Perimeter security with bollards, fences, and walls
A “bollard fence” or a “bollard wall”?
The new barrier going up along the border between the US and Mexico is often referred to either as a bollard wall or a bollard fence. Fences, walls, and bollards are all important parts of perimeter security, but they are usually separate items that work together. What is different about the southern border that makes it a combination of all three? It is likely a combination of sight lines and type of foundation that causes the barrier at the border to be referred to this way.
Bollards are, by design, short. They allow pedestrian traffic to cross through them. A “bollard wall” or “bollard fence” that stands 11 feet tall and does not allow easy pedestrian crossing has neither of these features.
However, crash-rated bollards are a permanent barrier installed to stop cars from crossing a boundary. For cars they are permanent and impenetrable impediments. They are also poles: even in a line, you can see through them. Their crash-readiness comes from a combination of concrete and steel.
The border barrier design features close vertical poles, made of steel and concrete, deeply set to prevent vehicle access. It is these features that make the comparison come to mind. Because the poles are tall and interlinked above grade, they’re not truly bollards, but their ruggedness creates the comparison.
The US-Mexican barrier is a palisade-style security fence using the concrete-and-steel engineering popular in modern bollards. Fortifications of posts driven deep and held together with ties along top and bottom have a long history of installation around battlements and along borders.
Traditionally palisades were made of logs. The word “bollard” likely comes from the root word, “bole,” used to describe the trunk of a tree. It was originally used in a Scottish paper to describe a mooring post on a dock. Bole-ards, then, also have a history of being made of logs. Today’s bollards are often metal and reminiscent of cannons—a shape that grew popular in the 17th and 18th centuries, when spent cannons were buried for bollards rather than being shipped back to the foundry for recycling.
From patios to secure perimeters, fences, bollards, and walls are found together protecting and defining space. These elements of boundary design complement each other. Each offers different options and aesthetics.
What is a bollard?
Bollards are short posts generally used to guide traffic and define perimeters. They don't create a physical obstacle to pedestrians. Even when bollards are linked by chains, they suggest pedestrian behavior, rather than barring the way. However, bollards can be installed with fortifications that create a physical barrier to vehicles; they can act like a fence to four-wheeled traffic.
What is a fence?
Fences are upright structures of varying heights that work as a railing or barrier and define an area. They’re made of two elements: posts that go into the ground and a “fence fabric” that stretches between these posts. The fence fabric usually allows airflow and is translucent. Fences guide vehicle or pedestrian traffic to particular access points: gates or spaces in the fences. Like bollards, fences can be installed as “civil suggestions” or as impenetrable security barriers.
What is an outdoor wall?
Outdoor walls are upright structures often made of wood, concrete, or stone. They differ from fences in that they have a continuous foundation. Walls can be many different heights—retaining walls, for example, might be short and installed to help support an embankment. However, most outdoor walls are not built to support structures. Non-structural walls are usually tall, opaque, impenetrable to wind or water, and used to define or enclose an area.
Fences, bollards, and walls in perimeter security
Although fences, bollards, and walls are united as good tools to create perimeter security they differ in major ways. Height, foundation type, sight lines, and physical permeability all factor into why one is chosen over another.
A fence differs from a wall in that it does not have a solid foundation along its whole length. In a security installation, fences may have close-placed posts that would make it hard to tunnel under them.
A bollard differs from both in that each bollard stands alone above grade. Most stand alone below grade as well. (Some bollard installations are connected by hydraulics or security frames below ground.)
Security walls, bollards, and fences are usually built to withstand the force of vehicle impact. Non-security versions of all three of these items exist. Impact resistance is often provided by engineered foundations as well as the material strength of the barrier.
Foundations are an important part of impact resistance but they can have a fairly small footprint. Bollards with small individual foundations can be strong enough to withstand incredible impact forces. Fence posts can be engineered to the same resilience.
A wall, featuring a continuous foundation, may therefore be over-engineering. If the wall’s opacity and wind-resistance are not needed, a fence or line of bollards may be enough.
A bollard is generally a short traffic post. Even impact-protective security bollards are most commonly between 28-36 inches. Security bollards of five feet tall exist, but are unusual. Part of the utility of bollards has been to be effective at stopping cars without drawing too much attention to themselves.
Fences come in a wide range of heights, but security fences are required to have a minimum “fence fabric” height of 7 feet.
Like fences, walls can be short. For secure perimeters, walls are generally 7 feet or taller, to make them less scalable.
Sight lines and views
Bollards are generally short enough to fall below common sight lines. They’re also usually spaced apart. Part of their task is to be relatively unobtrusive and often unseen. When they are installed as part of a secure perimeter—perhaps complemented by target-hardened planters, benches, or trees—they can almost disappear, or even be part of the overall view.
Fences, especially security fences stretching 7 feet tall and above, generally offer more visual interruption. However, a fence is often made of a light and wind permeable “fence fabric,” like chain link, wrought iron bars, or horizontal cording. Most fencing allows some movement of light and air through the fabric between fence-posts.
Walls may have windows or doors within them that allow for a larger sight line, but the wall itself is usually opaque. Walls offer a third type of security beyond fences and bollards. Walls are often installed around sites that may be vulnerable to weapons attack because bollards or fences don’t stop gunfire.
Bollards normally create a barrier that people can walk through but cars cannot drive through. In comparison, fences and walls generally must have entrances to allow passage through them.
Security fences and security bollards
Security fences are a visual notification of a legal boundary or protected area. Where a bollard invites pedestrians while excluding vehicles, a fence suggests only authorized people should cross the perimeter through established access points.
Fences make entering an area more challenging. A security fence, even one with climbable fabric like chain link, will take time to scale. It can be barbed wired on top or alarmed to help discourage intruders. In security installations, fences are a delaying tactic. Most people will respect a fence as a boundary, and people who might casually cut through an area will be discouraged. A determined intruder may well find their way past the fence, but it will slow them down, and may make them easier to detect.
Unlike walls, fences are normally see-through. This gives a security team time to evaluate incoming vehicles and pedestrians.
Security bollards are also notification of a legal boundary. Unlike fences, they invite foot traffic, strollers, wheelchairs, and bicycles. Bollards create safe spaces where people move freely without the threat of vehicle accidents. Like curbs, they delineate pedestrian space as separate from vehicle space; unlike curbs, they offer protection against accident or attack.
Fences and bollards are often used together. A security bollard can protect a standard “civil” fence. In this instance, the fence may encourage people to use a particular entrance but does not need to be fortified against attack. Bollards might be placed around parts of the fence near the road or parking lot. Like bollards around utility boxes, they're installed to protect against accident.
“Bollard fencing” may be designed by placing bollards with chains between them, where the bollards stand for the fence posts and the chains between them stand in as the fence fabric. Bollard fences like this can be effective in communicating preferred ways of walking: for example, around a garden they suggest to keep off the plants while allowing someone to hop over them to retrieve an errant frisbee.
Yet standard fencing creates more of a visual and psychological deterrent. If fencing is what’s preferred, there are a wide range of options available on the market.
Bollards can support or enhance the installation. Bike bollards can offer parking outside the fence. Lighting bollards might provide guidance at night to the entrance. Architectural bollards can provide a classical counterpoint to wrought iron fences. Security bollards can help protect fence posts. In general, bollards and fences work well together rather than instead of each other: they're designed for different jobs.
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