Investigation of rolling shear properties of cross-laminated timber (CLT) and comparison of experimental approaches

Abstract

The structural behaviour of a cross laminated timber (CLT) system is dictated in-part by the rolling shear properties (both strength fr and modulus Gr) of its laminates. To investigate the rolling shear properties of CLT, short-span four-point bending tests, modified planar shear (MPS) tests with timber sandwich plates, and MPS tests with steel sandwich plates were conducted on Australian grown Eucalyptus nitens and radiata pine, as well as European grown Norway spruce. A strong positive relationship between aspect ratio and strength was found, which agrees with previous research. However, the annual ring orientation distribution rather than the aspect ratio was shown to have the most pronounced effect on the shear modulus. The load direction, which has not been accounted for by researchers, was shown to have a substantial effect on the measured rolling shear strength. When the applied load direction was parallel to the annual rings, the Eucalyptus nitens, radiata pine and Norway spruce specimens exhibited rolling shear strengths of 4.1, 2.7 and 1.7 MPa respectively. These average recorded strengths were found to decrease by 16%, 40% and 52% respectively when the load direction was changed to run perpendicular to the rings instead. By directly measuring the shear deformations of two specimens tested under different MPS approaches using Digital Image Correlation (DIC), the impact of the plate material on the recorded shear modulus was isolated. This revealed that the MPS tests with steel plates recorded average rolling shear modulus values which were as much as 42% higher than their counterparts with timber plates, as well as identifying substantial errors in laser and LVDT-like measurement tools. It is recommended that a DIC approach is employed to complement or replace a UTM or laser sensor deformation measurement approach to mitigate these potential errors.