EXPERIMENTAL ANALYSIS OF THE DEFORMATION AND FAILURE BEHAVIOUR OF SIGNIFICANT KNOT CLUSTERS

Timber is a widely available natural resource but highly complex due to its material anisotropy and inhomogeneity. Thus, compared to other building materials, timber demonstrates higher variability of the mechanical properties. The variability is pronounced not only between different structural elements but also within single elements [1]. A major reason for this is the presence of knots and knot clusters in structural timber. Nordic spruce timber components are commonly characterised by as sequence of knot clusters divided by sections without any disturbance by knots. The knot clusters are distributed over the length of the board with rather regular longitudinal distances. Considering the trunk of a tree the average distance between the clusters is directly related to the yearly primary growth of the tree. Within one cluster knots are growing horizontally in radial direction. Every knot has its origin in the pith. The change of the grain orientation appears in the areas around the knots. In Figure 1a the knots (black area) and the ambient area with deviated grain orientation (grey area) within one cross section of the tree are illustrated. Since the individual boards are cut out of the natural shape of the timber log during the sawing process the well-structured natural arrangement of the knots becomes decomposed due to different sawing patterns. As a result, numerous different knot arrangements within the timber boards are occurring (Figure 1bc). The strain distribution around single knots is analysed in several studies; e.g. Buksnowitz [2], Oscarsson [3]. The present paper is more focused on several knots within one knot cluster and the grain orientation around the knots. Thus, the deformation and failure behaviour of significant knot clusters within commercially used timber boards is analysed. Thereby it is particularly focused on the commonly occurring knot arrangements.