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

Stage I

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.                

Stage II

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.

New Barrier Technology Development

To avoid deep excavations, some new concepts have been recently proposed, including shallow foundation mounted bollards, which typically employ large strong steel frames cast into a base concrete slab. If a vehicle crashes into it, the huge rigid impact forces will spread out over large areas, thereby limiting foundation damages. However the vast foundation work may create many other construction and cost issues.

Another innovative idea is the exertion of a relatively constant “calibrated force” by cushioning energy-dissipaters, such as hydraulic systems or crushable aluminum Hexcel® blocks. Such energy dissipaters allow the barrier to stop an explosive-laden truck differently than traditional methods.

Figure 5 Vehicle Momentum versus Barrier Resistance

Stopping the momentum of a terrorist vehicle requires changing its impact speed to zero forward velocity, without permitting significant penetration towards the target structure. Based on Newton’s second law of motion, the vehicle’s momentum (the vehicle’s mass times velocity) can be successfully brought to rest at a zero velocity if a relatively low and constant deceleration force can be exerted for sufficient duration of time without failure, as shown in Figure 5.

Precast reinforced concrete barriers containing energy absorbers become ideal because of the flexibility of both structural and geometric design, large stiffness and strength, compatibility to connections, and its secure nature against normal destructions. The barrier’s decelerating energy dissipaters can be accurately set at any force and stroke required by analytic studies and crash validations. Such “calibrated” deceleration forces assure the deceleration of a vehicle to zero velocity outside the secured perimeter, while at the same time controlling the forces imposed on the foundation. The controlled shear force is transmitted to the foundation, either a sidewalk or deck, using mechanical interlocking at its underside as well as optional soil anchors that lock the existing diaphragm to the earth below. The proposed barrier technology eliminates any deep foundation needs by employing an effective load transfer mechanism with calibrated decelerating forces. 

Barrier Evaluation and Comparisons

It should be noted that both DoS and ASTM perimeter barrier crash test standards only consider right-angle impact scenarios, excluding any uncertainties inherent in the barrier and vehicle characteristics as well as potential impacts in the real world. Such facts need to be taken into account by the clients or professionals. Simply picking off-the-shelf products with universal solution imaginations may not work for all site-specific requirements. Comprehensive comparisons and evaluations of barriers have to be made to achieve the final decisions. The authors have made side-by-side comparisons for stationary barriers. Table 2 (attached below) summarizes the comparisons on multiple aspects covering performance, constructability, cost, and appearance. 

Numerical Crash Simulations and Prototype Crash Validations

Both high-tech computer-based numerical simulations and full-scale prototype crash validations can be used to analyze and evaluate the proposed perimeter security barrier technologies.

Numerical simulations of a truck crashing into a physical barrier can be executed using advanced computer programs such as LS-DYNA3D, one state-of-the-art software in analyzing real-time dynamic behaviors considering actual geometry and material characteristics for both truck and barriers.  It is always meaningful for the barrier developers to perform numerical simulations before directly jumping into field tests. High-tech expertise and powerful computer software as well as hardware make it possible to cover different barrier design options and many vehicle crash scenarios. The developers can then fine-tune the technical design based on results in the virtual world. The clients can also gain a good understanding of the barrier’s performance under real attack predicted from the simulations.

For full-scale prototype crash validations, these are the primary objectives: 1) to verify the barrier’s actual anti-crash performance, following ASTM F 2656-07 Standard and 2) to further optimize the barrier design through observed crash results, recorded data, pictures, and videos. One crash validation only illustrates one impact scenario; however in the real world, potential terrorist attacks could happen in situations different from the crash validation. There are many possibilities in combining vehicle impact speed and angle or location, which can not be covered in one crash validation. Therefore, the worst crash scenario shall be investigated beforehand and followed in both design and necessary validations to assure the robust functionalities of proposed barriers in any future applications. 

Figure 6 Field Crash Validation Compared with Computer-Based Numerical Simulations

Figure 6 shows the representative results from both numerical simulations and prototype validations during a recent barrier development, following the ASTM F 2656-07 Standard M50 Designation. More than 20 simulations were analyzed in LS-DYNA3D, and two crash tests were performed considering two stringent impact scenarios. During the crash, the new barrier successfully stopped the 15,000 lbs Ford F-800 truck crashing into barriers at 50 mph. The simulations have accurately predicted the behaviors of both barrier and truck. The side-by-side comparison in Figure 6 showed that numerical engineering simulation is an effective tool that supplements physical crash testing and can be used to optimize barrier design before an expensive test is performed. The fact that actual crash observations and analytic results match one another so closely enables potential owners to modify security and decoration requirements without additional prototype crashes. 

Conclusions

Because the science behind choosing the right protective barrier system directly relates to protecting human life and property, it must be a thorough process. To provide the most reliable and effective security solutions for clients, security professionals should utilize multi-disciplinary expertise to perform a systematic and comprehensive risk assessment, scientific structural analyses and barrier evaluations, and then recommend the right security barrier type and arrangement based off their client’s particular needs. 

As terrorism concerns underline the necessity of more and more anti-crash and anti-blast barriers throughout city landscapes, architects and designers have found many innovative ways to blend them into the urban environment based off their stakeholders’ demands. Nevertheless, security professionals will need to work ever more closely with architects and city planners in the design and implementation of vehicle control barriers to address more pressing functional requirements.

♦ Marc Caspe, PE, SE, is Manager of Engineering at KKCS, Oakland, CA 94612. He can be reached at 415-299-9914 or [email protected].

♦ Jun Ji, PhD, PE, is Senior Engineer at KKCS. He can be reached at 217-721-2501 or [email protected].

♦ Lin Shen, PhD, PE, is Engineer at KKCS. He can be reached at 217-493-3418 or [email protected].

♦ Qian Wang, PhD, PE, LEED AP, is Engineer at KKCS. He can be reached at 319-331-5734 or [email protected].

Source Materials for this article included:

U.S. Department of State – DS 9. SD-STD-02.01 Specification for Vehicle Crash Test of Perimeter Barriers and Gates, 1985.U.S. Department of State – DS 9. SD-STD-02.01, Revision A Test Method for Vehicle Crash Testing of Perimeter Barriers and Gates, 2003Unified Facilities Criteria (UFC), DoD Minimum Antiterrorism Standards for Buildings, 2003ASTM Standard F 2656-07, Standard Test method for Vehicle Crash Testing of Perimeter Barriers, 2007

 Table 2_Detail Comparison of Stationary Barriers.pdf