Comparing the effect of presence of the knot and the size of the knot on the rolling shear properties in cross laminated timber (CLT) by modified planar shear test and fem analysis

One specific characteristic of Cross-laminated Timber (CLT), as an orthogonal laminar structure compared to a uni-directional timber composite such as glued laminated timber (GLT), is its intra-laminar shear flexibility due to rolling shear in the cross layers. Rolling shear (RS) properties in CLT are critical for the composite flexure of the layered panels. The applied out-of-plane loads cause a shear stress in the cross layers. The shear properties of cross-layers influence the overall bending and shear capacities of CLT panels because of the low shear modulus and strength in the rolling shear direction. Cross-laminated Timber (CLT) has the potential to beneficially use lower grade lumber for inner lamina for planar elements. Often these lower grades contain defects such as knots. The study utilized a modified planar shear test on 33 panels with 297 mm × 140 mm × 85.5 mm (length, width, and depth respectively) dimensions, as well as finite element analysis (FEA) simulations to determine the rolling shear properties of small CLT specimens fabricated from eastern white pine (Pinus strobus) consisting of cross layers with knots of different sizes. Testing and simulations compared the presence and size of knots to determine their effect on the rolling shear properties of the lamina. The test results showed a p-value of 0.002 confirming that the rolling shear strength and rolling shear modulus of the samples without any knots in the cross layer are significantly less than the ones with knots, with the differences of 24% and 38%, respectively while having larger and/or more knots in the cross layers can increase the rolling shear stiffness and ultimate strength. The FEM simulation results showed a strong confirmation to the experimental results.