Description of inhomogeneities in wooden structures: modelling of branches

In order to apply numerical methods to analyse the load bearing behaviour and failure characteristics of timber structures, suitable and realistic numerical descriptions of the mechanical properties of the material wood are needed. The present work investigates selected aspects of the mechanics of wood with respect to inhomogeneities and the appropriate numerical treatment in the scope of the finite elements method. All material properties depend on the species-specific anatomy of wood. This holds true especially for inhomogeneities like varying orientation of wood fibres or the presence of branches or knots as they are called in sawn timber. In the present work, two 3-D approaches to numerically determine the fibre orientation in the area of inhomogeneities are introduced. Each method is based on a specific property of the fibres. Basis of the first model is the load transfer of the dead load and external loads like wind within a tree. It is assumed that the fibres are always orientated in direction of the principal stresses during growth. As a second model, a fluid mechanical approach regarding the transport function of the tracheids, i.e. the fibres, is presented. The methods are discussed and compared.