Investigation Of The Structural Behaviour And The Design Calculation Methods Of Cross-Laminated Timber Structures

The multi-storey and high-rise timber structures using cross-laminated timber(CLT)as roofs,floors and shearwalls are called cross-laminated timber structures.Research of CLT and the structures can be traced back to the 1990 s.CLT as an engineered wood product is now covered in many national standards,including Standard for Design of Timber Structures GB 50005—2017.The build-ups of CLT can be different,the grade,geometric dimension and number of layers are veriable,so it is more complicated to calculate the load-carrying capability and stiffness compared with homogeneous material.In order to investigate the structural behaviour and calculation methods of the load-carrying capability of CLT members,the doweltype connections and CLT structures,a series of tests were conducted,including those of hemlock CLT members in flatwise bending,CLT members in axial compression,bolted and screwed connections of CLT and CLT shearwalls.Equations for calculating the load-carrying capability of CLT members and connections of CLT using the mechanical properties of the laminations were proposed;kinematic models and methods for calculating the lateral load-carrying capability of CLT shearwalls were developed based on the structural behaviour of the CLT connections;pushover analysis was also conducted on CLT structures.The main content of this research are as follow.Experimental study of hemlock CLT members in flatwise bending was conducted,and the failure modes,load-carrying capability and bending stiffness of the members were obtained.Rolling shear strength and rolling shear modulus of the transeverse laminations were measured.Finite element analysis of CLT bending members showed that increasing the shear span-to-depth ratio of the members and the width-tothickness ratio of the transverse laminations can avoid rolling shear failure,thus increase the load-carrying capability and stiffness of flatwise bending members;increasing the shear span-to-depth ratio of the members and the number of laminations can increase the load-carrying capability and stiffness of edgewise bending members.Based on the mechaniacally jointed beam theory,the calculation methods of load-carrying capability and stiffness of CLT bending members with any number of laminations were derived.Buckling tests of hemlock CLT members in axial compression were carried out,the bucklng capacity and failure machanism were revealed.Elasto-plastic buckling occurred to members with small slenderness ratio,elastic buckling occurred to members with large slenderness ratio.Rolling shear failure of transverse laminations was observed on members with a medium valued slenderness ratio.The γ-method is suggested in calculating the effective stiffness and effective slenderness of CLT member.Equations for calculating the stability coefficient of CLT members in axial compression were proposed,in which the material parameters were determined by regression analysis.The correctness and feasibility of the proposed equations were verified with the test results.The lateral load-carrying capability of bolted timber-to-CLT connections and that of screwed steel-to-CLT connections were tested.The bolted connections exhibted better plasticity than the screwed connections,test results showed that the mechanical behaviour of CLT connections was signifcantly affected by the mechanical properties of dowels.The concept of equivalent embedment stress distribution was proposed,the equations for lateral load-carrying capability and yield mode of the bolted and screwed conenctions in CLT were derived based on the EYM.The test results verified the correctness of the proposed equations.Experimental work was conducted on single-panel,double-panel and triplepanel CLT shearwalls,the mechanical properties,failure modes and hysteretic curves of the specimens were obtained.The hysteristic model of the dowel-type connection was developed based on the test results,and it was input into the finite element models of CLT shearwalls using the user defined element(VUEL)in ABAQUS.Parameter analysis of the CLT shearwalls was conducted,the effects of the fastener types,the stiffness of hold-down connections,and the aspect ratio of shearwalls on the lateral performance of CLT shearwalls were obtained.Based on the test results and results of the FEA,kinematic models of CLT shearwall were proposed,and the calculation methods of the lower bound and upper bound of the lateral load-carrying capability of single-panel and multi-panel CLT shearwalls using load-carrying capability of connections were proposed.Finite element models of CLT structures were developed and were used to analyze the dynamic characteristics of CLT structures,and the vibration modes and natural periods of platform CLT structures were obtained through the FEA.Pushover analysis of CLT structures was conducted,which revealed the load-transfer paths and working mechanism of the CLT structures under lateral load.Seismic performance of the structures in different intensity levels of earthquakes,site groups and types were evaluated based on the improved capacity spectrum method.Suggestions for engineering seismic design of CLT structures were also provided.