Study On Mechanical Properties Of Cross Laminated Engineered Wood

Cross laminated engineered wood is a new type of engineered wood product,which can bear in-plane and out-of-plane loads,and can be used as floors and walls in middle and high-rise timber structures.The adjacent laminates are perpendicular to each other,and the laminates can be connected together by adhesive,nails or wooden dowels.When the adhesive is used,it is called Cross laminated timber（CLT）,when the nails or wooden dowels are used,it is called Adhesive-Free cross laminated timber（AFCLT）.CLT bending members often fail prematurely due to poor rolling shear performance.In order to improve the bending property of CLT,this study is focused on the rolling shear performance of CLT with different materials as cross layer.The rolling shear performance of8 groups of CLT specimens was tested using a modified shear test method.The outer layer material of all specimens is Spruce-Pine-Fir（SPF）,and the middle layer materials are SPF,red birch,compressed wood,glued laminated bamboo,Parallel strand lumber（PSL）,Laminated veneer lumber（LVL）,Oriented strand board（OSB）and plywood.Compared with SPF,glued laminated bamboo has higher rolling shear strength,compressed wood has good ductility,and other materials do not show better rolling shear performance than SPF.In order to reduce the use of adhesives,the develop of adhesive-free engineered wood products is one of the research focus in timber structures.Dowelled cross laminated timber is easier to cut than nailed cross laminated timber and can avoid thermal bridges.Therefore,in this paper the four-point bending tests were adopted to study the influence of the arrangement of compressed wood dowels and the number of dowels on the effective stiffness of the dowelled cross laminated timber,and the CLT with the same size were also tested as a contrast.The stiffness of dowelled cross laminated timber is lower than that of CLT,and the arrangement of compressed wood dowels has no obvious effect on its bearing capacity and stiffness,but the increase of compressed wood dowels can significantly improve its bearing capacity and stiffness.Three theoretical calculation methods on bending stiffness of CLT（shear analogy method,K method and Gamma method）were reviewed.The effective bending stiffness of CLT was calculated using three methods respectively and compared with the experimental results.The maximum normal stress and rolling shear stress at the time of CLT failure were calculated using the shear analogy method.The outer laminate firstly reaches its tensile strength,and the specimen undergoes bending failure.At this time,the calculated rolling shear stress has not yet reached its rolling shear strength.Therefore,it can be inferred that the CLT firstly undergoes bending failure,and then undergoes rolling shear failure.It is consistent with the phenomenon observed in the experiment.In addition,the effective bending stiffness of the dowelled cross laminated timber was calculated using the Gamma method,and then it was compared with the experimental value.Using the stiffness of timber joints with compressed wood dowel obtained from the push-out test,the Gamma method can accurately predict the effective bending stiffness of dowelled cross laminated timber.