6.1.3 Physical Protective Barriers

There are two basic categories of perimeter anti-ram barriers; passive (or fixed) and active (or operable). Each is described below.

Passive Barriers

Passive barriers are those that are fixed in place and do not allow for vehicle entry. These are to be used away from vehicle access points. The majority of these are constructed in place.

For lower-risk buildings without straight-on vehicular access, it may be appropriate to install surface-mounted systems such as planters, or to use landscaping features to deter an intrusion threat. An example of a simple but effective landscaping solution is to install a deep permanent planter around the building with a wall that is as high as a car or truck bumper.

Individual planters mounted on the sidewalk resist impact through inertia and friction between the planter and the pavement. It can be expected that the planter will move as a result of the impact. For a successful design, the maximum displacement of the planter should be less than the setback distance to the building. The structure supporting the weight of the planter must be considered prior to installation.

To further reduce displacement, the planter may be placed several inches below the pavement surface. A roughened, grouted interface surface will also improve performance.

The traditional anti-ram solution entails the use of bollards (see Figure 6-1). Bollards are concrete-filled steel pipes that are placed every few feet along the curb of a sidewalk to prevent vehicle intrusion. In order for them to resist the impact of a vehicle, the bollard needs to be fully embedded into a concrete strip foundation that is several feet deep. The height of the bollard above ground should be higher than the bumper of the vehicle. The spacing of the bollards is based on several factors including ADA (American Disabilities Act) requirements, the minimum width of a vehicle, and the number of bollards required to resist the impact. As a rule of thumb, the center-to-center spacing should be between three and five feet to be effective. The height of the bollard is to be at least as high as the bumper of the design threat vehicle, which is taken typically between two and three feet.

An alternative to a bollard is a plinth wall, which is a continuous knee wall constructed of reinforced concrete with a buried foundation (see Figure 6-2). The wall may be fashioned into a bench, a base for a fence, or the wall of a planter. To be effective, the height needs to be at least as high as the vehicle bumper.

For effectiveness, the barriers need to be placed as close to the curb as possible. However, the property line of buildings often does not extend to the curb. Therefore, to place barriers with foundations near the curb, a permit is required by the local authorities, which can be a difficult time-consuming effort to obtain. To avoid this, building owners are often inclined to place bollards along the property line, which significantly reduces the effectiveness of the barrier system.

The foundation of the bollard and plinth wall system can present challenges. There are sometimes vaults or basements below the pavement that extend to the property line, which often require special foundation details. Unless the foundation wall can sustain the reaction forces, significant damage may occur.

Below-ground utilities that are frequently close to the pavement surface present additional problems. Their location may not be known with certainty, and this often leads to difficulties during construction. This also can be a strong deterrent to selecting barriers with foundations as a solution. However, for high-risk facilities, it is recommended that these issues be resolved during the design phase so that a reliable anti-ram barrier solution can be installed. For lower-risk buildings without straight-on vehicular access, it may be more appropriate to install surface-mounted systems such as planters or to use landscaping features to deter an intrusion threat. An example of a simple but effective landscaping solution is to install a deep permanent planter around the building with a wall that is at least as high as a car or truck bumper.

Active Systems

At vehicular access points, active or operational anti-ram systems are required. There are off-the-shelf products available that have been rated to resist various levels of car and truck impacts. Solutions include:

• crash beams;
• crash gates;
• surface-mounted plate systems;
• retractable bollards; and
• rotating-wedge systems.

The first three systems listed above generally have lower impact ratings than the last two listed. Check with the manufacturer to ensure that the system has been tested to meet the impact requirements for your project.

It is important that the installation of hydraulically operated systems be performed by a qualified contractor to ensure a reliable system that will work properly in all weather conditions.