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Climatic conditions for wildlife overpasses built in wood

Updated: Dec 9, 2023

WHFF project: 2020.08

Project leader: Prof. Andreas Müller (BHF), Dr. Karim Ghazi Wakili (BFH)

Partners: Timbatec Holzbauingenieure Schweiz AG, Häring AG/Roth Burgdorf AG, terra vermessungen AG, Lignum Holzwirtschaft Schweiz

The short video about the project on YouTube can be watched at the following link (only available in German):

The most important in brief

  • The Rynetel Suhr (AG) wildlife overpass over the A1 motorway is a lighthouse project built using structural timberwork. The moisture content of the timber was monitored and analysed over a period of 20 months using various sensors.

  • Particular attention was paid to the effects of the spray mist wetting the surfaces by the passing vehicles, as well as the weather conditions.

  • No correlations were found between precipitation and moisture in the timber structure, and even high precipitation and high traffic volumes had no significant effects on moisture.

  • The results on the moisture behaviour of the impregnated timber structure of the wildlife overpass under consideration suggest that even untreated timber would not reach critical moisture levels under the given climatic conditions.

  • The simulations carried out show that certain construction features can affect the air circulation in the tunnel. However, with the help of computational fluid dynamic calculations, the optimal spacing for the beams in relation to the tunnel size can be determined.

Project description

The aim of the present project was to record and scientifically analyse the climatic conditions in the wildlife overpass Rynetel Suhr (AG) above the A1 motorway (Zurich-Bern) by means of comprehensive climate and humidity monitoring. Particular attention was paid to the influence of traffic. This is because the spray generated by traffic in rainy weather wets the wooden structure with water. Traffic also has positive aspects, namely it has a positive effect on the exchange of air and the drying of the construction. In a further step, it was investigated how the shape and construction of the tunnel-like bridge influences the moisture absorption and release of the wooden construction.

The results are to serve as recommendations for the design in the planning and implementation of future wildlife crossings with timber structures.


During the observed period, no significant correlations between precipitation and the increase of moisture in the timber structure could be found. Even with heavy precipitation and high traffic, there was no significant increase in moisture in the timber structure, provided that the influence of boiler pressure impregnation was neglected.

The values measured on site for air temperature, humidity and air velocity deviated slightly from the data from the nearby weather station. Therefore, it is advisable to also carry out future measurements directly at the structure in order to obtain accurate monitoring results.

The impregnation of the wooden parts tended to result in lower moisture absorption in dry air, but approached the values of untreated wood as the air humidity increased.

Simulation calculations showed that high construction heights of the primary structure and small spacing of the components negatively influenced the air circulation inside the tunnel. However, the optimal spacing could be determined with the help of computer-aided fluid dynamic calculations (CFD calculations).

The monitoring system and data loggers were taken over from BFH (AHB laboratory) and should be continued for long-term data collection in view of climate changes.

Hydrophobisation of the primary and secondary supporting structure is effective in slowing down moisture absorption in the first 2 to 3 years. Continuation of monitoring would provide additional assurance regarding wood moisture content.

The full report to download (DE):

WHFF 2020.08 Holztragwerke Wildtierbrücken
Download PDF • 5.80MB

You can find more information about the project on ARAMIS.


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