DRYING OF BOXED HEART SQUARE TIMBERS .1. CHANGES OF SURFACE STRESS AND CROSS-SECTIONAL SHRINKAGE OF DEHUMIDIFIER KILN-DRIED TIMBERS OF SUGI

In Japan, boxed-heart square timbers of sugi (Cryptomeria japonica D. Don) with cross-sectional dimensions of 10.5 x 10.5 cm have been used widely as structural members of wooden houses. Not only the unstabilit y of dimensions but checks and splits often occur during the drying an d practical use of these members. Therefore, these are occasionally cu t vertically (grooved) on one face to preclude the appearance of undes irable checks and splits. However it is difficult to make them complet ely free from these defects. This investigation was conducted to explo re the relationships between stabilities in qualities and the finished moisture content for sugi square timbers dried in dehumidifier kilns. Results obtained were as follows; (1) During drying in a dehumidifier kiln with heated air at 35 to 40-degrees-C and 78 to 41% relative hum idity (RH) and seasoning in a room at 20-degrees-C and 65% RH, the wid th shrinkage increased more remarkably for boxed-heart square timbers with grooves than for those without grooves. (2) For boxed-heart squar e timbers with grooves kiln-dried to 32% in moisture content, their wi dths shrank by 2.5 to 3.0% during seasoning in the room. However, they shrank by only 1.0 to 1. 5% for those kiln-dried to 17% in moisture c ontent. (3) For boxed-heart square timbers with grooves and pithless s quare timbers, the tensile stress acting on a face almost did not appe ar in the case of green conditions, but it became large at 40 to 50% m oisture contents. Then it shifted to compression stress at about 20% m oisture content. For boxed-heart square timbers without grooves, the t ensile stress acting on a face remained when at less than 20% moisture content.