Lateral Impact Behaviour of Square-Tube-in-Circular Hollow Sandwich CFST Composite Members

Literature Overview

This paper, published in the China Civil Engineering Journal (Vol. 52, No. 12, 2019, pp. 11-21) by Shi Yanli, Xian Wei, Wang Rui, and Wang Wenda from Lanzhou University of Technology and Taiyuan University of Technology, presents an experimental investigation into the dynamic response of square-tube-in-circular hollow sandwich concrete-filled steel tube (CFST) composite members subjected to lateral impact loading. The study was supported by the National Natural Science Foundation of China (Grant No. 51768038), the Gansu Provincial University Collaborative Innovation Team (2018C-08), and the Lanzhou Science and Technology Plan Project (2019-1-61). Eight specimens with varying hollow rates were tested using a drop hammer impact method, examining the influence of impact height, boundary constraints, and axial compression ratio on the dynamic behaviour of these hybrid members.

Core Technical Findings

The research addresses a critical gap in structural engineering concerning the impact resistance of innovative composite cross-sections. The square-tube-in-circular hollow sandwich CFST member combines the advantages of a square outer steel tube, a circular inner steel tube, and a hollow core, creating a multi-layered composite system that distributes impact energy across multiple structural interfaces. The hollow rate of 0.69 was found to produce predominantly local denting deformation with minimal overall member displacement, which is a significant finding for engineers designing impact-resistant structural systems.

The following table summarises the key experimental parameters and their effects:

Interpretation of Impact Force Time-History Curves

One of the most valuable aspects of this study is the detailed characterisation of impact force time-history curves. The authors identified distinct phases in the impact process: an initial loading phase, a plateau phase, and a decay phase. The axial compression ratio was found to be a critical factor governing the duration and magnitude of the plateau phase. As the axial compression ratio increases, the plateau phase progressively shortens until it vanishes entirely, indicating that the member transitions from a ductile energy-absorption mode to a more brittle response under high pre-compression conditions.

From a welding and fabrication standpoint, this finding has direct implications for the design of composite structural members that may be subjected to both axial and lateral impact loads simultaneously, such as those found in bridge piers, offshore platforms, and industrial facilities. The presence of a significant axial pre-load reduces the effective energy-absorption capacity of the composite system, which must be accounted for in both the design phase and the quality assurance of welded connections between the square outer tube, the circular inner tube, and the concrete core.

Engineering Practice Implications

For engineers involved in the fabrication of such hybrid composite members, several practical considerations emerge from this study. First, the local denting behaviour at a hollow rate of 0.69 suggests that the hollow core contributes significantly to energy absorption through controlled deformation of the steel tubes. This means that weld quality at the junctions between the square outer tube and the circular inner tube becomes critical, as these welds must withstand significant plastic deformation without cracking during impact events.

Second, the linear relationship between impact height and local dent depth indicates that the composite system behaves predictably within the tested range, which is reassuring for engineers who need to extrapolate design parameters. However, the non-linear behaviour observed at higher axial compression ratios warns against assuming linearity across the full range of service conditions.

Third, the influence of boundary constraints on residual deflection and impact force plateau values highlights the importance of accurate boundary condition modelling in both experimental testing and finite element analysis. Engineers must ensure that the support conditions in their analytical models faithfully replicate the actual structural constraints, particularly at welded or bolted connections.

Key Questions and Reflections

Several important questions arise from this research that warrant further investigation. The study focuses primarily on quasi-static impact with a drop hammer, but real-world impact events such as vehicle collisions, falling objects, or blast loads may involve different strain rate regimes that could alter the material behaviour of both the steel tubes and the concrete core. Additionally, the study does not address the effect of steel grade on the dynamic response, which is particularly relevant given the wide range of structural steel grades available for CFST applications.

Another area requiring further study is the long-term durability of the composite system after impact damage. The local denting observed in the tests may compromise the corrosion protection of the inner surfaces, particularly at the hollow core interfaces where moisture ingress could lead to accelerated degradation. For engineers specifying such members in aggressive environments, additional consideration should be given to the post-impact integrity and repairability of the composite system.

Study Insights and Implications

This research contributes meaningfully to the understanding of impact-resistant composite structural systems. The square-tube-in-circular hollow sandwich CFST member represents an innovative approach to combining the torsional rigidity of a square outer section with the efficient load-bearing capacity of a circular inner section, while the hollow core provides additional energy absorption capacity through controlled deformation. The findings suggest that this hybrid configuration offers a viable alternative to conventional CFST members in applications where lateral impact resistance is a primary design consideration.

For the steel pipe and welding industry, this study underscores the importance of developing fabrication and welding procedures that can accommodate the complex geometry of multi-tube composite members. The welds between dissimilar cross-sections (square to circular) require careful attention to fit-up, welding sequence, and post-weld heat treatment to ensure adequate ductility and toughness. Engineers should also consider the residual stress distribution introduced during fabrication, as these residual stresses may interact with impact-induced dynamic stresses to reduce the effective impact resistance of the member.

In conclusion, the experimental data presented in this paper provides a solid foundation for the design and fabrication of square-tube-in-circular hollow sandwich CFST members in impact-prone environments, and the identified relationships between impact parameters and dynamic response offer practical guidance for engineers seeking to optimise both the structural performance and the fabrication quality of such innovative composite systems.