Impact Resistance of Circular Concrete-Filled Steel Tube Members with Internal Steel Tubes

Literature Overview

The paper by Shi Yanli and colleagues (2019, Vibration and Shock, Vol. 38, No. 9, pp. 123-132) investigates a novel composite structural member: a circular concrete-filled steel tube (CFST) with an additional concentric inner steel tube, where concrete is cast both inside and outside the inner tube. This configuration creates a sandwich-type CFST section that leverages the confinement effect of two steel shells to enhance impact resistance. The study employs ABAQUS finite element simulation validated against existing experimental data on conventional CFST and hollow sandwich CFST specimens subjected to lateral low-speed impact.

Core Technical Content and Methodology

The research addresses a critical engineering challenge: improving the crashworthiness of tubular members used in bridge piers, marine structures, and industrial facilities exposed to accidental impacts. The sandwich CFST concept introduces an internal steel tube that partitions the concrete core into two annular zones, each independently confined by its adjacent steel shell. The finite element model employs shell elements for the steel tubes and solid elements for the concrete, with appropriate material models including strain-rate-dependent concrete behavior and von Mises plasticity for steel.

Validation was performed against experimental data from prior researchers on both conventional CFST and hollow sandwich CFST specimens under lateral impact loading. The agreement between simulated and experimental load-displacement curves and failure modes confirmed the model's reliability before parametric studies were conducted.

Key Findings from Parametric Analysis

The parametric study systematically varied several design parameters to identify their influence on impact performance:

Interpretation of Results

The finding that concrete strength has negligible effect on impact performance is particularly instructive. Under low-speed lateral impact, the dominant energy dissipation mechanism is steel tube plastic deformation and local buckling rather than concrete crushing. The steel tubes absorb energy through membrane stretching, bending, and progressive local buckling, while concrete primarily serves as a core material that prevents inward collapse and distributes contact forces. This insight has direct implications for material selection in impact-resistant design.

The optimal inner-to-outer diameter ratio of 0.6 to 0.8 represents a balance between two competing effects. A larger inner tube (ratio approaching 1.0) provides greater confinement but leaves insufficient concrete thickness for effective load transfer between shells. A smaller inner tube (ratio below 0.5) reduces the number of confinement layers and the beneficial interaction between inner and outer concrete zones. The optimal range ensures adequate concrete annular thickness while maintaining meaningful dual-confinement action.

Engineering Practice Implications

From a steel pipe manufacturing perspective, this research has several practical implications. The inner tube geometry requires precise concentricity control during fabrication, typically achieved through internal jigs or guide rings positioned at regular intervals along the member length. The inner-to-outer diameter ratio of 0.6 to 0.8 translates to specific dimensional tolerances that must be maintained throughout the fabrication process.

For welded pipe construction, the inner tube fabrication demands careful weld quality control since the inner surface welds are difficult to inspect. Ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) may be required to verify weld integrity in this configuration. The impact loading scenario also demands attention to weld heat-affected zone (HAZ) properties, as the strain rates during impact can activate dynamic embrittlement mechanisms in HAZ regions.

The finding regarding end support conditions suggests that connection design is as important as section design for impact performance. In practical applications, moment-resisting connections at column bases should be designed to provide rotational restraint, which effectively increases the effective length for impact energy dissipation.

Study Insights and Reflections

This research demonstrates that structural topology optimization can achieve significant performance improvements without proportionally increasing material consumption. The sandwich CFST concept achieves better impact performance than a solid CFST of equivalent outer dimensions, suggesting that material efficiency is improved through intelligent section design rather than simply increasing steel thickness. The parametric study methodology, while thorough, could be extended by incorporating explicit strain-rate effects in the steel material model and by examining the influence of tube length and slenderness ratio on impact behavior.