Since the infamous act of terrorism on American soil on Sept. 11, 2001, precast concrete has been protecting citizens and buildings as both attractive architectural elements and as impact-rated security barriers.
By Marc Caspe, P.E., S.E.; Jun Ji, Ph.D., P.E.; Lin Shen, Ph.D., P.E.; Qian Wang, Ph.D., P.E., LEED AP; and Yanzhi Zhai, P.E.
Anti-terrorism physical barrier techniques and applications have attracted serious attention in recent years due to increasing threats to critical facilities and to human life. In this article, the authors review existing anti-crash test standards and the physical security barrier selection criteria as they relate to protection from terrorist attacks with truck-borne explosives.
Widely used physical perimeter security systems, including recently developed designs for reinforced concrete, metal and precast perimeter barriers, are surveyed, evaluated and compared. Highly secured barrier systems can effectively dissipate impact energy and simultaneously offer flexible designs for diverse cityscape styles. Concrete and metal barriers are compared to provide valuable references for security professionals, owners and precast manufacturing professionals (see the Summer 2010 issue of Precast Solutions magazine for an article on security barriers written for the engineering specifier and architect).
Terrorist targets not limited to military sites
Terrorist attack has become a world-wide threat, not just in war zones but in urban communities toward civilians not involved in armed conflict. Targets are no longer limited to military facilities but have expanded to include important political, social or commercial areas in the cities (for example, the devastating 1995 bombing attack in Oklahoma City). Hotels, tourist centers, emissaries and other international civilian venues have come under the cross hairs of terrorist intent. Vehicle bombing has become the major massive attacking style for taking down targets in urban areas.
In addition to proactive intelligence work to prevent such disasters in advance, the main tasks for security professionals and stakeholders are to perform thorough risk assessments for target facilities and to build reliable passive perimeter barrier systems to secure vulnerable access points. Designs that prevent building collapse and protect occupants are the ultimate goals in development and implementation of passive perimeter security systems.
Guidelines for selecting security barriers
According to the U.S. Federal Emergency Management Agency (FEMA) 430: “Perimeter security is designed to protect employees, visitors, and building functions and services from threats such as unauthorized vehicles approaching close to or penetrating high-risk buildings. Complete security systems shall include both perimeter barriers and controlled access points for pedestrian as well as authorized vehicles.”
In contrast to military security designs, perimeter barriers for urban environments need to satisfy unique requirements of both functionality and acceptable visible aesthetics. While providing robust defense, urban and municipal defenses must also ideally serve as city architectural amenities and functional street furniture. The selection of appropriate barriers follows these two steps:
Step 1: Risk assessment for possible attack:
a) Perform facility site investigation to identify vulnerabilities
b) Conduct traffic analyses to identify potential attacking scenarios and authorized vehicle access
c) Derive minimum blast stand-off distance
d) Determine level of security protection
Step 2: Select and implement the most appropriate perimeter
a) Locate barrier perimeter to assure minimum stand-off distance
b) Investigate site conditions for feasible foundation types
c) Choose appropriate anti-crash criteria/ratings based on desired level of protection
d) Select aesthetic barrier design to accommodate urban environment
These two steps define the optimization process for selecting the most-suitable barrier system for site conditions. The critical selection factors are: anti-terrorism functionality; decoration/aesthetic flexibility in urban areas; environmental impacts; constructability; and cost.
Understanding crash test standards
To retain the minimum blast stand-off distance after a vehicle crash, the perimeter barriers must fully stop any impacting vehicles and keep explosives out of the security perimeter. Systematic crash test standards have been developed to physically verify and quantitatively certify barrier performance (see Table 1). These standards include those from: the U.S. Department of State (DoS); the U.S. Bureau of Diplomatic Security Standards SD-STD-02.01 (1985) and SD-STD-02.01 Revision A (2003); and the more recent ASTM F 2656-07, “Perimeter Barrier Vehicle Crash Test Standard.” (For current information on security barrier ratings from the U.S. Army Corps of Engineers, visit https://pdc.usace.army.mil/library/BarrierCertification.)
Table 1 shows the typical specifications from the DoS and ASTM. The left half of Table 1 lists the DoS K-rating criteria and their latest equivalents, the M-designations, from the ASTM standard. These standards rate crashes with different combinations of vehicle weight and impact speed.
Measurement of a vehicle’s forward movement is the criteria for a barrier’s actual protection performance, specifically the movement of a truck bed laden with explosives. The right half of Table 1 shows DoS Standard L-ratings and ASTM Standard P-ratings that 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 impact. The shorter the truck bed’s penetrating distance, the higher the barrier’s performance level will be to stop vehicular intrusions and the longer the blast stand-off distance.
Materials and types of barrier systems
Many types of perimeter barriers have been researched, designed and implemented to fulfill the increasing need for civilian defense. Ideal perimeter barriers need to be highly effective, easy to install and flexible in design to suit different environments and architectural needs.
Barrier materials. Common fabrication materials for
• Metal: cast iron; carbon steel; stainless steel; high strength low-alloy steel; and aluminum, bronze (sculptures) or other alloys
• Reinforced concrete: either cast-in-place or precast incorporating steel bar or steel/fiber mixed reinforcement
• Other materials: large boulders or natural stones; wood or trees; and industrial plastic like high density polyethylene (HDPE)
Two barrier types. Considering the structural nature of perimeter barrier systems, there are two essential categories of barriers: stationary and operable.
• Stationary barriers are equivalent but not identical to “fixed barriers” as defined in FEMA 430 and are attached to the ground or base diaphragms to block vehicle entry. These barriers shall not move or deform unless within allowable magnitude under attack.
• Operable Barriers are not anchored and can be moved as necessary for authorized vehicle access. Operable barriers are beam barricades or wedge barriers. Typically hydraulic or electrical power equipment is needed to reposition an operable barrier, making its entire design and construction (lift points) more complicated than that of stationary barriers.
Only stationary barriers are discussed herein. Stationary barriers, however, are often employed in conjunction with operable barrier systems to serve as heavy anchorage or to provide other critical supplemental defense features.
Typical stationary barriers can be of several types including:
• Bollards and posts – traditional anti-crash barrier types with the advantages of slim shapes and simple applications
• Sculptures and other heavy objects
• Metal fencing
• Fixed or anchored concrete
• Median barriers, also known as Jersey barriers – conventional precast concrete structures for anti-crash systems, with typical lowrating applications such as the K4 rating defined in DoS standards (see Table 1)
Median (Jersey) barriers
Jersey barriers were originally developed by the New Jersey State Highway Department to divide highway lanes. A concrete Jersey barrier stands 2 to 3 feet (0.6 to 0.9 m) tall with the original design intent of minimizing damage and reducing the likelihood of vehicles crossing into oncoming lanes in the event of a collision. There have been variations from the original shape including the constant slope barrier, the F-shape barrier and the California K-rail.
The easy constructability, mobility and efficiency of the Jersey barrier make it applicable in the anti-terrorism security field. U.S. military forces use them extensively in Iraq to fortify roadblocks and public infrastructure with taller variants. In addition, Jersey barriers have been employed domestically as perimeter security barriers since the Oklahoma City bombing and the Sept. 11, 2001, terrorist attacks to enforce standoff distances from federal buildings and monuments such as the Washington Monument in Washington, D.C., the Willis Tower in Chicago and the Library
Tower in downtown Los Angeles.
Advantages of the Jersey barriers are their ability to:
• Stop the impacting vehicle quickly with a large mass of concrete
• Reflect vehicle collisions at angular impact
• Dissipate kinetic energy by the lifting vehicle’s front and engaging its undercarriage
• Integrate easily into continuous walls or combine with other types of barriers like steel fencing
• Install in various flexible patterns for temporary or permanent protection purposes
The downside of Jersey barriers is mainly related to its original design purpose: to deflect the crashing vehicle rather than completely damage or totally stop a vehicle’s forward movement. In defense against terrorist attacks, Jersey barriers normally cannot qualify for high ratings (like the DoS K12) if directly impacted by heavy trucks due to:
• Insufficient barrier height
• Relatively weaker anchorage to ground
• Sloped front lower portion causes lifting of the truck bed
When installed near city buildings, the disadvantage of the Jersey barrier is its plain utilitarian appearance, particularly in areas of high pedestrian traffic where street features are usually expected to be visually appealing.
Fixed or anchored concrete planters
Fixed or anchored planters are either cast-in-place, precast concrete or concrete masonry units (CMUs). Precast planters are typically designed to serve as urban architectural elements, or street furniture, and as impact energy absorbers when filled with soil and plants. To fully prevent high-speed vehicle intrusion, planter barriers must be either embedded into the ground or anchored into a base diaphragm like a sidewalk slab.
Fixed planters can cover security levels from DoS K4 to K12 with variations in height and mass. Two types of fixed planters are typically used as security barriers:
1. Large cast-in-place concrete continuous planters, suitable for protection of critical structural points such as building corners at street crossings where vehicles may crash at higher speeds
2. Individual cast-in-place or precast concrete planters along linear roadsides
Concrete planters, embedded in ground or anchored to the base diaphragm, can stop a 15,000-pound (6,800 kg) truck traveling at a velocity greater than 30 mph (48 km/h), even if the planters are arranged in a scattered pattern of individual pieces. Concrete planters are usually 3 feet (0.9 m) high or more, and larger than 3 feet in plane dimensions.
Advantages of these popular perimeter barrier types is that they:
• Effectively stop a high-speed impacting vehicle
• Destroy the vehicle and prevent any potential further motion
• Dissipate kinetic energy with the combined mass of concrete and soil
• Beautify urban environments and complement landscape design
Some disadvantages of fixed planters are due to:
• Obtrusive large dimensions if certified for high anti-crash rating
• Higher cost and more construction time compared with median (Jersey) barriers
• Layout difficulties in dense urban areas with limited rights-of-way
Concrete or CMU walls
Short retaining walls and tall freestanding walls are constructed of precast concrete, CMUs or other CMU materials typically reinforced with steel. Walls may be perforated or discontinuous to enhance visual appearance or allow pedestrian passage while still satisfying security requirements. Similar to fixed planters, security walls must be either embedded in the ground or anchored to an expanded base slab to resist the significant momentum of vehicular impact.
Short retaining walls are normally intensively reinforced and maintain a height of at least 3 feet (0.9 m) above grade. The heavy reinforcing in short concrete walls can resist large shear impact forces and maintain stability due to the mass of passive soil pressure. Tall security walls are designed to block the force of incoming vehicles as well as the truck bed’s payload. Walls of greater height will engage more contact area of the truck’s front grille/ engine hood assembly and cause a larger overturning moment. Tall security walls, therefore, require a wider foundation slab than shorter walls. To minimize cost and construction time, precast concrete wall panels are generally preferred to cast-in-place structures and precast units are easily integrated into other perimeter site systems.
Advantages of using concrete walls as perimeter barriers are:
• High anti-crash capability to stop vehicles crashing from all angles
• Tall walls can destroy the vehicle and block a majority of catapulted objects
• Tall walls serve as both anti-crash and anti-blast barriers
Main disadvantages of concrete walls are:
• Obtrusive large overall dimensions for walls with high anti-crash ratings
• Higher cost and more construction required compared with shorter barriers
• Unfriendly appearance for dense urban areas
• Difficulties for layout in dense urban areas with limited rights-of-way
Innovative barrier designs
As discussed previously, deep foundations are almost always required for conventional metal and concrete barriers. To avoid extensive excavations in urban areas with congested underground utilities, some new designs have been proposed. One design calls for shallow-foundation mounted bollards that use large steel frames cast into a base concrete slab. In the event of a vehicle crash, the significant rigid impact forces will spread out over a large area and minimize foundation damages. The extensive foundation work required, however, may increase project cost and create constructability issues.
Another innovative idea is that of a relatively constant “calibrated force” resistance provided by cushioning energy dissipaters incorporated into the design. Such energy dissipaters allow the barrier to stop an explosive-laden truck with a relatively low and constant deceleration force that is exerted for a sufficient duration.
Reinforced precast concrete barriers containing energy absorbers can be ideal security barriers because of the flexibility of both structural and geometric/architectural design; large stiffness and strength; compatibility to connections; and its inherent mass to resist destructive impact. The barrier’s decelerating energy dissipaters can be accurately set at any force and stroke required by analytic studies and crash validations.
Calibrated deceleration forces were validated in real crash tests to assure the deceleration of a vehicle to zero velocity outside the secured perimeter while simultaneously controlling the forces imposed on the foundation. The controlled shear force is transmitted to a foundation slab or concrete sidewalk, using mechanical interlocking at the base to lock the existing diaphragm into the earth below. This kind of precast barrier technology eliminates the need for a deep foundation.
Barrier evaluation and comparisons
The apparent advantages of metal barriers are high strength; light weight and relatively small or slim shapes; and ease of handling during construction. However, some obvious disadvantages of metal include high maintenance requirements; routine anti-corrosion painting/coating over the product service life; and less architectural compatibility (color, shape, texture) with adjacent building materials.
Compared with metal alternatives, precast concrete is a superior barrier material because of higher design compatibility (color, shape, texture) with the urban environment; high corrosion resistance; superior durability; low maintenance needs; and a typically lower cost.
Other security barrier materials include but are not limited to natural stones and boulders; wood or planted trees; and industrial plastic or HDPE. Stone and plastic materials usually cannot provide the high strength of metal or concrete and therefore require very large overall dimensions or specific structural combinations to effectively stop inbound vehicles. Combinations of these other materials can be designed with metal or concrete barriers to act as supplemental or decorative features, and offer the advantage of architectural enhancement or landscape complementation in city environs.
When performing risk analysis for a terrorist attack, all vulnerabilities need to be investigated as completely as possible. Choosing off-the-shelf products as a universal solution strategy may not be advisable for all site-specific project requirements. Comprehensive comparisons and evaluations of the various barriers available in the marketplace should be done before reaching any final product selection. Due to its many advantages, precast concrete barriers provide architects, designers and security professionals many innovative ways to blend them architecturally into the urban environment while reliably addressing the more critical defense function
About the Authors
Marc Caspe is a registered civil and structural engineer with more than 45 years of professional experience. He has published numerious technical papers and developed several patents on technical innovations.
Jun Ji, Ph.D., is a registered civil engineer with 15 years of professional and research experience. His research interests include risk assessment and engineering responses to terrorism and natural hazards; earthquake engineering; experimental research and advanced analysis; and development of innovative security and retrofitting techniques.
Lin Shen, Ph.D., is a registered civil engineer. His research interests include experimental research and structural retrofit; advanced cementitious materials; non-destructive testing; and structural rehabilitation strategy and design.
Qian Wang, Ph.D., LEED AP, is a registered civil engineer. His research interests include numerical optimization-based analysis and design, and experimental and numerical studies with applications to structural, seismic, security and retrofitting engineering.
Yanzhi Zhai is a registered civil engineer with 12 years of professional and research experience. She has intensive experience in highway and bridge design, advanced structural analysis, vehicle traffic analysis, concrete barrier applications and site planning.
U.S. Department of State – DS 9. SDST D-02.01, “Specification for Vehicle
Crash Test of Perimeter Barriers and Gates,” 1985
U.S. Department of State – DS 9. SDST D-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
Federal Emergency Management Agency (FEMA) 430, “Perimeter Security Design,” 2007