Ultimate Bearing Capacity Calculation Method for Round-Ended Steel Tube Concrete Short Columns

Literature Overview

This paper by Ren Zhigang, Liu Chuang, Wang Dandan, and Wei Wei from Wuhan University of Technology, published in Building Structures in 2021 (Vol. 51, No. 7, pp. 38–45), addresses a significant gap in the structural engineering literature: the lack of quantitative expressions for calculating the ultimate bearing capacity of round-ended steel tube concrete (CFST) short columns under axial compression and eccentric compression. The research was supported by the National Natural Science Foundation of China (Grant 51778512) and the Wuhan University of Technology Graduate Excellent Thesis Cultivation Project (2018-YS-046). The authors conducted 8 axial compression tests and 6 eccentric compression tests on round-ended CFST short columns, then developed three theoretical calculation methods based on the double-shear unified strength theory, limit equilibrium theory, and fiber model method.

Core Technical Contributions

The round-ended cross-section represents a hybrid geometry combining the circular and rectangular section characteristics, which introduces additional complexity in stress distribution, confinement effects, and failure mechanisms compared to conventional circular or rectangular CFST columns. The authors derived corresponding axial compression bearing capacity formulas from three theoretical frameworks and compiled a fiber model calculation program using Python language.

Three Theoretical Calculation Approaches

Method	Theoretical Basis	Accuracy	Applicability
Double-shear unified strength theory	Multi-shear failure criterion	Highest precision	Axial compression
Limit equilibrium theory	Plastic limit analysis	Good precision	Axial compression
Fiber model method	Discretized cross-section integration	Good precision	Axial and eccentric compression

The study concluded that all three theoretical formulas for axial compression demonstrate satisfactory accuracy meeting engineering requirements. However, the formula based on the double-shear unified strength theory provides the highest precision for axial compression calculations. The unified bearing capacity formula for both axial and eccentric compression was validated, with the eccentric compression applicable range determined as e/R ≤ 1.55.

Comparison with Existing Code

A notable finding is that the bearing capacity calculated according to the DBJ/T 13-51 code specification yields conservative results with the poorest accuracy among all methods compared. This suggests that current design codes for CFST structures may not adequately account for the enhanced confinement effect and composite action specific to the round-ended cross-section geometry.

Engineering Practice Integration

For engineers designing structures incorporating round-ended CFST columns, this research provides several actionable insights:

The round-ended section offers a balance between the high confinement efficiency of circular sections and the practical advantages of flat surfaces for connection detailing.
When designing for eccentric loading, the e/R ≤ 1.55 limit should be observed to ensure the validity of the proposed unified formula.
The conservative nature of code-based calculations means that utilizing the refined theoretical methods can lead to more economical designs without compromising safety.
The fiber model approach, while computationally more intensive, provides the most versatile tool for analyzing complex loading conditions and can be readily adapted for parametric studies of different geometric configurations.

Study Insights and Implications

This work exemplifies the rigorous approach needed to extend structural design methodologies to novel cross-sectional geometries. The round-ended CFST column represents an innovative section shape that has been adopted in practice but lacked a corresponding theoretical foundation. By systematically comparing three theoretical approaches and validating against experimental data, the authors have established a reliable design basis. The finding that code-based calculations are overly conservative is particularly important for cost-conscious engineering projects, as it suggests potential for material optimization. The Python-based fiber model program represents a practical computational tool that can be adapted by other researchers and practitioners for similar cross-sectional analyses. This research reinforces the principle that theoretical development must keep pace with innovative structural forms to ensure both safety and efficiency in engineering practice.