Effective stiffness prediction of GLT beams based on stiffness distributions of individual lamellas

The mechanical properties of structural timber—particularly in terms of stiffness and strength—are subject to high variability, which also affects the properties of timber products made from structural timber, e.g., glued laminated timber (GLT). In this paper, the influence of the longitudinal stiffness variability of wooden lamellas on the effective stiffness of GLT is investigated. In a first step, the local fiber orientation on the surfaces of 350 lamellas of Norway spruce was determined by an optical scanning device. This fiber angle information in combination with a micromechanical model for wood was used for the generation of a longitudinal stiffness profile of each lamella. Recording the position and orientation of each lamella, a total number of 50 GLT beams were assembled (with 4, 7, and 10 laminations) and tested under four-point bending. Knowing the stiffness profile of each board and its location within the GLT beam allowed for an accurate numerical finite element model, which is able to predict the effective GLT stiffness with high accuracy. Interesting insights into the relation between the stiffness of lamellas and the resulting GLT beams could be gained, and finally, a numerical simulation tool which is able to reproduce the experimental results appropriately was obtained.