Influence of wood moisture content on bending and shear stiffness of cross laminated timber panels

The bending and shear stiffness of cross-laminated timber (CLT) made from softwood depend on the moisture content of the panel. In principle, the stiffness properties drop with increasing moisture content within the hygroscopic range. However, swelling provokes closing of open gaps of the middle layers and leads to an apparent increase of stiffness due to internal friction. This increase in stiffness is only relevant for low deflections and can provoke misinterpretation of measuring results when deriving stiffness properties of CLT by means of dynamic methods such as modal analysis. Drying, on the other hand causes cracking within CLT, which manifests itself in a distinct reduction of the bending stiffness perpendicular to the grain direction of the face layers. Calculating bending stiffness of CLT perpendicular to the grain direction of the face layers by means of the compound theory not taking into account the face layers therefore is a common and correct procedure.