The Seismic Performance Analysis Of Ancient Multi-layer Timber Structures Considering The Loose Straight Tenon Mortise Joints

The ancient Chinese timber architecture has formed its own architectural system in the ancient world because of its high level of artistic value,historical value and scientific value.However,due to the degradation of wood body,natural disasters and man-made damage factors,the existing ancient timber structures have various forms of damage phenomenon and mainly occur at the important joints,which makes the structure in a potential state of destruction.Therefore,it is of great significance to study the seismic behavior of damaged joints and timber structures with damaged joints,and to protect and repair them,combined with the experimental subject,the seismic behavior of the loose damaged straight tenon joint and the multi-layer timber structure with loose joints is analyzed preliminarily.Taking the Embrasured Watchtower of Yong Ning Gate of Xi'an City Wall as research object,establish four calculation and analysis models of timber frame with different loose damaged mortise tenon joints and conduct horizontal cyclic loading simulation tests.The failure patterns,hysteretic curves,skeleton curves,stiffness degradation laws,energy dissipation capacity of the intact joint F,the horizontal loosening joint S,the vertical loosening joint H and the rotating loosened joint R around the column are obtained through the simulation tests.The results show that the failure patterns of the loose and damaged joints are similar to the test model,mainly for the tenon,the joints are extruded seriously and the tenons are pulled out,the column body and the Fang body are basically in good condition.The hysteretic curves of all loose and damaged joints show obvious pinch effect of Z shape,and the stiffnesses of all joints under different loading levels are smaller than that of intact joints,in which R joint and intact joint have the least difference between H joint and R joint,and the degradation of H joint is significant;compared with F joint,H joint energy consumption decreased by 28%,S joint decreased by 16%,and R joint decreased by 7.5%.In order to study the effect of loose damaged joints on the seismic performance of the whole structure,a multi-scale modeling technique is used to simulate and analyze the seismic behavior of the whole structure.Considering the complexity of the materialization of all tenon joints and the cost of numerical calculation,the geometric distribution of the most adverse effects on the mechanical properties of multi-layer timber structures caused by local loose and damaged joints is determined by using the uniform defect mode method.First of all,a multi-scale model with no loose joints is established,modal analysis and dynamic time-history analysis are carried out.The results show that: the multi-scale is more consistent with the natural vibration characteristics of the local model,it can not only simulate the dynamic response of the whole structure with high precision,but also reflect the damage mechanism of the local key joints directly.However,with the increase of the intensity of the input ground motion,the precision of the model is reduced.On the basis of selecting loose form of loose damaged joints and parameters of ancient timber properties,a multi-layer timber structure model with loose damaged joints(SD model)is established,and its modal analysis and dynamic time history analysis are carried out.According to the time history result,the hysteretic curve of second story structure and the accumulated hysteretic energy consumption curves of each main component are obtained.The results show that the structural cycle is longer and the natural vibration frequency is shorter because of the degeneracy of wood and loose damaged joints;the loose joints increase the displacement response of the structure relative to the column foot,weaken the upper transmission of the larger amplitude seismic movement,and reduce the acceleration response of the second,third and fourth layers;the hysteretic curve of two-layer structure is "long flat",and the energy dissipation capacity between layers decreases;compared with the experimental model,the energy dissipation capacity of SD model is obviously weakened,the energy consumption ratio of column layer decreased and the total hysteretic energy consumption decreased 10.7~21.0 percent.