By: Mike Abrahams and Paul Lutkevich
Over the past 10-15 years, increasing attention has been paid to bridge aesthetics. Motivated by community pride, urban renewal plans and promotion of tourism, public agencies are committing resources to support designs that are not only functional and cost-effective but solutions that also will enhance their community.
As part of this development, aesthetic lighting has become an important issue in bridge design. At the same time, there is growing awareness about the environmental impacts of bridge lighting. These types of issues have spurred developments in bridge lighting design and technologies.
A read on lamps
Metal halide lamps and high-pressure sodium vapor lamps, which are typically used for roadway lighting, remain effective light sources for aesthetic bridge lighting effects when a constant light source is desirable. These powerful light sources can be manipulated to provide maximum visual impact of features such as cables and towers. Using gels, or colored filters, they also can be used in designs featuring color effects.
Rated at 10,000-16,000 hours, they are relatively economical to operate. However, metal halide and high-pressure sodium lamp technologies require two to four minutes to re-strike and start up again. Therefore, they have limited utility in designs that depend on a rapid on and off cycle, such as those that feature chasing (the illusion of movement along the structure), as well as variations in color and intensity.
The new induction (electrodeless) and light-emitting diode (LED) technologies are effective alternatives for lighting effects that require rapid on and off cycles or variations in color and light intensity. Induction lamp technology uses a wire coil wrapped around the outside of the lamp to generate a high-frequency electrical field that excites the gas within the lamp.
Unlike conventional fluorescent technology, high-frequency, non-strike induction technology is less destructive to the lamp, resulting in a rating of 60,000-100,000 hours. Capable of instant on-off cycling and dimming, they can be used to create a variety of movement and color effects.
LEDs, which are illuminated by the movement of electrons in a semiconductor material, are even more versatile. In aesthetic lighting applications, red, green and blue LEDs are packed together on circuit boards enclosed in fixtures and their relative intensity controlled by computer to generate literally millions of colors and rapid movement effects. LED lamps are rated at approximately 60,000 hours.
Although the initial cost of induction and LED technologies is higher, their long lives result in substantial reduction in maintenance costs when compared with metal halide or high-pressure sodium vapor lamps. However, the lower wattage of induction and LED lamp technologies makes these unsuitable for certain applications, such as flood lighting.
Controlling a spill
The environmental impacts of light spill on the night sky include effects on human observation of astronomical features; interference with bird flight, including collisions with structures and interference with migration patterns; and interference with migration patterns and reproductive behaviors of sea animals, such as sea turtles.
States vary in regulating this effect. For example, some states allow only the use of full cut-off luminaires for roadway lighting, but make exceptions for lighting monuments and structures, including bridges. Other jurisdictions require that aesthetic lighting be dimmed or shut off during certain hours, for example from midnight to 5 a.m., and many impose time-of-year restrictions as well.
In any case, it is simply good practice to consider the impact of lighting both from an environmental and aesthetic standpoint. Imagine a simple aesthetic bridge lighting design—standard deck-mounted, high-wattage floodlights aimed up at the structures and cables running at full intensity from sundown to sunup. Admittedly an extreme example, it serves to illustrate a point: uncontrolled uplighting is reflected off the atmosphere, creating a veil of light that obscures the night sky; at the same time, it is ineffective from an aesthetic standpoint because it fails to focus the light for maximum impact.
An effective solution combines proper lamp, wattage and fixture selection for the application and an operational plan that minimizes impacts on humans and wildlife in the area surrounding the bridge.
Beauty by computer design
Beautiful, functional and cost-effective bridge lighting design is facilitated by use of new computer software that allows the designer to create three-dimensional still and moving (video) images that accurately depict the lighting effects of various solutions. Not only are these images being used to enable designers to fine-tune their lighting schemes, they also are being used by consultants, owners and other public agencies to illustrate proposed lighting schemes to communities and to facilitate selection and approval of a lighting design.
A long tune-up
Sophisticated computer-generated 3-D images enabled designers to fine-tune a lighting design intended to maximize visual impact and minimize the effects of stray light on the reproductive behavior of sea turtles. The $531 million Cooper River Bridges Replacement Project will result in a new cable-stayed crossing of the Cooper River in Charleston, S.C. The new Arthur Ravenel Jr. bridge, which will replace two obsolete Cooper River bridges, will have a main span of 1,546 ft.
The structure will feature two lighted diamond-shaped concrete towers and lighted cables. One fixture is positioned between each cable at the roadway level, aimed at the center point where the cable meets the tower, illuminating both the cable and tower; another set of lights positioned at the water level of the towers illuminate the lower section of the tower.
The fixtures are narrow-beam (i.e., 4° distribution) fixture fitted with 250W metal halide lamps, which minimizes stray lighting. In addition, the lights will be turned off between 11 p.m. and 4 a.m. so as not to interfere with sea turtle reproductive behavior.
Computer-generated 3-D graphics were used to fine-tune the design effects, in particular, to assure uniformity over the structure and to assess the design for stray lighting. In fact, the designers initially considered using a wider distribution fixture; however, when these were placed in the 3-D model, there was insufficient lighting at the towers, especially at the tops, as well as excessive stray light.
Boston elegance
The $105 million Leonard P. Zakim Bunker Hill Bridge over the Charles River in Boston is a key element of the Massachusetts Turnpike Authority’s Central Artery/Tunnel (CA/T) project. As management consultant for the CA/T project, the Bechtel/PB joint venture developed a preliminary bridge design and concepts for roadway and aesthetic lighting.
All aesthetic lighting consists of white metal halide yoke-mounted 277-V lights, except as noted. Towers are lighted both from above and below the span with fixtures mounted on the exterior face of the tower, with 400-W lights shooting up from deck level and 250-W lights shooting down from deck level. Main and back span cables are lighted with 250-W lights, with main-span cable lights located outside at the cable planes and back-span cable lights located in the median along with the cables. The cable lighting effect is heightened with white high-density polyethylene cable covering.
The interior of the “Y-shaped” tower legs are lighted with 1,000-W, 277-V blue lights, with fixtures mounted at roadway level outside the main line barriers and shot toward the point at which the two inclined legs meet, producing a gateway effect at night.
About The Author: Abrahams is technical director for major bridges for Parsons Brinckerhoff, New York, N.Y. Lutkevich is a project manager with PB.
