (FEA) and validated the models against experimental results, confirming the accuracy of the FEA models. The FEA was then used for parametric studies to investigate the effects of various geometric and material parameters on the ultimate bearing capacity.
Evaluation of Existing Design Formulas
A significant contribution of this study is the evaluation of existing ultimate bearing capacity calculation formulas from Chinese codes, European codes (Eurocode), American codes (AISC), and existing literature. The evaluation revealed that none of the existing formulas are applicable to irregular octagonal cold-formed thin-walled high-strength hollow steel pipe short columns. This is a critical finding because it highlights the need for new design methods for this novel cross-section.
Proposed New Calculation Formula
Based on the experimental data and FEA results, the authors proposed a new ultimate bearing capacity calculation formula for irregular octagonal high-strength steel pipe short columns. The proposed formula was validated against both experimental data and FEA results, demonstrating good predictive accuracy. This is a significant contribution to structural design practice, as it provides engineers with a reliable method for calculating the ultimate capacity of this novel cross-section.
Discussion on Local Buckling and Design Implications
The local buckling failure mode observed in this study is consistent with the well-established behavior of cold-formed thin-walled steel members. The local buckling stress depends on the plate slenderness ratio (width-to-thickness ratio) of each flat plate segment of the octagonal cross-section. For high-strength steel, the higher yield strength results in a higher local buckling stress, but the higher strength also means that the material is more sensitive to imperfections and residual stresses introduced during cold forming.
The irregular octagonal shape introduces additional complexity compared to regular octagonal or circular sections. The varying plate widths and the presence of internal angles (both acute and obtuse) create non-uniform stress distributions and varying buckling resistances across the cross-section. This non-uniformity is likely the primary reason why existing design formulas, which are typically developed for regular sections or circular tubes, do not apply to irregular octagonal sections.
Design Recommendations for Irregular Octagonal Steel Pipe Columns
Based on the findings of this study, the following design recommendations can be made:
- Cross-section design: The plate width-to-thickness ratios of all flat plate segments should be controlled to minimize the risk of local buckling. The internal angles should be designed to minimize stress concentrations.
- Material selection: High-strength steel grades should be selected based on their cold-forming properties, not just their yield strength. The material should have adequate ductility to accommodate the plastic deformation associated with local buckling.
- Manufacturing quality: The cold-forming process should be controlled to minimize residual stresses and geometric imperfections, which can significantly reduce the local buckling resistance.
- Design formula: The proposed new calculation formula should be adopted for the design of irregular octagonal cold-formed thin-walled high-strength steel pipe columns, as existing formulas are not applicable.
Study Insights and Future Research Directions
This study makes a significant contribution to the understanding of irregular octagonal cold-formed thin-walled high-strength steel pipe columns. The identification of local buckling as the governing failure mode and the development of a new design formula are both important contributions to structural engineering practice.
The finding that existing design formulas are not applicable to this novel cross-section is a reminder that the development of new structural systems requires corresponding development of design methods. The proposed new formula provides a starting point for practical design, but further research is needed to establish a comprehensive design framework.
For future research, I would recommend: (1) investigation of longer columns to study the interaction between local and overall buckling; (2) study of columns under combined axial and bending loads, which is more representative of actual structural behavior; (3) parametric studies on the effects of cross-section geometry (plate widths, angles, thickness) on the ultimate capacity; (4) investigation of the effects of cold-forming process parameters (forming speed, die geometry, lubrication) on the material properties and structural behavior; and (5) development of a complete design methodology, including stability design, connection design, and fabrication guidelines, for irregular octagonal steel pipe columns.
In summary, this study provides a rigorous investigation of the axial compression behavior of irregular octagonal cold-formed thin-walled high-strength steel pipe short columns, offering both experimental evidence and a new design formula that addresses a gap in the existing design codes and standards.