The second key stage is to select and implement the most appropriate physical perimeter barriers based on the outcome of Stage I. When selecting the perimeter barrier, the security professional needs to consider many key factors, including:
1) Minimum stand-off distance versus allowable property lines;
2) Possible maximum vehicle size and speed versus existing traffic;
3) Site conditions versus barrier’s foundation needs;
4) Security level of protection versus accessibility for authorized people and vehicles;
5) Flexibility of aesthetic design versus surroundings, especially in urban areas.
Such selection is actually an optimization process going through some inevitable tradeoffs in the real world. The selection of the most suitable barrier is always driven by the combination of anti-terrorism functionality, decoration flexibility (very important in urban area), environmental impacts, construction efforts, and, of course, the owner’s budget.
Vehicular Crash Test Standards for Physical Barriers
As discussed previously, to keep the target facility far enough from devastating blast wave loads and to prevent similar tragedies as the Oklahoma City Bombing, a sufficient blast stand-off distance needs to be maintained, especially after terrorists try to crash their truck through any secured perimeter. Hence, physical perimeter barriers should fully stop any impacting vehicles while keeping explosives farther away from the target than any required minimum stand-off distance.
US federal agencies have developed systematic test standards using real crash tests to quantify, verify, and certify barrier performance. Such test methods were initially published and maintained by the U.S. Department of State (DoS), Bureau of Diplomatic Security, in 1985 as SD-STD-02.01. It was revised in 2003 as SD-STD-02.01 Revision A, which has been gradually replaced since 2007, with ASTM F 2656-07 Perimeter Barrier Vehicle Crash Test Standard.
Table 1 Comparison between Anti-Crash Standards
Table 1 shows the typical requirements and regulations from the DoS and ASTM standards. The first basic idea here is to define certain threat levels according to the weight of the vehicle and its maximum speed when it crashes into the barrier. A 15,000 lbs truck such as Ford F750 truck is normally selected as the predominant vehicle type due for two reasons: 1) its large truck bed can carry enough explosive ( anywhere from 1000 to 4000 lbs of TNT equivalent, and 2) its overall medium size makes it ideal for driving within a typical city at a high speeds. The left half of Table 1 describes the DoS K-Rating criteria and their latest equivalents, the M-Designations, in the ASTM standard. The new standard follows the same basic idea of the DoS K-Rating system with different combinations of the vehicle’s weight and its impact speed. Say a 15,000 lbs truck crashing into the barrier’s front at speeds of 30, 40 or 50 mph.
The second basic idea is to evaluate a barrier’s actual protection performance according to the measured forward movement of the truck bed after crash. A truck bed with explosives shall always be kept as far as possible from the target facility so that sufficient blast stand-off distance could be maintained after a vehicle crash. Comparisons between the two past examples, the 1995 Oklahoma City Bombing and the 1996 Khobar Towers Bombing in Saudi Arabia, have clearly demonstrated the ultimate importance of this evaluation criterion based on a truck bed’s final location when a potential explosion happens. The right half of Table 1 describes the L-Ratings in DoS Standard and P-Ratings in ASTM Standard, which define different levels based on the maximum penetrating distance measured from the front most edge of a vehicle’s truck bed to the backside face of the barrier after a crash. The shorter the truck bed’s penetrating distance is, the higher the barrier’s performance level will be to stop the vehicle intrusions, hence the longer the blast stand-off distance will be.
The two highest anti-crash ratings in Table 1, DoS K12-L3 rating and ASTM F 2656-07 M50-P1 rating (marked in red in Table 1), are essentially equivalent with slightly different truck bed penetrations allowed. They represent very high level of protections for perimeter barriers.
Traditional Barriers Overview
The primary focus when selecting a perimeter barrier system isn’t rocket science. Security professionals want a barrier that’s highly effective, simple for application, and flexible for different environments. Because of limited space between the target and adjacent roadways in cities, a barrier system has to be placed as close to the roadway curb as possible, while keeping explosive detonation as far from the target structure as possible to maintain required minimum blast stand-off distance .
Many barriers have been developed and implemented following the perimeter barrier crash test standards by both DoS and ASTM in the past. Usually there are two categories of permanent perimeter barriers. “Stationary Barriers” attach to the ground or base diaphragms, without the need to change or move them once they’re installed except under attack. “Operable Barriers,” on the other hand, move occasionally for authorized vehicle access, such as beam barricades or wedge barriers. In general, hydraulic or electrical power units and electrical control systems are needed for an operable barrier, which makes its entire design and construction more complicated than stationary barriers. In this article, only stationary barriers are deliberated. Some common stationary barriers include:
1) Bollards and Posts: Bollards are widely used as a traditional anti-crash barrier type with advantages of slim shapes and simple applications. However, traditional bollards usually require deep foundations to assure a fixed bottom condition in order to resist large vehicle impact loads.
2) Jersey Barriers and Retaining Wall: Such conventional engineering structures can also be used as crash barriers. Nevertheless, they are typically efficient for low-speed, low- rating applications only, such as the K4 or K8 ratings defined in DoS Standards.
3) Planters: The use of planters serves two purposes: one is to beautify city streets, and the other is to use filled soil and plants to absorb a crash.
4) Walls and Fences: Security walls can effectively protect against both crashes and explosive blasts. However, security walls generally obstruct vision, which makes fences another good alternate option for anti-crash applications.
Configuration of a barrier’s dimensions can be set to engage an attacking Ford F-750 truck’s tires, engine block, and its steel chassis rails. Most stationary barrier systems are compatible with landscape architecture design and are fabricated as street furniture, such as benches, planters, bike racks, parking meters, and lamp posts. Street furniture barriers, as shown in Figure 4, can be further enhanced with surface finishes, such as visual embellishments and attractively growing floral displays.
Figure 4 Some Beautification Schemes for Street Furniture Applications
Widely used traditional stationary barriers, such as bollards or thick walls, are developed by providing a nearly rigid structure to withstand the extremely high impact load caused by a rigid-to-rigid impact. Therefore, deep or large foundations are required for these barriers. Their unattractive appearance, however, may undermine the applicability of these barriers in urban areas. Another noteworthy disadvantage of traditional barriers such as bollards is that, even though an attacking truck might be stopped, its explosive-laden bed could approach very close to the security perimeter since most of the truck cabin is crushed and pushed through the bollards. This may reduce the facility’s safety factor if the permitted minimum stand-off distance is already critical.