Strength grading of northern hardwood species for structural engineered wood products : identification of the relevant indicating properties

Strength grading of hardwoods is a prerequisite to use them in structural engineered wood products. However, hardwood strength grading is considerably less developed than it is for softwood species. Previous study has shown that white ash and yellow birch are promising species for the manufacture of glued-laminated timber. However, no strength grading procedure dedicated to hardwoods is available in Canada. The objective of this study was to identify the relevant indicating properties for predicting the ultimate tensile strength of the investigated species. A model selection approach allowed to identify the most performing models and to compare, for each species, the relative impact of the indicating properties. The indicating properties included in the final models were the density of the specimens, the dynamic modulus of elasticity, the sinus of the maximum local grain deviation (SGDmax) as well as the knot area index (KAI), derived from the knot area ratio. The final models revealed important differences between the two species, indicating that it may be relevant to grade them separately to ensure the most efficient utilization of the resource. The coefficients of determination between the actual and model predicted UTS were 0.82 for white ash and 0.78 for yellow birch