Dynamic Response of Concrete-Filled Steel Tube Piers Under Blast Loading

Literature Overview

The paper by Sun Shanshan, Zhao Junhai, He Shuanhai, Cui Ying, and Liu Yan, published in Engineering Mechanics (Volume 35, Issue 5, 2018, pages 27–35), investigates the dynamic response and failure modes of concrete-filled steel tube (CFST) pier columns subjected to TNT blast loads. The study combines experimental static blast tests with finite element analysis to characterize the behavior of both conventional and double-skin hollow CFST piers. This work is significant for bridge engineering and civil infrastructure protection, where blast resistance of support piers is a critical safety consideration.

Core Technical Findings

The researchers designed and fabricated four specimens: three conventional CFST piers and one double-skin hollow CFST pier. Static blast tests were conducted using TNT charges of 3 kg and 50 kg across three shots and four loading conditions. The key findings can be summarized as follows:

• Under a 50 kg TNT charge at a scaled distance of 0.14 m/kg^(1/3), the conventional CFST pier (outer diameter 273 mm, wall thickness 7 mm) demonstrated superior blast resistance deformation capacity compared to the double-skin CFST pier (inner diameter 50 mm, wall thickness 4 mm).
• Three typical failure modes were identified: bending failure under low overpressure peak with high duration, shear failure under high overpressure peak with low duration, and a combined flexural-shear failure mode in between.
• When the scaled distance exceeds 0.3 m/kg^(1/3) with a 50 kg charge, residual deformation of the pier is negligible, indicating that the structure can recover functionally.
• Increasing core concrete strength and steel ratio effectively reduces mid-span horizontal residual deformation.
• Raising the steel tube yield strength reduces residual deformation, but when the steel grade reaches or exceeds 345 MPa, further increases in yield strength provide diminishing returns in blast performance improvement.

Blast Resistance Parameters and Design Implications

The following table summarizes the critical parameters and design thresholds identified in this study:

Flow-Solid Coupling Simulation Methodology

The study developed a multi-material flow-solid coupling numerical simulation method based on experimental results. This approach effectively models the dynamic response of CFST piers under blast loading by accounting for the interaction between the air-blast wave, the steel tube, and the concrete core. The simulation captures the complex phenomena of shock wave propagation, reflection, and the subsequent quasi-static pressure phase that drives structural deformation.

From a materials and welding engineering perspective, the blast resistance performance of these piers is intimately connected to the quality of the steel tube manufacturing and welding. The steel tubes used in CFST piers are typically manufactured as ERW (electric resistance welded) or LSAW (longitudinal submerged arc welded) pipes, and the integrity of the weld seam directly affects the tube's ability to confine the concrete core and resist blast-induced deformation. Any weld defect—such as lack of fusion, porosity, or undercut—could serve as a stress concentration site under the extreme dynamic loading of a blast event, potentially initiating premature failure.

Engineering Practice Integration

In practical bridge design, the findings of this study have direct implications for pier column specifications:

1. For applications where blast protection is a concern, designers should ensure that the scaled distance between potential explosive sources and pier columns exceeds 0.3 m/kg^(1/3) to maintain negligible residual deformation.
2. The optimal steel grade for CFST piers appears to be in the range of 300–345 MPa yield strength; using higher-grade steels (such as Q420 or Q460) does not proportionally improve blast performance and may introduce challenges in welding and fabrication.
3. The double-skin hollow configuration, while offering potential weight savings, may not always outperform conventional CFST designs under blast loading, particularly when the inner tube wall thickness is reduced.

Study Insights and Reflections

After reviewing this paper, I find the identification of three distinct failure modes based on overpressure peak and duration particularly valuable for practical engineering judgment. In my experience with steel pipe and welding quality assessment, understanding the loading regime is essential for predicting failure modes. Similarly, in blast engineering, distinguishing between quasi-static pressure-dominated loading and impulse-dominated loading is critical for appropriate design responses.

The observation that steel yield strength beyond 345 MPa provides diminishing returns is consistent with what we observe in welding metallurgy—higher strength steels often exhibit reduced ductility in the heat-affected zone, which can compromise the overall structural performance under dynamic loading. This reinforces the importance of selecting appropriate material grades rather than simply maximizing strength.

The study's emphasis on residual deformation as a key performance metric aligns with modern performance-based design philosophy, where the ability of a structure to return to serviceable condition after an extreme event is often more important than ultimate failure resistance. For engineers involved in steel pipe fabrication and welding inspection, this means that maintaining weld quality and material integrity is not merely a compliance exercise but a fundamental requirement for structural resilience.

This paper provides a solid foundation for the design of blast-resistant CFST piers and highlights the importance of integrating materials science, welding quality, and structural dynamics in achieving robust infrastructure protection.