Knots and their effect on the tensile strength of lumber

When assessing the strength of sawn lumber for use in engineering applications, the sizes and locations of knots are an important consideration. Knots are the most common visual characteristics of lumber, that result from the growth of tree branches. Large individual knots, as well as clusters of distinct knots, are known to have strength-reducing effects. However, industry grading rules that govern the allowable arrangements of knots are informed by subjective judgment to some extent. Thus, the spatial interaction of knots and their relationship with strength properties has not been fully understood. This paper reports the results of a study that investigated and modelled the strength-reducing effects of knots on a sample of Douglas Fir lumber. Experimental data were obtained by taking scans of lumber surfaces and applying tensile strength testing. The modelling approach presented extends current methodology by incorporating all relevant knot information in a Bayesian framework.