CHARPENTE CONCEPT - Ingénieur bois et civil - Architecture - Palais de l'équilibre - Genève

Timber and architecture: a world premiere!

Under the leadership of its dynamic Chairman Mr Jean Bianchi and Director Mr Daniel Touffait, SIVOM Val Cenis decided, after discussions with the local administrations, to go ahead with the project developed by Architect Philippe Barbeyer and Timber Engineer Thomas Büchi.
Before we go on to describe the project, it should be said that the construction of a bridge for skiers to crossover needs to meet very specific requirements. First, the structure was designed to support heavy snow loads equal to half a tonne per square metre, as well as the extra weight of the skiers themselves and of the snow clearance equipment.

To summarize, the bridge was designed for distributed loads higher than those supported by a motorway bridge designed for the crossing of 40 tonne trucks. Given these facts, the designers proposed a project which combines architectural lightness and structural boldness, whilst advancing the various aspects of sustainable development.
The main material is wood, namely local larch, which forms the main part of the structure, i.e. the bridge deck, surface course, barriers and balustrades as well as the masts to which are attached the cables supporting the pedestrian part of the structure. The connections are made with the Ferwood system, which links the timber pieces together with rods glued and sealed with a special resin. This type of connection for timber structures is like welding for steel construction! This is the most spectacular innovation of this structure which creates totally watertight connections and provides a 30% saving of raw material use compared to traditional construction methods.

To design a structure spanning over 52m and able to support such heavy loads, it was necessary to incorporate a tie a under the bridge deck; the latter being made of posts and steel bars in tension.

To reduce raw material consumption and thus reduce the cost, whilst gaining structural elegance, designers Philippe Barbeyer and Thomas Büchi thought of incorporating the “Tensairity Airlight” system, which consisted literally of covering the tie with a glass fibre and silicone membrane and pumping 200 hPa of air into it. This is only just equivalent to the difference in air pressure between fine and bad weather but it was enough to provide stability to the whole bridge and save a considerable amount of raw material when compared to traditional construction.
In order to achieve their objectives, the designers employed the services of the famous Swiss Doctor and Engineer Mauro Pedretti, the inventor of this reliable and particularly leading-edge technology.

TENSAIRITY CONCEPT

TENSAIRITY is a revolutionary technology in the field of long span low weight construction.
TENSAIRITY is a new type of pneumatic beam with mechanical properties far greater than any other type of structure.

TENSAIRITY beams are much lighter and span over greater distances using less raw material than traditional systems.
TENSAIRITY innovatively combines a flexible tube under very low pressure, cables in tension and a compression element. In this type of structure, the air is only used to pre-tension the cables and prevent the compression element from buckling and does not have a load-bearing role. Pressure and span are independent.

TENSAIRITY consists of a non-combustible glass fibre membrane coated in a layer of silicone, with a long life expectancy, high resistance to UV rays and excellent mechanical properties. The beams are self-supporting even in the event of an accidental loss of air pressure.

Pre-fabrication and assembly:
In accordance with the law on public contracts, a call for tender was issued for the first stage of the works consisting of the foundations and civil engineering works. The engineering firm MDP was responsible for the calculations and allocated the works to the company Locatelli. The works were particularly difficult since the conditions of the ground were bad as it consisted mainly of fills. For this reason the bridge abutments were stabilized with foundation piles sunk several metres underground. The involvement of a land surveyor meant that the builders were able to attach the anchorage shoes with a gap of only 6 mm over 52m!

The second stage of works incorporating the timber structure, ties and Airlight membranes, was allocated to the company Labat & Sierra, based in Annecy and headed by Mr. Barrouilhet and Mr. Darnauguilhem. Given the extremely tight deadlines, a very precise schedule was implemented. The involvement of several specialists from all parts of Europe was required:
. a successful cranage company which committed a 500 tonne crane (there are only 4 such cranes in France!),
. the processing of timber parts using the only robot of its size in Europe, by the company Ducret in Orges (French-speaking Switzerland),
. the cutting of Tensairity glass fibre membranes by the company Canobbio based in Milan (Italy),
. site preparations, construction coordination and assembly by the carpenters of the company Labat & Sierra based in Annecy.

Lifting method:
The method used to assemble the bridge was to pre-assemble two 4m wide and 52m long half-bridges on the right bank of River “l’Arc”, and then to lift them one after the other, using two 200 tonne cranes and one 500 tonne crane, across the river in one single operation.
Because of this spectacular operation, it was possible to gain several weeks on the construction schedule, but it required considerable accuracy.
Because of the choice of combined state-of-the-art technologies, the constructed dimensions were very close to the designed dimensions (to the nearest millimetre), thus justifying the choice of such a bold construction method.

A few figures:
Bridge span: 52m without intermediate supports
Working width: 8m
Height of the balustrades: 2m
Wood used: ~ 220m3 of glulam larch, which is the amount of timber growing in the French forests every 2 minutes!
Ties and steel connection elements: 30 tonnes
Tensairity membrane surface area: 850m2
Total hours spent for planning, architects' calculations and drawings and hours of work by foundations, timber and Airlight engineers: 2,800 hours

Conclusion:
Val Cenis ski resort has always been a pioneer in the development of skiing facilities and innovated once again by building the first bridge in the world to include a light and elegant timber structure made stable through the use of an Airlight compressed air membrane.
By taking up this challenge, Val Cenis has demonstrated that it is a modern ski resort paving the way for the advancement of a state-of-the art technology which will radically change the design of linking structures in Europe.

Projects participants:

Client:
SIVOM in Val Cenis
Mr. Daniel Touffait, Director
Mr. Jean Bianchi, SIVOM Chairman and Lanslevillard Mayor
Mr. Jean-Pierre Jorcin, Lanslebourg Mayor
Tel. 04 79 052 96 48
Email : rmvalcenis@wanadoo.fr

Architectural design and timber engineering
Project consortium:

Philippe Barbeyer – Architect in Chambéry
Tel. 04 79 75 20 67 – barbeyer.dupuis@wanadoo.fr

Charpente Concept: represented by Mr. Thomas Büchi, Timber Engineer and Designer in Geneva, Paris and Annecy.
Tel. 04 50 46 07 28 – tbuchi@charpente-concept.com

Airlight System:
Airlight Ltd: Mr. Andrea Pedretti
Tel. 004191 873 05 05 – andrea.pedretti@airlight.biz

Foundations and civil engineering:
M D P: Mr. Emmanuel Blanc and Alain Meot
Tel. 04 76 90 91 00 – emmanuel.blanc@mdpsa.fr

Carpentry company:
Labat & Sierra SA, represented by Mr. Francis Barrouilhet
Tel. 04 50 68 70 61 – labat.et.sierra@wanadoo.fr

Masonry and foundations company:
Locatelli, represented by Mr. Arbaretaz
Tél. 04 79 28 11 81 – omnis.structures@free.fr

Building control :
Passera & Pedretti: Doctor Engineer Mauro Pedretti, Director
Lugano (Switzerland) – +41 91 993 05 33 – ppeng@ppeng.ch
Varese (Italy) - +39 0332 28 19 39 – varese@ppeng.com