In the case of the new type of wooden bridge by Aachen University of Applied Sciences, the stone slabs on the carriageway also provide rain protection for the load-bearing beams

Graphic representation of the new wooden bridge.

The construction thus achieves a long service life and, due to the natural building materials, the best CO2 results

Pedestrian bridges made of wood used to be covered with roofs to protect the building material from moisture and thus increase the lifespan of the structure. Famous examples are the Chapel Bridge in Lucerne, which is the city’s landmark, or the Ponte Vecchio in Bassano del Grappa, for which Andrea Palladio provided the planning.

However: a roof makes such constructions expensive.

But: could the floor covering of the bridge be taken in consideration together with the roof and could such a construction thus achieve even greater durability at reduced costs? This was the one of the questions that a team researchers from the Aachen University of Applied Sciences asked themselves.

The answer the scientists found is very simple: natural stone slabs are used as the pavement for the footpath. If you provide for the accumulation of rainwater, these keep the moisture away from the wooden construction underneath and are also extremely durable as a pavement for the traffic area.

Unlike comparable constructions made entirely of wood and without special protection, which usually do not even reach 30 years of age, the researchers have estimated a lifespan of at least 80 years for their innovative bridge idea.

And they need 80 years to make the concept pay off, because the wood-stone combination is more expensive than conventional materials.

However, it pays off over its entire service life. And, last but not least, its results of C02-emission are unsurpassed in comparison to constructions made of steel, reinforced concrete or aluminum.

Graphic representation of the new wooden bridge.

In addition, both the wood and the stone can be recognized by passers-by as domestic materials.

As already mentioned: in order to achieve the desired durability, moisture must be kept away from the wood.

Constructive wood protection was therefore the objective of the research project. What is meant by this is that the type of construction method and the materials used play the essential role in protecting the load-bearing parts of wood.

The occasional rain is not the problem – only if wood remains wet for a long time will it begin to rot destroying its stability. This is especially true for those points where individual parts are joined together.

The researchers have developed new types of connections and other innovative solutions for this purpose.

Special attention was paid to the drainage of rainwater from greywacke but this was easily solved with a gentle slope and weather grooves. The stone slabs also protrude the wooden construction which is to be protected.

Of course, metal was not completely avoided. For example, the screw connections and parts of the railing are made of steel.

Two different types of bridges were developed: 3 types of deck bridges, which are mentioned here, and a trough bridge.

The standard version is designed for a maximum length of 16 m and a width of 3 m. Son, a half-width prototype will be completed in the Mühlenpark in Mechernich in the Eifel not far from Cologne.

It now has to prove itself as a completely normal bridge for a footpath and cycle path, but is fitted with sensors. Above all, the main task is to measure the moisture in the wood over the course of a whole year. “We want to achieve an average value of no more than 20%,” says Christian Bedbur, a research assistant in the project. More than 26 to 28% permanently would not do the wood any good.

FH Aachen, Fachbereich Bauingenieurwesen, Lehr- und Forschungsgebiet Holzbau

Photos: Aachen University of Applied Sciences

Greywacke paving for the roadway on the wooden bridge.

(07.09.2020, USA: 09.07.2020)