Dynamic Response of Precast Segmental Double-Skin CFST Piers Under Blast Loading

Literature Overview

This 2024 study by Li Minghong et al., published in the Journal of Southeast University (Natural Science Edition), presents a comprehensive numerical investigation of a novel precast segmental double-skin concrete-filled steel tube (PS-CFDST) composite pier designed for blast-resistant highway bridge applications. Funded by the National Natural Science Foundation of China (Grant 52208469), the China Postdoctoral Science Foundation (Grants 2020M681459, 2022T150120), and the Jiangsu Provincial Natural Science Foundation (Grant BK20220850), this research was conducted at Southeast University's Ministry of Education Engineering Research Center for Blast Protection. The work represents a state-of-the-art contribution to the field of blast-resistant structural engineering, combining precast construction methodology with double-skin composite design philosophy.

Technical Framework and Modeling Approach

Structural Configuration

The PS-CFDST pier concept integrates three key design features:

1. Precast segmental assembly: The pier is constructed from prefabricated segments joined at interfaces, enabling rapid construction and field assembly.
2. Double-skin CFST configuration: An inner and outer steel tube encloses the concrete core, providing enhanced confinement, redundancy, and blast resistance.
3. Pre-stressed connections: The segment interfaces are connected using pre-stressing, which enhances the integrity of the assembled pier under dynamic loading.

Numerical Modeling

The finite element model was developed using LS-DYNA, a nonlinear explicit dynamic analysis program well-suited for blast and impact simulations. The model was validated against existing experimental data to ensure accuracy before parametric studies were conducted. The modeling approach accounts for:

• Material constitutive models for steel tubes, concrete core, and pre-stressing tendons.
• Contact algorithms for segment interface interactions.
• Air-blast loading conditions based on standard blast load formulations.
• Large deformation and material failure criteria.

Key Findings and Technical Analysis

Damage Mechanisms and Failure Progression

The numerical simulation reveals a complex damage progression sequence under blast loading:

1. Initial elastic response with stress wave propagation through the double-skin tube system.
2. Outer tube yielding and local buckling at the blast-facing side.
3. Concrete core crushing and spalling in the high-strain-rate zone.
4. Inner tube yielding providing secondary containment.
5. Segment interface sliding and debonding under sustained loading.
6. Progressive damage accumulation leading to potential structural failure.

The double-skin configuration provides significant redundancy: even after the outer tube fails, the inner tube continues to confine the concrete core, maintaining structural integrity beyond the point of outer tube collapse.

Energy Dissipation Mechanisms

A particularly significant finding is the quantification of energy dissipation contributions:

The segment interface sliding mechanism contributes over 40% to the total energy dissipation capacity. This finding is of paramount importance for the design of precast blast-resistant structures, as it demonstrates that the interfaces—traditionally considered as potential weak links—can be deliberately designed as energy dissipation elements. This represents a paradigm shift in precast structural design philosophy, moving from minimizing interface effects to actively exploiting them for energy absorption.

Parametric Study Results

Blast Distance Effects

Increasing the distance between the blast source and the pier significantly enhances the pier's blast resistance. The blast pressure decays with distance following well-established blast scaling laws, and the numerical results confirm that proximity is the dominant factor governing damage severity. Design recommendations emphasize maximizing standoff distance as the most effective and economical blast mitigation strategy.

Number of Segments Effects

The number of precast segments influences both the dynamic response and the energy dissipation capacity. More segments provide more interface surfaces for sliding energy dissipation but also introduce more potential failure locations. An optimal balance must be struck between energy dissipation capacity and structural integrity.

Pre-stress Level Effects

Increasing the pre-stress level at segment interfaces yields two beneficial effects:

• Reduction of maximum horizontal displacement of the pier.
• Reduction of shear sliding at the pier base.

The pre-stressing effectively clamps the segments together, preventing premature separation under blast loading and ensuring that the interfaces remain engaged throughout the loading cycle. This finding validates the use of pre-stressing as a key design parameter for enhancing the blast performance of precast composite piers.

Engineering Practice Integration

Blast Design Recommendations

Based on the parametric study results, the following design recommendations emerge for PS-CFDST piers in blast-prone environments:

1. Maximize standoff distance: The most effective and cost-efficient blast protection measure is to increase the distance between potential blast sources and the structural element.
2. Optimize pre-stress levels: Pre-stressing should be designed to maintain interface engagement under the design blast load while allowing controlled sliding for energy dissipation beyond the design event.
3. Segment interface design: Interfaces should be designed with sufficient friction capacity to resist sliding under service and moderate blast loads, while allowing controlled sliding under extreme events for energy absorption.
4. Double-skin tube design: The inner tube should be designed to maintain concrete confinement even after outer tube failure, providing structural redundancy.

Welding and Fabrication Considerations

The PS-CFDST pier concept presents unique challenges for steel pipe fabrication and welding:

• The double-skin tube configuration requires welding of both inner and outer tubes, with potential interaction effects between the two welding sequences.
• Segment interface connections may involve welded or bolted joints, each with distinct quality control requirements under blast loading conditions.
• The high strain rates experienced during blast events demand that weld metals possess adequate strain rate sensitivity and ductility at elevated strain rates.
• Welding procedures must be qualified for the specific steel grades used in the double-skin configuration, with particular attention to the toughness requirements at the welding consumable/base metal interface.
• Non-destructive testing (NDT) requirements should be enhanced for blast-critical structures, potentially incorporating phased array ultrasonic testing (PAUT) for comprehensive weld volume inspection.

Standards and Code Considerations

The design of blast-resistant precast composite piers intersects with multiple standards:

Study Insights and Reflections

The most intellectually stimulating aspect of this research is the reframing of precast segment interfaces from potential structural weaknesses into deliberate energy dissipation mechanisms. This perspective aligns with the broader engineering philosophy of designing for controlled failure rather than preventing all damage. The finding that interface sliding can contribute over 40% of total energy dissipation fundamentally changes how precast connections should be designed for blast-resistant applications. However, this also introduces complexity in the design process, as the interface behavior must be carefully calibrated to provide sufficient sliding for energy absorption without excessive sliding that would compromise structural integrity. The parametric study results regarding pre-stress levels provide a practical design tool, but the optimal pre-stress level will depend on the specific blast scenario, pier geometry, and performance objectives. Future research should address the cumulative damage effects of multiple blast events on precast interfaces and the long-term durability of pre-stressing systems under cyclic thermal and environmental loading.