Durability of timber members : Moisture conditions and service life assessment of bridge detailing

Abstract: The longevity of timber structures in exposed outdoor environments is often perceived to be unreliable as compared with that of concrete and steel structures. This situation is exacerbated to some extent by the fact that the European commission for standardization (CEN) currently provides no quantitative method for service life assessment of timber structures. To counteract this, the present thesis deals with methods for the quantitative service life assessment of timber in outdoor above-ground construction.Wood moisture content and temperature, i.e. the material climate involved, are two fundamental indicators of decay activity of wooden members. Performance based service life assessment of wood is largely comprised of two parts: (1) establishing the relationship between the environmental conditions and the material climate, and (2) establishing the relationship between the material climate and the rate of deterioration. The former depends on factors such as weather parameters, material properties and the design of details.Two experimental trials were conducted in order to acquire data concerning the relationship between the environmental conditions and the moisture content of the wood. The aim of the first experiment was to study typical details utilized for timber bridges with respect to their variation in moisture content. The type of detailing employed was shown to have a great effect on the moisture conditions, with the exposed contact areas and the end-grain being subject to increased levels of moisture content. The second experiment focused on the effects of long-term superficial damage on the moisture conditions of wood. It was shown that superficial damage caused by photo degradation and checking lead to increased moisture absorption and consequently to a longer duration of surface wetness. The relationship between environmental conditions and material climate was estimated through numerical modelling, which involves the explicit modelling of moisture transport within the wooden member and the exchange of moisture at the surfaces that are exposed. The model was applied to one-dimensional transverse moisture transport as well as to multi-dimensional transport in the case of simple details, such as side-grain to side-grain contact areas. The numerical model was then coupled with decay prediction models so as to map the relative decay hazard in European climates.