Zhuojin Pipe Fitting Co., LtdLateral Impact Response of Square-in-Circle Hollow Sandwich Steel Tube-Concrete Composite Members
Literature Overview
The study by Shi Yanli and colleagues, published in the China Civil Engineering Journal (2019, Vol. 52, No. 12), presents a comprehensive experimental investigation into the dynamic response of square-in-circle hollow sandwich steel tube-concrete (SC-HSCFST) composite members subjected to lateral impact loading. This research is particularly significant for engineers working in the field of protective structures, bridge columns, and industrial equipment supports where impact resistance is a critical design parameter. The work was supported by the National Natural Science Foundation of China (51768038), the Gansu Provincial University Collaborative Innovation Team (2018C-08), and the Lanzhou Science and Technology Program (2019-1-61). Eight specimens with large hollow ratios were tested under drop-weight impact, with systematic variation of impact height, boundary constraints, and axial compression ratio.
Core Technical Content
Specimen Configuration and Test Parameters
The specimens feature a unique sandwich geometry where a square steel tube encases a circular steel tube, with the annular space filled with concrete. The hollow ratio of 0.69 represents a significant void fraction, which fundamentally alters the impact energy absorption mechanism compared to solid CFST members. The test matrix examined three primary variables: drop hammer impact height (representing impact energy level), boundary constraint conditions (simulating different support scenarios), and axial compression ratio (reflecting combined loading states typical in structural columns).
Key Experimental Findings
| Parameter | Observation | Engineering Implication |
|---|---|---|
| Local deformation | Significant local denting at hollow ratio of 0.69 | Local buckling dominates failure mode |
| Impact height increase | Linear growth in dent depth, residual deflection, and impact duration | Energy absorption scales linearly with impact energy |
| Boundary constraints | Strong influence on residual deflection, force plateau, and impact duration | Support conditions critically affect structural performance |
| Axial compression ratio increase | Force plateau shortens and eventually disappears | Combined loading accelerates failure progression |
| Mid-span deflection rate | Increases with axial compression ratio | Higher pre-stress reduces structural ductility |
Dynamic Response Characteristics
The impact force-time history curves exhibit distinct phases: initial peak force, a plateau phase representing stable energy absorption, and a terminal phase where resistance drops. As the axial compression ratio increases, the plateau phase progressively shortens and ultimately vanishes, indicating that the member transitions from a ductile energy-absorbing behavior to a more brittle failure mode. This finding has profound implications for the design of impact-resistant columns in combined compression and lateral impact scenarios.
Engineering Practice Integration
From a structural engineering perspective, the hollow sandwich configuration offers an attractive balance between weight reduction and impact resistance. The hollow ratio of 0.69 represents a substantial material savings compared to solid CFST members, while the dual-tube geometry provides multiple load paths for impact energy dissipation. However, the local denting behavior observed at this hollow ratio suggests that local buckling of the inner circular tube wall becomes the primary failure mechanism, which must be accounted for in design calculations.
The linear relationship between impact height and response parameters (dent depth, residual deflection, impact duration) simplifies the prediction of structural performance under varying impact energies. This linearity implies that the energy absorption mechanism remains fundamentally unchanged across the tested range, which is beneficial for design extrapolation.
Key Questions and Reflections
Several critical questions arise from this research that warrant further investigation:
- The hollow ratio of 0.69 is quite high, and the transition from local denting to global failure likely occurs at lower hollow ratios. Determining the critical hollow ratio threshold is essential for practical design applications.
- The boundary constraint effects on impact response are substantial, yet the study does not fully explore the spectrum of possible constraint conditions encountered in real structures. Parametric studies covering intermediate constraint conditions would provide more complete design guidance.
- The interaction between axial compression and lateral impact involves complex stress states that may trigger different failure modes at different stress levels. The observation that the force plateau disappears at high axial compression ratios suggests a potential brittle failure risk that must be carefully managed in design.
Study Insights and Implications
This research provides valuable experimental data for the design of impact-resistant steel tube-concrete composite structures, particularly in applications such as highway bridge columns, industrial plant supports, and protective structures. The findings suggest that the square-in-circle hollow sandwich configuration offers promising impact resistance characteristics while achieving significant weight reduction. However, the sensitivity to boundary conditions and axial compression ratio highlights the importance of accurate modeling of structural boundary conditions and service loads in impact-resistant design. Engineers should consider implementing local stiffening measures to delay the onset of local denting and ensure more uniform energy absorption across the member cross-section.