Load-Bearing Capacity of Defective Steel Tube Concrete Members: Experimental and Numerical Investigation

Literature Overview

This paper, published in Industrial Construction (2020, Vol. 50, No. 8, pp. 138-144), authored by Hao Zhaofeng, Zhang Rongling, Ma Lina, Alan Kwan, Ning Guixia, and Li Zhiyang from Lanzhou Jiaotong University and Cardiff University, investigates the axial compressive behavior of steel tube concrete (CFST) members with manufacturing defects. The study combines experimental testing of nine CFST specimens with defects of varying types and locations with finite element analysis to quantify the impact of defects on load-bearing capacity and propose design guidance.

Experimental Program and Test Configuration

The experimental program was designed to systematically evaluate how different defect configurations affect the compressive performance of CFST members. The test matrix included variations in defect type, defect location (edge versus center), and defect severity (expressed as defect rate).

The specimens were loaded to failure, and the load-displacement curves were recorded to characterize the elastic, plastic, and post-peak behavior of each configuration. Strain measurements at critical locations provided insight into the deformation mechanisms and failure modes.

Key Experimental Findings

The study produced several significant findings regarding the sensitivity of CFST members to manufacturing defects:

1. Magnitude of Capacity Loss: Defects cause substantial reductions in compressive capacity, with minimum loss of 14% and maximum loss reaching 59% compared to defect-free reference members. This wide range indicates that the severity of capacity degradation is highly dependent on defect characteristics.
2. Defect Rate Proportionality: When defect location is held constant, the capacity loss is directly proportional to the defect rate. This linear relationship provides a useful basis for quantitative assessment of defect impact in engineering practice.
3. Location Sensitivity: Edge defects cause greater capacity loss than center defects of equivalent severity. This is attributed to the stress concentration effect at the steel tube boundary, where the confined concrete is most critical for providing lateral restraint and preventing local buckling of the steel tube.
4. Failure Mode Evolution: Defective members exhibit different failure modes compared to intact members. The presence of defects promotes localized buckling and premature concrete crushing at the defect site, reducing the overall ductility and energy absorption capacity of the member.

Finite Element Analysis and Model Validation

The authors developed finite element models using commercial FEA software to simulate the behavior of CFST members with defects. The numerical models incorporated material nonlinearity for both the steel tube and the confined concrete, geometric nonlinearity to capture large deformations, and contact interfaces to represent the steel-concrete interaction.

The FE models were validated against the experimental results, demonstrating good agreement in load-displacement response, failure mode prediction, and strain distribution. The validated models were then extended to cover additional structural configurations beyond those tested experimentally, enabling parametric studies on defect geometry, member slenderness, and steel-concrete strength ratios.

The FE analysis confirmed the experimental trends and provided additional insight into the internal stress redistribution mechanisms. Specifically, the analysis revealed that defects create localized stress concentrations that initiate premature yielding in the steel tube, which in turn reduces the confining pressure on the concrete and accelerates concrete failure.

Engineering Implications and Defect Management

The findings of this study have direct implications for quality control in CFST member fabrication and installation:

• Acceptance Criteria: The quantified capacity loss percentages provide a basis for establishing defect acceptance criteria. Defects that cause capacity loss exceeding a certain threshold (e.g., 20%) should be repaired or the member rejected.
• Inspection Focus: The greater sensitivity to edge defects highlights the importance of thorough inspection of weld seams, tube ends, and connection regions, which are the most common locations for manufacturing defects.
• Repair Strategies: For members with moderate defects, repair methods such as local reinforcement, additional concrete injection, or external steel jacketing can be evaluated using the validated FE models.
• Design Margin: The study suggests that design safety factors for CFST structures should account for the potential presence of manufacturing defects, particularly in applications where defect inspection is difficult or incomplete.

Study Insights and Reflections

This research addresses a practically important but under-studied topic: the structural consequences of manufacturing defects in CFST members. In practice, CFST members are often fabricated with field-welded connections, bolted splice plates, and transport-induced damage that may not be fully captured by idealized design assumptions. The systematic quantification of defect impact, combined with validated numerical models, provides engineers with the tools needed to make informed decisions about defect acceptance, repair, and design modification. The finding that edge defects are more detrimental than center defects is particularly significant for inspection planning, as it directs attention to the most critical defect locations.