Clarification of the resistance mechanism of timber joints is crucial to ensure the safety of timber structures. Therefore, the resistance mechanism of timber structures has been analyzed using theoretical approaches. The mechanisms were assumed and the mechanical behavior of the joints was simulated using theoretically derived equations. The consistencies between the simulated and experimental results were confirmed to verify the validity of the assumed mechanisms. For conducting this approach, (i) modeling of deformation appearing at the local part of the timber joint, and (ii) clarifying the basic mechanics of wood become necessary. This paper summarized the above two in line with the author's works.

Cuttings from Japanese cedar (Cryptomeria japonica D. Don) mating families, which were presumed to have high Young's modulus, were clonally propagated and planted. The stress wave propagation velocity (standing tree V_p) and annual fluctuations in growth characteristics were measured from 2 to 10 years after planting. In addition, the applicability of the evaluation method with V_p in early age was verified by comparing them with the Young's modulus of logs cut in the 6th to 10th years. The V_p of standing trees showed a difference among the lines due to the annual fluctuation from 2 to 10 years after planting the seedlings. The coefficient of determination was high even in the regression equation for each measurement year, and the difference among the lines continued to appear. Differences among the lines were also observed in the dynamic modulus of elasticity (E_fr) of logs cut down 6 to 10 years after planting the seedlings, and the coefficient of determination of linear regression between the standing tree V_p and the log E_fr of each line was also high. Therefore, V_p measurement was considered to be an effective method for non-destructively estimating Young's modulus. Furthermore, the regression equations of E_fr from the pith to the 5-year ring and from the 6-year ring to the outermost ring showed a high coefficient of determination. This suggested that clones with a high Young's modulus in the early stages would continue to form materials of similar qualities.

The objective of this study is to investigate the effects of various environmental conditions such as dry environment, intermittent wet environment, and continuous wet environment on the out-of-plane shear strength of 5-layer 5-ply CLT with Japanese larch (Larix kaempferi) or Sakhalin fir (Abies sachalinensis). In this study, we chose five methods of the accelerated deterioration treatment regulated in the Notification 1446 of The Ministry of Construction of Japan: the boiling method with 1 cycle or 2 cycles, the vacuum and pressure method with 1 cycle or 2 cycles, and the steaming and freezing method with 6 cycles. We carried out shear tests and delamination tests after the accelerated deterioration treatment. Results from these tests were as follows: (1) the residual ratio of shear strength was about 90% in dry environment, about 80% in intermittent wet environment, and about 60% in the continuous wet environment; (2) the rate of decrease in shear strength was considerably higher than the delamination rate; (3) the wood failure near the adhesive layer and the radial check of cross layer lamina due to accelerated deterioration treatment affected the decrease in shear strength.

Material properties of laminae sawn from mature parts of large diameter logs of Japanese larch (Larix kaempferi), such as Young's modulus obtained by the longitudinal vibration method and twist warp, were investigated. These laminae were sawn from Japanese larch logs from Hokkaido, Japan, and had small end diameters of 20 cm or more (≥20-cm logs) and 26 cm or more (≥26-cm logs). Laminae from ≥26-cm logs were sawn from mature wood parts of trucks. Subsequently, the effects of the rough sawn size and crosscut frequency on yield in the production of jointed laminae were examined. Additionally, simulations of stress grades capable of manufacturing glulam and cross laminated timber (CLT) were conducted. Laminae from ≥26-cm logs had higher Young's modulus and smaller twist warp than those from ≥20-cm logs. In this study, the yield in the production of a 30-mm-thick jointed laminae was maximized when the rough sawn thickness was 35 mm and crosscut frequency was once for each rough sawn lamina. The possible stress grade of glulam made of laminae from ≥26-cm logs was as high as E150-F435 in symmetrical mixed-grade according to the Japanese Agricultural Standards for glulam. The major stress grades of CLT made of laminae from ≥26-cm logs corresponded to Mx120 and S120 according to the Japanese Agricultural Standards for CLT. The laminae from mature wood parts of the Japanese larch have the potential to improve yields in the manufacturing of jointed laminae and produce high stress grade glulam and CLT.