Length Effects on Tensile Strength in Timber Members With and Without Joints

Strength properties in timber disperse remarkable. A regressive course of strength with increasing volume is observed. This phenomenon is known as size effect, dominated by the stochastic part, the dispersion in strengths locally. Although boundary conditions of this theory are violated, traditionally, size effects have been modelled by means of Weibull's brittle failure theory. We address stochastic length effects on the tensile strength parallel to grain of timber members with and without finger joints by means of a probabilistic approach. For jointed members regulations of minimum requirements on finger joint tensile strength are discussed. We assume lognormal distributed strengths and use a second-order hierarchical model together with equicorrelation to account for within and between members' strength variations. As outcome, the effect of length on the mean and 5 %-quantile of tensile strength is quantified. Simplified models and parameters for the design of timber structures are provided. Minimum requirements on finger joint tensile strength, relevant for modelling of timber products as well as factory production control, are defined.