Blast Mitigation Measures for Public Spaces
Executive Summary
As terrorist threats evolve, particularly the use of improvised explosive devices (IEDs) in public spaces, protecting urban environments from such attacks has become a critical priority. IEDs can be transported via various methods, including vehicles (VBIEDs), people (PBIEDs), cargo bicycles, or even unmanned aerial systems (UAS). A comprehensive blast mitigation strategy that integrates both structural and non-structural protective measures is essential to safeguard public spaces from such threats. This case study focuses on blast mitigation measures, specifically hardening structures, increasing standoff distances, and employing innovative technologies to minimize the risks associated with explosive attacks in public spaces. It highlights using empirical models, pressure-impulse diagrams, and advanced simulation tools to assess blast loads and implement appropriate countermeasures.
Blast Assessment Methodologies
Protecting structures from external explosions requires calculating the blast loads a building or urban infrastructure must withstand. The engineering approach to this process involves the following:
Empirical and Semi-Empirical Models: These models, such as the Kingery and Bulmash formulas (1984), are widely used to estimate blast pressures and impulse parameters based on the characteristics of the explosive material (e.g., TNT, ANFO). These models work under simplified conditions, such as assuming spherical or hemispherical blast propagation, and are helpful for preliminary assessments.
Pressure-Impulse Diagrams: Pressure-impulse (P-I) diagrams condense the results of numerous experiments or simulations into a single graphical representation. These diagrams map the behaviour of structures under varying blast load conditions and show the relationship between peak pressure, impulse, and damage. Tools like the Viper::Blast use P-I diagrams to facilitate the pre-assessment of structures under blast loads and calculate critical parameters such as peak overpressure and impulse at specific distances.
Numerical Blast Simulations: For more complex scenarios involving multireflection, channelling, and shadowing phenomena, finite element analysis (FEA) is used. Explicit finite element software like Europlexus can model the effects of blast waves interacting with urban structures, accounting for variables such as street layouts and building geometries. These simulations offer higher accuracy but are computationally intensive.
Blast Mitigation Strategy
Several factors must be considered, including the standoff distance, the charge type, and the risk analysis for the targeted structure, to design effective blast mitigation measures:
Standoff Distance: The standoff distance (R), or the distance between the potential explosion site and the protected structure, is the most critical factor in reducing the impact of a blast wave. By increasing this distance, the intensity of the blast wave decreases exponentially. Installing perimeter barriers (e.g., fences, bollards) can effectively increase this standoff distance, reducing the need for costly building hardening. In scenarios where increasing standoff distance is not feasible, additional hardening of structures, such as reinforced facades and windows, becomes necessary to minimize blast effects.
Structural Hardening: Reinforcing the building envelope (windows, doors, and facade) is crucial for blast mitigation. The weakest points in a building, such as windows and doors, require blast-resistant materials, including: Laminated safety glass or blast-resistant glazing systems prevent the glass from shattering and becoming secondary fragments. Blast-resistant doors and reinforced facades are designed to withstand high overpressure. Specialized safety films are applied to windows to prevent glass fragments from becoming dangerous projectiles.
Building geometry also plays a key role in distributing blast pressure. Convex-shaped buildings distribute pressure more evenly, reducing the amplification of blast loads on the facade.
Enhanced Perimeter Security: Installing structural barriers that prevent unscreened vehicles from accessing sensitive areas is essential. These barriers include: Reinforced bollards, Chicanes and tight road bends, Hardened street furniture & Surveillance and detection systems can also monitor potential threats, enhancing perimeter security.
Hazards and Effects of Explosions: The primary hazards from an explosion in public space include blast waves, primary and secondary fragments, and tertiary impacts (e.g., falling debris). These hazards vary depending on the distance from the explosion and the structural elements in place.
Blast Waves: The primary effect of an explosion is the propagation of high-pressure blast waves, which can cause severe structural damage and injury to individuals within a certain radius. The intensity of these waves decreases rapidly with increased standoff distance.
Primary and Secondary Fragments: Primary fragments (such as nails or casing parts) from the explosive device can cause significant injury. Secondary fragments include debris, such as broken glass or detached urban furniture, propelled by the blast wave. Securing street furniture to the ground can mitigate the risk of it becoming dangerous debris during an explosion.
Progressive Building Collapse: There is a risk of progressive collapse if critical structural elements fail for large explosions near buildings. Reinforcing load-bearing structures ensures that the failure of one element does not lead to the collapse of the entire building.
Innovative Technical Solutions: The Design of blast-resistant structures requires innovative materials and technologies. Key solutions include: Blast-resistant windows and doors that comply with EN 13541 and ISO 16933 & Structural retrofitting.
Cost vs. Benefit Analysis: Increasing standoff distance is typically a cost-effective method of reducing blast effects compared to structural hardening. However, despite the higher costs, investing in structural reinforcement (such as blast-resistant facades and laminated glass) may be necessary in dense urban areas with limited space.
Conclusion and Strategic Recommendations
Blast mitigation measures are an integral part of Security by Design, which aims to protect public spaces from the increasing threat of terrorist attacks. By employing a combination of standoff barriers, structural hardening, and advanced simulation tools, urban planners and security engineers can design public spaces that are both safe and functional. Integrating blast-resistant materials, strategic barriers, and early detection systems ensures a holistic approach to mitigating the effects of explosions while maintaining the aesthetics and accessibility of public spaces.28