The prominence of bollards has dramatically increased during the past decade due to heightened concerns about security. They are a simple, practical, and cost-effective means of erecting anti-ram perimeter defense without creating a visual sense of a fortified bunker. Bollards are widely used for traffic direction and control, and in purely decorative applications. However, bollards can serve many functions beyond security. They can be used for purely aesthetic purposes, functioning as landscaping elements. Bollards can create visible boundaries of a property, or separate areas within sites. They can control traffic and are often arranged to permit pedestrian access while preventing entry of vehicles.
Removable and retractable bollards can allow different levels of access restriction for a variety of circumstances. They frequently tell us where we can and cannot drive, park, bike, or walk, protect us from crime, shield vehicles and property from accidents, and add aesthetic features to our building exteriors and surrounding areas. Bollards can incorporate other functions such as lighting, security cameras, bicycle parking or even seating. Decorative bollards are made in a variety of patterns to harmonize with a wide range of architectural styles. The prevalence of the most common form of security bollard, the concrete-filled steel pipe, has encouraged the manufacturing of decorative bollards designed to fit as covers over standard steel pipe sizes, adding pleasing form to the required function.
What Is A Bollard?
A bollard is a short vertical post. Early bollards were for mooring large ships at dock, and they are still in use today. A typical marine bollard is produced in cast iron or steel and shaped somewhat like a mushroom; the enlarged top is designed to prevent mooring ropes from slipping off.
Today, the word bollard also describes a variety of structures used on streets, around buildings, and in landscaping. According to legend, the first street bollards were actually cannons – sometimes said to be captured enemy weapons – planted in the ground as boundary posts and town markers. When the supply of former cannons was used up, similarly shaped iron castings were made to fulfill the same functions. Bollards have since evolved into many varieties that are widely employed on roads, especially in urban areas, as well as outside supermarkets, restaurants, hotels, shops, government buildings and stadiums.
The most common type of bollard is fixed. The simplest is an unaesthetic steel post, about 914 to 1219 mm (36 to 48 in.) above-grade. Specially manufactured bollards include not only simple posts, but also a wide variety of decorative designs. Some feature square or rectangular cross-sections, but most are cylindrical, sometimes with a domed, angled, or flat cap. They come in a variety of metallic, painted, and durable powder coat finishes.
Removable bollards are used where the need to limit access or direct traffic changes occasionally. Both retractable and fold-down styles are employed where selective entry is frequently needed, and are designed so the bollard can be easily collapsed to ground level and quickly re-erected. Both retractable units may be manually operated or automated with hydraulic movements. Movable bollards are large, heavy objects – frequently stone or concrete – that rely on their weight rather than structural anchoring to stay in place. They are designed to be moved rarely, and then only with heavy machinery such as a fork-lift.
Bollards generally fall into three types of applications:
- Decorative Bollards – decorative bollards for architectural and/or landscaping highlights;
- Traffic and Safety Bollards – bollards that provide asset and pedestrian safety, as well as traffic direction; and
- Security Bollards and Post Covers – decorative, impact-resistant bollard enhancements
Some bollards are intended purely to be an ornament. As standalone architectural or landscaping features, they can border, divide, or define a space. They can also be accents, sentries, or supporting players to larger, more dramatic architectural gesture.
Decorative bollards are manufactured to harmonize with both traditional and contemporary architectural styles. The latter lean toward visual simplicity – often straight-sided posts with one or more reveals near the top. Styles made to match various historic periods usually have more elaborate shapes and surface details. These include flutes, bands, scrolls and other ornamentation.The post-top is a distinctive feature; traditional bollard design often includes elaborate decorative finials, whereas contemporary versions frequently feature a simple rounded or slanted top to deter passersby from leaving trash or using them for impromptu seating. On the other hand, they are sometimes made flat and broad specifically to encourage seating. Common decorative bollard materials include iron, aluminum, stainless steel, and concrete.
Ornamental designs with elaborate detail are frequently made of iron or aluminum casting. Aluminum bollards are desirable for applications where weight is an issue, such as a removable bollard. Aluminum units tend to be slightly more expensive than iron. For applications where a decorative bollard may be subject to destructive impact, ductile iron is a safer choice than more brittle metals, as force will deform the metal rather than shatter and transforming it into possible hazardous flying projectiles.
Iron and aluminum bollards are frequently manufactured by sand-casting – a traditional foundry technique that is economical and well-suited to objects this size. However, sand-cast objects frequently bear surface irregularities that tend to leave the finished product less appealing to the eye. If high-finish consistency is desired, seek a manufacturer that will machine 100% of the surface after casting to produce units with a uniform surface for maximum visual appeal.
Finish is an important consideration in a decorative bollard, from functional as well as aesthetic standpoints. Bollards are, by their nature, prone to being scratched or nicked by pedestrians and vehicles. Those located near roadways are exposed to a fairly aggressive environment; petrochemical residues and splashes of diluted road de-icing salts may compromise some painted finishes. Factory-applied powder coating – which is available on iron, aluminum, and steel – is an especially durable form of painted finish. The application process builds up a coating with very consistent coverage. During coating, any bare metal tends to attract the powder, eliminating pinholes in coverage. The baking process that completes the finish gives it additional toughness and abuse resistance.
In applications where greater physical abuse is predictable, decorative bollards made of aluminum may be a better choice than iron. If the finish coat is damaged, aluminum oxidizes to a color that is generally more acceptable than the red rust produced by iron. Aluminum and stainless steel are also available in a number of bare metal finishes. Functionality can be added to the otherwise decorative bollard. For example, common option is the chain eye – linking two or more bollards with chain, creating a simple traffic direction system. A large metal loop or arm on the side of the post allows parking and locking of bicycles, an increasingly popular choice as more people seek alternative green transportation. Bollards may also contain lighting units or security devices, such as motion sensors or cameras.
Traffic and Safety Bollards
The most common bollard applications are traffic direction and control, along with safety and security. The first function is achieved by the visual presence of the bollards, and to some extent by impact resistance, although, in these applications visual deterrence is the primary function. Safety and security applications depend on higher levels of impact resistance. The major difference between the two is safety designs are concerned with stopping accidental breach of a defined space, whereas security is about stopping intentional ramming.
Closely spaced lines of bollards can form a traffic filter, separating motor vehicles from pedestrians and bicycles. Placing the posts with 1 m (3 ft) of clearance between them, for example, allows easy passage for humans and human-powered vehicles – such as wheelchairs or shopping carts – but prevents the passage of cars. Such installations are often seen in front of the parking lot entrance to a store, and at the mouths of streets converted to outdoor malls or ‘walk streets’. In designing bollard installations for a site, care must be taken to avoid locating them where they will become a navigational hazard to authorized vehicles or cyclists.
Some applications for traffic guidance depend on the cooperation of drivers and pedestrians and do not require impact resistance. A line of bollards linked by a chain presents a visual cue not to cross the boundary, even though it may be easy enough for a pedestrian to go over or under the chain if they choose. Bollards designed to direct traffic are sometimes made to fold, deflect, or break away on impact.
Adding greater collision resistance allows a bollard to enforce traffic restrictions instead of merely suggesting them. Plain pipe bollards are frequently placed at the corners of buildings, or flanking lamp-posts, public phones, fire hydrants, gas pipes and other installations that need to be protected from accidental contact. A bollard at the edge of a roadway prevents cars from over-running sidewalks and harming pedestrians. Bell-shaped bollards can actually redirect a vehicle back onto the roadway when its wheels hit the bollard’s sloped sides.
They are employed where U-turns and tight-radius turns are frequent. This kind of usage is particularly common at corners where vehicle drivers often misestimate turns, and pedestrians are especially close to the roadbed waiting to cross. In some cities, automatically retractable impact-resistant bollards are installed to regulate the flow of traffic into an intersection. Internet videos of ‘bollard runners’ graphically demonstrate the effectiveness of even a low post at stopping cars.
Security Bollards and Post Covers
The aftermaths of the 1995 Oklahoma City bombing and the September 11, 2001, attacks saw a sharp rise in the installation of bollards for security purposes. Anti-ram installations include not only posts, but other objects designed to resist impact without presenting the appearance of a protective barrier, such as large planters or benches that conceal bollards. Once the design threat is determined, the resistance needed to stop it can be calculated. (See ‘Security Design Concepts’ – below). Specification of anti-ram perimeter takes into account both the mass and the speed of an approaching attack vehicle, with the latter being considered the more significant.
According to Weidlinger Associates principal, Peter DiMaggio – an expert in security design – careful assessment of the surrounding site is required. “Street and site architecture will determine the maximum possible approach speed,” he said. “If there are no approaches to the building with a long run-up, an attack vehicle cannot build up high speed, and the resistance of the anti-ram barriers can be adjusted accordingly.”
Anti-ram resistance is commonly measured using a standard developed by the Department of State, called the K-rating. K-4, K-8 and K-12 each refer to the ability to stop a truck of a specific weight and speed and prevent penetration of the payload more than 1 m (3 ft) past the anti-ram barrier. Resistance depends not only on the size and strength of the bollard itself, but also on the way it is anchored and the substrate it’s anchored into.
Videos of bollard crash tests are featured on a number of manufacturer’s Web sites. The truck impacts two or three bollards at high speed, and the front of the vehicle often crumples, wrapping completely around the centermost post. Part of the cab may fly off the truck, the front or rear end could rise several feet in the air, and front or rear axles might detach. The bollards and their footings are sometimes lifted several feet upward. In all successful tests, the payload on the back of the truck does not penetrate more than 1 meter past the line of bollards, thus satisfying the standard.
The simplest security bollard is a piece of 203-mm (8-in.), 254-mm (10-in.), or 305-mm (12 in.) carbon steel structural pipe. Some impact resistance is achieved even with a 102-mm (4-in.) pipe, depending on the engineering of its foundation. It is often filled with concrete to increase stiffness, although unfilled pipe with plate stiffeners inside may actually produce better resistance in the same diameter pipe. Without any form of internal stiffening, the pipe’s wall-thickness needs to be significantly greater. For fixed-type security bollards, simple pipe bollards may be functionally sufficient, if properly mounted. Undecorated pipe-type bollards are also specially manufactured.
The biggest disadvantage of a plain pipe is aesthetics. A piece of painted pipe does not truly blend into – much less enhance – most architectural schemes. However, this can be overcome by a decorative bollard cover. Many standalone bollards that do not have impact-resistance of their own are designed with alternative mounting capability to slip over standard pipe sizes, forming an attractive and architecturally appropriate impact-resistance system. These decorative covers may also be available to enhance specially designed (but non-decorative) pipe-type bollards.
Security Design Concepts
Much of modern security design focuses on the threat of bomb attacks. The most significant factor in protecting against explosions is the distance between the detonation and the target. The force of the blast shockwave diminishes as a function of the square of the distance. The more distance that can be placed between the detonation and the protected structure – referred to as standoff distance – the greater the threat resistance or, conversely, the less blast resistance needs to be built into the structure. Therefore, creation of secure perimeter is the first step in the overall design of blast resistance.
Standoff is valuable architecturally because it allows a building to be protected without having to look like a bunker. It also has economic impact, because it is frequently less costly to create standoff than to bomb-proof the structure itself. Security bollards and similar anti-ram installations are designed and positioned to create standoff by thwarting the delivery of explosives close to the target by a vehicle.
Any security design depends on an estimate of the size of threat to be resisted – the ‘design threat.’ The force of the explosion that can be expected is directly related to the weight- and volume-carrying capabilities of the delivery vehicle. Explosives are measured in relation to tonnes of trinitrotoluene (TNT). The most potent molecular explosives such, as Composition 4 (i.e. C-4), are approximately a third more powerful than TNT, whereas a fuel and fertilizer bomb – such as was used in Oklahoma City – is considerably less powerful than TNT. Reasonable approximations can be made about how much explosive power can be delivered by a person carrying a backpack, a passenger vehicle, a pickup truck, a flatbed truck, etc. based on its weight-and volume-carrying capacity.
There are three basic types of bollard mountings: fixed, removable, and operable (retractable or fold-down). Fixed bollards can be mounted into existing concrete, or installed in new foundations. Manufactured bollards are frequently designed with their own mounting systems. Standalone mountings can be as non-invasive as drilling into existing concrete and anchoring with epoxy or concrete inserts. Such surface-mounted bollards can be used for purely aesthetic installations and substantial visual deterrence and direction, but provide only minimal impact resistance.
Bollards designed to protect against impact are usually embedded in concrete several feet deep, if site conditions permit. Engineering of the mounting depends on design threat, soil conditions and other site-specific factors. Strip footings that mount several bollards provide better resistance, spreading the impact load over a wider area. For sites where deep excavation is not desirable or possible (e.g. an urban location with a basement or subway beneath the pavement), bollards made with shallow-depth installation systems are available for both individual posts and groups of bollards. In general, the shallower the mounting, the broader it must be to resist impact loading.
A removable bollard typically has a permanently installed mount or sleeve below grade, while the sleeve’s top is flush with the pavement. The mating bollard can be manually lifted out of the mount to allow access. This system is intended for locations where the change of access is occasionally needed. It can include a locking mechanism, either exposed or concealed, to prevent unauthorized removal. Both plain and decorative bollards are available for this type of application. Most removable bollards are not designed for high-impact resistance and are usually not used in anti-ram applications.
Retractable bollards telescope down below pavement level, and may be either manual or automatically operated. Manual systems sometimes have lift-assistance mechanisms to ease and speed deployment. Automatic systems may be electric or hydraulic and sometimes include a dedicated backup power installation so the bollard remains functional during emergencies. Retractable systems tend to be unornamented.
Bollards are as ubiquitous as they are overlooked. They speak to the need for defining space, one of the basic tasks of the built environment. Decorative bollards and bollard covers offer a versatile solution for bringing pleasing form to a variety of functions. The range of available options is vast in terms of both visual style and performance properties. For security applications, a design professional with security expertise should be included in the planning team.
Find review the original version of this bollards reference article complete with imagery and references.[ad_2]
Source by Brad Done