Structural design and architectural expression synthesise in NORD Architecture’s Bridgewater Bridge, writes Felix Mara
It is perhaps a truism that structural engineering rests on the principles of efficiency, minimising the weight, and therefore the cost, of construction and getting from point A to point B as quickly as possible. But NORD Architecture’s Bridgewater pedestrian and cycle bridge across the Caldon Canal in Hanley, Stoke-on-Trent, is predicated on a different set of criteria. The structural concept could be seen as idiosyncratic and the tectonics of the bridge, the way it’s put together and the way this is expressed, is an end rather than a means. Nevertheless, this bridge is very buildable. Bridgewater is a unique synthesis of structural engineering and craftsmanship.
Developer Urban Splash and RENEW North Staffordshire commissioned Glasgow-based NORD to propose a masterplan, comprising residential, retail and leisure facilities. The bridge, named after the neighbouring Bridgewater Pottery, is the first completed component of the masterplan. It improves access to the site and also enhances the canal-side environment.
NORD approached the design as both a crossing point and a canopy structure, providing clear physical and visual access to the space below the bridge. The sculpted soffit, an integral part of the 22m clear-span structural frame, defines what NORD describes as a ‘canopied space’. The underside is faceted and illuminated to create a visual spectacle for both boat users and pedestrians on the towpaths. The structural engineer is the Glasgow office of Buro Happold.
The 27m-long bridge spans between two concrete beams at the north end and cantilevered cross-beams at the south end, which it oversails by 4m. Adjacent to these cross-beams there is a ramp and a staircase. The support structure from which the beams are cantilevered, like the structure on the north side where there is another ramp, is clad in Staffordshire Blue engineering bricks. The oversailing area, removed from the main circulation route, is a place where people can rest or gather.
The bridge’s deck comprises a lattice of primary 406 x 178mm universal beams (UBs) at 700mm centres, spanning between perimeter 533 x 210mm UBs. Short 406 x 178mm UBs, at 700mm centres, span between these primaries to form a lattice grid at a 45o angle to the principal axis of the bridge.
At the north end of the structure, where there is no soffit, the diagonal grid deck connects to a more utilitarian structure, comprising 406 x 178mm UBs parallel with the main axis.
Subtly, when the bridge is viewed from below, the brick pier on the north side of the canal appears to support one end, but its south end looks as though it is either fixed to the side of the pier adjacent to the staircase or cantilevering 22m. NORD achieved this by concealing the cross beams in the coffered soffit. It was necessary to increase the depth of the UBs, and therefore the apparent thickness of the bridge, in order to do this.
As for the tectonics of the bridge, the diagonal grid is expressed on the soffit, with sheet-metal pyramids forming an array of coffers. The void above these coffers is filled with expanding Uretek resin, which forms the deck. This was chosen instead of steel to reduce the weight and, therefore, the depth of the bridge. Vertical plates span between the outer flanges of the perimeter rolled steel joists and this simplifies the appearance of the deck, which reads as a white slab. Thus, NORD concealed the structure at the perimeter but expressed it on the soffit, forming a decorative pattern.
Although the structural deck is not directly expressed on the top surface of the bridge, the configuration of block pavers shows a similar fascination with pattern-making and weaving that has a resonance with the tectonic investigations of Gottfried Semper. The paving pattern also emphasises the main axis of the bridge, the direction of pedestrian movement and, together with a perimeter gratings, it feels as tough as a Victorian public convenience or train station platform.
The perimeter balustrade comprises 1165 x 815 x 21mm laminated toughened glass panels, with Sunshield anti-graffiti film. These are grouted into a perimeter channel.Presumably, the vertical joints between these panels are at close centres for architectural reasons rather than to accommodate deflections in
the deck. Because the balustrade is glass and set back on plan from the bridge’s perimeter the slab structure supporting them remains visually intact.
The fabrication of the bridge was an art form in itself and its erection was an event. At the highlight of this sequence, the deck structure was lifted into place by a lofty pivoting crane.