Impact Resistance Performance of Circular Steel Tube Concrete Composite Members Under Transverse Drop Hammer Loading

Literature Overview

This paper by Hu Changming and Han Linhai (2016), published in the China Civil Engineering Journal, presents a comprehensive experimental study on the impact resistance of circular steel tube concrete (CFST) composite members. The research is funded by the National Natural Science Foundation of China (51378290) and conducted at Tsinghua University. The study involves drop hammer impact tests on CFST composite specimens, comparing their failure modes, overall deformation, and local deformation characteristics with those of conventional CFST specimens and reinforced concrete specimens under transverse impact loading. The research provides quantitative analysis of impact force, axial force, and mid-span deflection time-history curves, examining the effects of impact energy, axial preload, and steel tube thickness on structural response.

Core Experimental Findings and Failure Modes

The most significant finding of this study is that the outer reinforced concrete component and the core CFST component work in coordinated complementarity under transverse impact loading, resulting in excellent impact resistance performance. This composite action is fundamentally different from the behavior of either component acting alone. The outer reinforced concrete layer provides additional mass and energy absorption capacity, while the core CFST provides high confinement strength and ductility. The two components share the impact load through interfacial shear transfer, creating a synergistic effect that exceeds the sum of their individual contributions.

Parameter Influence on Impact Response

The time-history curves of impact force exhibit a characteristic pattern: a rapid initial peak followed by a plateau region and then a gradual decline. The peak impact force is governed primarily by the dynamic stiffness of the specimen, which is a function of the steel tube thickness and the concrete compressive strength. The plateau region represents the plastic deformation phase, where energy is dissipated through yielding of the steel tube and crushing of the concrete. The impact duration is inversely related to the dynamic stiffness, with thicker steel tubes producing shorter impact durations due to their higher stiffness.

Welding and Steel Tube Manufacturing Considerations

The impact resistance of CFST composite members is intimately related to the quality of the steel tube manufacturing and welding processes. The steel tube in a CFST member is typically manufactured from seamless pipe or longitudinally welded pipe. For impact applications, the following welding and manufacturing considerations are critical:

1. Steel tube grade selection: The steel grade should have adequate Charpy V-notch impact energy at the expected service temperature. For impact applications, grades such as Q355D or Q390D (per GB/T 1591) with minimum impact energy of 34 J at 0°C are recommended.
2. Weld process selection: For the longitudinal weld of the steel tube, submerged arc welding (SAW) or narrow-gap welding is preferred over ERW for thick-walled tubes, as SAW produces a more uniform weld metal with consistent impact toughness throughout the weld cross-section.
3. Weld post-treatment: Post-weld heat treatment (PWHT) may be required to relieve residual stresses in the weld and heat-affected zone, particularly for tubes with wall thickness exceeding 20 mm. The PWHT temperature should be controlled between 580°C and 620°C with a holding time of 1 hour per 25 mm of thickness.
4. Surface quality: The external surface of the steel tube should be free of welding spatter, undercut, and geometric irregularities, as these features can act as stress concentrators under impact loading and initiate cracks.

Steel Tube Specifications for Impact-Resistant CFST Members

Comparison with Conventional Members

The study's comparison of CFST composite specimens with plain CFST and reinforced concrete specimens reveals important structural engineering insights. The CFST composite specimen exhibits a failure mode that combines the ductility of the CFST core with the energy absorption capacity of the outer reinforced concrete layer. In contrast, the plain CFST specimen fails primarily through local buckling of the steel tube wall, with limited energy absorption beyond the initial yield. The reinforced concrete specimen fails through brittle concrete crushing with limited ductility.

This comparison has direct implications for the design of impact-resistant structures such as protective barriers, blast walls, and equipment housings. The CFST composite configuration offers a superior combination of strength, ductility, and energy absorption, making it an attractive option for applications where impact or blast resistance is a design requirement.

Key Questions and Reflections

The study raises an important question regarding the interface behavior between the outer reinforced concrete layer and the core CFST component under impact loading. The interfacial shear strength governs the load transfer between the two components, and this strength can be affected by the concrete mix design, surface preparation of the steel tube, and the presence of shear connectors. The study does not explicitly investigate the effect of interface treatment on impact performance, which represents a significant gap in the current understanding.

Another reflection concerns the scalability of the experimental findings. The drop hammer impact tests were conducted on laboratory-scale specimens, and the transition from laboratory to full-scale structural members involves size effects that can alter the failure mode and energy absorption capacity. The dynamic fracture toughness of concrete, which is size-dependent, may lead to different failure patterns in full-scale members compared to the laboratory specimens.

Study Insights and Implications

This research provides valuable experimental data on the impact resistance of CFST composite members, demonstrating the synergistic benefits of combining a CFST core with an outer reinforced concrete layer. For welding and pipe fitting engineers, the key takeaway is that the steel tube manufacturing quality and weld integrity are fundamental prerequisites for achieving the full impact resistance potential of the composite member. Any defect in the steel tube or its welds can compromise the dynamic response and lead to premature failure. The study's findings should inform the specification of steel tube grades, welding procedures, and quality control requirements for impact-resistant CFST applications in protective structures and equipment design.