Abstract
M.Ing.
Timber is an organic, orthotropic material of which the strength is currently not fully utilised in structural designs. Most design codes treat timber as an isotropic material with special clauses to compensate for the weaker strength across the grain. Several safety factors are used to accommodate the large variations in the strength of timber.
In the orthotropic approach presented here, the unidirectional properties of timber are
assumed as constant. It is also assumed that the large strength variation which is observed between samples of the same group, can mostly be attributed to the grain slope variation in a sample. The finite element method is employed to model grain slopes around a predetermined defect in a sample. The sample is then forced to fail at this defect, and the finite element model is then analyzed at the failure load to determine which stress combination led to the failure. Initial unidirectional material strengths are used in a failure criterion to evaluate the critical stress combination. The method of least squares is used to fit the predicted strength against the experimental strength. In this process new values are found for the unidirectional material strengths. The process is then repeated to determine whether another stress combination is not more critical than the original choice. As soon as the numerically acquired material strengths stabilise, it is compared with known values. The results indicate that the strength of a timber section can be predicted on the grounds of the observed fibre directions if the unidirectional material strengths are known.