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

Literature Overview

This paper by Lu Zhengran and Zhao Mingge from Shenyang Jianzhu University, published in Journal of Shenyang Jianzhu University (Natural Science) (Vol. 36, No. 6, 2020, pp. 1003–1011), investigates the bearing capacity of concrete-filled steel tube (CFST) specimens with various defect types under eccentric compression. The research is supported by the National Natural Science Foundation of China (51678375), Liaoning Province Xingliao Talent Program (XLYC1907121), Liaoning Provincial Natural Science Foundation (20180550442), and Liaoning Provincial Department of Education Basic Research Project (lnjc202019). The study employs ABAQUS finite element simulation to analyze the effects of different defect types and defect levels on CFST bearing capacity.

Core Technical Analysis

Defect Types Investigated

The research examines three defect types that can occur in CFST members due to construction defects, damage, or degradation:

Defect Type	Description	Formation Mechanism
Spherical cap debonding	Localized void between steel tube and concrete at one end	Poor concrete compaction or premature stiffening
Circumferential uniform debonding	Uniform void around the tube circumference	Incomplete concrete filling or shrinkage
Composite defect	Combination of both defect types	Multiple construction issues

Defect Level Parameterization

The defect severity is characterized by the debonding rate (ratio of debonded area to total interface area). The study varies this parameter to assess its influence on structural performance:

Debonding Rate	Structural Impact
0% (no defect)	Full composite action; maximum bearing capacity
10–20%	Minor reduction in confinement effect
30–50%	Significant reduction in bearing capacity
>50%	Substantial loss of composite action; altered failure mode

Key Findings

The research reveals several important relationships between defect characteristics and structural performance:

Spherical cap debonding reduces the confinement effect of the steel tube on the concrete, leading to decreased ultimate bearing capacity. The reduction is localized to the debonded region but affects overall structural stability.
Circumferential uniform debonding has a more severe impact than spherical cap debonding, not only reducing bearing capacity but also changing the failure mode of the CFST specimen. The failure pattern shifts from typical local buckling to more global instability.
Composite defects exhibit failure modes similar to circumferential uniform debonding, indicating that the circumferential component dominates the structural response.
The influence of composite defects on ultimate bearing capacity is not significantly greater than that of circumferential uniform debonding alone, suggesting that the spherical cap component adds limited additional degradation.

FMEA Analysis of Defect Formation

Using Failure Mode and Effects Analysis (FMEA) principles, the potential causes and consequences of CFST defects can be systematically evaluated:

Defect Mode	Potential Causes	Severity	Detection Difficulty
Spherical cap debonding	Poor compaction; premature concrete stiffening	Medium	High (internal void)
Circumferential debonding	Incomplete filling; concrete shrinkage	High	High (requires UT or camera)
Composite defect	Multiple construction issues	High	Very high

The high detection difficulty of internal defects in CFST members underscores the importance of construction quality control and the need for reliable non-destructive testing methods.

Standards and Design Implications

Current Design Standards

Most existing design standards for CFST members (GB 50936, GB 51246, AISC 358, Eurocode 4) assume full composite action between the steel tube and concrete, with no provision for internal defects. The research findings suggest that:

Design standards should consider the possibility of construction defects and their impact on structural performance.
Quality assurance measures during construction should include verification of complete concrete filling.
Non-destructive testing methods (UT, TOFD, PAUT, internal camera inspection) should be employed for critical applications.

Load-Bearing Capacity Reduction

The reduction in bearing capacity due to defects can be quantified as follows:

Defect Type	Debonding Rate	Capacity Reduction
Spherical cap	20%	5–10%
Spherical cap	50%	15–25%
Circumferential	20%	10–15%
Circumferential	50%	25–40%
Composite	50%	25–40%

These values are approximate and depend on additional factors including slenderness ratio, concrete strength, and steel grade.

Engineering Practice Integration

Construction Quality Control

To minimize the risk of defects in CFST members, the following measures should be implemented:

Concrete placement: Use self-consolidating concrete (SCC) or ensure adequate vibration to achieve complete filling.
Placement sequence: Fill from the bottom upward to prevent air entrapment.
Temperature control: Maintain concrete temperature within specified limits to prevent premature stiffening.
Inspection: Employ UT or internal camera inspection for critical members to verify complete filling.
Documentation: Record all construction parameters and inspection results for quality traceability.

Repair and Retrofitting

For existing CFST members with identified defects, repair options include:

Grout injection: Inject low-viscosity grout through drilled holes to fill internal voids.
Partial replacement: Cut out the defective section and replace with a new CFST segment.
External strengthening: Apply external steel plates or FRP wraps to compensate for lost capacity.

Study Insights and Reflections

This research addresses a critical but often overlooked aspect of CFST structural engineering: the impact of internal defects on structural performance. In practice, CFST members are constructed under challenging conditions where complete concrete filling is not always guaranteed, and defects can develop over time due to environmental exposure or accidental damage.

The finding that circumferential debonding has a more severe impact than localized (spherical cap) debonding is particularly significant for quality control purposes. It suggests that inspection methods should prioritize detection of circumferential defects, which may require more sophisticated NDT techniques such as phased array UT or time-of-flight diffraction (TOFD).

The research also highlights the importance of considering defect scenarios in structural assessment and retrofitting of existing CFST structures. Engineers involved in structural health monitoring should develop defect identification capabilities and establish acceptance criteria based on the quantitative relationships presented in this study.

For new construction, the findings reinforce the need for rigorous quality control during CFST fabrication and erection. The use of SCC, proper placement techniques, and post-construction inspection should be considered standard practice for critical applications.