Evaluation of a testing method for shear stiffness properties for cross laminated timber

Abstract: Cross laminated timber (CLT) is an engineered wood product that was first developed in the 1990s. Since then it has grown in popularity thanks to its many advantages such as: low environment impact, high degree of prefabrication and low weight. As the material is relatively new the standardization process is still in the early stages. The purpose of this thesis is to evaluate a testing method which is suggested to be part of the standardization of CLT. The testing method, which is described in the European standard EN 16351, is a four-point bending test with the purpose to determine, among other properties, the rolling shear modulus of the transverse layers in CLT. The test is performed by measuring the so called local- and global deflections. The local deflection is measured between the two loads and is assumed to depend only on bending and is used to estimate the bending stiffness of the beam. The global deflection is measured for the entire span and is therefore dependent on both bending and shear. The global deflection is used to estimate the so called apparent bending stiffness. By determining the local- and apparent bending stiffness the shear stiffness of the entire cross section can be determined. When the shear stiffness of the entire cross section is determined the shear modulus for the transverse layers (rolling shear modulus) can be determined by applying Timoshenko beam theory and subtracting the contribution of the shear modulus of the longitudinal layers. For this project no laboratory testing was performed, the testing method was instead evaluated with Finite Element-models (FE-models). When analysing with FE-models the rolling shear modulus is known beforehand since it is used as an input parameter to the models. The accuracy of the test method was evaluated by comparing the calculated rolling shear modulus to the input rolling shear modulus. An accurate result should result in the output and input being equal. The results indicated that the method used to determine the rolling shear modulus is largely influenced by assumptions made according to Timoshenko beam theory. One of these assumptions include the shear correction factor, which is used to correct a theoretical assumption that results in an overestimated shear stiffness. The shear correction factor according to beam theory results in inaccurate results, but the factor can be altered to correlate better with the expected rolling shear modulus. One of the problems with such a procedure is that the rolling shear modulus must be known beforehand to do an accurate alteration. Other deviations between beam theory and the FE-models affecting the results include: boundary conditions and shear strain distributions.