Well-lit Directions

March 1, 2006

Bridges are beautiful, and you don’t have to be a civil engineer to recognize that. Bridges represent a living testimony to man’s achievement in conquering nature’s obstacles. Philosophically, should bridges be illuminated at night? The answer is simple: If no one can see it at night or if the bridge will compete for visual attention with a more significant object in its field of view, don’t waste your time and money lighting it.

Bridges are beautiful, and you don’t have to be a civil engineer to recognize that. Bridges represent a living testimony to man’s achievement in conquering nature’s obstacles. Philosophically, should bridges be illuminated at night? The answer is simple: If no one can see it at night or if the bridge will compete for visual attention with a more significant object in its field of view, don’t waste your time and money lighting it.

Bridges are structures which span a man-made or natural obstacle to the movement of vehicles, pedestrians or materials. They possess certain elements which must be identified and coherently addressed by the designer. For example, approaches, roadways, structural support systems, portals, gateways, piers, towers, sidewalks and so on.

Bridges vary in size, shape, materials, structures system, color and location. In some cases the surrounding is so awe-inspiring that nothing man made can compete (nor should it) with the setting. Sometimes, competition is unavoidable. In the daytime competition is rarely a factor because daylight is so democratic. The irrelevant as well as the important are illuminated equally.

After dark, however, the situation changes. At that time the lighting designer is given a unique advantage: that which is deemed unattractive or undesirable can be erased from view simply by allowing it to remain in darkness. However, with light, that which is beautiful can be revealed, modeled, exaggerated and enhanced.

Constraints which must be addressed are building codes, site conditions, spill light, glare control and budgets. This last item consists of installation, operation and maintenance costs over the life of the bridge. Sustainability and maintainability are two very important issues. We have an unbreakable rule for all of our lighting projects. Unless a project can be maintained in a reasonable manner, and the maintenance is affordable, do not waste you and your client’s time and money installing it. Lighting doesn’t make a bridge beautiful; it simply expresses the natural beauty which is already there.

The florescent four

If a bridge warrants lighting, then there are at least four distinctly different approaches to the design process.

The first is Historical Precedent. Relighting may consist of restoration and renovation and electrification of existing historic fixtures and employing long life, high color rendering and energy-efficient light sources. Any new additional lighting added by the designer must be selected and located so that the fixtures are virtually invisible.

The remaining approaches vary primarily with the educational and professional background of the designer. There are primarily three:

Illuminating Engineering:

This is generally a cut-and-dry approach utilizing general floodlighting of the bridge structure. It works well for bridges with greater visual density as seen from either side. These include many old railroad, covered wood and even more modern concrete and steel-bent bridges. It requires great precision for lightly framed structures, such as suspension bridges with all the variations that exist. Care must be taken in the placement and concealment of fixtures and baffling of light sources to minimize glare. When glare is unavoidable to pedestrians or vehicles, another technique should be selected. Elements to be considered in the design of the bridge is the color of paint used (the lighter the better) and the intended purpose for the bridge, whether it is rail, auto, pedestrian or for industrial materials.

Theatrical Approach:

This has become increasingly popular in recent years. Techniques include visual projections onto the bridge or structural members; another is the use of colored floodlights. There are two types of these; the older technique employs color filters. While the filters themselves have a limited life, they lend themselves to simple replacement. An example would be special events or holidays. Internal rotating colored wheels allow for ease of color changes but require a great deal of maintenance to keep all the motor and moving parts functioning. A more contemporary technique employs xenon lamps and diachronic reflectors. These are far more expensive to purchase and relamp. However, color changes may be pre-programmed with electronic controls for virtually instant response and hundreds of different colors.

LED Systems:

Because of the narrow beam inherent in LED lamps, these fixtures are best used when they are in direct view or aimed at a surface to be lighted. These lamps are still in a state of rapid development and some claims by manufactures are open to question. At this time the life of white LEDs are between 30-50K hours and blue LEDs are between 40-50K hours (not over 100,000 hours as some manufacturers claim). The life of yellow and red LEDs are between 80-100K hours. Rated life is based on the time in use before the LEDs loses half of their initial output. To put this into context, there are approximately 4,300 hours of darkness at 39° North latitude (Washington D.C., St. Louis and San Francisco). By combining selected red, blue and green LEDs into one fixture and varying the intensity of each in an interrelated manner there is a claimed selection of approximately 1,500 different spectrum of colored light available.

Architectural Lighting:

Architectural lighting design is the fourth approach. It incorporates both the illuminating engineering and the theatrical lighting design technique. The architectural lighting design approach varies in its focus on the design approach, which is far more subjective. That is, an object oriented on its location within the surrounding area. An example of this approach can be seen in the eight bridges crossing the Schuylkill River in the city of Philadelphia. These bridges vary significantly in age, structural type, span, width, location and nature of use. In response, the lighting design approach varies. For example, sustainability and maintainability were of major concern. The bridge lighting approach distinguished specific tasks appropriate for each bridge. The first task was to determine which bridges lent to floodlighting. One newly designed fixture was selected which had excellent optics (to reduce glare) and a flexible “barn door” system (these failed within days because the supports were too weak to withstand the high winds channeled along the river). This is a common problem. A superior system was designed which has been adopted by the manufacturers. The fixtures were pole-mounted at varying heights to avoid blinding motorist and pedestrians.

A second fixture type was selected to illuminate the vehicular roadway. These were pole-mounted cutoff luminaries with flat lenses to minimize high-angle glare. Metal halide lamps, which are very high color rendering white lights, were selected. Unfortunately, the city standard is high-pressure sodium yellow- or peach-colored lamps and the city lighting engineers were unwilling to compromise on this issue.

A third fixture type was pole-mounted decorative, pedestrian scale lanterns. These varied in style with each bridge based upon the bridges shape and immediate surrounding environment. The light source and ballast were the same for all decorative lanterns to simplify maintenance for the city. The South Street Bridge occurs at the entry to the University of Pennsylvania campus. The standard university fixture and pole (with the new light source) was selected. The Spring Garden Street Bridge located adjacent to the Philadelphia Museum of Art uses decorative fixture and pole that also is used in front of the museum (with the new metal halide lamp source).

The SEPTA and Market Street bridges are masonry and multi-vaulted in which the arches on either side were outlined with low voltage long-life xenon lamps. The undersides of these bridges also were illuminated with a blue-white light to capture the magic of the reflected image in the river below.

All bridges have portal lighting at each end. Decorative fixture spacing along each bridge varies based on what was deemed appropriate in scale and form. The Market Street Bridge, built in the early 20th century, has two huge historic navigational fixtures mounted on the up river and down river side of the central pier of one of the masonry bridges, which were subject to extreme damage from debris and water during periodic flooding. These two monumental bronze fixtures were rebuilt, reinforced and reglazed with fiberglass reinforce acrylic. The vapor-tight light source housings were redesigned for simple replacement if ever required. Other changes are too numerous to describe but there are some guidelines which architectural lighting designers employ.

About The Author: Grenald is a Fellow of the IALD, AIA and IES.

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