Applications and Limitations of the Knot Area Ratio (KAR) in the Performance Prediction of Structural Timber

The knot is one of the defects that reduce strength and contribute to performance variability in structural timber. The knot area ratio (KAR) has been widely used to quantify its effects in grading and performance prediction. This review examines the definition of KAR; methods for measuring and estimating knot information; applications of KAR in structural timber performance prediction; and the use and limitations of KAR-based probabilistic models and indicating property (IP)-based models incorporating local fiber deviation. The reviewed studies show that non-destructive techniques can provide information on knot size, position, distribution, and geometry, but this information is often reduced to representative values for specific cross-sections, longitudinal segments, or local regions when used in prediction models. Consequently, three-dimensional knot geometry, longitudinal distribution, clustering, and local fiber deviation are not fully represented. KAR-based probabilistic models are useful for capturing material variability and performance distributions but remain limited in explaining local failure behavior and stress concentration in critical regions. IP-based approaches incorporating local fiber deviation provide complementary information. Future performance prediction models should integrate knot spatial information, local modulus of elasticity (MOE), and local fiber deviation while retaining the practical advantages of KAR.