Calculation of Load-Bearing Capacity of Circular Cross-Section Steel Tube Reinforced Concrete Members

Literature Overview and Research Background

This paper by Wang Tingwei, Gong Jinxin, Wu Zhiliang, and Zhu Xiaoyu, published in the Journal of Waterway and Port Engineering in 2014, presents experimental and numerical investigation of eight circular cross-section steel tube reinforced concrete (STRC) members under axial and eccentric compression. The research was funded by the National Natural Science Foundation of China (Grant No. 51278081) and conducted at Dalian University of Technology and CCCC Third Harbor Engineering Survey and Design Institute. From the perspective of steel pipe engineering, this study is highly relevant because it addresses the interaction between steel pipe and concrete fill, which is a fundamental concern in composite structural design where steel tubes serve as both structural elements and formwork for concrete.

Core Technical Findings and Methodology

The authors employed both finite element method (FEM) and fiber model method for calculating the load-bearing capacity of the tested specimens. The key finding is that FEM results tend to underestimate the actual capacity, while the fiber model method yields predictions that correlate well with experimental results. A simplified calculation method was proposed by treating the steel tube as ordinary reinforcing steel and the concrete as confined concrete within a double-layer steel reinforcement system, using the standard calculation approach for circular cross-section reinforced concrete members.

The experimental specimens were subjected to both axial compression and eccentric compression loading, which is critical for understanding the behavior under realistic structural loading conditions. The specimens represented different configurations of steel tube thickness, concrete strength, and internal reinforcement arrangement, providing a comprehensive dataset for validating analytical models.

Welding and Steel Pipe Engineering Implications

From a steel pipe fabrication and welding standpoint, this research has several important implications. First, the quality of the steel tube itself—its dimensional tolerances, wall thickness uniformity, and surface condition—directly affects the confinement effect on the internal concrete. In practice, seamless steel tubes conforming to GB/T 8163 or GB/T 17395 are commonly specified for such applications, and their manufacturing quality is paramount.

The welding quality at any joints connecting these composite members is equally critical. Longitudinal submerged arc welded (LSAW) tubes or electric resistance welded (ERW) tubes used in composite structures must exhibit full fusion welds with no lack of fusion, porosity, or slag inclusions that could compromise the tube's integrity under confinement stresses. The heat-affected zone (HAZ) of the longitudinal weld must be properly tempered to avoid brittle fracture initiation, particularly in low-temperature environments.

The study's finding that the steel tube acts as a confining element reinforces the importance of maintaining geometric accuracy in steel tube fabrication. Ovality exceeding the tolerances specified in GB/T 17395 can lead to uneven concrete confinement and premature local buckling of the tube wall. Engineers should ensure that incoming steel tubes meet the following minimum requirements:

• Ovality not exceeding 0.5% of the mean diameter for precision tubes
• Wall thickness tolerance within ±5% of nominal value
• Surface free from scale, pits, and cracks that could initiate corrosion or stress concentration

Key Questions and Study Reflections

One question that emerges from this study is whether the simplified calculation method adequately accounts for the actual weld quality at the tube-to-concrete interface. In practice, the bond between steel tube and concrete is influenced by surface roughness, which is itself affected by the tube manufacturing process. Seamless tubes typically exhibit a smoother surface than ERW or LSAW tubes, potentially affecting the frictional bond. This suggests that the simplified method may need adjustment factors depending on the tube manufacturing process.

Another reflection concerns the long-term durability of such composite members. The steel tube, when embedded in concrete, is protected from atmospheric corrosion. However, if the concrete cover is damaged or if chloride ingress occurs from aggressive environments, the steel tube could still corrode. This is particularly relevant for marine and port engineering applications, where CCCC Third Harbor Engineering Survey and Design Institute operates. Corrosion-resistant steel grades or additional protective measures should be considered for such environments.

Engineering Practice Recommendations

For engineers specifying circular STRC members in practice, the following recommendations emerge from this study and from general steel pipe engineering experience:

1. Prefer seamless tubes for critical load-bearing applications where uniform confinement is essential.
2. If welded tubes are used, ensure that the longitudinal weld is subjected to 100% ultrasonic testing (UT) conforming to GB/T 11345, with acceptance criteria per GB/T 2970.
3. The simplified calculation method is suitable for preliminary design and routine checks, but detailed fiber model analysis should be employed for critical or innovative structural configurations.
4. Concrete strength grade should be selected in coordination with the steel tube grade to ensure compatible deformation characteristics and avoid premature yielding of either component.
5. For eccentrically loaded members, attention should be paid to the local buckling behavior of the steel tube wall under combined compressive and bending stresses, which may require stiffening rings or additional internal reinforcement.

Summary

This study provides valuable analytical tools for the design of circular cross-section steel tube reinforced concrete members, bridging the gap between experimental observation and practical engineering calculation. The simplified method proposed by the authors offers a convenient design approach that treats the steel tube as a confining reinforcement layer, which aligns well with the fundamental understanding of steel tube composite behavior. For steel pipe engineers, the key takeaway is that the manufacturing quality of the steel tube—its dimensional accuracy, weld integrity, and surface condition—has a direct and measurable influence on the structural performance of the composite member. Ensuring compliance with relevant steel tube standards and implementing rigorous quality control at the fabrication stage is not merely a regulatory requirement but a structural necessity. The interplay between steel tube geometry, welding quality, and concrete confinement mechanics deserves continued attention in future research, particularly as composite structures find increasing application in bridge, offshore, and industrial construction.