| The 140-year old Clifton Suspension Bridge, a famous landmark in Bristol, UK, is to undergo a complete lighting transformation in time to memorialize the 200th anniversary of its designer, Isambard Kingdom Brunel, in 2006.
Architectural lighting designers Pinniger & Partners have developed a lighting scheme with four main elements, one involving the use of LEDs, which will emphasize the aesthetics of the bridge, cut down on light pollution and provide a visual connection between the bridge and the gorge below. The whole bridge will be lit with white light.
Advanced illumination of the Clifton Suspension Bridge dates back to the day it opened in 1864, when it was lit during the evening using a new invention - electricity.
The current lighting system consists of incandescent lamps in polycarbonate tubes that trace the outline of the bridge, and emit light indiscriminately in all directions. Bulb replacements and repairs to the system are required frequently, but are hazardous, given the exposed location, and are particularly hampered by wind and rain.
LEDs have been chosen to illuminate the chains from which the 214 meter (702 ft) span of the bridge is suspended. The bridge is so old that iron chains, rather than steel wires, are suspended between the two 26 meter (86 ft) high towers. Drop rods hang vertically from the chains to suspend the road and pedestrian walkways.
According to Dave Robinson, senior designer with Pinniger & Partners, two LED fixtures are located adjacent to each knuckle, which is the nut that secures each drop rod to the chain. "At each knuckle, two LED projectors point along the chain in opposite directions," he says. "These illuminate the face of the chain rather than projecting light away from it."
The LED fixtures are built around off-the-shelf modules comprising 3 one-watt LEDs. "The modules are in purpose-built housings with input from various parties to make them look as aesthetically pleasing as possible," says Robinson. The fixtures are also fitted with cowls to deflect stray light onto the chains, and also to cut down glare for pedestrians.
Why LEDs?
Different types of lighting will be used elsewhere on the bridge; for example, a line of linear asymmetric fluorescent lamps will be positioned on each walkway to silhouette the lattice work of the bridge. So why were LEDs chosen for the chains?
"The original brief was to have no electrical components on any part of the chains," says Robinson. He explains that the initial concept for lighting the chains was to use fibre-optics, with light projectors below deck level and fibres running up the suspension rods to light-emitting heads on the chains.
"At that stage, over 3 years ago, LED performance was not sufficiently good, but this has improved significantly," says Robinson. LEDs provided the opportunity to keep power consumption to an absolute minimum, and reliability also influenced the switch to LED technology, he says. "Fibre-optic technology would have required us to change the lamps every 6000 hours, so maintenance costs would have been unacceptably high."
The LEDs have a projected lifetime of 60,000 hours, which should allow the system to operate for a minimum of 20 years, given the limited hours of operation per day. The system is dimmable, providing the flexibility to reduce the brightness and maximize lifetime.
The LED drivers will be located together at the tops of the bridge towers, with cables concealed within the chains and hidden from view. "This provides better access compared with putting the drivers under the bridge deck, and will not affect the appearance of the drop rods," says Robinson. The 12 W drivers each supply 4 LED fixtures.
Making sure the fixtures all produce the same white colour is important, but relatively straightforward. White LEDs are supplied from the same bin, but even so there can be slight variations. "However, the output from the LEDs is mixed together and projected onto the chains so any variation is evened out," says Robinson.
Pinniger & Partners is also behind a project to light the much larger Humber Bridge, near Hull, UK. With a central span of 4626 feet, the Humber Bridge is presently the fourth longest suspension bridge in the world, and is the only one of the top ten suspension bridges that is not lit. |
