Anti-terrorism physical barrier techniques and applications have become highly desired to protect critical facilities and human lives in recent years. Based on in-depth review and practices, the systemic criteria are proposed here for selection of security barriers specifically against terrorist attacks with truck-borne explosives.
Motivated from the survey and evaluation of typical perimeter security systems, a highly secured Surface Barrier and associated technologies developed recently are presented in details. In contrast to most conventional barriers with a fixed base, the proposed new Surface Barrier technology employs an innovative “calibrated” energy-dissipation concept to decelerate a crashing vehicle gradually instead of relying on rigid impact. The impact force transferred to the base is controlled to such a low level that deep or massive foundations are not needed. In fact, the Surface Barrier can be made with shop-precast concrete that can be formed into a wide variety of decorative cityscape street furniture.
Both high-fidelity numerical simulations and prototype crash validations were utilized to validate the Surface Barrier, which is certified by ASTM F 2656-07 standard. During the development of this new technology, the correlation between actual crash observations and analytic results closely indicated high effectiveness and efficiency of the advanced numerical modeling and simulations. As a supplement to physical crash testing, high-fidelity finite element simulations enable potential owner modification of both security criteria and landscape architecture criteria from certified barriers without needing additional prototype crashes.
Terrorism has been recognized as worldwide threat, especially attacks with vehicle-borne bombs, not only for military targets but also to government buildings and facilities in urban areas, e.g., the devastating bombing attacks of the Murrah Federal Building in Oklahoma City in 1995 and Khobar Towers in Saudi Arabia in 1996. Security professionals must now completely assess potential vulnerabilities for critical facilities as well as human lives. Multi-disciplinary cooperation and technical integration 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 are in great needs to assure the safety of buildings and occupants.
This article starts with a review of existing anti-crash test standards. Referential guideline factors are proposed for clients and professionals to select the right physical security barrier systems against terrorist attacks with truck borne explosives. After the survey and evaluations on some widely used perimeter security systems, an innovative highly secured Surface Barrier technology is described, covering the motivation and concept, basic design features and mechanism, high-fidelity numerical crash simulations, and final prototype validations.
2. Crash Testing Standards in the U.S.
Perimeter barrier 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. That standard was revised in 2003 to SD-STD-02.01 Revision A, which was superseded by ASTM F 2656-07 Perimeter Barrier Vehicle Crash Test Standard in February 2009. The DOS K-rating criteria and latest equivalents in ASTM Standard, both based on the impact vehicle’s kinetic energy (KE), are listed in Table 1. The criteria specify impact (perpendicular to barrier) of a 15,000 lb vehicle at a speed of 30, 40, or 50 mph. L-ratings in DOS Standard and P-ratings in ASTM Standard are used to indicate the maximum actual penetration of the vehicle’s truck bed with explosives beyond inside face of the barrier. The two medium-duty anti-crash ratings, K12-L3 rating in DOS Standard and M50-P1 rating in ASTM Standard (marked in red in Table 1), are essentially equivalent with slightly different truck bed penetrations allowed.
Table 1. Comparison between DOS and ASTM Standards
3. Anti-Crash Barrier Selection Procedure
The generic framework to select appropriate physical anti-crash barrier types includes four major steps as shown in Figure 1.
Figure 1. Generic Framework for Appropriate Barrier Selection
The first key stage, risk assessment of possible attacks, starts with a site investigation and vehicle traffic analyses to detect vulnerable spots as well as a review of secured access points for authorized vehicles. Blast analyses can then be performed in series considering vulnerable spots and correlated stand-off distances, using state-of-the-art software like Abaqus/Explicit, LS-DYNA, and Blast/FX, etc. With potential facility damage to the target predicted through blast analyses, target protection criteria can be decided based on the owner’s need and attack consequences for the second stage: selection or development of right barriers. The fundamental parameters in determination of protection criteria are explosive amount and minimum stand-off distance, as illustrated in Figure 2, showing sample blast wave propagation.
Figure 2. Explosive Blast Characteristics beside a Target Structure
Researchers have found that with certain explosive quantity (TNT equivalent), the blast overpressure and reflected pressure decrease at a rate between square and cubic power along the stand-off distance. It is very important to assure the minimum blast stand-off distance between the target and explosive after a truck crashes into the barriers. This becomes even more critical for any urban buildings or facilities, where the allowable secured area is often limited by the surrounding structures or streets.
The second stage, final selection and implementation of the most appropriate anti-crash perimeter barriers, needs to consider the following factors:
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 construction needs;
4) Security level and barrier rating versus accessibility for authorized people and vehicles;
5) Flexibility of aesthetic design versus surroundings, especially in urban area.
The primary focus in security solutions is to select a perimeter barrier system which is highly effective, simple to apply, and flexible for different environments. Especially in urban areas, the usually limited stand-off distance between the target and adjacent roadways pushes the barrier as close to the curb as possible, while keeping any explosive detonation as far from the target as possible. Such selection is actually an optimization process, and the decision should always be driven by the goal to maximize the anti-terrorism functionality plus decoration flexibility while minimizing the environmental impact and demand of construction effort.
4. Innovative Barrier Technology Development
4.1 Survey of Existing Anti-Crash Barriers
The literature contains many kinds of permanent anti-crash perimeter barriers that have been developed and implemented over decades following the DOS and ASTM vehicle crash test standards. These permanent barriers can be categorized into two groups: 1) “stationary barriers,” which are attached to the ground or base diaphragms, with no need to change status once installed, and; 2) “operable barriers,” whose status must vary occasionally for authorized vehicle access. Typical widely used stationary barriers include:
1) Bollards and posts: Most conventional anti-crash barriers with relatively slim shapes, while deep foundations usually required to assure fixed-bottom boundary conditions.
2) Jersey barriers and retaining walls: Conventional civil structures used as low-rating crash barriers, such as DOS K4 or K8, to stop vehicle impact at lower speed.
3) Planters: Flexibly shaped planters installed to utilize filled soil for energy absorption resisting crash and to beautify city streets.
4) Walls and fences: Security walls serve as both anti-crash and anti-blast barriers. Walls’ disadvantage of blocking public view in urban areas makes transparent security fences another viable alternative.
Operable barriers include retractable bollards, beam barricades, cable barriers, wedge barriers, and operable fences. In general hydraulic or electrical power units and electrical control systems are needed for operable barriers, which makes their design and construction more complicated than stationary barriers.
4.2 Motivation of New Technology Development
Conventional stationary barriers, such as bollards or thick walls, are developed by providing a nearly rigid structure to withstand the extremely high load caused by a rigid-to-rigid impact. Most of the total kinetic energy will be transmitted to strain energy of crushed truck parts and resistant structure, including the foundation. However, their big disadvantages—deep foundation needs and unfriendly appearance—undermine the applicability of these barriers in urban areas. Another shortcoming of stationary barriers like bollards is that, even though a truck might be stopped, the truck bed containing the explosives could approach very close to the security perimeter, since most of the truck cabin would be crushed and sheathed through by the bollards. It may push the safety factor to be very marginal if the permitted minimum stand-off distance is already critical. To avoid deep excavations, shallow foundation-mounted bollards have been developed recently, which typically employ large strong steel frames cast into a base concrete slab. In this way the huge rigid impact forces will be spread out over large areas and hence the foundation damage could be limited. However, the vast foundation work creates almost as many constructability and cost issues as traditional bollards.
In response to the limitations of the stationary barriers currently available, one innovative perimeter security technology has been newly developed with a concept altered from the conventional rigid structure idea. In the development of a Surface Barrier to meet ASTM F 2656-07 Standard M50/P1 rating, the authors proposed a concept of controlling impact forces transferred to the system’s base by significant energy dissipation of the total kinetic energy. The details are described in following sections.
4.3 Energy Dissipation Concept and Devices
Stopping the momentum of a terrorist vehicle requires changing its impact speed to zero forward velocity, without permitting significant penetration toward the target structure. Based on Newton’s second law of motion, the vehicle’s momentum (vehicle mass M times velocity change dV) 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 (impact force F times impact time duration dT) without failure, as shown in Figure 3.
Figure 3. Vehicle Momentum versus Barrier Resistance
By exerting a relatively constant “calibrated force” either by hydraulic system or crushable aluminum Hexcel blocks, energy-dissipaters can stop a terrorist vehicle’s explosive bearing truck bed at a safe distance from an attacked building.
4.4 Barrier Design and Impact Resisting Mechanism
• Barrier Design
The basic concept for new barrier requires sufficient rigidity and dimensions of the structure itself to engage the truck’s major parts and be able to transfer the impact forces to energy absorbers and down to the base. Proposed reinforced concrete Surface Barrier containing energy absorbers has evolved to best suit limitations imposed by both highest possible security and Cityscape beautification, with advantages of the structural design flexibility, large stiffness and strength, compatibility to connections and its secure nature against normal destructions. Configuration of the Surface Barrier’s height is set to engage an attacking Ford F-800 truck’s standard tires, truck’s engine block and its supporting steel chassis rails, as shown in Figure 4. Minimum barrier width was set as 8 ft which is same as that of typical median-duty truck defined by ASTM F 2656-07 or DOS K12 Rating.
Figure 4. Barrier Dimensions Determined to Effectively Stop Attacking Vehicles
• Impact Resisting Mechanism and Foundation Design
The Surface 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 the vehicle to zero velocity outside the secured perimeter, while at the same time controlling the load imposed on the foundation. Impact forces are transmitted across the Surface Barrier and foundation as shown in Figure 5 below:
Figure 5. Impact Load Transfer from Barrier to Foundation
The controlled shear force is transmitted through the anchorage plate to the foundation, which can be either a sidewalk or deck’s structural diaphragm. Any secondary vertical forces will be directly distributed to the much larger slab-on-grade base with soil beneath, and the force values are greatly reduced.
Therefore the proposed barrier technology practically eliminates any deep foundation needs by employing such an effective load transfer mechanism with calibrated decelerating forces.
4.5 Advanced Numerical Crash Simulations
Finite element (FE) simulations of a 15,000 lb truck impacting the Surface Barrier were performed using LS-DYNA3D, a general-purpose transient dynamic FEA software with extensive capabilities in modeling nonlinear dynamic structural responses. Figure 4 shows the finite element model of a Ford F700 series truck. A series of simulations has been performed covering many different combinations of barrier design and energy absorber properties. The major energy absorptions of the total initial kinetic energy 1250 ft-kip between the barrier and the vehicle are shown in Figure 6, where IE means “Internal Strain Energy”, KE means “Kinetic Energy”, and HG represents “Hour-Glass Energy.”
Figure 6. Impact Energy Absorption Distribution Details
|(a) Impact Energy Absorption Distribution Chart||(b) Internal Energy Absorption Details upon Energy
Absorbers and Vehicle Parts
From Figure 7, it can be observed that proposed energy absorbers prove the functionality and effectiveness as predicted. Under vehicle impact, the barrier was accelerated to move and activate the energy absorber simultaneously, which is the fundamental difference from other stationary barriers using a rigid structure concept.
Figure 7. Barrier Velocity Time History Diagram
The performed numerical simulations indicated clearly that:
1) The energy absorbers can be used to limit the shear load transferred to the ground or deck and also effectively uses the inertia of the Surface Barrier to help stop the truck.
2) It is possible there exists a threshold for height and weight of the barrier above which changes do not matter very much with calibrated energy absorbers.
3) Eccentric impact simulations demonstrated the Surface Barrier’s sufficient functionality resisting most stringent attack scenarios with controlled barrier displacement and limited rotation.
4.6 Prototype Crash Validations
The primary objectives of prototype crash validations are: 1) to verify the Surface Barrier’s real performance, following ASTM F 2656-07 Standard M50 designation and 2) to obtain actual crash parameters to evaluate any further modification or optimizations. Two vehicular crashes were conducted at a crash test facility, to validate both concentric and eccentric impacts considering potential vulnerable situations.
1) Vehicle Preparation and Crashing
Following the ASTM F 2656 M50 requirements, two commercially available model test vehicles were selected, including one Ford F700 truck and one Ford F800 truck. The test vehicle was towed to within 25 feet of the barrier and then released to proceed under its own momentum before impacting the test article.
2) Concentric Impact
The vehicle approached and hit the front of barrier at 90° with both truck and barrier centerlines concentrically aligned, at a speed of 50 mph. Surface Barrier’s foundation was reinforced concrete slab on grade. The vehicle was completely arrested by the barrier within the ASTM F 2656-07 P1 penetration limits (less than 1 meter or 3.3 ft). The maximum penetration of truck bed leading edge from the inside face of barrier was -3 meters (-10.5 ft) after impact, indicating that the truck bed was far behind reaching the barrier; hence, a significant safety margin was gained beyond ASTM F 2656-07 P1 rating. During the impact the barrier sustained only minor damage, mostly .minor surface dents. There appeared to be no damage to the structure of the barrier, the foundation, or the energy absorbers used in the test.
3) Eccentric Impact
The vehicle approached and impacted the front of barrier at 90° and the vehicle centerline was centered on 2’-2” offset to the middle of barrier front face. Actual vehicle impact speed was 52.2 mph, slightly higher than the required 50 mph in ASTM. The Surface Barrier’s foundation was reinforced concrete slab on grade. The vehicle was again completely arrested by the barrier within the ASTM F 2656-07 P1 penetration limits (less than 1 meter or 3.3 ft). The maximum static and dynamic penetration of the tested barrier was -3.2 ft, which still indicates significant safety margin. During the impact the barrier sustained the very stringent eccentric hit successfully.
4.7 Surface Barrier Technical Evaluations
Both high-fidelity numerical simulations and prototype crash validations were performed to fully validate the proposed perimeter security barrier technology. Most stringent requirements from both DOS K12 rating and ASTM F 2656-07 M50 standards have been followed through the entire development process. Besides the obtained ASTM certificate, the fact that actual crash observations and analytic results showed strong correlations and similarities (Figure 8) enables potential owner modification of both security criteria and landscape architecture criteria without the need for additional prototype crashes.
Figure 8. Crash validation compared with numerical simulation of 15,000 lbs Ford F-800 at 50 mph
4.8 Architectural Design
As more anti-crash and anti-blast barriers are used in city landscape, it will become more the task of architects and designers to blend these devices into the urban environment in innovative ways to meet the needs of the many stakeholders involved. The new Surface Barrier is fully compatible with any landscape architecture desired when cast as concrete benches, planters, bike racks, parking meters, or lampposts to beautify any desired cityscape design. One option with visual embellishments and attractively growing floral displays, as shown in Figure 9, can be further enhanced with surface finishes and floral planters.
Figure 9. Architectural Configurations for Possible Street Furniture Applications
Terrorist threats with truck bombs can result in dramatic and widespread loss of both life and property. It is always crucial to employ the right technology for strategically protecting such vulnerabilities. Multi-disciplinary experts must work together to perform systematic risk assessment, scientific structural analyses, and barrier evaluations, in order to find right solutions for security barrier type and arrangement.
To protect against the event of a truck bomb attack on a civil structure, a new Surface Barrier has been developed to create a secure perimeter around strategic and irreplaceable infrastructure. A prototype Surface Barrier technology was proven to withstand the impact of a 15,000-lb truck crashing at 50 miles per hour. Crash validations certified this barrier to meet the high standards of DOS and ASTM International. Compared with other off-the-shelf barriers, the significant advantage of this Surface Barrier is that the impact force transferred to the barrier foundation is calibrated and controlled to such a degree that deep or massive foundations are not needed.
Notably, this surface barrier is made with environmentally sensitive shop-precast concrete that can be formed into a wide variety of decorative cityscape street furniture, so that it is not only effective in its strategic security mission, but also presents an attractive “green” exterior on city streets, compatible with any type of civic architecture.
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, 2003
Unified Facilities Criteria (UFC), DOD Minimum Antiterrorism Standards For Buildings, 2003
• ASTM Standard F 2656-07, Standard Test method for Vehicle Crash Testing of Perimeter Barriers, 2007.
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]