Orthotropic hygroscopic behavior of mass timber: theory, computation, and experimental validation

Recent rapid improvements in laminated timber technology have led to the increased use of wood in both mid- and high-rise construction, generally posed as a more carbon-friendly alternative to concrete. However, wood is significantly more sensitive to changes in relative humidity than concrete, which may impact the sustainability and durability of mass timber buildings. Moisture cycling in particular affects not only shrinkage and swelling but also strongly influences wood creep. This sensitivity is of high concern for engineered wood used in mass timber buildings. At the same time, wood, considered as an orthotropic material, exhibits varying diffusivity in all three directions, complicating efforts to characterize its behavior. In this work, an orthotropic hygroscopic model was developed for use in laminated timber. A species database for wood sorption isotherm was created and an existing model was used to fit species-based parameters. Diffusion behavior which considers the sorption isotherm was modeled through numerical simulations, and species-dependent orthotropic diffusion parameters were identified. A database of permeability in all directions for various species was created. The resulting model is able to predict diffusion behavior in glulam and cross-laminated timber (CLT) for multiple species of the lab tests. The model also predicts the moisture ranges for a CLT panel under environmental change with parameters from these sorption isotherm and diffusion databases.