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

Literature Overview

This paper by Hao Zhaofeng, Zhang Rongling, Ma Lina, Alan Kwan, Ning Guixia, and Li Zhiyang from Lanzhou Jiaotong University, Cardiff University, and Gansu Henglu Transportation Survey and Design Institute investigates the compressive load-bearing capacity of steel tube concrete (STC) members with manufacturing defects. Published in the journal Industrial Construction in 2020 (Volume 50, Issue 8, pages 138-144), the research is funded by the National Natural Science Foundation of China (Grant 51768433) and several other provincial and institutional grants.

The study is motivated by a practical concern: in the fabrication and construction of STC members, defects are inevitable to some extent. These defects may include incomplete concrete filling, voids or air pockets within the concrete, local debonding between the steel tube and concrete, steel tube local buckling, and weld defects. Understanding how these defects affect the load-bearing capacity is essential for quality control, fitness-for-service assessment, and the development of appropriate design safety factors.

Core Experimental Program

The experimental program consists of 9 groups of STC defect specimens subjected to axial compression loading. The specimens are designed to simulate various types and severities of defects that may occur in practice:

The load-displacement curves obtained from the tests provide comprehensive information about the load-bearing capacity, stiffness, ductility, and failure mode of the defective specimens.

Technical Findings

The experimental results reveal several important and quantifiable relationships:

1. Significant capacity reduction: Defects have a substantial impact on the compressive load-bearing capacity of STC members. Compared to defect-free specimens, the capacity loss ranges from a minimum of 14% to a maximum of 59%. This wide range reflects the sensitivity of STC members to defect severity and location.
2. Defect rate proportionality: When the defect location is held constant, the capacity loss is proportional to the defect rate. This linear relationship provides a practical basis for estimating capacity loss from measured defect sizes.
3. Edge vs. center defect: Edge defects (located near the outer surface of the steel tube) cause greater capacity loss than center defects (located near the center of the cross-section) of the same size. This is because edge defects directly compromise the confinement mechanism and the load transfer path between the steel tube and the concrete core.
4. Failure mode evolution: As the defect severity increases, the failure mode transitions from ductile crushing (characteristic of well-confined STC members) to brittle local buckling or spalling, with reduced post-peak deformation capacity.

Numerical Analysis and Verification

A finite element model is developed using commercial FEA software to simulate the behavior of defective STC members under axial compression. The model incorporates:

• Steel tube: Modeled with an elastic-perfectly plastic or elastic-plastic constitutive law, accounting for strain hardening.
• Concrete core: Modeled with a constitutive law that captures cracking, crushing, and confinement effects (e.g., Mander model or Kent-Park model).
• Defect region: Modeled as a void or reduced-stiffness zone, depending on the defect type.
• Steel-concrete interface: Modeled with appropriate contact or bond-slip elements to capture the interaction between the two materials.

The numerical results are compared with the experimental data, and good agreement is achieved, validating the model's ability to predict the load-bearing capacity of defective STC members. The model is then extended to cover additional structural configurations and defect scenarios not covered by the experimental program.

Engineering Practice Implications

The findings of this study have direct implications for quality control and engineering practice:

1. Acceptance criteria: The quantified relationship between defect severity and capacity loss can inform the development of acceptance criteria for STC members. For example, a defect rate below a certain threshold may be acceptable if the capacity loss is within the design safety margin.
2. Inspection and evaluation: The study provides a basis for evaluating the structural adequacy of existing STC members that may have undetected defects. Non-destructive testing methods (UT, RT, GPR) can be used to detect and quantify defects, and the results can be correlated with capacity loss using the relationships established in this study.
3. Design safety factors: The capacity reduction caused by defects should be considered in the selection of design safety factors. For applications where defects are likely (e.g., complex geometries, difficult concrete placement), higher safety factors or more rigorous quality control measures should be applied.
4. Repair and retrofit: For defective STC members that cannot be replaced, repair strategies such as grout injection, external strengthening, or internal reinforcement may be considered. The effectiveness of these repairs should be evaluated using the analytical framework developed in this study.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the study focuses on axial compression; how do defects affect the behavior under bending, shear, or combined loading?