Frame and Form

Switzerland, in a gorge called Viamala, built in 1996 , the Traversina Steg I footbridge perched its geometries. With a spectacular design of wood and steel cable, was short-lived because a rock avalanche destroyed it in 1999.

The design of the bridge is the work of Conzett Bronzini Gartmann. Structurally we can speak of a simple supported truss, but it is outstanding because two main things : the lower lattice work and the upper channel board.

The lattice is formed by 23 wooden struts that have a triangle with two vertices at the base. The height of triangular struts grows toward the center of the runway, marking a parabolic path to steel cables that connect and support those props.

At its apex the struts are connected by a wooden beam. Finally, to provide rigidity to the lattice, both the lower parabolic surface and the lateral faces are restrained by cables forming crosses (usually called crosses of San Andres).

The design of this wood is so light it could be constructed out of site and then transported by helicopter into position .

The board is made entirely of wood and its channel shape has a dual role:

On the one hand, has given a great structural height to the girder and turns it into a rigid beam that supports the distribution of loads on the struts. For example, a concentrated load at the center of the bridge will be shared between many props as the girder is rigid and its deformation, or saying it other way, its movement is very similar in all struts.

On the other hand, the channel shape materialises an integral design since the handrails are part of the beam itself.

The junction between the girder and the beam board lattice is done through a network of H-shaped wooden elements.

The end result of this reinvention of the lattice is a light and transparent footbridge: Steel cables are so thin they can barely be seen from a distance, and it seems that the bridge instead of relying on the lattice, has a family of beams hanging from his girder.

Meta-Information:

Photos taken from the article Wilfried Dechau and archinect. More photos Cultural Viamala.

Article on the runway in Zuschnitt.at. You can see a spectacular picture of the footbridge transport by helicopter. You can also find the elevation and section of the catwalk, and a interview with the designers. All in German.

The book “Masters of the structure” contains an interesting article of this piece.