Hysteretic Performance of Large-Hollow-Ratio Conical Hollow Sandwich Concrete-Filled Steel Tube Compression-Bending Members

Literature Overview

The paper by Shi Yanli, Ji Sunhang, Wang Wenda, Zhang Chen, and Fan Jiahao, published in the China Civil Engineering Journal (Volume 55, Issue 1, 2022, pages 75–88), investigates the hysteretic performance of large-hollow-ratio conical hollow sandwich concrete-filled steel tube (CHS-CFST) compression-bending members. Ten test specimens were fabricated and tested, including eight under reversed cyclic loading and two under monotonic loading. The study examines the effects of hollow ratio (0.6 and 0.8) and axial compression ratio (0, 0.2, 0.4, and 0.6) on failure modes, P-Δ relationship curves, and hysteretic behavior, and proposes a simplified calculation method for lateral load capacity.

Specimen Design and Test Parameters

The test program covered a comprehensive parametric range:

The conical hollow sandwich CFST configuration features a conical outer steel tube with a hollow inner core, creating a sandwich structure that provides both lateral load resistance and weight reduction through the hollow interior.

Failure Modes and P-Δ Relationship

Both hollow ratio specimens (0.6 and 0.8) exhibited similar failure patterns, characterized by varying degrees of local damage in the lower portion of the specimens. The P-Δ relationship curves for the reversed cyclic specimens were full and well-shaped, indicating good hysteretic performance. The monotonic loading specimens provided insight into the ultimate load capacity and deformation characteristics.

Effect of Hollow Ratio

The influence of hollow ratio on structural performance was found to be relatively modest:

The finding that increasing the hollow ratio from 0.6 to 0.8 results in only about a 3% reduction in peak load is practically significant, as it suggests that substantial material savings can be achieved with minimal loss of lateral load capacity. However, the more pronounced stiffness degradation at the higher hollow ratio indicates that the structure becomes more flexible under cyclic loading, which may affect the overall dynamic response.

Effect of Axial Compression Ratio

The axial compression ratio had a more pronounced effect on structural performance:

• As the axial compression ratio increased from 0 to 0.6, the ultimate load capacity decreased progressively.
• Ductility deteriorated with increasing axial compression ratio, as expected from the increased compressive stress in the concrete core.
• The interaction between axial load and bending moment was captured through the P-Δ relationship curves, which showed the characteristic reduction in lateral load capacity with increasing axial load.

Hysteretic Performance and Energy Dissipation

The reversed cyclic loading specimens demonstrated good energy dissipation characteristics:

The viscous damping coefficient of 0.322 indicates that the conical hollow sandwich CFST members can effectively dissipate energy through inelastic deformation, which is beneficial for seismic applications. The improvement of 15% in viscous damping coefficient when the hollow ratio increases from 0.6 to 0.8 is an interesting finding that suggests the larger hollow space may enhance the energy dissipation mechanism through increased deformation of the steel tube walls.

Simplified Lateral Load Capacity Calculation

The paper proposes a simplified calculation method for the lateral load capacity of conical hollow sandwich CFST compression-bending members. This method is based on the numerical analysis of monotonic loading specimens and provides a practical tool for structural design without the need for complex finite element analysis.

Engineering Practice Integration

For engineers involved in steel pipe fabrication, welding, and structural design, this study offers several important insights:

1. The conical hollow sandwich CFST configuration provides an efficient structural solution that combines weight reduction through the hollow interior with adequate lateral load capacity and energy dissipation.
2. The hollow ratio can be increased to 0.8 with only a 3% reduction in lateral load capacity, suggesting significant material and cost savings potential.
3. The conical geometry of the outer tube introduces fabrication challenges, including the need for precision forming of the conical shape and careful welding of the conical tube to the end plates and internal components.

From a welding quality control perspective, the conical hollow sandwich CFST member presents several challenges:

• The conical outer tube requires precise forming to maintain wall thickness uniformity, as any thinning at the apex or base of the cone could compromise structural integrity.
• The welding of the internal hollow tube to the conical outer tube must be executed with procedures that account for the varying geometry and potential for warping.
• Non-destructive testing of the welds in the conical region requires careful probe positioning and interpretation of indications, as the varying geometry can affect signal interpretation.

Study Insights and Reflections

The investigation of large-hollow-ratio conical hollow sandwich CFST members addresses an important structural engineering challenge—the development of efficient, lightweight, and ductile members for applications where both lateral load resistance and material economy are critical. The finding that hollow ratios up to 0.8 can be used with minimal loss of lateral load capacity is practically significant, as it opens the possibility of substantial material savings in structural applications.

The conical geometry of the outer tube is particularly interesting from a structural efficiency standpoint. The tapering wall thickness naturally provides a gradient of stiffness and strength that can be matched to the expected moment distribution in a compression-bending member, where the maximum moment typically occurs at the base. This geometric optimization is analogous to the tapered profiles used in aerospace and automotive structures, where material is distributed according to the stress distribution.

The improvement in viscous damping coefficient with increasing hollow ratio is a noteworthy finding that suggests the larger hollow space may enhance the energy dissipation mechanism through increased plastic deformation of the steel tube walls. This is consistent with the understanding that energy dissipation in CFST members is primarily achieved through the plastic deformation of the steel tube under confinement, and a larger hollow ratio may promote more uniform plastic deformation across the tube wall.

This paper contributes valuable experimental data and analytical tools for the design of conical hollow sandwich CFST members, which have potential applications in bridge piers, building columns, and other structural elements where lateral load resistance, ductility, and material efficiency are important design considerations. The simplified lateral load capacity calculation method provides a practical design tool that can be readily incorporated into engineering practice without requiring advanced computational resources.