Justification of output structural pine and spruce sawn timber from logs
The main factors affecting the strength of structural wood-based materials are its density and moisture, width of annual ring, type of wood (sapwood and heartwood) Sawn timber constructional purposes, used for the manufacture of glued laminated beams, make special demands. Their density at equilibrium moisture content should not be less than 500 kg/m3, in accordance with applicable Russian regulations. It is known that the density of the wood varies in different parts of the tree trunk, and the uneven distribution of moisture in it. This effect negatively influence on not only the mechanical properties of the sawn timber but also on the quality of its processing, primarily drying. To determine the density and moisture content of pine and spruce wood harvested in the Leningrad region, standard weight method was used. Moisture distribution was determined in green wood. Density distribution was studied in green and absolutely dry wood. Density at 12 % moisture content was determined by calculation. Assessment of the internal state of saw logs was performed by magnetic resonance imaging and computed tomography. Research results show that pine and spruce have a similar character of distribution moisture on the tree trunk. Wood with a moisture content less than 40 % concentrated predominately in the butt log. Density of freshly cut spruce varies over a wider range compared to pine. Density of spruce wood in a completely dry state is much lower compared with a pine. Both wood density in different parts of saw logs depend on the age of the tree with the increment trend from the center towards the periphery. For the sawn timber manufacturers, it is of the interest the wood density at equilibrium moisture content. Depending on the location of sawn timber in the trunk its’ density at 12 % moisture content may be different. Spruce sawn timber can have a density from 350 to 500, pines from 400 to 600 kg/m3. Consequently, in order to produce structural sawn timber, one should firstly determine which part of a saw log holds the required density. Pine sawn timber required for glued laminated beam, can be obtained from the butt log at not more than 35 %. Qualitative assessment of the distribution of density and moisture saw logs can be given through the use of computed tomography.