Knot influence on the strength of a spruce and fir structural sawn timber

Grče zmanjšujejo uporabnost lesa, saj je na mestih grč močno spremenjen potek oz. lokalni naklon lesnih vlaken, zato ima grčav les slabše mehanske lastnosti. V nalogi smo ugotavljali, kakšne so povezave med grčavostjo in mehanskimi lastnostmi lesa ter primerjali različna pravila ocenjevanja grčavosti. Grče smo ocenjevali po pravilih, ki so podana v DIN 4074-1 standardu in BS 4978 standardu. Položaj in velikost grč smo določali na 475 smrekovih in jelovih konstrukcijskih elementih dimenzij 50 mm x 150 mm x 4000 mm. S štiri-točkovnim upogibnim preskusom smo elementom določili globalni modul elastičnosti in upogibno trdnost. Ugotovili smo, da grčavost vpliva na trdnost, da pa je napovedovanje trdnosti samo na podlagi grč dokaj nezanesljivo (r = 0,27 do 0,56). Najzanesljivejše je napovedovanje trdnosti na podlagi ocene grčavosti po pravilih DIN 4074-1 standarda za razvrščanje nosilcev (parameter Ar = 0,56) in na podlagi ocene deleža grč v prerezu nosilca po BS 4978 standardu (parameter tKARr = 0,54). Najmanj zanesljiv kriterij za ocenjevanje grč je globina bočnih grč (parameter EBBr = 0,28). Izkoristek razvrščanja je bil višji pri razvrščanju na podlagi BS 4978 standarda, pri razvrščanju po DIN 4074-1 standardu pa je bil zelo visok delež (87,4 %) prenizko razvrščenih elementov.Knots have an adverse effect on the mechanical properties of wood, due to influence on slope of grain, therefore wood can be less suitable for particular use. The purpose of this thesis was to examine the correlation between knots and the mechanical properties of wood and also the evaluation of different knots measuring methods. A comparison of knot measuring methods listed in the DIN 4074-1 and BS 4978 standards was carried out. The knots positions and size were measured on 475 spruce and fir structural elements with the dimension 50 mm x 150 mm x 4000 mm. Then, the global modulus of elasticity and bending strength were determined, by a destructive four-point bending test. It was established that knots affect wood strengthhowever, predicting wood strength based only on knots is fairly unreliable (r = 0.27 to 0.56). More reliable strength predictions on the base of knots is grading in accordance with the DIN 4074-1 standard rules for beams (parameter Ar = 0.56) and by knot area ratio grading in accordance with the BS 4978 standard (parameter tKARr = 0.54). The least reliable criterion is the depth of lateral knots (parameter EEBr = 0.28). The yield of grading was higher when the knots were evaluated by BS 4978 standardwhen using the DIN 4074-1 standard, however, there was too high ratio (87.4%) of underestimated elements