TRANSBORDER NANTES

Huť architektury EN – Martin Rajniš, Tomáš Kosnar

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HAMR presents its project for a transporter bridge across the river Sèvre close to the town of Gorges. The basic question is: why a transbordeur in the first place? We can find three main arguments:

The first of them is, for us, the strength and charm of the idea behind the transbordeur. The ‘transbordeur’ is not only a clever invention, but indeed an amazing one. In the French contribution to the worldwide technological revolution, it played a significant role. Returning once again to this idea, yet on a much small, human scale, is amazing. A celebration and recollection of the great builders and engineers who created and developed this machine. At the same time, one of the last French ponts transbordeurs, as we all know well, originally stood in Nantes, crossing the Loire up until the 1960s.

Secondly – for the greater public, the transporter bridge is an attraction. A large toy for adults. Our Works fact realised not long ago another transbordeur for pedestrians in the Czech Republic, near the town of Chrastava. It replaced an earlier footbridge destroyed in a flood that was part of a hiking trail. All at once, the attraction of the trail rose sharply, and many people decided to follow this route simply because of this absolutely unique way of crossing the river – something playful and adventurous. There is no doubt in our minds that our Transbordeur across the Sèvre will be a similar tourist attraction. It’ll make an interesting report for the media, draw in the tourists, become an object of affection for the public, and not only those in the locality.

As for the third reason, and perhaps the most important one for any investor: the transbordeur is the best possible technical and economic solution in the conditions of Moulin du Liveau in Gorges. If we compare a footbridge and a transbordeur, the transbordeur has a gondola 2m wide and the strain per 1m2 is the same as a footbridge. But with the bridge, the surface under strain is the entire length, let’s say 2x ca. 40m, which gives us 80m2. The strain on a transbordeur is 2x 4m, giving 8m2. This means that the overall strain on the transbordeur when compared to the bridge is 1/10. A technically minded listener could object that we can use a diagram of transverse forces that would correct this calculation somewhat – yet still, we can be safe in saying that the strain on the load-bearing structure of the transbordeur is several times lower than on the footbridge, and this has a particularly unambiguous impact on the dimensions, the weight, and most finally the cost. Moreover, on the site where our transbordeur is to be constructed, it would be necessary in the case of a footbridge to construct embankments or ramps on both sides of the river, which would increase construction costs still further. Not to speak of how this plan, in terms of protecting the land from floods, would most likely prove completely unacceptable. The load-bearing structure of the Transbordeur stands high above the level of any possible flood, while the passenger gondola moves at the level of the surrounding terrain. And so there is no need to climb up an embankment or scale a height. It makes use of the very effect for which the first transbordeur bridges were built at the mouths of rivers in large port cities: to ensure that boats could sail into port, and travellers on land were not forced to climb up and down onto high bridges. In the event of a flood, it’s enough to use the simple pulley mechanism to raise the gondola above the level of the main beam. All that remains in the floodwaters will be the legs of the structure, and these will be protected against floating objects through a cover of large trees or built-in wedges.

Fourth of all: we designed our Transbordeur to be made out of wooden beams reinforced and connected with steel components. Our love and admiration for Jules Verne and his era is truly great – so we chose this intriguing, and technically still quite modern system of the Howe girder, – a truss structure with reversed placement of compressed and stretched elements. For its economy, it was widely used in the final decades of the 19th century on major rail projects across Europe, and even more widely in America. This design is a recollection of the heroic age of Belle Epoque engineering as we look back on it today. Of the work of engineers like Gustav Eiffel. And in fact, he as well designed a splendid transbordeur for the port in Marseille. During our studies, this was a major inspiration for us. We’re returning to the natural origins, returning to the romance of 19th-century engineering. Yet at the same time we remain strictly objective and economical. Part of these economies includes the protection measures, which significantly increase the expected lifespan of the wooden structure. The main girder will be covered with transparent foil, recalling the wings of the first Louis Blériot aircrafts. This foil will be pinned above the girder and keep off rain and water that would cause the wood to decay. With this, the lifespan of the construction rises from a mere few decades to well over a century. Wherever the structure is not protected by this wing, we’ve used local coverings of heat-treated zinc-steel sheet metal. In the viewing towers we’ve designed, this material has worked exceptionally well.

The gondola can carry up to 12 people comfortably, families with prams, up to 6 cyclists. The entire design is handicapped-accessible. Thanks to the extremely simple pulley mechanism, the gondola can be moved, even witha full load of passengers, by pulling on the rope – anyone can do it, regardless of strength, a small child or someone quite elderly. This pulley mechanism allows the gondola to be drawn from one side of the river to the other. In other words, the gondola is fully human-powered transport, not requiring any motor or any power source. It’s an environmentally sound means of transportation, with minimal operating costs and no risk of breakdown or injury. To protect the pulling rope from vandalism, we recommend using a hemp rope with a woven steel core, which resists any attempts to cut through it.

 

 

Technical specifications:

Span                                                                                                            41,16m
Lenght of the construction                                                                     67,52m + anchoring
Capacity of the gondole                                                                           12 people or 6 cyclists
Capacity per hour                                                                                     288 people in each direction (12 people / 1,25 minut per ride)
Accessibility for persons with reduced mobility:                               fully accessible

 

Construction:

truss from the tree trunks (larch or impregnated pine) – main trunks cca ca. 300mm in diameter, trunks connected and braced by the zinced steel plates, ropes and rodes
foundations: drilled piles
roof: Mylar / ETFE foil

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