Experimental Study on Axial Compression Mechanical Properties of Steel Tube Concrete Columns After Impact Loading

Literature Overview

This research by Fu Chaojiang, Zhang Ting, Chen Huayan, Gao Ying, Luo Caisong, Wang Bizhen, and Zhang Zequn (published in 2024, Volume 54, Issue 16, pp. 25-31 in the journal "Building Structures") investigates the post-impact axial compression behavior of circular steel tube concrete (CFST) columns. The study was supported by the Fujian Provincial Natural Science Foundation (Grant 2022J01928) and the Fujian Provincial Department of Housing and Urban-Rural Development Science and Technology Program (Grants 2022-K-71, 2022-K-305). Seven CFST column specimens were subjected to drop-weight impact tests followed by quasi-static axial compression testing, enabling evaluation of residual structural capacity after impact damage.

Core Technical Findings

Effect of Axial Compression Ratio

The axial compression ratio (the ratio of pre-applied axial force to the column's ultimate capacity) exhibits a non-monotonic influence on post-impact performance:

This non-monotonic behavior suggests an optimal axial compression ratio around 0.2 for post-impact performance, likely due to the transition from beneficial confinement effects at low ratios to detrimental effects of pre-existing damage at higher ratios.

Effect of Steel Tube Wall Thickness

The comparison between 5 mm and 7 mm wall thickness demonstrates substantial improvements:

Effect of Impact Energy

Increasing impact energy consistently degrades post-impact axial compression performance, indicating that the damage mechanisms (local denting, concrete cracking, steel tube yielding) are not fully recoverable through subsequent loading.

Failure Mechanism Analysis

The post-impact failure modes observed in the tests reveal several important structural behaviors:

1. Local denting of steel tube: Impact creates localized plastic deformation that reduces effective confinement capacity in the impacted region.
2. Concrete core cracking: Radial and circumferential cracks propagate from the impact zone, reducing the effective concrete cross-section.
3. Progressive buckling: Under subsequent axial compression, the pre-damaged region becomes the buckling initiation point, leading to accelerated failure.
4. Bond degradation: The impact may partially debond the steel tube from the concrete core, reducing composite action.

Proposed Calculation Formula

The authors developed a formula for the residual axial compression capacity of post-impact CFST columns, incorporating:

• The degree of impact damage (quantified through impact energy and dent geometry)
• The tube wall thickness and D/t ratio
• The concrete strength and confinement effects
• The axial compression ratio

This formula provides a practical tool for structural assessment and repair decisions following impact events such as vehicle collisions, projectile impacts, or equipment drops.

Welding and Structural Integrity Considerations

From a fabrication and integrity perspective, several aspects deserve attention:

• Weld integrity under impact: If the CFST column contains welded joints (such as splice connections or bolted flange welds), the impact may initiate fatigue cracks at weld toes. Post-impact inspection using magnetic particle testing (MT) or ultrasonic testing (UT) is essential before any continued service.
• Material selection: The steel tube material grade affects post-impact performance. Higher-strength steels (such as ASTM A500 Grade C or Q355) may exhibit reduced ductility in the HAZ, potentially limiting post-impact capacity. The wall thickness effect observed in this study may be partly attributed to the increased strain hardening capacity of thicker sections.
• Repair strategies: Post-impact repair may involve localized welding of steel tube patches or sleeves. The welding process must account for the existing residual stress field from impact, potentially requiring preheating and post-weld heat treatment to minimize additional HAZ damage.

Study Insights and Reflections

The non-monotonic relationship between axial compression ratio and post-impact performance is a particularly valuable finding. It suggests that moderate pre-compression may enhance confinement efficiency without introducing excessive pre-damage, while high pre-compression creates conditions where the already-stressed concrete core cannot sustain additional impact-induced damage. This finding has direct relevance to the assessment of existing structures after impact events, where the pre-impact axial load level significantly influences residual capacity.

The substantial improvement in capacity with increased wall thickness (35.46% for a 40% thickness increase) indicates a super-linear relationship, suggesting that thicker tubes not only provide more material for load resistance but also maintain more effective confinement over a larger strain range. This observation supports the use of heavier tube sections in impact-prone applications such as bridge columns, offshore platforms, and industrial facilities where impact loading is a credible threat.

The research methodology of combining impact testing with subsequent quasi-static compression testing is particularly rigorous, as it captures the real-world scenario of structural assessment after damage events. The proposed capacity formula should be validated against full-scale tests and field data before incorporation into design codes, but it represents a significant step toward rational assessment of post-impact structural integrity.