Hysteretic Performance of Conical Hollow Sandwich CFST Compression-Bending Members with Large Void Ratios

Literature Overview

This study, published in the China Civil Engineering Journal (2022, Vol. 55, No. 1, pp. 75-88) by Shi Yanli et al. from Lanzhou University of Technology, investigates the cyclic and monotonic behavior of conical hollow sandwich concrete-filled steel tube (CFST) compression-bending members under large void ratios. The research is funded by the National Natural Science Foundation of China (51768038) and the Gansu Provincial Construction Science and Technology Project (JK2021-42). A total of ten specimens were tested, comprising eight cyclic loading specimens and two monotonic loading specimens, with void ratios of 0.6 and 0.8 and axial compression ratios of 0, 0.2, 0.4, and 0.6. The study also develops a numerical model for monotonic specimens to analyze the load-bearing mechanism and proposes a simplified calculation method for lateral bearing capacity.

Key Technical Parameters and Test Configuration

The conical hollow sandwich CFST section represents an innovative cross-sectional form that combines the benefits of concrete-filled steel tubes with hollow sandwich construction. The conical geometry introduces a taper along the member length, which is relevant for applications such as column bases, transition zones in multi-story structures, and seismic isolation components where variable cross-sections are advantageous. The hollow sandwich configuration places concrete on both sides of a steel tube, creating a composite action that can be tailored through the void ratio.

Core Technical Findings

The experimental results reveal that both void ratio specimens exhibit fundamentally consistent failure modes, characterized by varying degrees of local damage concentrated at the lower portion of the specimens. This failure pattern is consistent with the expected behavior of compression-bending members where the base experiences the highest combined compressive and bending stresses. The P-Δ relationship curves of the cyclically loaded specimens are relatively full, indicating good hysteretic performance and energy dissipation capacity.

One of the most significant findings is that increasing the void ratio from 0.6 to 0.8 has a relatively minor effect on lateral bearing capacity and ductility, with the average peak load difference being approximately 3%. This suggests that the hollow sandwich CFST system retains its structural integrity even at high void ratios, which has important implications for material economy and weight reduction in practical applications. However, the stiffness degradation is more pronounced at the higher void ratio of 0.8, indicating that while strength is maintained, the elastic behavior is more sensitive to the void ratio.

The viscous damping coefficient shows a notable improvement of approximately 15% when the void ratio increases from 0.6 to 0.8. This counterintuitive result suggests that the higher void ratio may promote more plastic deformation in the steel tube walls, thereby enhancing energy dissipation through hysteretic mechanisms. The average displacement ductility coefficient of 3.56 and viscous damping coefficient of 0.322 indicate that these members possess good energy dissipation capacity, meeting the requirements for seismic-resistant structural components.

Influence of Axial Compression Ratio

The axial compression ratio emerges as a more critical parameter than the void ratio in governing the overall structural performance. As the axial compression ratio increases from 0 to 0.6, the ultimate load of the specimens decreases progressively, and ductility deteriorates gradually. This behavior aligns with classical column theory, where higher axial loads reduce the moment capacity and limit the plastic hinge formation at the critical section. The intensity degradation in cyclically loaded specimens is not significant, suggesting that the conical hollow sandwich CFST members maintain their load-carrying capacity through multiple loading cycles.

From a practical engineering perspective, this finding is particularly valuable for seismic design of compression-bending members. The axial compression ratio of 0.4 represents a reasonable upper limit for applications requiring good ductility, while ratios approaching 0.6 should be used with caution in high-seismicity regions. The relatively small effect of void ratio on strength means that designers can optimize the void ratio primarily for weight and cost considerations without significantly compromising structural safety.

Engineering Practice Implications and Reflections

The proposed simplified calculation method for lateral bearing capacity provides a practical tool for engineers designing conical hollow sandwich CFST members. The numerical model developed for monotonic specimens validates the experimental observations and offers insights into the stress distribution and load transfer mechanisms within the composite section. The conical geometry introduces additional complexity in terms of stress concentration at the taper transition, which the numerical model captures effectively.

From a manufacturing and welding perspective, the conical hollow sandwich CFST construction requires careful attention to the geometric tolerances of the conical steel tube and the placement of the inner and outer concrete layers. The taper angle must be controlled to ensure uniform concrete compaction and avoid voids at the transition zones. Welding connections at the conical tube joints require consideration of the variable wall thickness, which may necessitate specific welding parameters and post-weld heat treatment to maintain material properties in the heat-affected zone.

This research opens new possibilities for lightweight, high-performance structural members in seismic regions. The combination of conical geometry, hollow sandwich construction, and large void ratios offers a promising approach to reducing structural weight while maintaining adequate strength and ductility. Future work should address the long-term durability of the hollow sandwich interface under cyclic loading and the practical constructability of these members in large-scale projects.

Study Insights and Implications

The study demonstrates that the conical hollow sandwich CFST concept is technically viable for compression-bending applications with large void ratios. The key insight is that the void ratio and axial compression ratio play different roles in governing structural performance: the void ratio primarily affects stiffness and damping characteristics, while the axial compression ratio dominates the ultimate strength and ductility. This decoupling of parameters provides engineers with multiple degrees of freedom in optimizing the member design for specific performance targets. The approximately 3% difference in peak load between void ratios of 0.6 and 0.8 is within typical engineering tolerances, making the higher void ratio a viable option for weight-sensitive applications where the 15% improvement in damping capacity is beneficial.