The Finite Element Analysis Of The Hysteretic Behavior Of Mortise-tenon Joints And Frames In Ancient Timber Buildings

The ancient buildings have a high value of culture, history and society. But during the long history, the situation is worsening for them and can become critical any time, because of large deformation of some wood components, and even damage due to compression perpendicular to grain and bending, and property-deteriorations of wood due to aging. Therefore, it is of urgent need to evaluate the structural safety and to undertake repairing accordingly. The members in Chinese ancient timber buildings were mainly connected with mortise-tenon joints, which is one of the main features of these buildings. Under seismic action, the squeezed formation and friction between mortise and tenon could dissipate the energy. But the destruction of the joints usually causes serious damage such as tilt and even collapse of the structure, so the study of hysteretic behavior is significant for the seismic assessment and maintenance of ancient timber buildings.Based on the result of existing test, the typical one-way straight mortise-tenon joint, through mortise-tenon joint and frame connected with through mortise-tenon joint were selected as research objects in the paper. A finite element model(FEM) which can simulate the mechanical mechanism and hysteretic behavior of the objects had been built. Comparing to the result of test, the failure mode, moment-rotation hysteretic curves and skeleton curves(when the joint rotation is greater than 0.02rad) of the one-way straight mortise-tenon joint FEM agree well with the test results; the two indicators—the stiffness degradation and equivalent viscous damping coefficient are close to the test results, so that, the validity of the FEM is verified. Through the analysis of the contact stress and the contact state between the components of tenon and mortise under cyclic load, change law of them are found, and then the change law of the contact pressure and friction between components are get, which reveals the joint's mechanical mechanism that resisting force through the contact, friction and sliding between components. The hysteretic behavior of through mortise-tenon joint and frame, such as skeleton curve and hysteretic curve have been simulated, and good agreement between FEM and tests results is also obtained.Based on the verified model of the one-way straight mortise-tenon joints, effect laws of the factors on the hysteretic behavior of the joints, such as material properties, friction coefficient, and the extraction amount of tenon were analyzed. The result show that, with the increase of timber modulus of elasticity perpendicular to grain, the initial stiffness and the load-bearing capacity of joints is increased, while the energy dissipation capacity is improved. However, the longitudinal elastic modulus has little influence on the hysteretic behavior of the joints. The higher the friction coefficient is, the fuller hysteretic curve, and the bigger stiffness and bearing capacity which are increased slightly. When the tenon is extracted with different degrees, the bearing capacity is reduced obviously, and the stiffness and the deformation ability of joint decrease, and the situation become worse with the increasing extraction amount; but the energy dissipation capacity has little change.