DURABILITY OF MASS TIMBER STRUCTURES: A REVIEW OF THE BIOLOGICAL RISKS

The effect of wood decay on the fully reversed cyclic performance of nailed oriented strand board (OSB) sheathing to Douglas-fir framing member connections was investigated. The connection geometry evaluated in this study was representative of lateral force resisting systems of light-framed wood structures, including shear walls and horizontal diaphragms. Maximum loads, slip at maximum loads, yield loads, initial stiffnesses, and cumulative energy dissipation of nailed connections exposed to increasing intervals to the brown rot fungus, Postia placenta, were characterized using fully reversed cyclic loading. After the destructive connection tests, portions of the sheathing and framing member from the samples were further evaluated for specific gravity. The OSB sheathing specific gravity was the best descriptive variable for the mechanical properties measured in this study. Cumulative energy dissipation was the connection property most affected by decay damage.

Development of Dinoderus minutus was studied under laboratory conditions at five constant temperatures (15, 18, 25, 28, and 30°C) to estimate developmental thresholds and thermal requirements of the egg, larva, pupa, and egg-to-adult stages. Adults began to burrow along the grain into bamboo pieces within 24 h after cutting and continued across the grain. Female beetle oviposited an average of 9.1 eggs in the ensuing 24 h, all in the metaxylem. The average period for development of eggs, larva, and pupa were 5.4, 43.8, and 4.6 d at 30°C, respectively. The development rate data were regressed through the x-axis to derive the thermal constants of 4.3, 6.8, and 4.5°C and thermal requirements of 113.6, 909.1, and 96.2 DD to predict D. minutus egg oviposition to hatch, larva-to-pupa development, and adult emergence from pupa, respectively. The thermal threshold for egg-to-adult development was 6.4°C, with total thermal requirements of 1,111.1 DD. Oviposition, emergence, and thermal requirements for development of the various stages of D. minutus may be used for postharvest pest management of bamboo.

The potential for using pectinase as a pre-treatment to improve the properties of bamboo or eucalyptus fibers for use in a woody fiber/polypropylene (PP) composite was compared with the addition of a silane coupling agent or NaOH pre-treatment. Pectinase pre-treatment reduced the amount of pectin in both fibers (p < 0.0001) and this reduction was associated with improved tensile strength, modulus of rupture and modulus of elasticity (p < 0.001) of bamboo, or eucalyptus fiber-based composites. Pre-treatment had no consistent effects on the thickness swell or water absorption. The addition of a coupling agent produced similar improvements in panel properties (p < 0.05), but also markedly reduced moisture absorption and thickness swell (p < 0.05). Sodium hydroxide pre-treatment had no consistent effects on panel properties, although it did appear to affect thickness swell. Short pectinase pre-treatment might be useful for enhancing the properties of woody tissue/PP composites while reducing but not completely eliminating the need for coupling agents.

Effects of Thermal Modification on Physical and Mechanical Properties of Douglas-Fir Heartwood

The resistance of three naturally durable heartwood species and a stranded giant bamboo product to fungal and termite (Coptotermes formosanus) attack was evaluated at a test site located near Hilo, Hawaii. Merbau (Intsia bijuga or I. palembanica) and ipe (Tabebuia spp.) were both exceptionally resistant to fungal and termite attack, while western juniper (Juniperus occidentalis) heartwood was slightly less resistant to degradation. The presence of heartwood on western juniper samples had no noticeable effect on the performance of adjacent sapwood. The bamboo decking proved to the least durable of the materials and experienced substantial termite and fungal attack over the 32-month test period.