Zhuojin Pipe Fitting Co., LtdCompression-Bending Capacity of Square CFST Short Columns with Constraining Tie Rods
Literature Overview
This paper by Long Yueling, Wang Yingtao, Cai Jian, and Chen Qingjun from Guangdong University of Technology and South China University of Technology investigates the compression-bending bearing capacity of square steel tube concrete (CFST) short columns with constraining tie rods. The study was published in Industrial Construction (2016, Vol. 46, No. 3, pp. 142–148) and was supported by the State Key Laboratory of Subtropical Building Science. A total of 10 specimens were tested: 2 without constraining tie rods and 8 with constraining tie rods of varying configurations.
Core Technical Findings
The research employed the finite strip method (FSM) to numerically calculate the skeleton curves of the specimens under cyclic loading, using a modified constitutive relationship for the core concrete that accounts for the constraining effect of the tie rods. The calculated results showed good agreement with experimental data. Parameter studies were conducted to evaluate the influence of steel tube yield strength, steel ratio, concrete strength, and tie rod constraint coefficient on the N/Nu–M/Mu interaction curves. A simplified calculation formula for the compression-bending bearing capacity was then proposed.
| Parameter | Effect on N/Nu–M/Mu Curve |
|---|---|
| Steel tube yield strength | Higher yield strength increases both axial and bending capacity |
| Steel ratio | Higher steel ratio shifts the interaction curve outward |
| Concrete strength | Higher concrete strength increases axial capacity more than bending capacity |
| Tie rod constraint coefficient | Higher constraint coefficient significantly improves both axial and bending capacity |
The study also compared the simplified formula with the CECS 159:2004 standard (Rectangular Steel Tube Concrete Technical Specification), finding that the existing standard does not account for the constraining effect of tie rods and therefore underestimates the compression-bending bearing capacity of such columns.
Interpretation of Technical Points
The constraining tie rods represent an innovative approach to enhancing the confinement effect in square CFST columns. In conventional square CFST columns, the flat faces of the steel tube are prone to local buckling under compressive loading, particularly in the mid-span region where bending moments are maximum. The constraining tie rods provide lateral support to the flat faces, effectively increasing the effective buckling length and delaying local buckling. This is analogous to the concept of internal stiffening rings used in cylindrical shells to enhance buckling resistance.
The finite strip method is well-suited for this type of analysis because it can efficiently model the thin-walled behavior of the steel tube while capturing the nonlinear interaction between the steel tube, core concrete, and tie rods. The modified concrete constitutive relationship that incorporates the tie rod constraint coefficient is a key innovation of this study, as it allows the numerical model to accurately represent the enhanced confinement pressure on the core concrete.
Standards and Code Comparison
The comparison with CECS 159:2004 reveals an important gap in the current Chinese standard for rectangular CFST structures. The standard's compression-bending design formula was developed based on conventional CFST columns without additional constraining elements. When tie rods are used, the actual bearing capacity exceeds the code prediction, which means that structures designed according to the current code are conservative but not optimally efficient. From an engineering economics perspective, this conservatism leads to unnecessary material usage and increased construction costs.
| Standard/Code | Considers Tie Rod Effect | Bearing Capacity Assessment |
|---|---|---|
| CECS 159:2004 | No | Underestimates capacity |
| Simplified formula (this study) | Yes | Good agreement with test results |
| Eurocode 4 (EN 1994-1-2) | No explicit provision | Not directly applicable |
| AISC 360 | No explicit provision | Not directly applicable |
Integration with Engineering Practice
From a steel pipe and welding perspective, the use of constraining tie rods introduces additional welding details that must be carefully designed and executed. The tie rods are typically welded to the interior or exterior of the steel tube, creating attachment welds that must be designed for both tension and shear loading. The weld design should follow the requirements of GB 50017 (Steel Structure Design Standard) or AWS D1.1, with particular attention to the weld throat thickness, weld length, and weld quality requirements.
The welding of tie rod attachments to the steel tube creates a complex stress state at the weld root, where the tie rod weld, the steel tube wall, and any internal stiffeners intersect. This is a critical location for fatigue and fracture initiation under cyclic loading. The weld procedure should be qualified using appropriate qualification tests, including macrographic examination, hardness surveys, and preferably fatigue testing of weld coupons. Post-weld stress relief may be required to reduce residual stresses in the tie rod attachment welds, particularly if the steel tube material has a high carbon equivalent.
The steel tube itself must be manufactured to ensure adequate wall thickness uniformity and geometric accuracy. For square CFST columns, the wall thickness variation should be within ±10% of the nominal value according to GB/T 6728 or EN 10219. The flat faces of the square tube should be free from excessive waviness, as this can reduce the effective buckling resistance and interfere with the tie rod attachment.
Key Questions and Reflections
One important question is the long-term durability of the tie rod attachment welds under cyclic loading. The tie rods experience repeated tension and relaxation during seismic events, which subjects the attachment welds to cyclic fatigue loading. The fatigue life of these welds should be evaluated according to the relevant fatigue design rules, and the weld detail should be designed to achieve the highest feasible fatigue category.
Another consideration is the constructability of the tie rod system. In practice, the tie rods must be installed before the concrete is poured, which requires careful coordination between the steel fabrication, welding, and concrete placement sequences. The tie rods should be positioned with sufficient accuracy to ensure uniform confinement pressure on the core concrete, and the welding of the tie rod attachments should be completed before the concrete is placed to avoid disturbing the concrete during welding operations.
Study Insights and Implications
This research demonstrates that constraining tie rods are an effective means of enhancing the compression-bending capacity of square CFST columns, and that the existing Chinese standard does not adequately account for this enhancement. The proposed simplified calculation formula provides a practical tool for designers, but it should be validated through additional experimental work before being incorporated into design codes. From a welding and steel pipe perspective, the tie rod system introduces additional welding details that require careful design, fabrication, and quality control to ensure long-term structural performance under seismic loading.