Strength grading of Norway spruce structural timber: revisiting property relationships used in EN 338 classification system

Solid timber for structural applications has to be strength graded prior to its use. In order to remain economic the grading process usually focuses on the most important physical and mechanical properties: density, modulus of elasticity (MOE) and bending strength. Based on respective limits given in standards, the timber is assigned to strength classes. Additional mechanical properties such as tensile and compression strength parallel to the grain are derived from the basic property values by empirical relationships. The objective of this study was to review some of these property relationships based on recently compiled large data sets as a contribution for a future revision of the grading standards. Based on mechanical tests of Norway spruce structural timber with different cross-sections, the following characteristic values and property relationships were evaluated: (a) strength and MOE in bending, (b) in-grade characteristic values of bending strength, bending MOE and density, (c) relationship of characteristic values of tension and compression strength parallel to the grain with respect to the corresponding characteristic value of bending strength, (d) ratio of fifth percentiles and mean values of density and MOE, as well as (e) the ratio of MOE in bending, tension and compression. Mechanical tests were accompanied by measurements of density and ultrasonic wave speed. Resulting dynamic MOE was partly used as an indicator of timber quality.