Method for predicting tension capacity of sawn timber considering slope of grain around knots

Abstract

This study introduced a new parameter, the area reduction factor (ARF), to consider the effect of knots on the tension strength of timber. It is an improved version of the knot area ratio (KAR). ARF considers both the projected area of knots and the effect of the slope of grains around the knots. The tension capacity of a tested structural timber was predicted as a product of ARF, clear wood tension strength parallel to the grain, and the area of the cross section. ARF was determined as the minimum value obtained when a knot measurement window of 100 mm was slid along the plank. The prediction method was examined with 11 planks. The average ratio of the predicted capacity to the actual value was 1.11 with a coefficient of variation of 0.26. The average ratio obtained by using a KAR-based parameter, the clear wood area ratio (CWAR), was 2.15 with a coefficient of variation of 0.23. To study the reliability of ARF and CWAR as single parameters, the correlations of ARF and CWAR with the tension strength were determined for 57 planks. The coefficients of determination for ARF were slightly better than those for CWAR, although both of them seemed to be quite poor predictors of tension capacity when used alone. Therefore, a multiparameter model is preferred and should be a subject for further studies.