The mechanical properties and stability of radiata pine structural timber

Investigating stiffness, strength and stability in radiata pine structural timber is significant to the civil engineering, forestry industry and genetic selection in New Zealand due to the following reasons: 1. Radiata pine provides about 90% structural timber for industry and is one of the major incomes of the forestry industry in New Zealand. 2. Stiffness, strength and stability are major quality criteria of structural timber. 3. Radiata pine stem has large proportion of corewood that tends to be low in stiffness, strength and stability. This study first presents the maps of average stiffness, average strength and average warp in radiata pine structural timber, obtained from sixty-two 27-year old, unpruned radiata pine trees. From these maps, one can conclude that the butt logs are problem logs, because the butt logs displayed the lowest stiffness, as well as the maximum bow and spring within the stems. The stiffness and strength distributions were compared with the typical distribution of wood density, which reveals that the wood density alone does not reflect the mechanical properties of radiata pine structural timber, because: 1) the denser butt logs exhibited the lowest stiffness among all log types; 2) the butt logs were not the strongest logs compared with other log types. Knots are found to be the most important factor weakening the mechanical properties and causing extra distortion in structural timber. 99% of the boards broke at a knot that is associated also with lower local stiffness. 70% of maximum bows and 40% of maximum springs occurred off the expected mid-span, which may be attributed to the deflections introduced by larger margin and edge knots. In order to estimate the failure strength non-destructively, this study examined the failure features of the weakest point in detail, including the local stiffness at the failure zone, the failure pattern of knots and the failure frequency in relation to growth and features of knots. The results of this study reveal that non-destructive estimation of strength at the likely weakest point in structural timber is possible