Evaluating the shear stiffness of asymmetric angle-ply layers in diagonal cross-laminated timber

The perpendicular orientation of the cross-layer (s) makes Cross-laminated Timber (CLT) susceptible to reduced stiffness in out-of-plane bending. Orienting the cross layers at angles other than 90 degrees could provide cost-effective layup customization for fabrication. This customization improvement has the potential to allow production of a panel with an optimized fiber content from regional resources for a requested structural performance and span. Under out-of-plane bending, shear deformation occurs in the cross layers along the radial-tangential plane. This reduced stiffness in CLT panels can be improved by varying the grain orientation of the cross-layers away from perpendicular-to-grain (90 degrees), and closer to parallel-to-grain (0 degrees). This paper studies the shear stiffness of a pair of asymmetric angle-ply layers rotated at angles from 90 to 70, 40, 20, 10, and 0 degrees. These two layers with varying grain orientations were assumed to be representative of the inner layers of a four-ply Diagonal Cross-laminated Timber panel. For this purpose, a theoretical model based on Hankinson's formula was developed, and compared to empirical testing from two-plate shear tests. These predicted and observed behaviors were then compared to numerical modeling using the Finite Element Method. The results of the three methods showed an agreement in predicting an increase in shear stiffness when the angle-ply orientation of the layers decreases from 90 to 0 degrees. The results indicated that Hankinson's formula seems to be a conservative theoretical predictor for the shear stiffness of diagonal layers. Regarding the experimental results, orienting the cross layers to an angle between 70 and 40 degrees was shown to improve the shear stiffness by 10 and 46% respectively. This research provides insights into innovations towards CLT development with improved shear stiffness using diagonally-oriented layers.