Zhuojin Pipe Fitting Co., LtdBlast Resistance Performance of FRP Confined Steel Tubular Concrete Columns Under Axial Compression
Literature Overview
This study, published in Industrial Construction (2021, Vol. 51, Issue 1, pp. 179-186) by Liu Lan, Wang Lijing, Guo Hong, and Cheng Zhi from the Department of Civil Engineering at North University of China, investigates the dynamic response of fiber-reinforced polymer (FRP) confined steel tubular concrete (STC) columns subjected to blast loading. The research employs finite element simulation using ANSYS/LS-DYNA to analyze 16 axially loaded column specimens under explosion loads, with parametric analysis covering axial compression ratio, slenderness ratio, FRP wrapping methods, and FRP material types. The work is funded by the Shanxi Provincial Applied Basic Research Program (grants 201601D202048, 201801D221233, 201901D111169).
Core Technical Findings
The parametric study reveals several critical relationships governing the blast resistance of FRP-confined STC columns. The axial compression ratio emerges as the most influential parameter, with a threshold behavior observed at 0.5. Below this threshold, the presence of axial compression enhances blast resistance by pre-stressing the composite system and promoting confinement effectiveness. Above 0.5, however, the axial load significantly degrades blast performance due to reduced ductility reserve and accelerated concrete crushing under combined compressive and dynamic loading.
Key Parametric Effects
| Parameter | Effect on Blast Resistance | Critical Threshold |
|---|---|---|
| Axial compression ratio (n) | Positive when n ≤ 0.5; strongly negative when n > 0.5 | n = 0.5 |
| Slenderness ratio (λ) | Larger λ results in greater mid-span displacement and poorer blast resistance | No sharp threshold |
| FRP wrapping method | Partial wrapping can approach full-wrapping confinement effect | Depends on location |
| FRP type (CFRP vs GFRP) | CFRP provides superior blast resistance enhancement | Material-dependent |
The slenderness ratio directly correlates with mid-span displacement magnitude under blast loading, indicating that stockier columns inherently possess better blast resistance characteristics. Regarding FRP application methods, the study demonstrates that strategically placed partial wrapping can achieve confinement effects approaching those of full-column wrapping, which has significant implications for retrofit cost optimization in existing structures.
Technical Interpretation and Engineering Practice
From a steel pipe manufacturing and structural engineering perspective, this study carries several important implications. First, the interaction between axial pre-compression and dynamic blast loading in confined concrete systems mirrors the behavior observed in pressure vessels and thick-walled pipe components subjected to shock loading. The threshold at axial compression ratio 0.5 aligns with established design codes for reinforced concrete columns under seismic and impact loading, suggesting consistency across loading regimes.
The finding that CFRP outperforms GFRP in blast resistance enhancement relates to the fundamental mechanical properties of these composite materials. CFRP possesses a higher elastic modulus (typically 200-250 GPa versus 70-85 GPa for E-glass) and ultimate tensile strength (3500-5500 MPa versus 1500-2000 MPa), which translates directly to superior strain confinement capacity under dynamic conditions. The dynamic strain rate effect further amplifies this advantage, as CFRP maintains a higher stress-strain response at elevated strain rates typical of blast loading scenarios.
Practical Considerations for FRP-Constrained Pipe Systems
When translating these findings to pipe-based structural applications, several factors warrant attention:
- The steel tube itself provides inherent lateral confinement to the concrete core, and the FRP wrap adds a secondary confinement layer that becomes particularly effective after the steel tube buckles or yields
- The bond interface between FRP and steel tube surface is critical; surface preparation methods (grinding, primer application) significantly affect wrap effectiveness
- For pipe columns with thin wall ratios (D/t > 50), the steel tube's confinement capacity diminishes rapidly, making FRP wrapping more beneficial
- Blast loading induces both axial and lateral dynamic forces, creating complex stress states that differ substantially from quasi-static conditions
The partial wrapping strategy identified as effective in this study has direct parallels in pipeline repair practices, where localized reinforcement of damaged or corroded pipe sections is commonly employed. The principle that strategic placement can approach full-circumference effectiveness suggests optimization opportunities in retrofit programs.
Key Questions and Reflections
Several questions arise from this research that merit further investigation. The study focuses exclusively on axial compression columns, yet in practical blast-protected structures, columns frequently experience biaxial or even triaxial bending combined with axial loads. The interaction between eccentricity and FRP confinement under blast conditions remains an open research question. Additionally, the long-term durability of FRP wraps under cyclic blast exposure—particularly the progressive debonding and matrix cracking mechanisms—deserves systematic study.
The numerical model validation against experimental data is essential for confidence in these parametric results. While the authors employed ANSYS/LS-DYNA, the accuracy of concrete damage models (such as the Concrete Damaged Plasticity model or Johnson-Holmquist model) under high strain rates remains a recognized challenge in computational structural mechanics. Future work should incorporate validated material models with rate-dependent constitutive relationships.
Study Insights and Implications
This research provides valuable guidance for the design and retrofit of steel tubular concrete columns in blast-prone environments such as offshore platforms, military facilities, and critical infrastructure. The identification of the 0.5 axial compression ratio threshold offers a clear design boundary for engineers, while the partial wrapping optimization strategy provides cost-effective retrofit solutions. For the steel pipe industry, these findings reinforce the importance of wall thickness selection in pipe column applications, as the tube's confinement capacity directly influences the system's blast response characteristics. The comparative performance of CFRP versus GFRP also informs material selection decisions in structural strengthening projects involving steel pipe components.