Development of decay performance models for outdoor timber structures

Performance based building (PBB) and design is closely connected to various needs and requirements: Performance levels need to be defined, test methods for verification of performance need to be developed, and reliable performance data are needed for materials, products, constructions, and different design solutions. In contrast to other building materials, PBB and thus service life prediction of timber and wood-based products requires particular consideration of wood-degrading organisms and their physiological needs. For the most relevant group of wood-destroying organisms, which are the different decay fungi and bacteria, wood moisture content and temperature need to be considered as key factors. This study aimed on the development of performance models on the basis of hard data obtained in field trials performed under most realistic conditions. Dose-response relationships, which can serve as essential parts of a performance based design model, were derived from material climatic data and corresponding decay development in the field. Different dose-response models are proposed and evaluated for predicting onset (and progress) of decay when wood is exposed to a dynamic and arbitrary climate exposure described in terms of time series of coupled temperature and moisture content. A logistic dose-response model was primarily focused on describing the relation between exposure and decay rating for moisture traps with long periods of high moisture contents. A two-step linear engineering model was more focused on predicting the behavior in a wider, more simplified, sense where periods of high moisture content are interrupted by periods of drier and/or colder climate.