Improving the total stiffness of laminated timber beams

The market for laminated timber products, such as Glue Laminated Timber (Glulam), Cross Laminated Timber (CLT) and Laminated Veneer Lumber (LVL), has increased in recent years as the construction industry seeks more sustainable structural alternatives to steel and concrete. As a result of timber's lower modulus, beam design is generally limited by serviceability requirements, in particular static deflection and vibration. Improving the serviceability performance of laminated timber beams could therefore lead to wider adoption within the construction industry. In bending, a beam is generally subject to combined flexural and shear stresses, with the resultant principal stresses varying along its length. To investigate the potential for improving beam stiffness, four-point bending tests were carried out on laminated softwood timber beams with straight and curved laminations. Nine beams had curved laminations aligned to the principal stress paths and four were conventional straight-laminated beams of the same material. An improvement in the total stiffness in bending of 9.2% was observed in the beams with the novel curved laminations. Experimental results are compared with an elastic analysis accounting for the variation in timber grain. The potential for further improvements, for example by using a higher-grade timber in the outermost tension principal stress path, is discussed.