Bending Mechanical Behaviour of Curved Stainless Steel Pipe Concrete Members

Literature Overview

This study by Zheng Lianqiong et al. (2023), published in Industrial Construction (Vol. 53, No. 8, pp. 118-126), investigates the bending performance of curved stainless steel pipe concrete (SSC-CFST) members under flexural loading. The research was supported by the National Natural Science Foundation of China (Grant 51308119) and the Fujian Provincial Natural Science Foundation (Grant 2020J01887). Five specimens were tested: three curved SSC-CFST members, one straight SSC-CFST member, and one curved hollow stainless steel tube member. The primary variables were initial curvature (u₀ = 0 to 120 mm) and whether the tube was filled with concrete.

Core Technical Findings

The experimental programme reveals several important mechanical behaviours that deserve careful engineering attention:

• Initial curvature effect on stiffness and capacity: As the initial curvature of the member increases, both the bending stiffness and bending capacity of the SSC-CFST member increase. This is counter-intuitive at first glance, but can be explained by the fact that the initial curvature effectively reduces the effective span of the member under three-point or four-point bending, thereby increasing the apparent stiffness and capacity.
• Concrete infill effect: Compared with the corresponding hollow stainless steel tube member, filling the tube with concrete significantly improves both bending capacity and bending stiffness. This confirms the well-known confinement and composite action effect in CFST members.
• Comparison with carbon steel CFST: The curved SSC-CFST members show bending capacity improvements within 10% and bending stiffness reductions within 15% compared with corresponding carbon steel CFST members. The capacity improvement is attributed to the higher yield strength and strain-hardening capacity of stainless steel, while the stiffness reduction is due to the lower elastic modulus of stainless steel relative to carbon steel.

Finite Element Modelling and Analytical Development

The authors employed ABAQUS software to establish finite element (FE) models that reproduced the load-midspan deflection curves and the full bending process of the curved SSC-CFST members. The FE models were validated against experimental results and showed good agreement.

A notable contribution of this work is the derivation of an analytical expression for the elastic bending stiffness of curved SSC-CFST members based on an elastic stiffness model. The expression incorporates the initial curvature parameter and demonstrates good agreement with test results. This analytical formulation is significant because it provides a practical design tool that avoids the need for full FE analysis in routine design.

Engineering Practice Implications

From a pipe manufacturing and structural engineering perspective, this study has several important implications:

1. Material selection: Stainless steel pipe concrete members offer enhanced bending capacity compared with carbon steel counterparts, making them attractive for applications where corrosion resistance and fatigue performance are critical. The 10% capacity improvement is meaningful for design optimisation.
2. Geometric considerations: The beneficial effect of initial curvature on bending performance suggests that pre-curved pipe geometries could be deliberately employed in certain structural applications, such as arch-type bridge components or curved structural shells, to improve load-bearing efficiency.
3. Design code applicability: The study concludes that existing code provisions for calculating the ultimate bending capacity of straight CFST members, combined with the corrected bending stiffness expression, can be applied with reasonable accuracy to curved SSC-CFST members. This is a practical finding that reduces the need for code modifications.

Key Reflections and Critical Analysis

The finding that initial curvature increases bending stiffness and capacity warrants careful interpretation. In engineering practice, initial curvature is often considered a defect or an unfavourable condition. However, in this specific bending test configuration, the initial curvature effectively shortens the unsupported length, leading to higher apparent stiffness. This effect would not necessarily translate to all loading configurations, and engineers should exercise caution when extrapolating these results to other structural scenarios.

The 15% stiffness reduction of stainless steel CFST members relative to carbon steel CFST members is a critical design consideration. In serviceability limit state design, where deflection limits govern, this reduction may require larger member dimensions or additional stiffness measures. The higher cost of stainless steel pipe must be weighed against the benefits of corrosion resistance and enhanced capacity.

The analytical stiffness expression derived in this study represents a valuable contribution to the design methodology. However, its applicability beyond the tested parameter range should be verified through additional parametric studies or experimental validation.

Summary

This study provides valuable experimental and analytical data on the bending behaviour of curved stainless steel pipe concrete members. The key findings—that initial curvature enhances bending performance, concrete infill significantly improves capacity and stiffness, and stainless steel offers modest capacity advantages over carbon steel—offer practical guidance for structural engineers. The derived analytical stiffness expression is particularly useful for design applications. However, engineers should carefully consider the specific loading configurations and serviceability requirements when applying these results to real structures.