Wooden bridges

Wooden bridges

Various bridges of many different sizes can be built out of wood, even for carrying road transport. Prefabricated wooden bridges can be assembled quickly, something which reduces the deleterious effects of construction on traffic flow. Wooden bridges have been shown to have long-term durability and low maintenance and repair costs.

The prime advantage of wood in bridge building is its lightness and strength. The change from using solid wood to using lam­inated wood has made it pos­sible to manufacture large beams in wood.

Because of the lightness and strength of wood, wooden bridges can be prefabricated, transported and fixed in the form of finished elements. Simple jointing technology speeds up assembly of the parts which, on comple­tion, are ready for waterproofing and finish­ing prior to use.

Assembly overcomes perhaps the most difficult phase, as it may be possible to lay the bridge deck as a single unit. A high de­gree of prefabrication and dry jointing tech­nology also help to make winter building easier.

According to research and experience in other countries, wooden bridges are very competitive in terms of both construction and life-cycle costs. Furthermore, wood is a renewable resource and, here in Finland, a home-grown material. While growing, wood sequesters the carbon dioxide in air and acts as a carbon sink.

Between 2010 and 2014, a total of 584 bridges were constructed in Finland, but only 17 of them were in wood. Of all road bridges, wooden bridges represent a figure of 4%. All in all, Finland has around 900 wood­en bridges out of a total of 20,000. In Swe­den and Norway, wooden bridges are much more common than in Finland, with Nor­way building some 10% and Sweden around 20% of all new bridges in wood. 

Wooden-bridge building in Norway

In Norway, around 140 to 160 new bridges are built every year, some 10% being in tim­ber. For about twenty years there has been a good deal of interest in Norway in developing and promoting the use of wooden bridges. The public sector is keen to show an example and at the same time increase the availabil­ity and competitiveness of wooden bridges.

Norway is a land of wood and the use of wood is of major economic importance to employment and the regional economy. Ex­ports are another major consideration. The aesthetics of wood, its renewa-bility and environmental matters related to construc­tion, all support the use of wood. The sim­ple architectonic approaches used in wood­en bridges are considered to be interesting.

Behind the second coming of wooden bridges, there is a Nordic research project. Small wooden bridges are normally stand­ard types, while large ones are usually indi­vidual, one-off designs. The bridge deck is typically pre-stressed, with steel parts being used for joints, ties and balustrades. Wooden bridges are financially competitive, especial­ly basic designs. Wooden bridges have done well in competitive tendering.

Wooden bridges have been shown to have long-term durability. When problems have occurred, there has usually been a specific and clear reason for them and it has been a simple matter to correct the fault. In Nor­way, creosote or mechanical protection is used to protect the wood. The longest wood­en bridge is currently 70 metres, but bridg­es with spans of up to 140 metres can eas­ily be built in wood. As a whole, people in Norway have been extremely pleased with wooden bridges

Wooden-bridge building in Sweden

In Sweden, the market share represented by wooden bridges has risen to around 20% over the last 15 years. Development has been driv­en by the strength of the industrial firms con­cerned, a good range of standard-type bridges and an active marketing approach. The initial impetus for wooden-bridge building was the Nordic research project in Sweden, too.

The principal material for wooden bridges is laminated timber, which is used for both heavy and lightweight traffic. A 15 to 20-me­tre laminated-timber road bridge is about 20 to 30 times more economical than an equiv­alent concrete bridge. Service and mainte­nance costs are as small as for other bridges.

Standard-type bridges incorporate a high degree of prefabrication. Surface treatment, pipe installa-tion, balustrade fixing and pre-drilling can all be done in the factory. Making a timber deck requires neither the construction nor demolition of any form­work for pouring concrete, nor any rein­force-ment, nor any drying time. Nor is heavy lifting equipment required. Installa­tion can be carried out even in tricky plac­es and, because of the speed of installation, traffic disturbance caused by installation can be minimised. Construction of abutments need not disturb other traffic.

Technically, the life of a bridge in Sweden is calculated to be 80 years. Wooden bridges can be assumed to last considerably longer. Creosote is not used for timber preservation due to EU regulations, so untreated timber members can be used to provide bio-energy after demolition.

Waterproofing of the deck has a key influ­ence on durability and is normally checked every six months. Spot checks are also carried out on pre-stressed steelwork to ensure the stress moment is correct.

Checking for moisture damage is done by eye as leaks can be easily spot­ted. Wood swells when wet and this can be seen by the nuts and washers on the pre-stressing steelwork sinking into the surrounding timber.

The waterproofing of the deck should be renewed in its entirety every 25 years. The most severe weathering affects the outer laminated timber beam which acts as a wearing layer. This can be re­placed if necessary.

Photo: Versowood Oy