Impact Resistance of Circular Steel Tube Concrete Members with Internal Circular Steel Tube

Literature Overview

This paper by Shi Yanli, He Jiaxing, Wang Wenda, Xian Wei, and Wang Rui, published in "Vibration and Shock" (Vol. 38, No. 9, 2019, pp. 123-132), investigates the impact resistance of a novel composite structural member: a circular steel tube concrete (CSTC) column with a concentrically placed internal circular steel tube, where concrete is cast on both the inner and outer sides of the inner tube. The study employs finite element analysis using ABAQUS to simulate low-speed lateral impact scenarios and validates the numerical model against existing experimental data from steel tube concrete and hollow sandwich steel tube concrete specimens. The research is supported by the National Natural Science Foundation of China (Grants 51768038, 51268035) and the Gansu Provincial University Collaborative Innovation Team Program (2018C-08).

Core Technical Points

The composite member described in this study represents an evolution of the hollow sandwich steel tube concrete concept, where the internal steel tube serves dual purposes: it provides additional structural confinement and acts as a conduit for utilities or service pipes. The key innovation lies in the concentric arrangement of two circular steel tubes with concrete infill on both sides, creating a layered composite cross-section that distributes impact energy across multiple structural layers.

The numerical simulation approach focuses on low-speed lateral impact, which is a critical scenario for bridge columns, building columns in urban environments, and industrial facilities where vehicle collisions or falling objects are credible threats. The model was validated by comparing numerical results with experimental data from two distinct specimen types: conventional steel tube concrete columns and hollow sandwich steel tube concrete specimens. This dual validation strategy strengthens confidence in the model's predictive capability for the novel internal-tube configuration.

Dynamic Response and Failure Modes

The full dynamic response process under lateral impact was analyzed, capturing the evolution of stress waves, local buckling initiation, and progressive damage accumulation. Typical failure modes include local denting of the outer steel tube at the impact point, progressive crushing of the concrete core, and potential internal tube deformation when the impact energy is sufficiently high. The energy absorption mechanism involves both plastic deformation of the steel tubes and crushing of the concrete layers, with the internal tube contributing to energy dissipation through its own plastic deformation and by confining the inner concrete layer.

Parameter Analysis Results

The parametric study identified several key factors influencing impact resistance performance:

The finding that concrete strength has minimal influence on impact resistance is particularly noteworthy from a materials engineering perspective. This suggests that under low-speed impact conditions, the ductile steel tubes dominate the energy absorption mechanism, and the concrete primarily serves as a confinement medium rather than a primary load-bearing component during the impact event. This insight has significant implications for material selection and cost optimization in impact-resistant structural design.

Process and Engineering Practice Integration

From a steel pipe manufacturing and welding quality control standpoint, the construction of such composite members imposes specific requirements on the steel tubes used. The outer steel tube must meet stringent dimensional tolerances and surface quality requirements, as any local defect—such as a weld seam imperfection in an HFW or LSAW pipe—could serve as a stress concentration site during impact loading. The internal steel tube, while potentially of smaller diameter, must also maintain geometric accuracy to ensure uniform concrete thickness on both sides.

The welding of end plates, stiffeners, and connection details to these composite columns must be designed with attention to impact load paths. Standard butt-welded connections (per ASME B31.3 or GB/T 20801) may require enhanced inspection protocols when used in impact-prone locations. The heat-affected zone (HAZ) properties of the welds should be verified through hardness mapping and microstructural examination to ensure adequate ductility and toughness at low temperatures, particularly if the structure is located in cold regions.

The diameter ratio recommendation of 0.6 to 0.8 provides clear guidance for pipe selection. For example, if the outer tube has an outer diameter of 600 mm, the internal tube should have an outer diameter between 360 mm and 480 mm. This ratio range ensures sufficient concrete thickness on both sides to provide adequate confinement while maintaining a reasonable steel ratio. From a fabrication standpoint, this means that pipe suppliers must be able to produce tubes within specific diameter ranges with controlled ovality and wall thickness tolerances.

Key Questions and Reflections

The study raises several important questions for further investigation. First, the analysis is limited to low-speed impact, but high-speed impact scenarios (such as vehicle collisions at highway speeds) may involve different failure mechanisms, including strain-rate effects on both steel and concrete materials. The strain-rate sensitivity of high-strength steel tubes under rapid loading conditions should be characterized through split Hopkinson pressure bar tests and incorporated into future design codes.

Second, the study does not address the effect of corrosion on long-term impact resistance. In aggressive environments, such as coastal or industrial areas, the outer steel tube may suffer from corrosion that reduces its wall thickness and compromises its impact resistance. Corrosion-resistant line pipe (CRA) materials or protective coatings should be considered for such applications, and the interaction between corrosion damage and impact response warrants further study.

Third, the numerical model validation relies on experimental data from other researchers, which raises questions about the consistency of material properties, boundary conditions, and impact energy levels between the referenced experiments and the parametric study. Future work should include dedicated experimental programs specifically designed for the internal-tube configuration to provide direct validation data.

Study Insights and Implications

This research contributes meaningfully to the understanding of impact-resistant composite structural systems. The finding that the diameter ratio of 0.6 to 0.8 provides optimal impact performance offers a practical design guideline that can be directly applied in engineering practice. The minimal influence of concrete strength on impact resistance simplifies material specification and can lead to cost savings, as lower-grade concrete can be used without compromising impact performance.

For steel pipe suppliers and fabricators, this work highlights the importance of maintaining high quality in pipe manufacturing, particularly in terms of dimensional accuracy, surface integrity, and weld quality. Any manufacturing defect that could initiate localized failure under impact loading should be identified and addressed through rigorous non-destructive testing protocols, including ultrasonic testing (UT), magnetic particle testing (MT), and visual examination (VT) of all welds and pipe surfaces.

Conclusion

The study by Shi et al. provides valuable insights into the impact resistance of a novel composite member combining internal and external circular steel tubes with concrete infill. The parametric analysis identifies steel tube strength, steel ratio, end constraints, and diameter ratio as the primary design parameters, while demonstrating that concrete strength plays a secondary role. These findings have direct implications for the design, fabrication, and quality control of impact-resistant steel tube concrete structures, particularly in applications where vehicle impact or falling object threats are credible. The recommended diameter ratio range of 0.6 to 0.8 and the emphasis on outer steel tube strength provide actionable guidance for engineers and pipe manufacturers working on such projects.