In recent years, terrorist attacks using vehicle-borne bombs have become a worldwide threat. These attacks have migrated from targeting military bases to densely populated urban areas, such as the devastating bombing attack in Oklahoma City in 1995. To protect critical facilities as well as human lives, security professionals need to completely assess potential vulnerabilities, which often is the most challenging task. Multidisciplinary cooperation and technical integrations as well as multi-resilience protection systems are always the key features. In addition to active prevention of such man-made hazards in advance, passive perimeter security barriers provide an additional way to assure life and facility safety.
This article proposes a generic and systematic framework to assess physical attacking risks so owners and security professionals have guidelines to select the most appropriate security measures. Widely-used as well as newly-developed barrier technologies are also reviewed, discussed, and compared in detail. Finally, computer-based numerical engineering simulations and prototype validations are examined to provide valuable tools for owners and professionals to evaluate various design options and optimize security solutions.
Framework to Select Barriers
The generic framework proposed here to select appropriate barrier types is illustrated in Figure 1.
Figure 1 Generic Framework to Select Appropriate Barriers
The first key stage is the risk assessment of possible attacks, which includes but is not limited to:
1) Facility site investigation to identify vulnerable spots surrounding the target.
2) Vehicle traffic analyses around the target, including two major components:
i. Vehicle attacking scenarios: Performing traffic and vehicle motion studies to determine possible attacking vehicle types, traveling paths toward the target, and maximum impact velocities along the paths.
ii. Authorized vehicle access: Clarifying client’s needs and rearranging allowable paths and secured accessible points for authorized vehicles.
3) Determination of security perimeter.
One key point in conducting risk assessments and determining security solutions is to define the minimum allowable blast stand-off distance away from the facility. The parameters are illustrated in Figure 2, showing typical blast wave propagation from an explosion near a building.
Figure 2 Explosive Blast Characteristics beside a Target Structure
Researchers have found that with certain explosive quantity, the blast overpressure and reflected pressure acting on the structure usually decays at a rate which is close to the reciprocal of cube of the stand-off distance. Some past events have demonstrated this concept. The attack with about 4,000 lbs TNT-equivalent explosive at a 15-foot stand-off distance caused most of the Alfred P. Murrah Federal Building in Oklahoma City to collapse. The 1995 attack killed 168 people. One year later, another attack with more than 20,000 lbs TNT equivalent at an about 80-foot stand-off distance only destroyed the front portion of the Khobar Towers in Saudi Arabia, killing 19 U.S. servicemen. It is therefore crucial to assure the minimum stand-off distance between a target structure and an explosive-laden vehicle. Three issues are essential to deal with:
i. Blast analysis of a target structure: Blast analyses consider all vulnerable spots and stand-off distances, using empirical equations as well as high-tech physical-based engineering software.
ii. Structural damage limit states and levels of protection: Limit states are defined as levels of damage which indicate, to certain extents, the potential loss of either structural functionality or occupants’ life safety. Structural damage limit states are typically classified as five categories as shown in Figure 3, ranging from immediate damages to partial or progressive major collapses.
Figure 3 Structural Damage Limit State Definitions
Levels of protection reflect the degree to which the facility needs to be protected against the threat based on its performance under attack and its value to the user. That is to say, level of protection refers to the damage limit state a facility or asset would be allowed to sustain in the event of an attack. The lower the damage limit state is allowed, the higher the level of protection shall be required. Hence the levels of protection will rule the design criteria based on allowed limit states and corresponding attack scenarios and direct professionals to find effective and efficient security solutions.
iii. Minimum stand-off distance: The minimum stand-off distance criterion can then be decided from the blast analyses results after owners choose what level of protection is adequate for their situation.