Influence of technical characteristics on the rolling shear properties of cross laminated timber by modified planar shear tests

Rolling shear properties of cross laminated timber are important mechanical properties for its structural application. To evaluate the influence of technical characteristics such as edge-gluing and gap size in the cross layers, on the measurement of rolling shear modulus and strength of cross laminated timber, three-layer spruce-pine-fire cross laminated timber shear block specimens with and without edge-gluing, with gaps of 2 mm, 4 mm and 6 mm, were tested by a modified planar shear test method. The mean values of rolling shear strength and modulus of No. 2 visual grade spruce-pine-fire cross laminated timber were 1,32 MPa and 111 MPa with coefficients of variance of 20% and 28%, respectively, regardless of technical characteristics. The characteristic rolling shear strength of all groups of three-layer cross laminated timber specimens was determined to be 0.88 MPa. The results indicated that the rolling shear strength and modulus values used in current design practice of spruce-pine-fire cross laminated timber were conservative. It was found that edge-gluing and gap size had a significant influence on measuring rolling shear strength rather than apparent rolling shear modulus by the modified planar shear test method. With the gap size larger than 2 mm, its influence on measuring rolling shear strength became negligible. The three major initial failure modes identified for the cross layer regardless of technical characteristics were rolling shear failure along the growth ring, tension perpendicular to grain failure in wood pith and tension perpendicular to grain failure along the wood ray.