Zhuojin Pipe Fitting Co., LtdSimplified Full-Process Torsion Calculation for Thin-Walled Centrifugal Concrete-Filled Steel Tubes
Literature Overview
This study by Jin Weilang, Qu Chen, Fu Jun, and Zhang Li, published in the Journal of Zhejiang University (Engineering Science) in 2003 (Vol. 37, No. 1, pp. 5-9), addresses the torsional behavior of thin-walled centrifugal concrete-filled steel tube (CFST) members. Based on the analysis of 22 experimental specimens, the authors proposed a three-stage concept for torsional failure and developed simplified formulas for elastic and plastic torsional moments. The combined modulus expression was formulated as a function of the steel tube hollow rate, steel ratio, and slenderness ratio. The combined stiffness and deformation calculations for each torsional stage were established using the direct bilinear method, consistent with international research and domestic codes.
Core Technical Points and Analysis
Three-Stage Torsional Failure Concept
The study identifies three distinct stages in the torsional response of thin-walled centrifugal CFST members: an elastic stage, an elasto-plastic stage, and a fully plastic stage. In the elastic stage, both the steel tube and the concrete core respond elastically to the applied torque, and the combined torsional stiffness is governed by the elastic moduli and geometric properties of both materials. As the torque increases beyond the elastic limit of the steel tube, the elasto-plastic stage begins, with plastic deformation developing in the steel tube while the concrete remains elastic. In the fully plastic stage, the steel tube has yielded throughout its cross-section, and the concrete core contributes additional resistance through shear strength.
| Stage | Steel Tube Behavior | Concrete Core Behavior | Combined Stiffness |
|---|---|---|---|
| Elastic | Elastic response | Elastic response | Sum of elastic torsional stiffnesses |
| Elasto-plastic | Partial plastic yielding | Elastic response | Reduced from elastic stage |
| Fully plastic | Fully yielded | Shear resistance | Governed by plastic torsional capacity |
Combined Modulus Expression
The combined modulus was expressed as a function of three key parameters: the hollow rate of the steel tube, the steel ratio, and the slenderness ratio. The hollow rate, defined as the ratio of the inner diameter to the outer diameter of the steel tube, directly affects the torsional resistance of the steel tube. A higher hollow rate reduces the tube wall thickness relative to the diameter, decreasing the torsional stiffness and strength. The steel ratio, representing the ratio of steel tube area to total cross-sectional area, determines the relative contribution of the steel and concrete to the overall torsional resistance. The slenderness ratio influences the buckling behavior of the steel tube under torsional loading.
Simplified Calculation Formulas
The elastic torsional moment formula was derived based on the existing code provisions for steel tubes and concrete, combined through the composite action model. The plastic torsional moment formula accounts for the full plastic utilization of the steel tube and the shear contribution of the concrete core. The direct bilinear method was adopted for the combined stiffness calculation, which provides a piecewise linear approximation of the torque-deformation relationship. This approach is consistent with the bilinear models used in international research and domestic codes, facilitating direct comparison and integration into existing design frameworks.
Validation Against Experimental Data
The simplified calculation results were compared against the experimental data from 22 specimens and demonstrated good agreement. This validation provides confidence in the applicability of the proposed formulas for practical design purposes. The consistency across a range of geometric and material parameters suggests that the formulas have reasonable generality and can be applied to various thin-walled centrifugal CFST configurations.
Integration with Engineering Practice
From a manufacturing perspective, thin-walled centrifugal CFST members are produced through a centrifugal casting process in which the concrete is cast into a rotating steel tube form. This process produces a dense, well-compacted concrete core with minimal voids, which is advantageous for torsional performance. The centrifugal force during casting ensures intimate contact between the concrete and the steel tube inner surface, promoting strong bond development. Engineers should ensure that the centrifugal casting parameters, including rotation speed, pouring rate, and vibration, are carefully controlled to achieve uniform concrete density and consistent bond quality.
The welding of end connections to centrifugal CFST members requires special attention due to the thin wall thickness of the steel tube. Thin-walled tubes are susceptible to warping and distortion during welding, which can compromise the geometric accuracy of the member. Low-heat-input welding processes, such as gas tungsten arc welding (GTAW), should be used to minimize thermal distortion. Preheating and interpass temperature control are essential to prevent cold cracking, particularly in higher-strength steel grades. Post-weld inspection using ultrasonic testing (UT) should be performed to detect any lack of fusion or porosity at the weld interface.
Key Reflections and Study Insights
This study makes a valuable contribution to the torsional design of thin-walled centrifugal CFST members, which are increasingly used in applications such as transmission towers, wind turbine towers, and marine structures where torsional loading is significant. The three-stage torsional failure concept and the simplified calculation formulas provide a practical framework for engineers to evaluate the torsional capacity of these members. However, the study is limited to thin-walled configurations, and the applicability to thick-walled CFST members may require modification. The direct bilinear method, while convenient, may not fully capture the nonlinear behavior of the concrete core under torsional shear. Future research should investigate the combined effect of torsion with axial compression and bending, as well as the fatigue behavior under cyclic torsional loading. Engineers applying the proposed formulas should verify the results with detailed finite element analysis for critical applications and should ensure that the centrifugal casting quality is rigorously controlled to achieve the assumed bond strength and concrete density.