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Modelling of moisture content in timber bridge details
2017 - IRG/WP 17-20613
The design of details is an important aspect in durability design of timber bridges. Details are often considered the weakest points in bridges due to their tendency to trap and accumulate moisture and thus facilitate decay. When designing maintenance plans and inspection intervals, it is important to know how design choices will come to influence the service life. Moreover, service life and wood moisture content are closely related. Therefore, the aim of the present study was to develop a method that can be used to estimate the moisture content in typical timber bridge details. This was done by considering the moisture content in details as the sum of two components: a reference moisture content (ideal drying conditions) and an amplified moisture content response (due to actual drying conditions). The focus of the present paper was on the latter component. The proposed approach consists of a set of simple equations and a detail-specific parameter, kdry, which describes the detail-specific drying conditions. The detail-specific parameter was fitted to four time-series from two different data sets. In all cases, the estimated moisture content time-series exhibited the same behaviour as the measured data. In all three wood-to-wood details, the best fit was obtained for kdry=1/10 which indicates some level of consistency. Although the approach was only tested against a few time-series, it has so far shown some promising features. More data will be added in future work.
J Niklewski, C Brischke, E Fr├╝hwald Hansson, L Meyer-Veltrup

Evaluation of timber bridges with special consideration of detail design
2017 - IRG/WP 17-40781
Since 1997 the use of wood in bridge construction has been increasingly criticized by the Hamburg federal state authority supposedly because of shortened service life due to decay. With regard to wood research, however, it is often suggested that use of timber is suitable for weathered structures as long as constructive protection measures will be observed. In order to unravel the reasons for the shortened service life in Hamburg, several bridges were examined by visual assessment focussing on decay and especially on design. Thus, details of design which affect the durability of individual bridge components and favour damage propagation as well as subsequent decay were primarily detected. In this respect, faulty planning measures and insufficient wood protection is consequently considered as the main cause of the short service life in Hamburg. Giving examples of pedestrian timber bridges, this practical report illustrates several essential faults of protective design which are still being made in planning and construction of naturally weathered timber structures. However, proven solutions are already available and emphasis is placed on constructive solutions with a positive influence on structure durability.
K Robbers, J Fromm, E Melcher