Blast Resistance Performance of Double-Layer Square CFST Piers

Literature Overview and Research Context

The paper by Li Minghong and colleagues (2023), published in China Journal of Highway and Transport, presents a pioneering experimental investigation into the blast resistance performance of double-layer square concrete-filled steel tube (CFST) bridge piers. This research, supported by multiple National Natural Science Foundation grants (52208469, 51678141), the Jiangsu Provincial Natural Science Foundation (BK20220850), and the China Postdoctoral Science Foundation (2020M681459, 2022T150120), addresses a critical safety concern for bridge infrastructure in conflict zones and areas susceptible to accidental explosions. Three double-layer square CFST pier specimens were subjected to close-range and contact blast tests, followed by static axial compression tests on the blast-damaged specimens and one undamaged control.

Experimental Configuration and Blast Parameters

The experimental setup involved precise control of blast parameters to systematically evaluate the effect of charge distance and explosive equivalent weight. The double-layer configuration consists of an outer square steel tube enclosing an inner square steel tube, with concrete filling the space between the inner tube and the outer tube. This configuration provides enhanced confinement compared to single-layer CFST sections.

Test Parameter	Specimen 1	Specimen 2	Specimen 3	Control
Explosive type	TNT equivalent	TNT equivalent	TNT equivalent	N/A
Charge weight (kg)	2.0	3.0	4.0	N/A
Charge distance (mm)	Contact	100	200	N/A
Section size (mm)	300×300	300×300	300×300	300×300
Outer tube thickness (mm)	6	6	6	6
Inner tube thickness (mm)	4	4	4	4
Concrete grade	C50	C50	C50	C50
Steel grade	Q345	Q345	Q345	Q345

The post-blast static axial compression tests measured the residual load-bearing capacity of each specimen, enabling quantitative assessment of damage severity through the remaining capacity ratio.

Damage Patterns and Residual Capacity Analysis

The experimental results revealed several important findings regarding the blast damage mechanisms:

Damage Patterns Observed:

Local indentation deformation in the region directly facing the blast center
Steel tube fracture and tearing at high-stress concentration zones
Concrete crushing in the compressed zone beneath the indented region
Penetration of the front wall in contact blast conditions
No significant overall deformation or global buckling observed

Residual Capacity Assessment:

Specimen	Charge Distance	Capacity Loss (%)	Damage Grade
Specimen 1	Contact	41%	Moderate
Specimen 2	100 mm	32%	Moderate
Specimen 3	200 mm	22%	Moderate
Control	N/A	0%	None

All three blast-tested specimens were classified as moderately damaged, maintaining relatively good post-blast load-bearing capacity. The charge distance demonstrated a significant influence on residual capacity, with closer charges causing proportionally greater damage.

Comparative Analysis with RC Piers and Protective Mechanisms

A particularly valuable aspect of this research is the comparative analysis with conventional reinforced concrete (RC) piers under identical blast conditions. The results demonstrate that double-layer CFST piers exhibit significantly superior blast resistance:

Smaller damage extent under the same explosive loading
Higher residual load-bearing capacity (22–41% loss vs. typically 50–70% for RC piers)
Effective suppression of concrete spalling by the steel tube confinement
Prevention of high-velocity concrete fragments that could cause secondary injuries to personnel and damage to surrounding facilities

The steel tube confinement mechanism is critical: it maintains concrete cohesion even under extreme compressive and tensile stresses induced by blast waves, preventing the explosive fragmentation of concrete that is characteristic of unprotected RC members. This protective effect is analogous to the confinement provided by spiral reinforcement in RC columns but is far more effective due to the continuous and uniform nature of the steel tube.

Engineering Implications and Design Considerations

From a bridge engineering and protective design perspective, this research provides critical data for the design of blast-resistant bridge piers. The double-layer CFST configuration offers several advantages over single-layer CFST:

The outer tube provides primary blast wave resistance and initial energy absorption
The inner tube provides secondary confinement and maintains structural integrity even when the outer tube is damaged
The concrete between the tubes serves as a buffer, absorbing and dissipating blast energy
The composite action between steel tubes and concrete provides high ductility and energy absorption capacity

For practical implementation, engineers must consider the increased material cost and fabrication complexity of the double-layer configuration. The inner tube must be precisely positioned within the outer tube with appropriate formwork to ensure uniform concrete placement. Welding connections between the outer tube and transverse stiffeners must be designed to distribute blast loads effectively. The research strongly supports the adoption of double-layer CFST piers for critical bridges in areas with elevated blast risk, including military installations, border crossings, and high-value infrastructure in conflict-prone regions.