Multiscale variability of stiffness properties of timber boards

In the present paper the variability of strength and stiffness related properties of timber is analyzed and described. Thereby it is particularly focused on the influence of knots and knot clusters on the timber stiffness. The analysis takes basis in material property assessments of a sample of 30 timber board specimen. Each board is subdivided into sections with and without knot clusters. For each of these sections the stress-strain relationship is measured and the stiffness is estimated. Based on the stiffness estimates a hierarchical model for the multi-scale variability of stiffness is developed that includes an explicit representation of the stiffness variability between boards and the stiffness variability within boards. The parameters of the model are presented together with the corresponding uncertainties. The presented model can be used e.g. as an input for a more refined probabilistic representation of the load bearing capacity of glued laminated timber. in previous studies to describe the within member variability of strength (Czmoch et al. 1991, Isaksson 1999, Kohler 2006).