First Swiss wildlife bridge with wooden cover

When animals cross the A1 highway near Suhr, they don't notice that they are walking across Switzerland's first wildlife bridge with a wooden cover. For them, it is important that their territories beyond the highway are made accessible again. Because traffic routes cut through the habitats of wild animals, the Federal Office for the Environment defines wildlife corridors of supraregional importance, which are restored with appropriate structures. Between Gränichen and Suhr, the 50-meter-wide wooden structure enables wildlife to cross the highway safely.

As early as 1998, a basic report using the example of the Chüsirain Bridge in Lucerne showed that wood as a local raw material is a cost-effective and ecological alternative for green bridges. More than twenty years later, the wildlife bridge Rynetel crosses the A1 freeway with the wooden covering. During the pre-project phase, the client and the planning team compared the concrete and timber designs. In addition to cost-effectiveness, the structure had to be low-maintenance and durable and last for 100 years. A later expansion of the highway from four to six lanes had to be taken into account and the traffic on the highway had to be able to continue rolling at all times during the construction period. In comparison, the timber decking performed better, so the light was green for the first wildlife bridge with timber decking in Switzerland.

Construction
The planning team arranged the supporting structure of the wildlife bridge higher than would have been necessary due to the clearance profile for road use. This minimizes the earth loads on the structure. In the crown area, 70 centimeters of soil and seepage gravel cover the structure, while the outer walls are around four meters high. This created an almost level usable surface for the wildlife. The two portals are sloped and closed off with embankments.

Interplay of materials
The 156 arch girders made of glulam with a cross-section of 240 x 760 millimeters and the concrete walls interact: lateral forces on the concrete walls are absorbed by the wooden arch structure. The arched trusses in turn support themselves horizontally as well as vertically on the concrete walls. Steel joints connect the wood trusses to the 80-centimeter-thick cast-in-place concrete walls. The supporting properties of the soil were modeled with spring supports and with calculated spring constants.

Exposure to water
Excessive moisture penetration, combined with poor drying behavior, can cause wood to be attacked by rot or insects after only a short time. Multi-layer waterproofing and a drainage layer protect the wood covering from seepage water. Surface water from vehicle spray must run off as freely and quickly as possible. For this purpose, a sufficiently large distance is formed between the concrete substrates and the timber components. This ensures that the support areas are well ventilated and can dry out. A replaceable and back-ventilated wooden cladding for the portal cladding protects the underlying structure from the effects of weather and vehicle spray.

Moisture monitoring
As part of a research project funded by the Federal Office for the Environment, the Bern University of Applied Sciences analyzed the climatic conditions and wood moisture content in wildlife crossings. It was shown that the wood cross-sections can dry out well in each case after moisture entry and are therefore durable.

Exposure to de-icing salts
Freezing de-icing salts are used to ensure safe travel on highways during cold temperatures. The use of de-icing salts increases the damage processes in steel and concrete components. For statically stressed components such as the bearing joints with a long service life, stainless steels of corrosion resistance class IV were used. Wood is not attacked by the salts.

Wood as a CO₂ store
The 850 cubic meters of timber used to cover the wildlife bridge grew back in the Swiss forest in three and a half hours. During this time, it removed 775 tons of CO₂ from the environment through photosynthesis. Constructing the cover in concrete would have caused CO₂ emissions of 520 tons. After deducting the emissions for the production of the glulam and the steel joints, a delta of 1,085 tons of CO₂ remains between the wood and concrete variants.

Building with a signal effect

The Council of States and the National Council have adopted the motion «Research into and innovation of wood as a material for use in infrastructure construction as a decarbonization contribution». Reinforced concrete should be supplemented or replaced by CO₂-storing materials in infrastructure construction wherever possible. Structures such as the Rynetel bridge can have a signal effect in this respect. To install the girders, two lanes were closed from 9 p.m. to 5 a.m. each night for 15 nights. Thanks to the prefabrication of the wooden girders, the bridge was built in record time and traffic was able to continue moving at all times.