Picture frame and diagonal compression testing of cross-laminated timber

Currently, no appropriate standard exists that describes how to determine the in-plane shear stiffness for cross-laminated timber (CLT) panels, meaning that, there is a lack of appropriate and reliable test methods. In this paper, two gross shear test methods are evaluated: a picture frame test and a diagonal compression test, which are intended to measure the shear stiffness of a whole CLT panel. This evaluation aimed to compare the shear modulus, the amount of compression/tension in the diagonal directions of the panels and the deformations of both sides of the panels. The picture frame test and diagonal compression test provides a bi- and uniaxial pre-stress, respectively. A total of 30 non-edge glued CLT panels were tested, 17 3-layer and 13 5-layer panels. The shear modulus for the 3- and 5-layer non-edge-glued panels were measured as 418 and 466 MPa, respectively, in the picture frame test. In the diagonal compression test, the shear modulus was measured to substantially higher values of 530 and 626 MPa for the 3- and 5-layer panels, respectively. In the picture frame test, panels were equally stretched along one of the diagonals as they were compressed along the other diagonal, which was not the case for panels in the diagonal compression test. The test results also showed that measuring only one side incurs a risk of over- or under-estimating the in-plane shear modulus. Compared with results from the literature, the picture frame test seems to be a more reliable test method than the diagonal compression test.