Zhuojin Pipe Fitting Co., LtdComparative Experimental Study on Mechanical Properties of Square Hollow Sandwich CFST Short Columns Under Axial Compression
Literature Overview
This paper by Huang Hong, Zha Baojun, Chen Mengcheng, and Guo Xiaoyu, published in Railway Construction in 2015 (Vol. 55, No. 10, pp. 85-89), presents a comparative experimental study on the mechanical properties of square hollow sandwich concrete-filled steel tube (CFST) short columns under axial compression. The research was supported by the National Natural Science Foundation of China (Grant Nos. 51378206, 51008122), the Jiangxi Provincial Youth Science Fund Program (Grant No. 20143ACB21020), and the Jiangxi Provincial Young Scientist Cultivation Program (Grant No. 20133BCB23015). The study was conducted at the School of Civil Engineering and Architecture, East China Jiaotong University, and the Jiangxi Provincial Key Laboratory of Construction Process Simulation and Control. Four different cross-sectional configurations were tested to evaluate the influence of hollow sandwich designs on the axial compressive behavior of square CFST columns.
Research Background and Sandwich Column Concept
The concept of hollow sandwich CFST columns represents an innovative approach to structural design that aims to reduce material usage and structural weight while maintaining or even enhancing the load-bearing capacity compared to solid CFST columns. By incorporating an internal steel tube within the outer square steel tube and filling the annular space between the inner and outer tubes with concrete, a sandwich-type composite section is created. The hollow core region reduces the overall weight of the column without proportionally reducing the load-bearing capacity, as the outer steel tube and the sandwich concrete layer together provide the primary resistance to axial compression.
The research investigated four different cross-sectional configurations: a solid square CFST specimen, a square-enclosing-circular hollow sandwich specimen, and two square-enclosing-square hollow sandwich specimens with different inner tube placement orientations. This systematic comparison enables the isolation of the influence of cross-sectional geometry on the mechanical behavior under axial compression.
Experimental Setup and Test Specimens
Four short column specimens were fabricated and tested under axial compression loading. Each specimen was designed with a specific cross-sectional configuration as described above. The specimens were instrumented with strain gauges at critical locations to measure the axial and lateral strain development during loading. The loading was applied monotonically until failure, and the load-displacement relationship was recorded throughout the test.
| Specimen Type | Configuration | Key Feature |
|---|---|---|
| Specimen 1 | Solid square CFST | Reference specimen, fully solid concrete core |
| Specimen 2 | Square-enclosing-circular hollow sandwich | Circular inner tube within square outer tube |
| Specimen 3 | Square-enclosing-square hollow sandwich (Type A) | Inner square tube in one orientation |
| Specimen 4 | Square-enclosing-square hollow sandwich (Type B) | Inner square tube in alternative orientation |
The hollow ratio, defined as the ratio of the hollow core area to the total cross-sectional area, was kept relatively small to ensure that the structural integrity of the sandwich column is maintained. The specimens were designed to be short columns to avoid the influence of buckling and focus on the material behavior and cross-sectional interaction effects.
Key Experimental Results
The experimental results revealed several important findings regarding the mechanical behavior of hollow sandwich CFST columns. All specimens, regardless of cross-sectional configuration, exhibited a similar failure pattern characterized by cracking of the outer tube welds and inward buckling of the inner tube after reaching the ultimate load-bearing capacity. This consistent failure mode suggests that the outer tube welds and the inner tube buckling represent the primary failure mechanisms for hollow sandwich CFST columns under axial compression.
A notable finding was that, when the hollow ratio is not too large, the axial compressive load-bearing capacity of the square hollow sandwich CFST columns can exceed that of the solid square CFST column. This counterintuitive result can be attributed to the enhanced confinement mechanism provided by the sandwich configuration, where the concrete layer between the inner and outer tubes experiences biaxial or triaxial compression that enhances its strength and ductility. The presence of the inner tube also provides additional direct load-bearing capacity and contributes to the confinement of the sandwich concrete layer.
Regarding lateral strain development, the solid square CFST specimen exhibited faster lateral strain development compared to the hollow sandwich specimens, indicating that the solid column undergoes greater lateral expansion under axial compression. Among the hollow sandwich specimens, the square-enclosing-circular configuration developed lateral strains faster than the square-enclosing-square configurations, suggesting that the circular inner tube provides less effective confinement of the sandwich concrete layer compared to the square inner tube.
Finite Element Validation
Finite element analysis was performed using ABAQUS software to simulate the full-range load-deformation behavior of the axially compressed specimens. The finite element models were calibrated to reproduce the experimental load-displacement curves, and the calculated ultimate load-bearing capacities showed good agreement with the experimental results. This validation confirms the accuracy of the finite element models and provides confidence in using numerical methods for the detailed analysis and optimization of hollow sandwich CFST column designs.
Engineering Practice and Design Considerations
The findings of this research have direct implications for the design of lightweight structural columns in buildings and bridges where material efficiency is a priority. The hollow sandwich CFST column concept offers a promising alternative to traditional solid CFST columns, potentially reducing material costs and structural weight while maintaining adequate load-bearing capacity. However, several design considerations must be addressed in practical applications:
- The hollow ratio must be carefully controlled to ensure that the load-bearing capacity is not compromised
- The welding quality between the inner and outer tubes must be ensured to prevent premature weld failure
- The concrete placement procedure must be adapted to accommodate the sandwich configuration, ensuring complete fill of the annular space without voids
- The inner tube orientation should be selected based on the desired lateral strain behavior and load-bearing capacity
From a welding and fabrication perspective, the hollow sandwich CFST column requires more complex fabrication procedures compared to solid CFST columns. The inner tube must be precisely positioned within the outer tube, and the connection between the inner and outer tubes must be designed to ensure proper load transfer. The weld quality at the inner-outer tube interface is critical and should be verified through non-destructive testing.
Study Insights and Reflections
This research demonstrates that the hollow sandwich CFST column concept can achieve load-bearing capacities comparable to or even exceeding those of solid CFST columns, provided that the hollow ratio is kept within appropriate limits. The consistent failure mode across all specimen types, characterized by outer tube weld cracking and inner tube inward buckling, highlights the importance of weld quality and inner tube stability in the design of sandwich CFST columns. The finding that the square-enclosing-square configuration outperforms the square-enclosing-circular configuration in terms of lateral strain control is particularly valuable for seismic design applications where ductility and energy dissipation are critical. The good agreement between finite element analysis and experimental results confirms the reliability of numerical methods for the design and optimization of hollow sandwich CFST columns. This research contributes to the ongoing development of innovative and efficient structural systems for modern construction.