Durable Timber Bridges

Most construction materials have a limited service life; concrete carbonates, steel corrodes and wood materials can be attacked by fungi and insects. Many of the built bridges in Europe over the past 50 years are characterized by decay and must either be repaired or renewed. Most of these bridges have fairly short spans in the range of 10 to 120 meters and enable crossing of rivers or plane less intersections. Bridges are important parts of our infrastructure and closure of bridges in connection with new constructions or repairs have high social costs. Timber bridges are very well suited to these ranges, allowing easy and quick installation and can in many cases use existing supports due to its low weight. Wood is one of our oldest building materials for bridges, but has been replaced by steel bridges and later by concrete bridges for the last hundred years. It was assumed that steel and concrete bridges had less maintenance costs and longer life, but today's state of our bridges indicates that this does not have to be the case. The life span of wooden bridges are most related to how the climatic conditions affect the wood materials, and especially the moisture content and exposure time are important. The project has focused on lifetime of wooden bridges, identification and classification of construction solutions with regard to service life and the development of new design and construction solutions with a long technical life. A model has been developed for estimating the technical life of unprotected wooden bridges. The model compares climate exposure with resistance to rot development, and is based on quantification of both exposure and resistance in the form of doses. A dose is a measure of development of damage and depends on moisture content, temperature and time. The model has been verified through comparisons with the life of built bridges. Improved details have been developed with regard to moisture protection of wooden decks on bridges. This work has been carried out both in the form of experiments and numerical models. The numerical models have been validated with comparisons of measured climate and moisture content in some instrumented wooden bridges. Furthermore, material data is provided for long-term behavior of stress-laminated wooden decks which takes into account the creep and moisture variation. This provides the basis for improved numerical models of wooden decks and a tool for further development of this type of decks. A parameterized model of network arch bridges has been developed for long spans especially suited for the use of timber in the main support system and decks. The model is optimized and provides an effective bridge type. Furthermore, jointing technique has been developed for large bridges, since bridges of greater length than about 35 meters are difficult to transport from factory to bridge site in one piece. Bridges are exposed to fatigue and experimental tests have been made to determine the lifetime with regard to the fatigue of attachment of hanging rods and joints. The project has prepared a joint report, where most of the results available at the reporting date are presented. Results that have been finalized afterwards are being published in scientific journals.

In the last decades it is experienced that most materials used for construction of bridges have limited lifetime. Concrete gets carbonized, steel corrodes and timber may be attacked by insects or fungi. A large number of concrete and steel bridges built a fter the Second World War was assumed to have little need for maintenance. However, the current state of many of these bridges does not support this assumption; we now face a vast gap between the needs for maintenance and repair of these bridges and the w ork actually performed. In many cases the bridges are beyond repair and new bridges are needed. Consequently, the number of bridges in European infrastructure that needs replacement is large. Most of these bridges have quite small spans, in the range 10 t o 120 m, crossing roads and rivers. Since the bridges are vital components of the transport infrastructure, the closing time in case of renovation or rebuild is an important issue. Timber bridges are well suited for this span range, they offer quick insta llation on site, and they can utilize existing foundation due to low weight. It is a common perception that the expected lifetime of a timber structure is only a fraction of that of a concrete or steel structure. In spite of this some of our timber struct ures like the Norwegian stave churches and the covered bridges in Switzerland are among our most durable structures. On the other hand we do have timber structures that show serious decay after only a few years in service due to elevated levels of moistur e and consequently growth of fungi and rot. This is also the case for many timber bridges in Europe. The proposed research aims to significantly improve the applicability of wood as a structural material in bridges and contribute to increased use of envir onmentally friendly timber bridges. The bridge design concepts to be developed shall be among the best alternatives with respect to environmental friendliness, initial costs and life-cycle costs and analyses.