Finger-Joints for Laminated Beams : Experimental and Numerical Studies of Mechanical Behaviour

Abstract: The introduction of laminated timber, glulam being an example, allowed many of the disadvantages associated with solid wood to be overcome. The disadvantages in question are mainly those related to the size and shape limits of structural elements made of solid wood and to the large variability in such material properties of solid wood as strength and stiffness.In the present thesis, which comprises both experimental and numerical studies,such phenomena like the laminating effect and to some extent the size effect are investigated and discussed using the concept of fracture mechanics.The experimental part involved the testing of bond line strength, stress vs. deformation and fracture energy of three adhesives (resorcinol phenol, polyurethane and polyvinylacetate) and tensile testing of finger-jointed laminations. The results from bond line tests were used as input data in a nonlinear fracture mechanics model based on the concept of a fictitious crack. The model was verified by simulating both the bond line tests and the lamination tests. Parameter studies on the influence of bond line characteristics on the strength of finger-joints were also performed, together with analyses of the sensitivity of a finger-joint to defects in the bond line. The influenceof finger-jointing pieces of lumber dissimilar in stiffness was also investigated. These parameter studies showed the brittleness of the bond line to be an important parameter in governing the strength of the finger-joint. For brittle resorcinol-phenol adhesive, the approach adopted, one based on nonlinear fracture mechanics, predicted the strength of a finger-jointed lamination to differ considerably from the strength predicted by such classical theories as single point maximum stress theory and perfect elastic plastic theory. The complete load displacement response of a finger-joint was determined using anumerical solution procedure that allowed so-called snap-back behaviour to be traced. The behaviour of a laminated beam in bending was also simulated. This showed the laminating effect to partly be explainable on the basis of fracture mechanics.