Bending Mechanical Performance of Curved Stainless Steel Pipe Concrete Members

Literature Overview

This study by Zheng Lianqiong and colleagues from Fujian University of Technology, published in Industrial Construction in 2023 (Vol. 53, No. 8), investigates the flexural behavior of curved stainless steel pipe concrete (SSPC) members. The research is supported by the National Natural Science Foundation of China (Grant 51308119) and the Fujian Provincial Natural Science Foundation (Grant 2020J01887). Five specimens were fabricated and tested: three curved SSPC members, one straight SSPC member, and one curved hollow stainless steel pipe member. The key experimental parameters were initial curvature (u₀ ranging from 0 to 120 mm) and whether the pipe was filled with concrete.

Core Technical Findings

The experimental results reveal several counterintuitive yet physically reasonable behaviors. As the initial curvature increases, both the flexural stiffness and flexural capacity of the curved SSPC members improve. This phenomenon can be attributed to the pre-existing geometric imperfection acting as a beneficial initial deformation that partially offsets the elastic buckling tendency under bending loads. Compared with the corresponding hollow stainless steel pipe member, filling the pipe with concrete substantially enhances both flexural capacity and flexural stiffness.

The authors developed a finite element model using ABAQUS to analyze the full-range load-deflection relationship and compared the behavior with conventional carbon steel pipe concrete members. Based on an elastic stiffness model, a formula was derived to predict the effect of initial curvature on the elastic flexural stiffness of curved SSPC members. The calculated results showed good agreement with experimental data.

Key Comparative Parameters

Interpretation of Technical Points

From a metallurgical and structural engineering perspective, the use of stainless steel pipe as a composite structural element offers inherent advantages in corrosion resistance and ductility. However, the study highlights an important engineering trade-off: while stainless steel provides excellent long-term durability, the flexural stiffness of curved SSPC members is approximately 15% lower than that of conventional carbon steel pipe concrete members. This stiffness reduction is primarily due to the lower elastic modulus of austenitic stainless steels (typically 190–200 GPa for grades such as 304 and 316, compared to 200 GPa for carbon steels) and the nonlinear stress-strain behavior of stainless steels, which exhibit no distinct yield plateau.

The derivation of the elastic stiffness formula incorporating initial curvature represents a valuable contribution. In practice, curved pipes are often used in pipeline systems to accommodate thermal expansion and misalignment. The finding that pre-existing curvature can enhance flexural capacity suggests that engineers should not dismiss curved pipe segments as inferior to straight segments in composite structural applications.

Engineering Practice Considerations

Integration with Engineering Practice

In pipeline engineering, curved stainless steel pipe segments are routinely fabricated using cold bending or hot bending processes. The study's findings have direct relevance to the design of composite structural members in marine environments, chemical processing plants, and other corrosive service conditions where stainless steel piping is preferred. The recommendation that existing codes can be applied with a modified stiffness formula provides engineers with a practical design pathway.

However, from a welding quality control standpoint, the fabrication of curved stainless steel pipe concrete members introduces additional challenges. Cold bending of stainless steel pipe can induce work hardening and residual stresses, particularly in the outer fiber of the bend. These factors may affect the subsequent welding quality of connection joints and the long-term fatigue performance of the member. Engineers should ensure that post-bending stress relief or appropriate heat treatment is considered, and that weld inspection protocols account for the modified microstructure in bent regions.

Key Questions and Reflections

The study raises an important question regarding the interaction between the stainless steel pipe and the infill concrete under bending loads. Unlike carbon steel pipes where the bond strength between steel and concrete is well-established, the bond characteristics of stainless steel pipe concrete interfaces may differ due to the smoother surface finish typical of stainless steel tubes and the potential for galvanic considerations in certain environments. Future research should quantify the bond-slip behavior at the stainless steel-concrete interface under cyclic and sustained loading conditions.

Another reflection concerns the practical limits of initial curvature. While the study demonstrates benefits up to 120 mm of initial deflection, the geometric nonlinearity at higher curvatures may introduce secondary bending moments that could compromise serviceability limits related to deflection and vibration. Engineers adopting this design approach should conduct serviceability checks in addition to ultimate strength verification.

Study Insights and Implications

The research provides a solid foundation for the design of curved stainless steel pipe concrete members, bridging the gap between experimental observations and practical design methodology. The derived stiffness formula, validated against experimental data, offers a tool that can be incorporated into design software and hand calculations. For engineers involved in the design of composite structures in corrosive environments, this study demonstrates that stainless steel pipe concrete members are viable alternatives to carbon steel counterparts, provided that the stiffness reduction is properly accounted for in serviceability design. The work also underscores the importance of geometric imperfections in structural analysis, reminding practitioners that real-world pipe segments are never perfectly straight and that these imperfections can be either detrimental or beneficial depending on the loading configuration.