Zhuojin Pipe Fitting Co., LtdMechanical Properties of Hollow CFST Composite Columns Under Eccentric Compression
Literature Overview
This paper by Ren Qingxin, Ding Jinan, Li Minglun, and Lou Heqing, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2021 (Vol. 37, Issue 1), investigates the mechanical behavior of hollow concrete-filled steel tube composite columns under eccentric compression loading. The research is supported by the National Natural Science Foundation of China (Grant 51678373) and combines finite element analysis with theoretical modeling to propose a simplified bearing capacity formula.
Structural Configuration and Test Parameters
The hollow CFST composite column consists of an outer steel tube, an inner steel tube, and concrete filling the annular space between the two tubes. This configuration offers the advantage of reduced self-weight compared to solid CFST columns while maintaining adequate load-bearing capacity and ductility. The study varies two primary parameters: the outer steel tube diameter (60 mm, 100 mm, and 140 mm) and the eccentricity ratio (0.2 to 0.5).
| Parameter | Range | Effect on Ultimate Load |
|---|---|---|
| Outer tube diameter | 60 mm to 140 mm | 0.4% reduction (60 to 100 mm), 5.8% reduction (60 to 140 mm) |
| Eccentricity ratio | 0.2 to 0.5 | 33% reduction in ultimate bearing capacity |
Working Mechanism and Failure Modes
The paper defines five characteristic points on the load-deflection curve to analyze the progressive failure mechanism of the column. These points correspond to distinct stages of structural response:
- Initial elastic stage: The column behaves linearly under low load levels, with both steel tubes and concrete contributing to stiffness.
- Concrete cracking stage: Microcracks initiate in the concrete under the compressive stress concentration on the loaded side.
- Steel tube yielding stage: The outer tube begins to yield on the compression side, and the load-deflection curve deviates from linearity.
- Plastic deformation stage: The inner tube's confinement effect on the concrete becomes more pronounced as the eccentricity ratio increases.
- Ultimate failure stage: Progressive crushing of concrete and buckling of the steel tube wall lead to the final collapse.
The confinement mechanism is particularly interesting. As the eccentricity ratio increases from 0.2 to 0.5, the concrete's confining action on the inner steel tube gradually improves. This is because the increased bending moment causes the concrete to be more uniformly compressed between the two steel tubes, enhancing the triaxial stress state that improves concrete strength and ductility.
Bearing Capacity Calculation Method
The simplified bearing capacity formula is derived using the superposition theory and limit equilibrium theory. The approach decomposes the total resistance into contributions from the outer tube, inner tube, and confined concrete, then combines these using an interaction formula that accounts for the eccentricity effect.
The calculation results show good agreement with the finite element simulation results, validating the proposed formula. The formula is particularly useful for preliminary design and rapid assessment of hollow CFST composite columns, as it avoids the need for complex numerical analysis while capturing the essential mechanics of the composite action.
Engineering Practice Considerations
From a steel pipe manufacturing perspective, the hollow CFST composite column design places specific requirements on the fabrication of both the outer and inner steel tubes. The inner tube diameter must be precisely controlled to ensure proper concrete placement and curing within the annular space. Welding connections between the tubes and end plates must be designed to accommodate the differential thermal expansion and contraction during the concrete curing process. The wall thickness of both tubes should be selected to provide adequate confinement without excessive material usage.
The study's finding that increasing the outer tube diameter from 60 mm to 140 mm results in only a 5.8% reduction in ultimate load is significant for design optimization. It suggests that the hollow configuration can achieve a favorable strength-to-weight ratio, making it suitable for applications where self-weight reduction is critical, such as high-rise buildings, offshore platforms, and transportation infrastructure.
Study Insights and Implications
The research confirms that hollow CFST composite columns offer a viable alternative to conventional solid CFST columns, particularly when self-weight reduction is a design priority. The simplified bearing capacity formula provides a practical tool for engineers to evaluate these columns without resorting to computationally intensive finite element analysis. However, the formula should be applied with caution outside the parameter range studied, as the confinement mechanism and failure mode may change significantly at very high eccentricity ratios or with different steel tube geometries. The 33% reduction in bearing capacity when the eccentricity ratio increases from 0.2 to 0.5 underscores the sensitivity of these columns to load eccentricity, which must be carefully considered in structural design to ensure adequate safety margins.