Tensile properties of lengthwise split boards and their application in engineered timber products

Timber is a naturally grown material featuring large uncertainties in its properties. For the application in engineered timber products it has to be strength graded. The main strength and elastic properties of structural timber products, such as glued laminated timber (glulam; GLT) and cross laminated timber (CLT), are usually described via load-bearing models which are derived from the tensile properties parallel to the grain of the base material boards and finger joints. These load-bearing models assume that the boards are strength graded and their hole cross section remains in the finished product. However, in some applications or use cases the total structural timber product gets split, e.g. split/resawn glulam, or during their production process parts of their components, the lamellas, are split, i.e. significantly reduced in their cross section. For these split boards/lamellas, the material properties assigned during the grading process done in full cross sections are not valid anymore. Such specific residual cross sections occur also in a new product called flex_GLT-beams, were CLT-factories are used to produce unidirectional, large-dimensional, multi-layer plates from which beam elements are cut off in required dimensions. In order to derive load-bearing models for flex_GLT-beams, a probabilistic model for characterisation of the base material board as well as finger joints has been developed. This model takes also the random splitting of lamella’s cross sections into account. It allows to quantify the influence of splitting on the natural growth characteristics and hence their impact on the board strength and elastic profiles in longitudinal direction. Furthermore, this probabilistic board/lamella model has been implemented within a stochastic numerical finite element model which allows to derive bending properties of flex_GLT-beams and corresponding serial and parallel system effects. The probabilistic board model as well as selected outcomes of the stochastic numerical finite element model will be presented and validated by recently conducted destructive tests.