Experimental stiffness investigation of finger joints in glued laminated timber beams using digital image correlation

Glued laminated timber (GLT) is an engineered wood product widely used in structural applications. The mechanical properties of the GLT beams significantly depend on the mechanical properties of local weak sections such as knots and finger joints (FJs). Conventionally, the mechanical behavior of the local weak sections has been mainly investigated in the individual lamellae. In the present study, their mechanical behaviors within the GLT beams are investigated. 22 GLT beams with well-known beam setups in four-point bending tests were studied. Digital image correlation was used to measure displacements and strains in the region of the beams with the constant bending moment. This paper presents the strain distributions in the GLT beams and discusses the influence of the timber board arrangements and, accordingly, the knots and the FJs. As expected, the strain distributions of the GLT beams vary significantly. Depending on the arrangement of the knots, they can cause strain concentrations in the beams, which can be distributed to the adjacent lamellae. FJs do not cause significant strain concentrations; however, they can influence the strain distribution along the lamellae. Furthermore, a reduced stiffness of the FJs, compared to the connected timber boards, is identified.