Blast Resistance of Circular Hollow Sandwich Steel Tube UHPSFRC Columns Under Explosive Loading

Literature Overview

This paper by Xu Shenchun, Liu Zhongxian, and Wu Chengqing (2017), published in Explosion and Shock Waves (Vol. 37, No. 4, pp. 649-660), presents a comprehensive investigation into the blast resistance behavior of circular hollow sandwich steel tube ultra-high performance steel fiber reinforced concrete (UHPSFRCFDST) columns. The study combines six full-scale blast tests with LS-DYNA finite element simulations, addressing a critical need in structural engineering for protective design against blast threats. The work was supported by the National Natural Science Foundation of China (51278326), the National Science and Technology Support Program (2012BAJ07B05), and the Tianjin Science and Technology Support Program (14ZCZDSF00016).

Core Technical Content and Key Findings

The research examines the dynamic response and damage patterns of UHPSFRCFDST columns under explosive loading, with particular attention to the influence of axial compression ratio, scaled distance, hollow ratio, and blast-facing surface geometry. The authors established a validated finite element model and then conducted parametric studies on axial compression ratio, hollow ratio, steel fiber content, inner and outer steel tube diameter-to-thickness ratios, and steel tube strengths.

Key Experimental Parameters

Principal Conclusions

The most important finding is the existence of a critical axial compression ratio. Below this threshold, increasing axial compression improves blast resistance by pre-stressing the composite system and delaying local failure initiation. Beyond this critical value, further increases in axial load aggravate damage because the column is already near its compressive capacity limit, making it more vulnerable to additional dynamic loading. This non-monotonic relationship is crucial for design optimization.

Reducing the hollow ratio or the diameter-to-thickness ratio of either tube layer enhances blast performance by increasing wall stiffness and delaying local buckling. Increasing steel fiber content improves energy absorption through crack bridging in the UHPSFRC layer. Increasing outer tube strength yields significant gains, whereas increasing inner tube strength produces negligible improvement—likely because the inner tube is shielded from direct blast loading by the concrete and outer tube layers.

Interpretation of Technical Points

From a metallurgical and materials engineering perspective, the use of UHPSFRC as the infill material is significant. Ultra-high performance concrete with steel fibers achieves compressive strengths exceeding 150 MPa and exhibits remarkable post-peak ductility. The steel fibers, typically 6-13 mm in length with aspect ratios of 50-100, bridge micro-cracks and prevent catastrophic spalling under explosive pressure pulses. The hollow sandwich design introduces an innovative topology: the inner steel tube provides additional confinement and structural redundancy without the full mass penalty of a solid section.

The LS-DYNA modeling approach requires careful attention to material constitutive models. The authors likely employed Johnson-Cook or similar strain-rate-dependent models for steel, while the UHPSFRC required a concrete damage model with appropriate rate-effect parameters. Validation against experimental data—particularly in terms of peak displacement, damage extent, and failure mode—is essential before parametric studies can yield reliable conclusions.

Engineering Practice Integration

For engineers designing blast-resistant structures, this study provides actionable design guidelines:

1. Axial load management: Ensure that the service axial load ratio remains below the identified critical value to maintain a favorable safety margin under blast events.
2. Hollow ratio optimization: While hollow sections reduce self-weight, excessive hollowing compromises blast resistance. A balance must be struck based on structural efficiency and protective requirements.
3. Steel fiber specification: Higher steel fiber content directly correlates with improved energy dissipation capacity, though economic constraints may limit practical fiber volumes to 2-4% by volume.
4. Tube thickness design: Prioritize outer tube thickness over inner tube strength upgrades for optimal cost-effectiveness in blast protection applications.

The findings have direct relevance to protective design in petrochemical facilities, military installations, and critical infrastructure where blast threats are credible. The methodology of combining full-scale testing with validated numerical models provides a replicable framework for evaluating other sandwich composite structural systems.

Key Questions and Reflections

Several questions merit further investigation. First, the critical axial compression ratio identified in this study likely depends on specific material properties and geometric proportions; a generalized design formula would be valuable. Second, the study focuses on single-sided blast loading—what about multi-directional or sequential blast events? Third, the long-term durability of UHPSFRC under cyclic loading and environmental exposure remains an open question for practical applications.

The observation that inner tube strength has minimal influence on blast performance is counterintuitive but physically reasonable. In a sandwich configuration subjected to external blast, the load path is outer tube → outer concrete layer → inner concrete layer → inner tube. The outer tube and outer concrete bear the brunt of the blast pressure, and the inner tube primarily provides secondary confinement. This load path understanding should guide material allocation decisions in design.

Study Insights and Implications

This paper represents a mature integration of experimental mechanics, computational modeling, and structural engineering design. The systematic parametric approach—validating the model first, then exploring design variables—is methodologically sound and provides engineers with confidence in the recommendations. For the steel pipe industry, the study highlights the potential of hollow sandwich steel tube sections in protective applications, suggesting market opportunities for specialized seamless or welded pipe products designed for blast-resistant construction. The emphasis on diameter-to-thickness ratio control also underscores the importance of dimensional accuracy in pipe manufacturing for structural applications.