Use of poplar (Populus euramericana cv. I-214) as cross layer to manufacture cross-laminated timber (CLT) was examined in this study. For comparison purpose, Douglas fir (Pseudotsuga menziesii) and Monterey pine (Pinus radiata D.Don) were used as well to produce five layups of CLT panels. The mechanical properties tested in this study included the bending strength in the major direction, modulus of elasticity in the major direction, shear strength parallel to the major direction and shear strength perpendicular to the major direction. It was found that the mechanical properties of CLT panels containing poplar were similar to those made of non-poplar wood. The major failure modes found were joint failure, shear failure and delamination. It could be feasible to use poplar as a cross layer to fabricate CLT without decreasing its strength properties.
Hybrid cross laminated timber (HCLT) was fabricated using lumber (Spruce-pine-fir,SPF) and laminated veneer lumber (LVL), the bending and shear performances of which were evaluated. Three types of CLT panels, one generic CLT (used as control) and two types HCLT, were fabricated. The failure modes of CLT and HCLT were visually examined and recorded. The mechanical properties measured included the bending properties (in the major direction) and shear properties (in both major and minor directions). It was found that the planar shear failure of cross layer was the key and primary failure mode of CLT and HCLT under bending. Lumber and LVL had different direction of crack propagation in planar shear, and LVL had lower planar shear properties than SPF lumber. The weak zones in the radial-tangential (RT) section of wood including earlywood/laterwood boundary and wood ray were easy to occur shear failure. The HCLT having LVL as the outer layer had the highest bending and shear strengths. The modulus of elasticity (MOE) of the HCLT having LVL as the outer layer and lumber as the cross layer was 17.6% higher than that of generic CLT. However, the HCLT having lumber as the outer layer and LVL as the cross layer had the lowest mechanical properties.
Planar (rolling) shear properties in cross laminated timber (CLT) is an important factor that should be considered for CLT structural components with short span or openings. The planar shear properties of SPF (Spruce-pine-fir) dimension lumber and laminated veneer lumber (LVL) were measured using a modified approach in this study. The failure modes in planar shear tests were investigated by visual inspection and optical microscope. Hybrid CLT (HCLT) was fabricated using lumber and/or LVL, and its bending properties (in major strength direction) were evaluated. It was found that SPF had higher planar shear strength and modulus than LVL. The failure mode of HCLT with LVL as cross layer in planar shear test was shear failure along the glue line direction, however, the typical failure modes of HCLT with SPF as cross layer in planar shear test included rolling shear failure along the growth ring direction, shear failure along the wood ray direction and shear failure along glue line direction. The bending properties of the generic CLT had been improved by using LVL as parallel layers and been reduced by using LVL as cross layer. The planar shear failure of cross layer in the zone between loading point and support point was the typical failure mode of CLT and HCLT in bending test.