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Thursday24 July 2014

Infinity footbridge, Stockton-on-Tees, by Expedition Engineering and Spence Associates

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Speirs & Major’s interactive kinetic lighting makes Stockton-on-Tees’ dramatic new landmark come to life.

Location
Stockton-on-Tees, County Durham

Lighting designer
Speirs & Major

Lead consultant
Expedition Engineering and Spence Associates

Completed
January 2010

The deck of the Infinity footbridge in Stockton-on-Tees is cloaked in a mysterious, sombre blue glow. Venture across it, though, and the user is accompanied by a reassuring pool of white light moving at the same pace. This interactive kinetic effect is just one of several unique features the bridge has to offer.

The 273m-long footbridge is suspended from an asymmetrical pair of slender bowstring arches. One arch has twice the span and height of the other, and they blend together into a sinuous, flowing form that skips across the River Tees like a pebble skimming water or, less poetically, like an anorexic dinosaur taking a drink.

From a distance, however, the bridge reads not as a double arch but a double loop, thanks to its reflection in the water. The double loop is also the symbol for infinity – hence the bridge’s name.

The £15 million footbridge was commissioned by Stockton Borough Council to link the Tees Business Park to its new extension on the opposite North Shore.

In 2003, the council sponsored an RIBA design competition for the bridge, won by Expedition Engineering and Spence Associates with Speirs & Major as lighting consultant. The subsequent design was carried out by the team led by Expedition.

The bridge prominently advertises the new business district on two fronts: the lofty double arch stands as an eye-catching landmark visible from afar; while the moving pool of white light effectively pulls people across the bridge to the promised land. At night, of course, neither of these publicity stunts would work without suitable lighting.

Speirs & Major co-director, Jonathan Speirs, says the arches and the deck have been disconnected in the way they are lit.

“The arches are an icon and symbol, whereas the deck and water are about people, boats and interaction,” he says. Accordingly, the arches are lit in up a constant, cold, radiant white light. The deck and the water below it, by contrast, are bathed in a blue half-light, interrupted by pedestrians and boats passing above and beneath.

Lighting the Bridge Deck

In addition to tracking a user’s movements across the bridge by a white light, the moving spotlight also signals the approach of oncomers.

The deck is lit by 200 tiny LEDs which have been neatly tucked out of sight behind and beneath the lower handrail. Two white lights and two blue LEDs, each one consuming only 1.5W of energy, are combined in each fitting. The four LEDs all point downwards at a slight angle to each other, and they are contained within a compact stainless-steel casing purpose-made to fit between pairs of the bridge’s steel balusters. Just below the LEDs in the same steel casing sits a circular electronic sensor which points horizontally across the bridge.

The downlighters and sensors work in unison to animate the bridge, entirely in response to live human activity. “When a person walks across the bridge, each sensor in turn picks up the movement and switches the blue lights off and the white lights on,” explains Jonathan Speirs.

“The white lights take one second to grow in strength and, after the movement stops, another four seconds to fade. It’s as if the person is creating a comet’s trail across the bridge in his or her wake. Kids and adults both love it.”

The blue light continues beneath the deck, where it is bounced off the water surface to illuminate the underbelly and create a blue zone above the water. The light here is emitted from metal halide luminaires fitted with blue filters, and these sit on the outriggers alongside the white uplighters of the same type.

“We’re keen to connect the bridge to the water,” says Speirs. “And here again the blue light interacts with people using the river. Many rowing boats, fishing boats and cruise ships sail up and down the river, and when they pass beneath the bridge, they create a dark shadow across the blue light. We like it when there is a narrative to a bridge and its lights.”

Lighting the Arches

At night, the bridge’s pair of slender steel arches is picked out as looping ribbons of white light. “Cold white light has been used to reveal the structural form and create the sense of a floating wave hovering just above the deck,” explains Jonathan Speirs.

Revealing the structural form accords with one of Speirs & Major’s abiding principles.

fAs Speirs puts it: “The light and the lit are what people enjoy seeing, not how the lighting is done.” The corollary is that the light fittings should be concealed as much as possible.

The footbridge’s arches are illuminated by 150W metal halide spotlights. Though not exactly hidden, the spotlights are housed inconspicuously on outriggers running along either side of the deck, without causing glare beyond the balustrades. The outriggers perform as structural tie rods that restrain the outward thrust of the arches. And the spotlights are fixed at the end of hinged arms that can be swung inwards for maintenance.

The spotlights have been precisely focused on the slender arches to give a crisp image in which the light is evenly distributed and does not overspill as a fuzzy halo. This is achieved by fitting each spotlight with an elliptical prismatic lens that spreads the light beam along one axis only.

Where this neat lighting formula does not work is at the two springing points of the double arch at either end of the bridge, where light beams projected from the outriggers would be interrupted by the balustrading. Instead uplighters have been housed directly at the base of the arch and concealed beneath metal mesh.

Project Team

Client Stockton Borough Council, Lighting designer Speirs & Major, Lead consultant and engineer Expedition Engineering, Concept designers Spence Associates and Expedition Engineering, Main contractor Balfour Beatty, LEDs and casings supplier ACDC Lighting Systems, Electronic sensor supplier Lee Engineering, Arch uplights manufacturer Meyer

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