Hysteretic Behaviour of Steel-Concrete-Filled Tube Members with Circular Arch-Shaped Debonding Defects under Combined Compression-Bending-Torsion Loading

Literature Overview

This study, published in Engineering Mechanics (Vol. 36, No. 12, 2019, pp. 121–133) by Zhang Weijie, Liao Feiyu, and Li Wei from Fujian Agriculture and Forestry University and Tsinghua University, investigates the seismic performance of circular steel tube-confined concrete (CFST) members containing circular arch-shaped debonding defects between the steel tube and the infill concrete. The research was supported by two National Natural Science Foundation of China grants (51578154, 51878176) and a Fujian Provincial Science and Technology Department university-industry collaboration project (2018H6005). The experimental programme comprised 16 specimens subjected to constant axial compression with cyclically applied bending-torsion coupled loading, representing a rigorous and systematic approach to quantifying the degradation caused by construction defects in seismic structures.

Core Technical Content

Specimen Configuration and Test Parameters

The test matrix was designed around three key variables: debonding ratio, axial compression ratio, and bending-to-torsion ratio. The specimen group included 8 specimens with debonding defects, 6 specimens without defects for comparison, and 2 empty steel tube specimens as baseline references. The circular arch-shaped debonding defect simulates a realistic construction scenario where concrete does not fully adhere to the inner surface of the steel tube, creating a void that compromises composite action.

Key Findings

The research produced several technically significant conclusions that are directly relevant to engineering practice:

1. Hysteresis loop shape: The circular arch-shaped debonding defect has minimal influence on the overall shape of the hysteresis curves, indicating that the fundamental loading-unloading mechanism remains largely unchanged despite the presence of the defect.
2. Post-ultimate stiffness degradation: While the shape is preserved, the specimen with debonding defects exhibits reduced stiffness in the strain-hardening stage after reaching the ultimate load, which is a critical observation for seismic design.
3. Capacity degradation with debonding ratio: Both load-carrying capacity, stiffness, and energy dissipation capacity decrease monotonically as the debonding ratio increases.
4. Axial compression ratio insensitivity: Changes in the axial compression ratio have no significant effect on the mechanical performance of the defective specimens, which is somewhat counterintuitive and warrants further investigation.
5. Bending-to-torsion ratio sensitivity: As the bending-to-torsion ratio increases, the influence of debonding defects on ultimate load capacity, stiffness, and energy dissipation becomes progressively more pronounced. This indicates that debonding is more detrimental to flexural performance than to torsional performance.

Engineering Practice Implications

Construction Quality Control

From a steel pipe fabrication and erection perspective, this research underscores the importance of ensuring full concrete-steel interface contact in CFST construction. In practice, debonding can arise from inadequate concrete placement, insufficient compaction, or the presence of construction joints. The finding that debonding primarily degrades post-peak behaviour rather than initial stiffness means that visual inspection during construction may not detect the defect, but its consequences will manifest during seismic events.

Seismic Design Considerations

The observation that flexural demand amplifies the detrimental effects of debonding is particularly important for tall structures where bending moments dominate the seismic response. Designers should consider applying higher safety factors or requiring additional interface treatments (such as mechanical interlocking or chemical bonding agents) in regions of high flexural demand.

Defect Assessment

For existing structures, the relatively mild effect on hysteresis shape suggests that non-destructive evaluation methods focused on detecting debonding (such as ultrasonic pulse-echo testing of the steel tube wall) could be incorporated into structural health monitoring programmes. The fact that post-peak stiffness is the primary degradation metric implies that dynamic testing methods sensitive to stiffness changes could effectively identify affected members.

Study Insights and Reflections

This research addresses a gap in the literature concerning the combined effects of construction defects and multi-axial seismic loading on CFST members. The choice to study bending-torsion coupling rather than pure bending or pure torsion represents a more realistic seismic loading condition for structural members. The finding that the axial compression ratio has limited influence on defective specimens is notable and suggests that the debonding defect dominates the response mechanism more than the gravity load level, at least within the tested parameter range. This has practical implications for damage assessment of existing CFST structures, where the severity of the debonding defect should be prioritised over the axial load level in determining seismic vulnerability. The study provides valuable data for calibrating numerical models that incorporate interface debonding in finite element analysis of CFST structures under multi-axial loading.