Crossing the river

The Pùnt de Suransus on the Via Mala in the Grisons is perhaps the most obvious starting point for this short description of crossings, paths, stones, steel cables, concrete, routes along rivers and passages between villages and cities designed by the Grisons architects. The Pùnt was completed in 1999 to a design by Conzett and was the first pedestrian stressed ribbon bridge built in Switzerland in nearly two decades. There is an approximately four-metre difference in level between the two ends of the structure, designed to span one of the deepest gorges in the Grisons. The walkway is made up of granite slabs laid along two long stainless-steel cables and it passes lightly over the river to reach the projecting rock on the other side. Like a line drawn from one end to the other, it is a delicate and almost invisible sign at the bottom of the narrow Alpine valley.

Its structural system is as simple as it is ingenious and Conzett reveals that it was inspired by an unbuilt 1954 project by the Swiss engineer Heinz Hossdorf for a new Ponte del Diavolo, intended for the north side of the Gotthard Pass. The ribbon walkway moves with the wind and requires weight to be stable, which prompted the use of slabs of gneiss that, when stressed and joined together, act as a stable monolithic plate between the two bridge supports. The static type seems a perfect choice for the refined principle of camouflaging light infrastructures in nature explored by Conzett Bronzini and Gartmann since they first established their Chur office in 1999.

The three crossings over the River Aar, built between 2005 and 2011, exploit the same principle adopted for Suransus but with different materials and longer lengths. The first walkway is near Brugg and the new Müllimatt sports centre, designed by the Ticino architect Vacchini. The pedestrian and cycle bridge stretches across the river and between the islands like a strip of undulated concrete. Four spans, measuring 35, 78, 35 and again 35 metres, form the longest stressed ribbon structure in Switzerland. Routes and paths wind along the two banks, parallel to the river. The morphology of the bridge and the sequential waves of the structure allow the pedestrian routes to slip naturally beneath its curves and continue along the watercourse. The Müllimatt walkway is an efficient response to the demands of the brief and extends between the earthen banks of Brugg like a perfectly proportioned sculpted architectural body. This time, the actual walkway is concrete—cast in situ on four steel plates running the full length of the structure. Stressed to create one long brace, it has a total thickness of just 48 cm and descends gently close to the water.

The gradient is slight and walking or cycling over the walkway is not just a quick way to pass through the sports centre and cut diagonally across the suburbs of Brugg, it also offers a close-up on the nature around the river. A new viewpoint of the city and surrounding scenery has been provided by an infrastructure that is totally immersed in and integrated with nature. A few kilometres farther south, people can cross the river near Auenstein and Rupperswill on two new bridges over the River Aar that rest on the two banks and on a natural island halfway across respectively, designed and completed last autumn by Gianfranco Bronzini and his office. Forming part of the regeneration plan for the nature on the river and its banks, the two walkways once again adopted the structural type of the stressed ribbon bridge, despite the span being too wide for such a solution.

Many birds nest at this point of the River Aar and the designed structures had to be as slender as possible and avoid the use of metal cables that might obstruct the birds' flight. The idea of putting pylons in the water was rejected because the river habitat was not to be invaded so the only possible solution was to reduce the river span, which in the case of the longer of the two pedestrian bridges is approximately 100 metres.

The Chur office excluded other static solutions and the stressed ribbon bridge meets the need to blend with the context and also have a low impact. This type of bridge is physically constructed by resting the walkway on supports like a tape, which is then stressed. Conceptually and physically, it is the least invasive approach. Two inclined reinforced-concrete pylons were built on the river's two gravel banks to reduce the span to 74 metres. The cost of constructing the pylons—the presence of the gravel required the design of superstructures for the foundations—was partially offset by redesigning a slender steel walkway. All that remained was to overcome the vibrations typical of such a lightweight structure. The ribbon consists in two rectangular bars running from one side to the other. The structure was then stressed using horizontal and diagonal crosspieces. The walkway surface is wood.

The architectural object and its morphology immediately reveal how it works: the two concrete pylons simply reach out towards the centre of the river, reducing the distance from one bank to the other. Them stretched between them are two tensioned ribbons of the lightest possible design and materials, suspended over the crystal-clear water of the river.
Francesco Garutti