This calls for structural timber of high quality with guaranteed strength and stiffness values. Machine strength grading of timber can contribute substantially to the fulfilment of these requirements. Accordingly, the grading machines used in Europe to date are approved for softwoods only, so far the new multi-sensing grading machines employed in Germany, Austria and Sweden have only been approved for the grading of spruce timber. Moreover, various combinations of these grading methods were also investigated. For comparison, all specimens were graded visually according to DIN 4074 to determine their characteristic bending and tensile strength and stiffness properties.
Results show that machine grading is suitable for the grading of all the above listed species.
Compared to visual grading, they allow structural sawn timber to be upgraded to higher strength classes. This could improve the chances for timber to be used in construction. The performance of the vibration and ultrasonic grading methods can be improved by including density measurements. Moreover, determination of knot ratio, e.g. by scanning or radiation, improves accuracy for all the grading methods listed above.