Zhuojin Pipe Fitting Co., LtdBending-Torsion Performance of Circular CFRP-Steel Tube Concrete Members
Overview and Research Motivation
The paper by Wang Qingli, Peng Kuan, and Shao Yongbo (2022) investigates the bending-torsion behaviour of circular cross-section CFRP-steel tube concrete (CFRP-ST/C) members through both experimental testing and finite element analysis. Nine specimens were designed and tested under combined bending and torsion loading. The study examines torque-rotation (T-θ) and shear stress-shear strain (τ-γ) curve characteristics, validates finite element simulations against experimental results, and conducts parametric analyses on CFRP layer count, material strength, bending moment ratio, and steel ratio. A torsional bearing capacity expression was proposed and validated with a mean ratio of 0.998 against experimental results.
This research is particularly relevant to pipe engineers because the steel tube serves as both a structural component and a permanent formwork for the concrete, while the CFRP wrapping provides additional confinement and torsional resistance. The interaction between these three materials under combined loading is complex and demands careful attention to fabrication quality at every interface.
Experimental and Analytical Results
The torque-rotation curves exhibited a linear elastic stage followed by a yield plateau and eventual failure, with the CFRP layers delaying the onset of steel tube local buckling and enhancing post-peak ductility. The shear stress-shear strain curves demonstrated the progressive mobilisation of shear resistance from the concrete core, the steel tube, and the CFRP layers in sequence.
| Parameter | Effect on Torsional Capacity | Effect on Ductility |
|---|---|---|
| CFRP layer count (1–3 layers) | Increases capacity proportionally | Moderate improvement |
| Steel tube wall thickness | Increases capacity significantly | Substantial improvement |
| Concrete compressive strength | Moderate increase in capacity | Slight improvement |
| Bending moment ratio (M/Mmax) | Higher ratio reduces torsional capacity | Accelerates failure |
| Steel ratio (ρs) | Increases capacity with diminishing returns | Improves post-peak behaviour |
The finite element model, built in ABAQUS using cohesive elements to simulate the CFRP-concrete and CFRP-steel interfaces, reproduced the experimental T-θ curves with good accuracy. The proposed torsional capacity formula achieved a mean ratio of 0.998 (experimental) and 0.973 (finite element) with standard deviations of 0.038 and 0.051 respectively, demonstrating high predictive reliability.
Fabrication and Welding Considerations for CFRP-ST/C Members
The steel tube in CFRP-ST/C members is typically a seamless or longitudinally welded circular tube. For torsion-critical applications, the tube should be manufactured to ensure uniform wall thickness and circularity, as eccentricity or ovality introduces bending-torsion coupling that reduces effective torsional capacity. The longitudinal weld, if present, should be ground flush with the tube surface to facilitate uniform CFRP wrapping and to prevent stress concentrations at the CFRP-tube interface.
CFRP wrapping quality is critical. The CFRP sheets must be applied with proper surface preparation (cleaning, shot-blasting, or grinding) to ensure adequate bond strength. The resin system must be compatible with both the CFRP and the steel tube surface. Defects such as voids, wrinkles, or incomplete impregnation in the CFRP layers can significantly reduce confinement effectiveness. Quality control should include visual inspection, ultrasonic testing for void detection, and pull-off tests for bond strength verification.
The concrete filling process for CFRP-ST/C members requires special attention. The CFRP layers are vulnerable to damage during concrete pumping, so protective measures such as internal formwork or controlled filling rates are necessary. The bond between concrete and the steel tube inner surface, as well as between concrete and CFRP (if CFRP is applied internally), must be ensured through appropriate surface treatment.
Analytical Formula and Design Implications
The proposed torsional capacity formula integrates the contributions of the concrete core, the steel tube, and the CFRP layers using a unified stress-strain framework. The formula accounts for the interaction between bending and torsion through a bending moment ratio parameter, which effectively captures the reduction in torsional capacity as bending demand increases. This interaction effect is well-established in reinforced concrete design but is less straightforward in composite members due to the different failure modes of each constituent.
The high accuracy of the formula (mean ratio of 0.998) suggests that the underlying assumptions regarding the stress distribution and failure criteria are well-calibrated. However, engineers should note that the formula is based on a limited database of nine experimental specimens and should be supplemented with additional testing for applications involving unusual geometric proportions or material combinations.
From a pipe manufacturing perspective, the study underscores the importance of tube geometry control. Variations in diameter, wall thickness, and circularity directly affect the torsional section properties (St. Venant torsion constant J and warping constant Iw). For circular tubes, J = πD³t/4, so a 5% variation in diameter produces a 15% variation in torsional stiffness. Tight dimensional tolerances are therefore essential for torsion-critical applications.
This research contributes valuable data and analytical tools for the design of composite members subjected to combined bending and torsion, a loading condition commonly encountered in spiral staircases, helical ramps, and certain industrial equipment supports. The integration of CFRP reinforcement with steel tube concrete offers a practical solution for enhancing torsional capacity without increasing member size, making it attractive for retrofit applications where space is constrained.