Photochemical degradation is a key process of the weathering that occurs when wood is exposed outdoors. It is also a major cause of the discoloration of wood in indoor applications. The effects of sunlight on the chemical composition of wood are superficial in nature, but estimates of the depth at which photodegradation occurs in wood vary greatly from 80μm to as much as 2540μm. Better understanding of the photodegradation of wood through depth profile analysis is desirable because it would allow the development of more effective photo-protective treatments that target the surface layers of wood most susceptible to photodegradation. This paper briefly describes fundamental aspects of photodegradation of wood and reviews progress made in the field of depth profile study on the photodegradation of wood.

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Bending creep properties of wooden I-beams with Todomatsu (Abies sachalinensis) lumber and Karamatsu (Larix kaempferi) plywood were examined by load-duration tests at different initial moisture content. Four I-joists of 235 mm depth were fabricated with lumber flanges and plywood webs glued by aqueous vinyl polymer solution isocyanate adhesive. These joists were divided into two pairs of specimens and each pair was conditioned at 20°C and 65% or 85%RH to adjust their initial moisture content. The creep tests for six years were carried out in a laboratory under uncontrolled air conditions. Test results showed that the mid-span deflections of specimens conditioned under 85%RH obviously increased with desorption at the start of loading. Following this immediate creep behavior, the deflections of all specimens fluctuated in response to changes of moisture content so that the deflections decreased in humid summer and increased in dry winter periodically. The deflections and moisture content of specimens conditioned at 85%RH fluctuated considerably more than those conditioned at 65%RH, and the prior conditioning in high humidity affected the creep behavior during the whole test period. The relative creep after 50 years of specimens conditioned at 85%RH was estimated to be greater than that of specimens conditioned at 65%RH. This result showed that wooden I-beams should be kept away from unexpected moisture or high humidity during shipment and application as floor framing to minimize creep deflection.

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In order to improve the bonding of fiberboard, the penetration of phenol formaldehyde (PF) resin was prevented. The means of preventing penetration were acetylation of wood fibers and the addition of filler to PF resin. Acetylation improved internal bond strength (IB) due to the resulting lack of penetration. In addition, the IB of fiberboard made from acetylated wood which had been ozonized (acetyl-ozonized board) was much higher than that of merely acetylated fiberboard due to improved wettability by the ozonization. The durability of the acetyl-ozonized board was significantly increased, resulting in 0.84 MPa for IB and 74% retention of IB after an ASTM 6-cycle accelerated aging test. In another test, fiberboard was made from untreated wood bonded with PF resin to which strained soybean meal was added as a filler, and the IB showed a roughly 40% increase. The retention of IB after the ASTM 6-cycle accelerated aging test was lower than that of the acetyl-ozonized fiberboard, but the addition of the filler was effective for improving properties for conventional uses.

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The crosslinking reaction between isocyanate compounds (pMDI) and acetoacetylated PVA (AAPVA) with various degrees of acetoacetylation or partially saponified PVA were investigated in a dimethylsulfoxide (DMSO) solvent system, and compared with those in a water solvent system. The following results were obtained on the reactivity of pMDI with AAPVA and the formation of crosslinking. The crosslinking densities calculated from storage modulus (E') at the rubbery plateau regions of cured films for the DMSO solvent system were almost equal to those calculated from the temperature differences of the peak temperatures of the loss modulus (E"). However, for the water solvent system the former values were higher than the latter ones. The degree of weight swelling of cured films of pMDI and PVA for DMSO solvent systems were generally smaller than for aqueous systems. The degree of weight swelling was less for AAPVA than for pMDI and PVA. From these results, it was found that a higher crosslinking density can be attained in the DMSO solvent system rather than in the aqueous system, and that AAPVA affords higher crosslinking density than ordinary PVA.

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We investigated local wooden houses with traditional structures to contribute to the popularization of locally suitable wooden houses built with regional lumber. Based on the results of the investigation, racking tests of traditional wooden structures with sashigamoi, nuki, and koshigumi joints and their frames were conducted. Sashigamoi, nuki, and koshigumi structures did not fail up to a maximum shear deformation angle of 0.15 rad, and showed great toughness. The shear load factors of sashigamoi, nuki, and koshigumi specimens were 0.46, 1.31, and 0.96, respectively. These resulting factors are low in value, contrary to the great toughness of traditional wooden structures. Then, the rotational properties of sashigamoi, nuki, and koshigumi joints were analyzed with the results of racking test of traditional wooden structures and their frames, and ultimate moment and rotational stiffness values of sashigamoi, nuki, and koshigumi joints were obtained.

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The ratio of working stress to allowable stress on columns and sill plates in three wooden houses, which were build in a general area, was investigated. It was found that : 1) The load-bearing ratio (working stress/allowable stress) on columns at each of durations of load was sufficiently small with some exceptions. 2) For long term load (dead and live loads) and short term load from snow, there was relatively smaller safety margin than that for the other loads in first floor columns which support a balcony and a long beam for a large space. 3) For short term load (seismic and wind loads), the safety margin of columns which are part of a shear wall showed a tendency to decrease with increasing strength of the shear wall. 4) The features on sill plates were very similar to them on columns.

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