Evaluating the Effect of Inner Layer Grain Orientation on Dimensional Stability in Hybrid Species Cross- and Diagonal-Cross-laminated Timber (DCLT)

Abstract

Wood deformation due to moisture adsorption and desorption in the cell wall components challenges the dimensional stability of multi-layer wood-based panels. This is a common issue in timber products with single board products and parallel glued layers (such as Glued-laminated Timber; GLT). Orienting the inner layers of timber products at an angle, like plywood and Cross-laminated Timber (CLT), with the perpendicular (90°) orientation of the inner layers, helps minimize overall shrinkage and swelling of the composite panel. However, due to the perpendicular orientation of the layers, Cross-laminated Timber (CLT) exhibits a lower out-of-plane bending stiffness than a parallel-layered Glued-laminated Timber (GLT) panel. An optimized modified orientation of the inner layers in a diagonal direction could improve dimensional stability compared to GLT while developing a better structural performance than CLT. This study evaluates the dimensional stability of small specimens of four-layer Diagonal-Cross-laminated Timber (DCLT) with a hybrid layup consisting of a high-density hardwood species, black locust, in the top and bottom layers, and a low-density softwood species, eastern white pine, in the asymmetric inner layers. The results show that increasing the inner layer fiber orientation angle improves the dimensional stability of the panels. While the dimensional change in the length of the panel (longitudinal) is negligible, by varying the inner layer angle-ply from 0° to 90°, the percentage changes in width (tangential), thickness (radial), and volume reduced respectively from 3% to 0.8%, 1.6% to 0.4% and 5.1% to 1.3%. Since the dimensional tolerances of the CLT due to the moisture content in the manufacturing process and the in-service conditions have already been established in the design and building standards, the result of this study can be helpful for the construction industry to predict the potential dimensional change (%) and corresponding required tolerances for DCLT panels when used in building structures.