The Ribble coast, the wide estuary, acrobatic flocks in the air – all along Douglas River visitors and locals can admire the wonder of Nature in its `grandeur`. But give yourself a moment, take your time and have a closer look: you’ll find an entire new world that lives unseen by the passer-by and that makes this habitat buoyant with life – the Golden Stem bridge aims at celebrating this hidden world.
The Concept - aka `When Nature does it all on your behalf`
Early morning on the riverbanks, a long stem of grass bent by a night of drizzle. Here a spider chose to tailor its web but, so far, the only things trapped in it are drops of dew - it looks like a piece of fine jewellery.
The stem is an arch and its shape is defined by nature and gravity. In elevation it displaces according to the Golden Section rule, also called Fibonacci sequence: two quarters of circles with radii in the `Golden proportion` (1:1.618033989…) with a common tangent at the top. In cross section the stem naturally bents inward and assumes a triangular configuration that increases its stiffness and better resists gravity.
When Structure enhances Architecture
Learning from nature allows the structure of the bridge to be efficient and fit for purpose: the steel arch spans the whole distance between the existing buttresses without intermediate supports, allowing maximum clearance for navigation and the enjoyment of the riverbanks; in section the arch is composed by caisson boxed steel sections in a triangular configuration that generates the maximum structural efficiency and best use of materials, which is translated in visual slenderness.
As in a spider web the structural cables are disposed radially towards the virtual centre of the arches: this cables configuration best suits the circular shape of the arches because it tends to generate mostly compression and not bending, combining geometry with the best use of materials. Irregularly placed non-structural tubular elements intersect the cables in various locations with a pattern that strengthen the reference to the spider web concept.
At both ends of the bridge, where loads from the arches are at their highest, the structure becomes reinforced concrete. On the East side a tapered prop that follows the shape of the steel arch, here at its sharpest point – where it flows into the open spaces of the countryside. On the West side a very different scenario - a massive gate is created for the people to walk through, a reminder of the stone gates that in the past marked the entrance to the towns of Britain – going out of it the evening jogger will leave his daily occupations behind him and will run into the wild; coming back to it the cycling commuter will recognize home from a distance and will fell embraced, welcomed and protected.
A Beacon for Biodiversity
The aim of the Golden Stem bridge is to raise awareness about the importance of biodiversity and the effects of human activity on the environment: we realise that habitats are compromised by pollution only when major species are affected. In reality, well before that, the first species to suffer are those at the margins of the food chain – `the forgotten`: spiders, snails, worms. The birds that make the Ribble Estuary famous hugely depend on them.
The tall stem-inspired shape legibly marks the crossing point and, combined with the shiny golden shingles inspired by the seeds of a wheat ear, forms a recognisable landmark that will be associated with the Regional Park’s commitment to the environment.
The deck section itself is conceived to provide an educational window for schools and general visitors: slightly lowered and offside but still very visible from the deck is an animal crossing made of sedum mat: a Polyester base layer on which is laid a shallow growing medium onto which is sprinkled sedum cuttings. Wind transported pollens and birds dropping will make this corridor thrive to maturity without any maintenance. Commonly used for `brown roofs` the technique has proved to enhance biodiversity: spiders, snails and worms and many species of flowers and small plants find this an ideal habitat to colonize and this in turns attracts insects and birds. The crossing also allows mice and other small mammals to populate both sides of the river.
Informative posters along the bridge help visitors to spot and recognise the rich wildlife at their feet, literally.
At night time the Golden Bridge radically transforms, nonetheless remaining a legible beacon: a specially conceived lighting strategy makes the arch disappear while point lights sparkle along the whole elevation of the bridge, like drops of dew trapped by the spider web. These LED lights, in different sizes and randomly located, are concealed at the nodes where cables and tubes intersect and also along the tubes.
Analysis of the geotechnical data and of the surveys suggests that a piled foundation, totally independent from the existing railway abutments is the most appropriate foundation option; piles both working in shaft friction in the Glacial Till and end-bearing in the relatively shallow bedrock may be considered. The presence of boulders in the Glacial Till stratum would suggest the use of bored piles, rather than CFA or driven piles.
The bridge sits on reinforced concrete piled raft foundation that resist only vertical loads: any horizontal thrust from the arch is taken away from tubular ties underneath the deck which therefore behaves like a bow string. These tubular ties are also curved in plan to provide lateral stability to the deck.
Like a stem of grass across the seasons the initial green colour of the TECU Gold copper shingles will go through characteristic changes when exposed to weathering. After the initial matting a yellow-green oxidation layer gradually emerges, finally changing into a soft, bright and lasting golden colour.
The finished deck surface is a combination of concrete planks and slats of several varieties of local timber: these materials, colours and textures combine in a patchwork that resembles the typical pattern of the Lancashire agricultural landscape.
The God of Small ThingsThe Ribble coast, the wide estuary, acrobatic flocks in the air – all along Douglas River visitors and locals can admire the wonder of Nature in its `grandeur`. But give yourself a moment, take your time and have a closer look: you’ll find an entire new world that lives unseen by the passer-by and that makes this habitat buoyant with life – the Golden Stem bridge aims at celebrating this hidden world. The Concept - aka `When Nature does it all on your behalf`Early...
- Year 2008
- Client Lancashire County
- Status Competition works
- Type Urban Furniture / Bridges and Roads / Landscape/territorial planning / Lighting Design / Leisure Centres