Comparative Study of Uniaxial Bending Capacity Calculation Methods for Rectangular Concrete-Filled Steel Tube Columns

Literature Overview

This paper by Fu Bo, Cheng Zhishu, Jia Shuhua, Sun Hao, and Tong Genshu (2022), published in Progress in Steel Building Structures, provides a comprehensive comparison of three different calculation methods for the uniaxial bending capacity of rectangular concrete-filled steel tube (CFST) columns as specified in current Chinese design codes. The research was funded by the National Key R&D Program of China (Grant No. 2016YFC0701201). The study compares methods from GB 50936-2014, CECS 159:2004, and JGJ 138-2016, and proposes new formulas derived from structural stability theory.

Core Technical Findings

The study identifies significant discrepancies between the three existing calculation methods, which can lead to inconsistent and potentially unsafe design outcomes:

The study systematically evaluates:

• Axial compression strength capacity
• Axial compression stability capacity
• Weak and strong axis bending capacity
• Weak and strong axis uniaxial bending capacity (combined loading)

Interpretation of Key Technical Points

The H/B Ratio Effect on Weak Axis Capacity

The finding that GB 50936-2014 produces large deviations for H/B ratios greater than 2.0 is particularly significant. Rectangular CFST columns with high aspect ratios are increasingly common in modern structural design due to architectural requirements and efficient use of material. The weak axis bending capacity of such columns is governed by:

• The narrower dimension (B) controlling the section modulus
• The confinement effect being less effective in the narrow direction
• Potential for local buckling of the narrow tube walls

The current code formula likely assumes a more uniform confinement distribution that does not account for the geometric asymmetry of high H/B sections.

Slenderness Ratio Sensitivity

The CECS 159:2004 method's unconservative predictions for slenderness ratios above 50 highlight an important design consideration. At high slenderness ratios:

• Flexural buckling becomes the governing failure mode
• The interaction between axial force and bending moment becomes more complex
• The confinement effect may be partially lost due to tube buckling before concrete crushing

This finding suggests that the CECS method's underlying assumptions about composite action validity break down at high slenderness ratios.

Conservative Bias in JGJ 138-2016

The conservative nature of JGJ 138-2016 for strong axis bending capacity, while structurally safe, may lead to uneconomic designs. In practical engineering, excessive conservatism translates to:

• Over-sized steel tubes (increased material cost)
• Excessive concrete volumes (increased self-weight)
• Potentially limiting the use of CFST columns in competitive design scenarios

Proposed New Formulas

The authors derive new formulas based on structural stability theory, which provides a more rigorous theoretical foundation. The stability-theory-based approach considers:

• The equilibrium of the column under combined axial force and moment
• The interaction between flexural buckling and material yielding
• The actual stress distribution in the composite section

Connection with Steel Pipe Engineering Practice

Impact on Steel Tube Specifications

The discrepancies between calculation methods directly affect steel tube specifications in engineering practice:

Quality Assurance Implications

The uncertainty in calculation methods has implications for quality assurance:

• Dimensional tolerance: When different codes give different required tube sizes, the manufacturing tolerance becomes more critical
• Material certification: The steel grade and concrete strength must be accurately verified, as calculation methods are sensitive to input parameters
• Weld quality: For LSAW tubes used in rectangular CFST columns, the weld seam quality directly affects the actual capacity, which may differ from calculated values

Rectangular Tube Manufacturing Considerations

Rectangular steel tubes for CFST columns are typically manufactured by:

• Cold-rolled forming: For smaller sections (up to approximately 400mm × 400mm)
• Hot-rolled forming: For larger sections
• Welded fabrication: For very large sections or special geometries

The study's findings regarding H/B ratios above 2.0 suggest that rectangular tubes with high aspect ratios (e.g., 600mm × 250mm) are increasingly relevant, and manufacturers should ensure:

• Flatness of narrow faces to prevent premature local buckling
• Corner radius consistency to maintain predictable bending behavior
• Wall thickness uniformity across the entire section

Engineering Practice Recommendations

Code Selection Guidance

Based on the study's findings, the following selection guidance is recommended:

1. For H/B ≤ 2.0: All three codes provide acceptable results; GB 50936-2014 is generally preferred for its comprehensive coverage
2. For H/B > 2.0: Use the newly proposed formulas or apply safety factors to GB 50936-2014 results
3. For slenderness ratio > 50: Avoid CECS 159:2004; use the new stability-theory-based formulas
4. For strong axis design where economy is critical: Consider the new formulas rather than JGJ 138-2016

Verification Procedures

Engineers should implement the following verification procedures:

• Cross-check calculations: Always verify results from one code against another to identify potential issues
• Finite element analysis: Use nonlinear FEA for critical members where code discrepancies are significant
• Test verification: For innovative or extreme geometric configurations, commission model-scale tests

Study Insights and Conclusions

This comparative study reveals a significant gap in the current design methodology for rectangular CFST columns. The existence of three codes with substantially different predictions for the same structural member is concerning from a safety and consistency perspective. The proposed new formulas based on structural stability theory represent a step toward rational design, but their adoption requires validation through additional experimental programs and code revision processes. For steel pipe manufacturers, this study highlights the importance of providing accurate geometric and material data to designers, as the sensitivity of capacity calculations to tube dimensions means that manufacturing precision directly affects structural safety margins. The study ultimately calls for harmonization of design methods and suggests that the structural stability approach offers the most reliable theoretical foundation for future code development in this area.