Bearing Capacity of Defective Concrete-Filled Steel Tube Specimens under Eccentric Compression

Literature Overview

This paper by Lu Zhengran and Zhao Mingge, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) (Vol. 36, No. 6, 2020), investigates the bearing capacity of concrete-filled steel tube (CFST) specimens containing three types of defects under eccentric compression. The research was supported by multiple funding sources including the National Natural Science Foundation of China (Project 51678375), the Liaoning Province Xingliao Talent Plan (XLYC1907121), the Liaoning Province Natural Science Foundation (20180550442), and the Liaoning Provincial Department of Education Basic Research Project (lnjc202019). The authors employ both experimental testing and ABAQUS finite element simulation to analyze the effects of different defect types and defect levels on CFST structural performance.

Core Technical Analysis

The paper addresses a critical practical issue in CFST engineering: the presence of defects, particularly voids or delamination between the steel tube and the internal concrete, which can significantly reduce the composite action and structural capacity. The authors identify three defect types: spherical crown void (spherical cap delamination), circumferential uniform void (circumferential delamination), and composite defect (combination of both types).

Defect Type Classification and Characteristics

The authors' experimental and numerical results demonstrate that circumferential void defects have a more significant impact on the ultimate bearing capacity than spherical crown void defects. This finding is consistent with the physical understanding that circumferential delamination eliminates the confinement effect over a large area, fundamentally altering the load transfer mechanism between the steel tube and the concrete core.

Effect of Defect Level on Bearing Capacity

The defect level, expressed as the void ratio (percentage of the interface area that is delaminated), was varied systematically in the numerical simulations. The results show that:

• Spherical crown voids reduce the bearing capacity progressively as the void ratio increases, but the reduction is relatively gradual.
• Circumferential voids cause a more abrupt reduction in bearing capacity, particularly at higher void ratios, because they fundamentally change the failure mode from a composite failure to a more localized steel tube failure.
• Composite defects exhibit failure behavior similar to circumferential voids, suggesting that the circumferential component dominates the structural response.

Failure Mode Analysis

The failure mode analysis reveals important differences between defect types. Intact CFST specimens typically exhibit a ductile failure mode characterized by progressive concrete crushing and steel tube local buckling, with significant load redistribution. Spherical crown void specimens show similar failure modes to intact specimens but with reduced capacity. Circumferential void specimens, however, exhibit a more brittle failure mode with sudden loss of load-carrying capacity, as the concrete core is no longer effectively confined by the steel tube.

Finite Element Modeling Approach

The authors employ ABAQUS for the finite element analysis, which requires careful attention to several modeling aspects:

• Material constitutive models: The concrete material model must capture the triaxial compression behavior, including the confinement effect provided by the steel tube. The steel material model should account for strain hardening and the Bauschinger effect.
• Contact interface modeling: The steel-concrete interface must be modeled with appropriate friction and bonding characteristics. For defect simulation, the contact area must be reduced to represent the void ratio.
• Mesh convergence: The mesh density must be sufficient to capture the localized stress concentrations at defect boundaries and the progressive failure of the concrete core.
• Boundary conditions: The eccentric compression loading must be applied correctly, with appropriate eccentricity ratios to match experimental conditions.

Engineering Practice Implications

From a quality control and inspection perspective, this research underscores the importance of detecting and characterizing voids and delamination in CFST structures. Traditional non-destructive testing methods such as ultrasonic testing (UT) and radiographic testing (RT) can be used to identify voids, but the interpretation of results must account for the different severity levels of different defect types.

For fabrication and construction practices, the following measures can help minimize defects:

• Ensuring proper cleaning and surface preparation of the steel tube interior before concrete placement.
• Using appropriate concrete mix designs with adequate workability for tube filling.
• Implementing proper compaction methods, whether by vibration or self-compacting concrete technology.
• Conducting post-placement inspection to verify complete filling and bond quality.

Key Questions and Reflections

The research raises several important questions for the engineering community. First, the threshold defect level at which CFST structures become unacceptable for service requires further investigation, as current codes do not provide clear acceptance criteria for interface defects. Second, the repair strategies for CFST structures with identified defects remain largely undeveloped, and the feasibility of in-situ repair using epoxy injection or other methods warrants further study. Third, the interaction between defects and other damage mechanisms, such as corrosion of the steel tube or concrete degradation, could lead to progressive deterioration that is difficult to predict.

Study Insights and Implications

This paper provides valuable insights into the structural behavior of CFST specimens with interface defects under eccentric compression. The finding that circumferential voids are more critical than spherical voids has direct implications for inspection priorities and acceptance criteria. For engineers involved in CFST design and construction, the key takeaway is that interface quality is a critical factor in structural performance, and rigorous quality control measures must be implemented during fabrication and erection. The finite element methodology presented in the paper provides a useful tool for evaluating the structural consequences of identified defects and for developing repair strategies. The research contributes to the broader understanding of CFST structural behavior and supports the development of more refined design and inspection standards.