CHARACTERISATION OF CROSS LAMINATED TIMBER PROPERTIES

Sammanfattning: This project concerns an analysis of a hitherto untested method for determining the modulus of elasticity (MoE) and shear modulus parallel to grain, as well as the rolling shear modulus of cross laminated timber (CLT) through 3-point bending tests. A review of analytical models and methods is included, followed by an evaluation of the effect of the annual ring orientation and shear stress distribution at shear loading of individual board cross-sections. Lastly a validation of the results using 2D and 3D finite element (FE) models are presented. A total of nine prismatic beams with a square cross section of 100×100 mm were cut centrically from a CLT plate with respect to the longitudinal boards. The beams were cut in two directions with respect to the outermost boards, resulting in two different layups and lengths. Six beams were cut with the outermost board in the longitudinal direction (with a length of 1500 mm), and three beams were cut with the outermost board in the transverse direction (with a length of 900 mm). Each beam was tested at in-plane loading, and out-of-plane loading for two different spans, respectively, resulting in four tests for each beam and 36 tests in total. By using the test data within the linear-elastic range and performing a linear regression the equivalent stiffness could be evaluated. The results from the in-plane bending tests were used to characterise the MoE and shear modulus parallel to grain by means of Timoshenko theory. The results from the out-of-plane bending tests were used to characterise the rolling shear modulus by means of the Gamma method or Timoshenko theory. The results were evaluated by comparing the calculated equivalent stiffness of FE-models (with homogenised material properties corresponding to the characterised stiffness properties) to the equivalent stiffness determined experimentally. The results show a promising reliability and validity for the beams with a length of 1500 mm when characterising the stiffness properties using Timoshenko theory, with respect to the shear stiffness being calculated for the gross area of the cross section. Thus, consideration of the shear stress distribution in the transverse boards should be reflected by modification of e.g. the effective area. The rolling shear modulus was characterised in the range of 64–89 MPa for the beams with a length of 1500 mm, with respect to Timoshenko theory, indicating realistic values and that the effect of annual ring pattern is accounted for. The Gamma method, however, resulted in characterised values of the rolling shear moduli that were considered too low with respect to the effect of annual ring pattern.