Non-destructive methods in strength grading of structural timber

Good knowledge of the characteristics of building material is of the utmost importance in static calculations in civil engineering. The characteristics of a piece of timber may vary depending on the growth area, log, and cutting. Therefore, each piece of timber should be examined separately. The examination is based on non-destructive testing, distinguishing – as in our case – between different strength grades. Eurocode 5 refers to the EN 14081 standard, which allows visual and machine grading. In order to identify the differences between the two, we carried out several measurements and compared them. We used several non-destructive methods (visual grading, longitudinal wave propagation, ultrasound, bending test and longitudinal frequency). Results of each non-destructive method were compared to the bending strength obtained with the destruction method. We made some correlations and evaluated them. The specimens were classified into optimal strength grades and the results compared with strength grades obtained by selected nondestructive methods. We also tested a prototype device developed during the project and compared it with other devices and visual standards. The device did well. Assuming that our sample is representative for Slovenia, we can conclude that the majority of Slovene timber can be classified into strength grade C30. Considering that in civil engineering the mark most widely used in practice is the old mark from the JUS Standard – class II or today's C24 –, structural design is on the safe side despite the lack of knowledge concerning the actual characteristics of wood. However, this is certainly not economical, since the timber is not used to its full potential. With the use of grading machines, most construction timber can be classified into strength grade C30 without major reject