Zhuojin Pipe Fitting Co., LtdShear Performance of Stainless Steel Pipe-in-Pipe Concrete Members
Literature Overview
This study by Zhang Jigang, Liu Feifei, and Zhao Tiejun, published in the Journal of Harbin Engineering University (2019, Vol. 40, Issue 7, pp. 1311-1318), investigates the shear performance of stainless steel pipe-in-pipe concrete members, which are designed for marine platform leg applications in severe cold regions where ice loading is a governing design load. Eighteen pipe-in-pipe concrete shear specimens were fabricated and tested to evaluate the influence of outer pipe material, concrete strength, void ratio, and shear span ratio on shear capacity. The research is supported by multiple National Natural Science Foundation grants and the 111 Program, reflecting its significance in offshore engineering structural design.
Experimental Program and Specimen Configuration
The experimental program was designed to isolate and evaluate four key variables affecting shear capacity:
| Parameter | Levels Tested | Rationale |
|---|---|---|
| Outer pipe material | Stainless steel grades | Material strength and ductility variation |
| Concrete strength | Multiple grades | Concrete contribution to shear resistance |
| Void ratio | Multiple levels | Effect of internal hollow space on shear behavior |
| Shear span ratio | Multiple values | Flexural-shear interaction |
The pipe-in-pipe configuration consists of an inner stainless steel pipe and an outer stainless steel pipe with concrete filling the annular space between them. This design provides enhanced confinement of the concrete and improved overall structural performance compared to conventional single-pipe CFST members. The inner pipe also serves as a formwork during construction, eliminating the need for separate formwork materials.
Key Experimental Findings
The study reveals clear trends in shear capacity behavior:
- Void ratio effect: As the void ratio decreases (i.e., more concrete fills the annular space), the shear capacity increases. This is consistent with the understanding that concrete contributes significantly to shear resistance through aggregate interlock and direct shear transfer.
- Concrete strength effect: Higher concrete strength leads to increased shear capacity. The concrete core acts as a strut in the truss analogy model, and stronger concrete provides greater strut capacity.
- Shear span ratio effect: A larger shear span ratio results in lower shear capacity. This is expected, as a larger shear span ratio shifts the failure mode from shear-dominated to flexure-shear interaction, reducing the contribution of concrete to shear resistance.
Empirical Formula and Finite Element Verification
Based on the experimental results, the authors proposed an empirical formula for the shear capacity of pipe-in-pipe concrete members. The formula was subsequently validated through ABAQUS finite element modeling, and the simulation results showed good agreement with experimental data. This dual verification approach—experimental testing combined with numerical simulation—strengthens the reliability of the proposed design methodology.
The finite element modeling approach provides additional insight into the internal behavior of the members that is difficult to obtain from physical testing alone, including stress distribution, strain localization, and progressive failure mechanisms. Engineers should note that the accuracy of such models depends critically on the constitutive models used for both stainless steel and concrete, particularly under multiaxial stress states.
Engineering Practice Considerations
For marine platform applications in ice-prone regions, the pipe-in-pipe concrete configuration offers several advantages over conventional designs:
- The stainless steel material provides excellent corrosion resistance in marine environments, reducing long-term maintenance requirements
- The dual-pipe configuration enhances confinement of the concrete, improving ductility and energy dissipation capacity under cyclic ice loading
- The construction method is relatively simple, as the inner pipe serves as formwork, reducing on-site construction complexity in harsh offshore environments
However, engineers should be mindful that the empirical formula developed in this study is based on a limited number of specimens (18 total). For practical design applications, particularly for critical offshore structures, the formula should be validated against additional test data and used in conjunction with appropriate safety factors. The interaction between ice loading and structural response is complex, and the quasi-static assumptions underlying the empirical formula may not fully capture dynamic ice loading effects.