Axial Compression Bearing Capacity Analysis of Square Steel Tube Concrete Short Columns with Edge Delamination Defects

Literature Overview

This paper by Zhang Kai from Fujian Expressway Datong Testing Co., Ltd., published in Journal of Qingdao University of Technology (2025, Vol. 46, Issue 4, pp. 56-65), investigates the effect of edge delamination defects on the axial compression performance of square steel tube concrete (STC) short columns. The research is supported by Fujian Transportation Science and Technology Projects (Grant Nos. 202046 and 202252). The study employs validated finite element analysis (FEA) to examine how delamination along the edges of square steel tubes affects load-displacement behavior, load allocation between steel and concrete, and the steel-concrete interaction mechanism.

Background and Problem Context

Square steel tube concrete columns are widely used in bridge piers, building frames, and industrial structures. During service, these columns are subjected to temperature variations (diurnal and seasonal), material incompatibility, and differential shrinkage between steel and concrete, all of which can lead to delamination—the separation of the concrete core from the steel tube wall. This delamination typically occurs along the edges or corners of square tubes and progressively worsens over time.

The presence of delamination defects fundamentally alters the load-bearing mechanism of STC columns. In an undamaged STC column, the steel tube and concrete core work together through composite action, with the steel tube providing lateral confinement to the concrete and the concrete providing bearing support to the steel tube. Delamination disrupts this interaction, potentially leading to significant reduction in load-bearing capacity.

Methodology

Finite Element Model Validation

The FEA model is validated against experimental test data, ensuring that the numerical predictions accurately reflect real structural behavior. The model incorporates:

• Material models: Concrete is modeled with a damage plasticity model, while steel is modeled with an elastic-plastic constitutive law.
• Contact definition: A penalty-based contact algorithm is used to simulate the steel-concrete interface, with friction and bond parameters calibrated from experimental data.
• Boundary conditions: Axial compression loading is applied with appropriate constraints to simulate realistic boundary conditions.

Parametric Study Variables

The study investigates five key parameters:

Key Technical Findings

Overall Bearing Capacity Reduction

Compared to undamaged STC columns, columns with edge delamination defects exhibit reduced bearing capacity. The magnitude of reduction depends on multiple factors:

• Increases with: number of delaminated edges, delamination ratio, and steel yield strength.
• Decreases with: concrete strength.

This pattern is physically intuitive. More delaminated edges and higher delamination ratios mean less effective composite action, while higher steel yield strength means the steel tube bears a larger share of the load, making the loss of composite action more critical. Higher concrete strength provides more inherent bearing capacity, partially compensating for the loss of composite action.

Effect of Steel Ratio

An important finding is that when the number of delaminated edges is fixed and varies, changes in steel ratio have minimal effect on the bearing capacity reduction. However, when all four edges are delaminated, high steel ratio columns show significantly larger bearing capacity reductions as the delamination ratio increases.

This observation suggests that for columns with extensive delamination, the steel ratio becomes a critical design parameter. High steel ratio columns rely more heavily on the composite action between steel and concrete, making them more vulnerable to delamination-induced capacity loss.

Load Allocation and Steel-Concrete Interaction

The FEA results reveal that delamination significantly alters the load allocation between the steel tube and concrete core. In undamaged columns, load is shared proportionally based on stiffness and strength. With delamination, the concrete core loses its lateral confinement and bears load independently, while the steel tube must carry a disproportionate share of the total load. This redistribution can lead to premature local buckling of the steel tube walls.

Simplified Calculation Method

Based on the FEA results, the paper proposes a simplified calculation formula for the axial compression bearing capacity of STC columns with edge delamination defects. The formula accounts for:

1. The contribution of the undamaged steel-concrete composite section.
2. The reduction factor due to delamination, which depends on the number of delaminated edges and the delamination ratio.
3. The material properties of both steel and concrete.

This simplified method provides a practical tool for engineers to assess the structural integrity of existing STC columns where delamination has been detected through non-destructive testing.

Engineering Practice Implications

Inspection and Assessment

The findings of this study have direct implications for the inspection and assessment of existing STC columns in bridges and buildings. Engineers should:

1. Conduct regular inspections for delamination using methods such as impact echo testing, ultrasonic testing, or infrared thermography.
2. Quantify the extent of delamination (number of affected edges and delamination ratio) for structural assessment.
3. Use the proposed simplified calculation method to evaluate the remaining bearing capacity.
4. Implement appropriate repair or strengthening measures based on the assessment results.

Repair Strategies

For columns with detected delamination, repair options include:

• Grouting: Injection of non-shrink grout into the delaminated zone to restore composite action.
• External steel jacketing: Addition of an external steel jacket to provide additional confinement.
• FRP wrapping: Application of fiber-reinforced polymer wraps to restore confinement and limit further delamination.
• Replacement: In severe cases, replacement of the damaged column may be necessary.

Critical Reflections and Questions

The study focuses on edge delamination, which is the most common type observed in practice. However, delamination can also occur at the corners of square tubes, which may have different structural implications due to the stress concentrations at corners. Future research should investigate corner delamination and its interaction with edge delamination.

Additionally, the study considers monotonic axial compression loading. In reality, STC columns in seismic regions are subjected to cyclic lateral loading, which may accelerate delamination and lead to different failure modes. The effect of delamination on the seismic performance of STC columns is an important area for future investigation.

The simplified calculation formula proposed in the paper should be validated against additional experimental data from different sources before being adopted in design codes. The current validation is based on the author's own experimental data, and broader validation is needed to establish the formula's reliability across different column geometries and material combinations.

Study Insights and Reference Value

This paper addresses a practically important problem in the assessment of existing STC structures. The systematic parametric study provides clear guidance on how delamination characteristics affect structural performance, and the simplified calculation formula offers a practical tool for engineers. For bridge inspection engineers and structural assessors, this work provides a basis for developing delamination-specific assessment protocols and repair guidelines. The findings underscore the importance of regular inspection and maintenance of STC structures to detect and address delamination before it compromises structural safety.