Rising to the challenge, carrying the load
Road lighting luminaires are generally post mounted to the top of a column or supported from bracket arms extended from a column. Wall brackets, suspension on catenary wires and high masts are other mounting possibilities.
In towns and city centres where road lighting columns at ground level can present an obstruction, wall brackets provide a good alternative solution. However, obtaining permission from building owners and overcoming legislative and practical complications can be a complex process.
Catenary wire suspension is widely used in Europe. This method has been used here on dual carriageways and at busy urban intersections. Where there are trees at the roadside obstructing light this method is a way of avoiding that obstruction. The visual appearance of overhead wires must be carefully considered especially in areas of historic or architectural importance.
High masts in excess of 20m are often used to support luminaires at complex intersections and interchanges. By using luminaires with high power lamps and controlled optics it is possible to limit the number of masts and reduce the visual complexity by day and night. Maintenance is carried out a ground level by lowering the luminaire mounting frame with an electrically powered winch.
Road lighting columns, available in a variety of materials, are chosen for aesthetic, economic and safety reasons. Economic considerations include purchase and installation, maintenance, durability and functionality, end of life disposal and recycling.
Reinforced concrete, still used in some countries, requires no maintenance but catastrophic failures have occurred owing to corrosion of the steel reinforcement. The effects on a vehicle and its occupants hitting a concrete column can be severe.
Steel columns with a galvanised finish can last in excess of 10 years without maintenance, thereafter further protection by painting may be required. Where vehicular access for maintenance is difficult, hinged columns are used, enabling the luminaires to be lowered to a convenient working height.
Aluminium is light weight, cheap to transport and easier to install. These columns require no maintenance and are much less damaging in the event of impact by a vehicle and are recyclable at the end of their service life.
Columns are also available in stainless steel, fibre reinforced polymer and natural wood.
Road lighting columns are fixed directly in the ground or bolted to a concrete foundation plinth, or a structure such as a bridge, by means of a flange plate welded to the base of the column. The integrity of the fixing method should be determined by qualified civil or structural engineers.
Protection against corrosion above ground, from dog urine and salt in the winter months, and below ground from the soil itself, is achieved with a heavy coating of bituminous or other suitable paint applied in the factory. Flange plates, nuts, washers and exposed threads of fixing bolts require similar protection. Where galvanic corrosion between dissimilar metals is a risk, particular care is needed. For aesthetic reasons the flange plate may be concealed with gravel or finished with a decorative cover.
The photometric performance of modern luminaires means that the lighting requirements can often be met with the fitting mounted directly on top of the column. This reduces costs and the weight and windage load on the column, however, a short bracket can help to damp oscillations caused by wind gusts.
Electrical supply connections
The base section of a column usually forms a compartment in which the terminations for the electrical supply and cut-outs are located, and may also contain control components. Access to the compartment is by a lockable door providing protection to the public, as well as protecting the electrical components from the elements and against interference.
The aesthetic appearance of road lighting is of particular concern during daylight, non-functioning, hours. In urban areas the column height of luminaires should not exceed that of nearby buildings. When viewed against the sky, it is preferable to use a slender column and luminaire in an effort to minimise visual intrusion. As appropriate to the architectural landscape or to reflect the historical importance of a location, futuristic or more decorative columns and luminaires can be considered.
Columns and brackets for road lighting must have good structural integrity to perform their function in demanding environments over long periods, sometimes in excess of 30 years. Specifications for structural design, manufacturing, testing, protection against corrosion and applications are to be found in EN 40, the European standard for lighting columns. It applies to columns with heights up to 15m for straight columns supporting floodlights, up to 18m for bracketmounted luminaires and 20m for post-top mounted luminaires. Passive safety requirements and test methods for support structures for road equipment are covered by EN12767.
The loads imposed on a column and bracket depend on several factors that must be taken into account to verify its fitness for the purpose. Factors include the most extreme wind forces to be expected, categorised into zones, as well as the degree of exposure or protection at the location. The column supplier should be consulted on all aspects of the lighting installation, with regard to the location, the ground conditions for the column’s root or concrete foundation, the proposed luminaire (windage (SCx) and weight), and any additional loads such as signs, banners, festive lighting or flower baskets that may be imposed on the column. Columns and brackets made of steel or aluminium must be CE marked.