Strength grading of timber with non-destructive methods

There are two main non-destructive methods of grading timber: visual and machine grading. The properties of timber as a natural material depend on the wood species, growth conditions, time of wood-cutting, storing, and the way of treating. In construction the structural and non-structural use of timber is distinguished and grading timber by visual or mechanical properties is used. Machine grading is non-destructive method, based on relationship between characteristic properties and strength of individual element. Visual grading is the oldest non-destructive method, which is still very common in practice and is based on visual inspection.. In European Union there is no unique standard for visual grading, only an European standard EN 518 with the grading requirements for national standards was adopted. It contains demands which national standards must follow as well as the limits for timber properties. The following properties should be controlled: knots, slope of grain, density, fissures, geometrical characteristics, biological and other characteristics. As main part of this work, the comparison between limits of timber properties and determination methods in Nordic, British and German standards were studied