Blast Resistance Performance of Square Hollow Sandwich Steel Tube UHPSFRC Columns

Literature Overview

This study by Xu Shenchun, Liu Zhongxian, and Wu Chengqing, published in Journal of Vibration and Shock in 2017 (Vol. 36, No. 1, pp. 45-54), investigates the blast resistance performance of square hollow sandwich steel tube ultra-high performance steel fiber reinforced concrete (UHPSFRC-FDST) columns through numerical simulation and experimental verification. The research was supported by the National Natural Science Foundation (Grant No. 51278326), National Science and Technology Support Program (2012BAJ07B05), and Tianjin Science and Technology Support Program (14ZCZDSF00016). This work addresses the critical need for blast-resistant structural elements in protection engineering.

Core Technical Content

The researchers developed a three-dimensional finite element numerical model to simulate the dynamic response and damage failure of UHPSFRC-FDST columns under blast loading. The model was validated against experimental blast test results. A parametric study was then conducted to investigate the influence of key parameters on blast resistance performance.

Key Parameters Studied

Parameter	Range Investigated	Effect on Blast Resistance
Hollow ratio	Variable	Decreasing hollow ratio improves performance
Steel fiber volume fraction	Variable	Increasing content significantly improves performance
Outer steel tube thickness	Variable	Increasing thickness improves performance
Outer steel tube strength	Variable	Higher strength improves performance
Inner steel tube thickness	Variable	Reducing thickness-to-height ratio improves performance
Inner steel tube strength	Variable	Minimal effect on performance

Interpretation of Technical Points

UHPSFRC-FDST Column Configuration

The UHPSFRC-FDST column consists of:

Outer square steel tube: Provides primary structural support and blast resistance
Inner square steel tube: Creates a sandwich configuration with the UHPSFRC core
UHPSFRC core: Ultra-high performance concrete with steel fiber reinforcement, providing exceptional compressive strength (typically 120-200 MPa) and ductility

Dynamic Response Mechanism

Under blast loading, the column undergoes:

Initial impact phase: Blast wave impinges on the column face, generating high-pressure loading.
Dynamic deformation phase: The column deforms under dynamic loading; the UHPSFRC core and steel tubes interact through confinement.
Damage accumulation phase: Micro-cracking initiates in the UHPSFRC; steel fibers bridge cracks and maintain load-bearing capacity.
Post-peak response: The column enters a ductile deformation regime; the sandwich configuration provides progressive energy absorption.

Parametric Study Findings

The parametric analysis reveals several important relationships:

Hollow ratio: Reducing the hollow ratio (increasing the solid UHPSFRC core proportion) within a certain range improves blast resistance by providing more material for energy absorption.
Steel fiber content: Increasing steel fiber volume fraction significantly enhances blast resistance through crack bridging and improved tensile strength of the UHPSFRC.
Outer tube strength: Higher strength outer tube directly improves blast resistance by providing greater confinement pressure to the UHPSFRC core.
Inner tube strength: Surprisingly, inner tube strength has minimal effect, suggesting that the inner tube primarily serves a geometric confinement function rather than a strength contribution.

Connection with Engineering Practice

Welding and Fabrication Requirements

The fabrication of UHPSFRC-FDST columns involves complex welding operations:

Weld Type	Process	Heat Input	Special Considerations
Outer tube longitudinal seam	SAW or FCAW	0.8-1.5 kJ/mm	Full penetration required
Inner tube longitudinal seam	SAW or FCAW	0.6-1.2 kJ/mm	Full penetration required
Outer tube circumferential weld	SMAW or FCAW	0.5-1.0 kJ/mm	Fit-up tolerance ±1 mm
Inner tube circumferential weld	SMAW or FCAW	0.5-1.0 kJ/mm	Fit-up tolerance ±1 mm
End seal welds (before concrete placement)	SMAW	0.5-1.0 kJ/mm	Leak-tightness critical
Spacer welds (between inner and outer tube)	Tack welds	0.2-0.5 kJ/mm	Spacing per design

UHPSFRC Placement and Quality Control

Concrete pumping: UHPSFRC has low workability; specialized pumping equipment and vibration methods are required.
Vibration: Internal vibration only; external vibration may damage the inner tube.
Curing: Extended curing period (minimum 14 days) required to achieve design strength.
Strength verification: Cube tests at 7, 14, and 28 days; design strength typically 120-200 MPa.

Blast Protection Design Standards

GB 50011-2010: Code for Seismic Design of Buildings (blast provisions)
GB 50779-2012: Code for Design of Blast-Resistant Buildings
UFC 3-340-02: Unified Facilities Criteria - Structures to Resist the Effects of Accidental Gas Explosions
MIL-STD-3007: Procedures for Performing Single Degree of Freedom (SDOF) Analyses
EN 1991-1-7: Eurocode 1 - Actions on structures - General actions - Actions due to accidental events

Key Questions and Reflections

The study raises several important engineering questions:

Optimal hollow ratio: What is the minimum hollow ratio that maintains structural efficiency while providing adequate blast resistance? The study suggests a range but does not establish a definitive optimum.
Steel fiber type: The study does not differentiate between steel fiber types (straight, hooked, crimped). Different fiber geometries may have different effects on blast resistance.
Multi-event loading: The study focuses on single blast events. How does the column perform under multiple sequential blast events?
Residual capacity: After a blast event, what is the residual load-bearing capacity of the column? This is critical for post-event assessment and repair.

Study Insights and Implications

This research provides valuable design guidance for blast-resistant structural engineering, demonstrating that the UHPSFRC-FDST configuration offers superior blast resistance compared to conventional CFST columns. The key insight is that the sandwich configuration, combined with ultra-high performance concrete, creates a highly effective energy absorption system under dynamic blast loading.

For steel pipe manufacturers and fabricators, this research highlights the following practical considerations:

Precision fabrication: The sandwich configuration requires precise dimensional control of both inner and outer tubes to ensure uniform UHPSFRC thickness.
Weld quality: All welds must achieve full fusion and be free of defects, as any weakness in the steel tube system compromises the entire blast resistance performance.
Material selection: High-strength steel grades (Q460, Q550, or equivalent) are recommended for the outer tube to maximize confinement effectiveness.

The practical applications of this research extend to:

Military and defense facilities requiring blast protection
Critical infrastructure (power plants, water treatment facilities) vulnerable to terrorist attacks
Industrial facilities handling hazardous materials
Underground and semi-underground structures

This work represents a significant advancement in blast-resistant structural design, combining advanced materials (UHPSFRC) with innovative structural configuration (sandwich steel tube) to achieve superior performance under extreme loading conditions. The numerical model developed in this study provides a powerful tool for optimizing UHPSFRC-FDST column design for specific blast scenarios, enabling engineers to achieve the required level of protection with efficient material usage.