18 November 2008

Swiss Bridges: 6. Pùnt da Suransuns

Throughout our second day of the bridge study tour, we were accompanied by Jürg Conzett. In the afternoon, we visited two of his bridges, both part of the Via Spluga hiking trail in Via Mala gorge near Thusis.

The gorge itself is absolutely stunning. At several points, you can look 80m down and struggle to see the water below in a dark crevice as little as 1m wide. And looking up there are tall cliffs for another 100m or more. Where it widens out, it's crossed by bridges including Christian Menn's Great Viamala Bridge.

We walked only a relatively short length of the hiking trail, under the Menn bridge to first see the Pùnt da Suransuns, a remarkable stressed ribbon footbridge spanning 40m across the river. I'm not entirely a fan of the stressed ribbon bridge - while they can be beautifully slender structures, the way they droop often looks unhappy - see the Maldonado Bridge in Uruguay for an example of this. They rarely meet the engineering or administrative constraints of most sites - the sag leads to slopes greater than are desirable for many users, and for the same reason they don't work where there is limited headroom below (which is the case for most footbridges). They are also almost never the most economic solution, with the costs of expensive foundation anchorages far outweighing the material savings to be made in the main bridge deck.

Suransuns, however, is a perfect example of just how a stressed ribbon bridge can work well. The river valley sides have plenty of stable rock, required for an efficient anchorage design, and the setting demands as minimal an intervention as possible. And Suransuns must be amongst the most minimal of stressed ribbon bridges you could get.

The bridge comprises granite planks, 60mm thick, 250mm wide, and 1100mm long, which sit directly on stainless steel strips only 15mm thick, and are held apart by 3mm thick aluminium inserts. That's basically the entire structural system of the bridge, and it is beautifully complemented by ultra-minimal steel handrails supported on 16mm diameter vertical rods.

Clearly, the location allowed Conzett to break many of the rules which normally bind footbridge design: minimum widths; maximum gradients; strength of parapets; susceptibility to vibration. However, it's the way he responded to some of the challenges of stressed ribbon design which was particularly impressive. Bending of the deck slab at its support abutments is a key consideration in design, which Conzett dealt with using a seemingly simple "leaf-spring" arrangement, just using more of the main stainless steel strips locally.

The elegance of the bridge's minimal silhouette is matched by the simplicity of the engineering, and for me, this was one of the best bridges we saw. Conzett, Bronzini and Gartmann are designing a very similar bridge (albeit multi-span) at Gemeinde Windisch, and it will be interesting to see how it compares.

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