Determination of the influence of size and position of knots on load capacity and stress distribution in timber beams of Pinus sylvestris using finite element model

The finite element method was used to investigate the influence of size and position of cylindrical knots on load capacity considering the elastic–plastic constitutive law of Scots pine timber. A finite element model for a four points bending test was generated considering four different knot conditions in the beams: without knot; knot as a hole; live knot and spring contact between the knot and the beam. For knots placed in the compression side, the live-knot-model best simulates real behaviour; however, when located in the tension side, the hole-model was most reliable. The bending strength of the beam, including different sizes and positions of knots, were presented in simplified diagrams and compared with clear timber strength. The results showed the influence of knots and their local grain deviation on stress distribution. The model allowed the ranking of bending strength of the beams caused by knots as a combination of three quantified indexes: tension parallel and perpendicular to the grain and shear.