Residual Bearing Capacity of Composite Columns with Internal Circular Steel Tube Reinforced Concrete Under Impact Loading

Literature Overview

This paper by Tian Xuemei, Zhu Xiang, Li Wenbo, and Wang Weixu, published in Building Structures (2024, Vol. 54, No. 16, pp. 14-24), investigates the residual axial bearing capacity of square composite columns with internal circular steel tube reinforced concrete (CFST) cores after lateral impact loading. The research employs finite element analysis using ANSYS/LS-DYNA to model the impact and subsequent axial compression behavior. The study was funded by the National Natural Science Foundation of China (Grant No. 51778276), Shanxi Provincial University Science and Technology Innovation Project (2023L492), and Shanxi Provincial Science and Technology Department Key R&D Program (201903D311007).

Core Technical Findings

The study establishes that composite columns subjected to lateral impact exhibit significantly reduced axial bearing capacity compared to undamaged columns. The post-impact axial compression behavior is characterized by a tendency toward flexural-shear failure, with substantially reduced energy absorption capacity. The research proposes a damage degree evaluation index (D_dag) that enables rapid assessment of the structural integrity of impacted composite columns.

Key Results Summary

Damage Degree Evaluation Index

Technical Interpretation of Impact-Induced Damage Mechanisms

The lateral impact loading on composite columns creates a complex damage pattern that involves the interaction between the internal circular steel tube, the surrounding concrete core, and the outer square concrete section. The damage mechanisms include:

Damage Mechanisms by Component

Post-Impact Axial Compression Behavior

After lateral impact, the composite column exhibits the following characteristics during subsequent axial compression loading:

1. Reduced initial stiffness: The impact-induced damage creates initial cracks and deformations that reduce the initial stiffness of the column.
2. Flexural-shear failure mode: The damaged column tends to fail in a flexural-shear mode rather than the pure compression mode of undamaged columns. This is because the impact-induced damage creates asymmetry in the cross-section, leading to eccentricity during axial loading.
3. Reduced energy absorption: The damaged column absorbs significantly less energy during axial compression compared to undamaged columns. This is because the impact-induced damage has already consumed a portion of the column's energy absorption capacity.
4. Reduced ductility: The damaged column exhibits reduced ductility, with a shorter post-peak load-deformation response.

Finite Element Modeling Approach

The ANSYS/LS-DYNA finite element model employed in this study incorporates several key elements to accurately simulate the impact and post-impact behavior:

Material Models

Mesh and Contact Modeling

• Mesh density: The mesh should be sufficiently fine to capture the local damage around the impact zone, with element sizes of 10-20 mm in the impact region and 30-50 mm in the undamaged regions.
• Contact modeling: The contact between the internal steel tube and the concrete core should be modeled with a penalty-based contact algorithm to capture the confinement interaction.
• Boundary conditions: The boundary conditions should be carefully defined to represent the actual structural constraints, including fixed, pinned, or rigid connections.

Engineering Practice and Structural Assessment

The findings of this study have direct implications for the structural assessment and post-disaster evaluation of composite columns in buildings and infrastructure. The damage degree evaluation index (D_dag) provides a practical tool for rapidly assessing the structural integrity of impacted columns and determining the appropriate response.

Structural Assessment Procedure

1. Initial visual inspection: Identify visible damage such as concrete spalling, steel tube denting, and reinforcement exposure.
2. Non-destructive testing: Employ ultrasonic testing, rebound hammer testing, or ground-penetrating radar to assess internal damage.
3. Load testing: Apply controlled axial loads to measure the residual bearing capacity.
4. Damage evaluation: Compare the measured residual capacity with the original design capacity to calculate the damage degree index.
5. Response determination: Based on the damage degree, determine whether the column can remain in service, requires repair, or must be replaced.

Comparison of Damage Assessment Methods

Key Questions and Reflections

A critical question that arises from this study is the generalizability of the damage degree evaluation index (D_dag) to different column geometries, material properties, and impact scenarios. The index was developed based on a specific set of parameters and boundary conditions, and its applicability to other scenarios requires further validation. Future research should develop a more comprehensive damage assessment framework that accounts for the full range of possible column configurations and impact scenarios.

Another important consideration is the effect of multiple impacts on the residual bearing capacity. In real-world scenarios, structures may be subjected to multiple impacts (e.g., vehicle collisions, explosions, or seismic events). The cumulative damage from multiple impacts may be significantly greater than the sum of the individual impacts, due to the progressive degradation of the material properties and the interaction between damage zones. Future research should investigate the cumulative damage effects and develop appropriate design methodologies for multi-impact scenarios.

From a steel pipe manufacturing perspective, the study highlights the importance of the steel tube wall thickness and material properties in determining the impact resistance and residual bearing capacity of composite columns. Thicker steel tubes provide better confinement and impact resistance, but also increase the weight and cost of the column. The optimal wall thickness should be determined through a cost-benefit analysis that considers the expected impact loading, the structural importance of the column, and the economic consequences of failure.

Study Insights and Implications

This research provides valuable insights into the post-impact behavior of composite columns and offers a practical damage assessment methodology for structural engineers. The identification of the key parameters that influence impact resistance and residual bearing capacity (impact velocity, axial compression ratio, and slenderness ratio) provides clear guidance for design and assessment.

For steel pipe manufacturers and structural engineers, the implications are that the impact resistance of composite columns is a system property that depends on the interaction between the steel tube, the concrete core, and the boundary conditions. The steel tube must be designed and manufactured with awareness of its role in the overall impact response, and the welding connections that join the steel tube to other structural components must be capable of maintaining structural integrity under impact loading.

The finite element modeling approach employed in this study provides a powerful tool for investigating the impact behavior of composite columns under a wide range of parameters and conditions. Future research should extend this approach to include more realistic impact scenarios, including multi-impact events, oblique impacts, and impacts on partially damaged columns.

The study ultimately contributes to the broader goal of improving the safety and resilience of structures against impact loading. By providing a quantitative understanding of the post-impact behavior and a practical damage assessment methodology, the research enables more informed decision-making in structural design, assessment, and post-disaster response.