Study On The Planar (Rolling) Shear And Bending Properties Of Cross-laminated Timber

Cross-laminated timber(CLT) is defined as a prefabricated solid engineered wood product made of at least three orthogonally adhesively bonded layers of solid sawn lumber. It has been recently gaining increased popularity in construction in Europe and North America because of its good dimensional stability and high in-plane stiffness and strength. However, CLT products are prone to the so-called planar(rolling) shear failure and excessive deflection when subjected to out-of-plane loading. This paper is mainly about the test and evaluation of the planar(rolling) shear properties of the solid sawn lumber and the laminated veneer lumber(LVL) and the materials modified by different means using a modified sandwich wood specimen. Besides, using hybrid CLT by putting Spruce-pine-fir(SPF) and LVL together instead of generic CLT, testing and evaluating the planar(rolling) shear, bending and shear properties of them. In this paper, the basic data of the planar(rolling) shear properties of poplar and LVL and its modified materials were obtained, and the structure of CLT was optimized, and the application of fast growing poplar in structural wood was expanded.The planar(rolling) shear properties of fast growing poplar with different modified treatments were studied in this paper. Set 3 groups of fast-growing poplar wood specimen: ordinary unmodified group and transverse compression group(compression rate of 28%) and impregnated with phenolic resin(PF) group(solid content is 20%), having a test and evaluation of the planar(rolling) shear properties and failure mode of fast-growing poplar with a modified sandwich wood specimen. The results indicated that: the planar(rolling) shear modulus of these 3 groups were 171.89 MPa, 102.69 MPa and172.32 MPa, and the planar(rolling) shear strength of them were 3.26 MPa, 3.30 MPa and 4.15 MPa. Transverse compression and impregnation modification treatment did not improve the planar(rolling) shear properties of fast-growing poplar. The planar(rolling) shear failure modes of poplar mainly included destruction in the early wood area of the boundary of earlywood and latewood, damage along the wood rays and adhesive failure.The effect of veneer impregnation on the planar(rolling) shear properties of the laminated veneer lumber was also studied in this paper. The test set up three groups of specimens: gelatinize LVL, 20% resin impregnated LVL(Type 20JL) and 30% resin impregnated LVL(Type 30JL), having a test and evaluation of the planar(rolling) shear properties and failure mode of them with a modified sandwich wood specimen. The results indicated that: the planar(rolling) shear modulus of these 3 groups were 61.78 MPa, 65.48 MPa and 103.91 MPa, and the planar(rolling) shear strength of them were 2.27 MPa, 1.02 MPa and 2.14 MPa. The planar(rolling) shear modulus of 30% resin impregnated LVL(Type 30JL) was greatly improved, and the improvement rate was 68.2%. The planar(rolling) shear failure modes of LVL were relatively single, the middle part firstly appeared cracks, then subsequent to the adhesive layer and spread throughout the direction of specimen length.Hybrid cross-laminated timber(HCLT) was fabricated using lumber(Spruce-pine-fir, SPF) and laminated veneer lumber(LVL), the mechanical performances of which were evaluated. Three types of CLT panels were fabricated(Type A: SPF-SPF-SPF, Type B:SPF-LVL-SPF, Type C:LVL-SPF-LVL). The failure modes of each kind of HCLT were visually examined and recorded. The properties measured included the bending properties(in the major direction) and planar shear properties(in both major and minor directions). The results indicated that: the modulus of elasticity(MOE) of 3 groups HCLT were 7908 MPa, 7222 MPa and 9299 MPa. The bending strength in the major direction of them were 28.6MPa, 25.7MPa and 30.1MPa.The shear strength parallel to the major direction of them were 2.25 MPa, 2.11 MPa and 2.40 MPa, and the shear strength perpendicular to the major direction of them were 1.05 MPa, 1.17 MPa and 1.17 MPa. It was found that 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. 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.At last, the paper summarized and compared the planar(rolling) shear properties of different materials, and the failure mode of the planar(rolling) shear of the SPF was observed by microscope. The comparison results showed that hardwood poplar was the best material for the planar(rolling) shear properties of the materials in this article. SPF and LVL were decreased in turn, indicating that the fast growing poplar can be used as the horizontal layer of CLT. Failure forms and microscopic observations showed that: early wood area of the demarcation of earlywood and latewood and wood ray direction was the two main areas where shear failure often occurred in SPF materials.