The quality of structural spruce wood is described with parameters: modulus of elasticity, modulus of rupture and wood density. They are determined by destructive bending tests according to EN 408. There is a dominant relationship between strength and predictive parameter representing a basis for strength grading of structural wood. Non-destructive methods based on various principles are used to determine the quality of structural wood. Correlations between parameters from destructive bending tests and non-destructive methods were evaluated. All dependences are statistically significant. This knowledge is used in two-stage prediction method for determination of modulus of rupture.
The current strength classification system for softwood in Europe is based on bending properties (C classes). To optimize the efficient use of timber, especially for engineered timber products, an additional strength class system based on tension properties (T classes) was defined and will be included in the next revision of EN 338. The strength profiles of both strength classes are compared. Today there are grading methods available which are optimized for both strength class systems.