Modelling knot size and location distribution for implementation in structural safety analysis of timber elements

The presence of local natural defects, such as knots, influence the global mechanical properties of timber elements. In this work, as low bending strength and stiffness are prone to coincide with the presence of knots or group of knots, the statistical analysis of the location and size of knots found in timber elements is presented aiming at the construction of a model for weak section definition. The main features of the model are given regarding the mean distance between clusters of knots with different values of diameter, its variation and definition of probabilistic distributions that describe the presence of these defects within the length of the elements. The probability of detection of defects using the proposed model is obtained regarding the variation of the assumed length for the weak section. Moreover, the reduction effect resulting from the presence of knots and of different levels of defect concentration are implemented in a Bayesian Probabilistic Network aiming at the retrieval of different strength classes for implementation within a structural safety analysis. The proposed models are based on the results of an experimental campaign made to 68 Picea abies boardsFEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT within the scope of the project POCI-01-0145-FEDER-007633. The financial support of FCT, through national funds, within the scope of the project Protimber, PTDC/ECM-EST/1072/2014