Transient prediction of the decay risk of wooden load bearing structures

Building constructions made of untreated wood contribute to carbon storage and improve both the carbon footprint and the recyclability of the constructions. However, if the construction is exposed to high moisture levels over longer periods, decay and biological degradation can counteract all efforts. Today, the moisture safety of building assemblies is often evaluated using hygrothermal simulations, which consider the combined influence of temperature and humidity over time. While even extreme short term moisture peaks may remain harmless concerning decay, long term impact can lead to damage. Based on decay tests in the lab, new steady state limit conditions for the coinciding temperature and humidity conditions in the critical area of the wood were defined. Furthermore, a transient wood decay prediction model was developed, considering the initiation phase, which means the establishment of the fungi in the wood, and the subsequent phase with decomposition of the material. The first validations of this model were conducted for the lab tests with transient conditions and some wood constructions with and without damages. This model allows a more detailed analysis of the conditions in the structure, to estimate the risk of wood decay more accurately. This model is of relevance in cases where design criteria from actual standards or guidelines or the new steady state limit curve for decay are not met or where constructions experience unplanned wet conditions, such as rain during construction work or flood damage. Here it is necessary to clarify whether and how freedom from damage can be guaranteed despite the moisture conditions that have occurred, or whether demolition of the component is actually necessary.