Axial Compression Performance of Damaged CFST Members with Open-Hole Damage

Literature Overview

Published in Industrial Construction in 2013 by Chen Zongping, Zhong Ming, and Chen Yuliang from Guangxi University, this paper presents experimental investigation of the axial compression behavior of circular steel tube confined concrete (CFST) members with open-hole damage. The research is funded by the National Natural Science Foundation of China (50908057), the Guangxi Science and Technology Program (Guikexiang 12118023-3), and the Guangxi Natural Science Foundation (2012GXNSFAA053203). Fourteen specimens were tested under axial compression with varying damage parameters.

Core Technical Content and Methodology

Open-hole damage in CFST members can occur during construction (such as openings for pipe penetrations, cable trays, or mechanical equipment connections) or during service (due to corrosion, impact, or fabrication errors). Understanding the structural capacity of damaged CFST members is essential for structural assessment, retrofitting, and damage tolerance design.

The experimental program includes:

The damage rate is defined as the ratio of the open-hole area to the total cross-sectional area of the CFST member. The study systematically varies this ratio along with concrete strength and slenderness ratio to establish quantitative relationships.

Key Findings and Technical Interpretation

The experimental results reveal several important characteristics:

1. Ductility in post-peak behavior: Damaged CFST members exhibit good ductility in the later stages of loading. This is attributed to the confinement effect of the remaining steel tube, which continues to restrain the concrete even after local damage has initiated.
2. Failure mode: The predominant failure mode is steel tube bulging followed by tearing rupture. The open-hole damage creates a stress concentration that initiates local buckling, which propagates until the steel tube tears.
3. Damage rate influence: The open-hole damage rate has varying degrees of influence on the failure mode and ultimate bearing capacity. At low damage rates (below 0.8%), the impact on capacity is relatively modest.
4. Code method safety: When using existing CFST design code methods for members with damage rates below 0.8%, the calculated results remain conservative (safe). This is a significant practical finding for structural assessment.

Engineering Practice Implications

The findings have important implications for structural engineers involved in assessment and retrofitting:

• Damage tolerance: CFST members demonstrate reasonable damage tolerance for small open-hole damage (below 0.8% of cross-sectional area). This means that minor construction damage or incidental penetrations do not necessarily require repair or replacement, provided the damage rate remains below this threshold.
• Structural assessment methodology: For existing CFST structures with known damage, engineers can use standard code methods for capacity assessment when the damage rate is below 0.8%. For higher damage rates, more sophisticated analysis methods (such as finite element modeling with explicit damage representation) are required.
• Repair strategies: When damage rates exceed acceptable limits, repair options include local reinforcement with steel plates, epoxy injection, or replacement of the damaged segment. The choice depends on the damage location, severity, and accessibility.
• Inspection protocols: Regular inspection of CFST members for open-hole damage should be incorporated into structural maintenance programs, particularly for members in aggressive environments or those subjected to impact loading.

Key Questions and Reflections

The study is limited to damage rates below 0.8%, which represents relatively minor damage. In practice, more severe damage may occur due to corrosion perforation, impact from vehicles or equipment, or construction errors. The behavior at higher damage rates is likely to be significantly different, with potential for sudden loss of confinement and catastrophic failure. Future research should extend the damage rate range to cover more severe scenarios.

Additionally, the study focuses on axial compression loading, which is the primary loading mode for CFST columns. However, in real structures, CFST members are often subjected to combined axial and bending loads. The interaction between open-hole damage and flexural capacity deserves investigation. Furthermore, the effect of damage location (mid-height versus near the end) on the structural response should be studied, as end damage may have a more pronounced effect on the overall member capacity.

Study Insights and Implications

This paper provides valuable experimental data for the assessment of damaged CFST members, filling an important gap in the technical literature. The finding that standard code methods remain safe for damage rates below 0.8% is practically significant, as it provides engineers with a clear threshold for when simplified assessment methods are applicable. The good ductility observed in damaged members is encouraging from a safety perspective, as it provides warning through visible deformation before ultimate failure. For engineers involved in structural health monitoring and assessment, this research supports the development of damage tolerance criteria for CFST structures, which is an area of growing importance as the existing stock of CFST buildings ages.