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V gradbeništvu je dobro poznavanje vgrajenega materiala zelo pomembno pri statičnih izračunih. Lastnosti kosa lesa se lahko razlikujejo glede na rastišče, glede na hlod in glede na razrez. Zato je treba vsak kos lesa pregledati posebej. Temu pregledovanju rečemo razvrščanje na osnovi nedestruktivnih meritev. V našem primeru razvrščamo v trdnostne razrede. Evrokod 5 se sklicuje na standard EN 14081, ki dopušča vizualno in strojno razvrščanje. Z namenom ugotovitve razlik med obema, smo naredili več meritev in jih primerjali. Opisali smo več nedestruktivnih metod, ki smo jih uporabili (vizualno razvrščanje, vzdolžno širjenje valov, ultrazvok, upogibni preizkus in vzdolžno nihanje). Vse nedestruktivne metode smo primerjali s trdnostjo, pridobljeno s porušno metodo. Naredili smo nekaj korelacij in jih ovrednotili. Preizkušance smo na podlagi standarda optimalno razvrstili v trdnostne razrede in te primerjali s trdnostnimi razredi, dobljenimi z nekaterimi nedestruktivnimi metodami. Preizkusili smo tudi prototip naprave, razvite med projektom, in jo primerjali z ostalimi napravami ter z vizualnim standardom. Naprava se je dobro izkazala. Ob predpostavki, da je naš vzorec reprezentativen za Slovenijo, lahko zaključimo, da spada večina slovenskega lesa v trdnostni razred C30. Glede na to, da se v praksi pri projektiranju še vedno večinoma uporablja stara oznaka iz JUS standardov, II. klasa oziroma današnja C24, je projektiranje kljub nepoznavanju dejanskih lastnosti lesa na zelo varni strani. Nikakor pa ni ekonomično, saj vgrajen les ni dovolj izkoriščen. Ob uporabi naprav za razvrščanje hitro ugotovimo, da lahko brez večjih ostankov pri razvrščanju pri projektiranju uporabljamo trdnostni razred C30. 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. |
