Razvrščanje lesenih konstrukcijskih elementov po trdnosti (Slovene)

In order to estimate material properties a number of experiments have to be performed. Since the timber is a natural material with high variability and relatively low correlation between non-destructive properties and strength a large number of tests need to be done. If the sample is not large enough the results may be misleading. Very careful selection of representative sample and accurate measurements are the basis for this important database on spruce timber properties in Slovenia. This data base has been and will be an important source for different research purposes. There are two main methods for the machine setting determination which are both described in standard: machine controlled and output controlled. Practical experiences show that both methods are to complicated and may lead to settings which don’t fulfill the requirements of strength grades. The settings may be too conservative and thus not economical for higher quality timber and non-conservative and thus potentially dangerous for lower quality timber. A considerable effort has been invested in the development of new methods. The prediction limit method proposed by colleagues in Sweden was used for the initial setting determination. The second part is an adaptive feature of the machine settings determination which changes the settings continuously according to the timber which is currently graded. The first results are promising since adaptive settings give both more reliable and safer settings. One very important advantage of the proposed method is its robustness.

It is well known that Slovenia doesn’t obtain the full value of its timber. All too often the logs are exported into foreign countries where the timber is sawn and graded only to be sold back to Slovenian customers for a considerably higher price. One of the reasons for this situation was certainly the fact that it was not allowed to grade and mark Slovenian timber. The project removed some of the main obstacles; the transformation table between visual and strength grades is now published in standards for Slovenian spruce. There are now some grading machines with settings valid for Slovenia. The results of the project indicate the high quality of Slovenian timber. The optimal grading of the sample shows that more than 40 % of the specimens are in strength class C40, more than 60 % in class C35 and about 90 % in class C30. These values are valid for the cases where the stated class was the highest in which the grading was done. Since most of Slovenian timber is used as C30 timber, we can conclude, that grading in higher grades could significantly effect the competitiveness of structural timber.