Picture frame and diagonal compression testing of cross-laminated timber

Abstract

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 underestimating 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.

Influence of laminate direction and glue area on in-plane shear modulus of cross-laminated timber

“…In the diagonal compression test method, a square CLT panel was compressed in one diagonal direction by a displacement controlled hydraulic jack. Please refer to [5,6] for more details on the test procedure. During a diagonal compression test, only one diagonal was compressed, and the second diagonal was left without control of the displacement.…”

“…Other studies have concluded that board dimensions affect G but not analysed how much. Some studies have calculated G from different variants of the picture frame testing method; see [6][7][8][9], and others have used the diagonal compression test method [5,6,10,11]. Recently, a method similar to the diagonal compression test has been used, but in a more sophisticated way [12,13].…”

“…The diagonal compression test method, which is analysed in this study, contends to define the shear stress occurring in the diagonally compressed sample. To calculate the shear stress in a diagonally compressed CLT panel, an isotropic model without any layers and gaps between boards has been used [6,11]. In [6], this was the reason for the less accurate result from the diagonal compression test.…”

“…To calculate the shear stress in a diagonally compressed CLT panel, an isotropic model without any layers and gaps between boards has been used [6,11]. In [6], this was the reason for the less accurate result from the diagonal compression test. The isotropic shear stress has been adjusted in [5] where the equation for calculating G was created based on FE (finite element) simulations.…”

“…The limited scope in [5] 0 resulted in a restricted equation which is only valid for the specific sizes and conditions occurring in the study and not in a general case. In [10], an equation suitable for a perfect shear condition has been used, but this situation does not occur during a diagonal compression test [6]. This method of assuming a perfect shear condition has been used earlier for other orthotropic materials like masonry [14], but the conditions for CLT panels are not equivalent.…”