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Survey of Typhoon-Damaged Coniferous Forests and Strength Properties of Lumber and Glued Laminated Timber Processed from Damaged Japanese Cedar and Japanese Cypress Trees

Toyoshima Isao, Noguchi Hiroshi, Yamasaki Mariko, Sasaki Yasutoshi
Forest products journal 2018 v.68 no.1 pp. 25-36
Chamaecyparis obtusa, Cryptomeria japonica, bark, bending strength, coniferous forests, lumber, modulus of elasticity, modulus of rupture, stand density, surveys, trees, typhoons, Japan
Typhoon Melor, which occurred in October 2009, damaged much of the forest in the eastern region of Japan's Aichi Prefecture. The damaged trees were primarily planted Japanese cedar (Cryptomeria japónica) and Japanese cypress (Chamaecyparis obtusa) between 40 and 60 years old and were intended for use as structural lumber. The purpose of this study was to investigate the usability of damaged trees for structural lumber. First, the state of the damaged forests was assessed, and the occurrence of compression failure (CF) in logs and lumber from damaged trees was surveyed. The strength properties of the lumber and glued laminated timber of both species were then determined, and usage-related issues for each species were investigated. The study found that the majority of damaged forests had a high stand density and indicated a high diameter at breast height-to-height ratio. The most common damage type was overturned. The rate of CF occurrence in damaged logs and lumber from damaged trees, in logs with the bark on, was highest in those that had suffered from bending. For lumber, the rate was highest in inclined and bending cases, and there were also a few instances in overturned cases. In lumber from damaged trees, a higher Young's modulus was slightly associated with a higher rate of CF occurrence. In terms of the strength properties of lumber from damaged trees, lumber in which the postprocessing surface showed no CF exceeded the standard strength for both species. The properties of lumber in which minute CF was visible on the surface were similar to lumber without CF for Japanese cedar, but Japanese cypress lumber with visible CF exhibited a failure morphology that failed within the elastic range, and most specimens fell short of standard strength. In addition, laminae cut from damaged trees were processed into glued laminated timber, using laminae on which CF could not be seen on the surface. These bending properties were compared with those of glued laminated timber processed from laminae made from normal trees. This comparison showed that the strength of both was similar in the Japanese cedar, whereas in the Japanese cypress, the bending strength (modulus of rupture) of the damaged glued laminated timber was significantly lower than that of normal trees.