Mechanical Performance Of Glulam Beam-column Moment-resisting Connection With Self-tapping Screws

Beam-column connections with moment resistance and rotation stiffness can effectively improve the mechanical performance of glulam frame structure,including but not limited to: optimizing moment distribution,improving the working performance of the structure in serviceability limit state and improving the lateral stability of the whole structure.With the development of mechanical fasteners in timber structure,self-tapping screws or threaded rods with sufficient lengths and optimized threads,applied as the fastener capable of load transfer along the fastener axis,provide a promising technical solution to achieve the beam-column connections with moment resistance and rotation stiffness.The methods of mechanical analysis,numerical simulation and full-scale test are adopted to carry out a systematic research on the glulam beam-column moment-resisting connections with self-tapping screws as fasteners in this work.The main research contents and conclusions are as follows.In order to take full advantage of the fasteners in axial load transfer,the withdrawal failure of self-tapping screws in wood should be avoided.Different from existing calculation methods based on the classic Volkersen theory,a type of―assembly unit‖ which contains threads is first introduced in a new model to research the withdrawal failure mechanism of self-tapping screws in glulam and predict the anchorage length of self-tapping screws in glulam.The model redefines the withdrawal failure surface according to the typical withdrawal failure phenomenon,considers the effect of the thread on the wood failure region,and reflects the local stress of the wood and the discontinuous transfer of shear stress/shear force on the withdrawal failure surface in the aspects of model simplification,formula derivation and algorithm design.When the theoretical model is verified by two groups of experimental investigation respectively for self-tapping screws and threaded rods,it is found that the prediction results of the current model can be in good agreement with the test results.The prediction results of the mechanical model also show that the withdrawal failure load of the self-tapping screw nearly has a linear relationship with the embedment length before the tensile failure of the screw occurs.With the aid of finite element program ABAQUS/Explicit,a numerical model for withdrawal failure load of self-tapping screw embedded in wood is constructed.The model sets the contact analysis between the thread and the engaging wood.Combined with the theory of fracture mechanics,the cohesive zone model(CZM)based on the―traction-separation‖ damage law is adopted to simulate the crack propagation of wood on the withdrawal failure surface.The numerical model focuses on the algorithm and related parameters in the contact analysis,as well as the material property parameters of the cohesive element.For the parameters without reference value,the numerical model conducts trial calculations under various working conditions.The simulation results of the numerical model basically coincide with experimental investigation,and also show that the friction coefficient between the screw and the wood in the contact analysis has little effect on the withdrawal failure load.On the premise that the critical strain energy release rate of the two fracture modes and the tensile strength perpendicular to grain/ shear strength parallel to grain of wood remain unchanged,the interfacial hardening coefficient of cohesive element will significantly influence on the withdrawal stiffness but negligibly on the withdrawal failure load.A series of static monotonic loading and low-frequency cyclic loading test are conducted on full-scale beam-column connections expected to fail in the mode of screw fracture.A total of 6 beam-column connections divided into 3 groups according to the angle between the screw axis and the beam grain(namely screw inclination: 0°,15°,30°)are carried out in static monotonic loading;and based on more test parameters including the screw inclination(0°,15°,30°),the screw number(4,6,8),the screw arrangement,the cross-section width(225mm,250mm)and the screws’ edge distance parallel to the direction of beam height(20mm,120mm),a total of 19 beam-column connections divided into 8 groups are designed to perform low-frequency cyclic loading test,in order to comprehensively investigate the mechanical performance of beam-column connections including moment resistance,rotation stiffness,deformability,hysteretic characteristic,stiffness degradation,energy dissipation.The experimental investigations show that the ultimate moment of all connections is in the range of 36.46 k Nm～92.14 k Nm.The influences of the screw number and the edge distance on the moment resistance are relatively obvious,and the influences of the screw inclination and the cross-section width can be basically ignored.The elastic stiffness of all connections calculated according to the equivalent energy elastic-plastic(EEEP)curve is in the range of 61.63 k Nm/°～124.93 k Nm/°.The influences of the screw inclination and the edge distance on the rotation stiffness are relatively obvious.There exists ―group effect‖ in the contribution of the increased screw number to the rotation stiffness of connections.The ductility ratio of all connections is in the range of 1.84～2.98,which implies a type of connection with low-ductility.The secant stiffness of connections in cyclic loading degenerates as the number of the cumulative cycles increases.The equivalent viscous damping ratio of connections in the first 11 cycles is small,and under every level of cyclic loading,the equivalent viscous damping ratio of the first cycle is always greater than that of the second and third cycles.Beginning with the 12 th cycle(corresponding to the displacement amplitude of 60% ultimate rotation),the connections enter a main stage of energy dissipation,reflected by the rapid growth of the equivalent viscous damping ratio.However,the energy dissipation capacity of connections is not so remarkble.The glulam beam-column moment-resisting connections with self-tapping screws as fasteners,in short,feature with the high moment resistance and rotation stiffness,but not very considerable deformation capacity and energy dissipation capacity.