Properties and Distortion of Douglas-fir with Comparison to Radiata Pine
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
The objectives of this study were to investigate stability related basic wood properties and to determine stability performance of New Zealand grown Douglas-fir and those of radiata pine wood. In this study, nine 60-year old Douglas-fir trees and thirteen 26-year old radiata pine trees were selected from forests in Canterbury region. From these trees, 36 discs of 200 mm thick (13 for Douglas-fir and 23 for radiata pine) and 388 boards with dimensions of 100mm x 50mm x 4.8m (210 fro Douglas-fir and 178 fr radiata pine) were prepared.
From the prepared discs, 515 specimens (204 for Douglas-fi and 211 for radiata pine) were prepared for green moisture content (MC) and basic density measurements, The same total number of 515 specimens (205 for Douglas-fir and 210 for radiata pine) were also prepared for shrinkage measurements. From the green MC specimens, green weight, volume and oven-dry weight were measured while for the shrinkage specimens, dimensions and weights were measured at equilibrium for nine humidity conditions. These data were used to analyse basic wood properties and moisture uptake characteristics for both Douglas-fir and radiata pine. Variation of shrinkage within a tree and between trees was also studied for both species. After this, selected specimens (36 for Doulas-fir and 36 for radiata pine) were further tested in water immersion for water repellence examination.
The 388 full size boards (100mmx50mmx4.8m) were used for studies on distortion and acoustic properties at a sawmill (Southland Timber Ltd.). Dimensions, weights and acoustic velocity were measured from each board before and after drying. These data were used to analyse distortion and strength characteristics for both Douglas-fir and radiata pine. Comparison of the relative stability of full sized Douglas-fir and radiata pine structural timber was investigated in this study.
The results from small sample study confirmed that Douglas-fir is much stronger, has lower longitudinal shrinkage and lower gradient in corewood which can be used to explain the better dimensional stability of Douglas-fir than radiate pine although there is significant variability in the shrinkage for both Douglas-fir and radiate pine. In water immersion tests, Douglas-fir has better water repellency property than radiata pine over 2000 hours period during water immersion.
Under the same commercial practice in sawing and kiln drying, it is clearly shown that Douglas-fir timbers were straighter with lower levels of distortion than radiata pine at similar final moisture content. It is also interesting to note that the final moisture content in a range of 13-18% for Douglas-fir did not have significant impact on timber distortion but a negative trend was observed for radiata pine with MC in a range of 9 -14%. Tree heights showed clear influence on twist for radiata pine timbers, but it was not clearly observed from Douglas-fir timbers. Corewood proportion is found to have negative impact on the timber distortion for both Douglas-fir and radiata pine. Douglas-fir timbers showed much higher average acoustic MOE value than radiata pine timbers at similar final moisture content.
Because of the various proportion of corewood, the shrinkage varied greatly along the stem height and along the disc radius direction for the two species. This variation caused the difference of distortion between corewood, outerwood and transition wood, but the difference between butt log, middle log and top log is inconsistent. Therefore, it is recommended that the corewood proportion to be a criterion for the timber pre-sorting.
Variation of stability performance between trees was also found to be significant for the two species, and methods need to be developed for log sorting as well to reduce the timber distortion degradation. Non-destructive testing method such as acoustic tool may be offered to be a new approach for sorting logs, but it is also necessary to be aware of the significant difference between species.
The outcome from this project includes better understanding of Douglas-fir for structural applications. The conclusion can be drawn that Douglas-fir has superior quality for its strength, durability and moisture resistance. Douglas-fir is also claimed to have uniform properties and thus to be more stable compared to radiata pine. Douglas-fir timbers showed much higher acoustic MOE value than radiate pine timbers as similar final moisture content.