Glulam from European White Oak: Finger Joint Influence on Bending Size Effect

Glued laminated timber consists today predominantly of softwoods as they are currently the main source of structural timber in the northern hemisphere. Due to several reasons, hardwoods will increasingly gain a more important role as a sustainable material resource which will of course therefore also impact glulam production. In Europe, the wood species white oak (Quercus Robur, Quercus Petraea) is the most important species other than beech, and is being increasingly used in structural applications. Recently, the first national and European technical approvals were issued for oak glulam. The design strength values were based on extensive testing campaigns, with emphasis on the differences inherent between growth regions of the raw material itself. The paper reports first on basic strength properties and requirements for oak laminations and beams as specified in the technical approvals. Further, essential test results which form the basis for the characteristic strength and stiffness parameters are given, showing a clear effect of size on the bending strength. In order to assess the beam characteristics based on basic lamination and finger joint properties, the applicability of the new European strength equations for softwood glulam is discussed. In the case where the important ratio of finger joint vs. lamination strength is at least one, as is typical for softwoods, the model applies for the considered hardwood databases as well. However, when the lamination strength reaches very high values with means in the range of 80 MPa to 120 MPa, finger joint strength vs. lamination strength drops significantly below one. It is then shown that the bending capacity and size effect can be well predicted for specific high strength hardwood glulams by a serial model, effectively including the bending stress gradient between adjacent laminations.