Variation of mechanical properties in oak boards and its effect on glued laminated timber

The renewable material wood and hereof derived structural engineered wood products (EWPs) is widely acknowledged as being the major pillar of sustainable building constructions. Due to an increasing availability and high mechanical performance the wood resource hardwoods has been gaining traction for the use in EWPs, typically dominated by softwoods. Wood, being a naturally grown material, exhibits a pronounced variation in its mechanical properties, presenting marked differences not only between species, but also between individual trees and locally throughout the stem of each tree. Glued laminated timber (GLT) reduces this variation by vertical glue-stacking previously finger-jointed boards to form a highly homogenized wooden beam. This configuration reduces considerably the variation of the observed global mechanical properties of GLT with respect to the individual boards. However, the rather complex material composition of GLT prohibits the derivation of an analytical relation between local (e.g. intra board and finger-joints) and global properties, thus numerical approaches need to be applied. This work addresses the need for an improved understanding and modeling approach of the variability of stiffness and strength along and between boards, and the resulting impact on the size-effect of GLT made of oak.