Research On Mechanical Properties Of Third-grade Timber Frame With Cushion

The column foot of the ancient wooden structure is a common failure node of the wooden structure.The high stress environment and biochemical erosion for a long time will cause the complicated damage at the foot of the column.In order to simulate this phenomenon,a three-class timber frame is built,and a cushion layer with variable thickness and elastic modulus is set at the foot of the column.With reference to the existing domestic and foreign seismic design regulations,simulation analysis of quasi-static loading and dynamic loading is carried out in Abaqus software for wooden frame models with cushions under different conditions.The quasi-static analysis adopts the displacement loading method,and the loading curves can be divided into two types: the loading curve with the linear increase of the peak displacement of each cyclic load and the CUREE loading curve.The overall deformation,P-△curve,skeleton curve,and stiffness degradation curve changes of the wooden structure with cushion under the first loading condition are analyzed,and the viewpoint that the ratio of the initial stiffness and the stiffness at the maximum displacement is used as the ductility coefficient of the wooden frame is proposed,and the influence of the cushion of the column on the overall ductility of the structure is analyzed.The results show that the proportional limit of the positive and negative loads decreases with the attenuation of the elastic modulus of the column bottom cushion in the elastic deformation stage of the structure,which indicates that the plasticity of the column foot will reduce the ultimate lateral resistance of the wooden structure in the elastic deformation stage.When the structure enters the sliding phase,the decrease in the elastic modulus of the cushion will slightly increase the lateral bearing capacity of the structure at the maximum displacement,but it will also significantly reduce the initial lateral stiffness and static ductility of the frame.This paper calculates the strength degradation coefficient of the frame with cushion under CUREE loading,and compares the influence of the cushion structure on the overall strength degradation of the frame.It is found that with the increase of the number of loading cycles,the lateral resistance of the structure increases first and then decreases.The smaller the elastic modulus is,the less obvious the strengthening section of the lateral resistance of the cushion structure is.The method of sinusoidal acceleration excitation is used in the dynamic analysis.This paper proposes a calculation index for evaluating the input energy of a single period of sinusoidal excitation.Based on the π theorem,the related physical quantities that affect the dynamic response of the timber frame are analyzed.The natural vibration frequency of the frame under different dynamic input,vertical load and cushion structure is calculated.It is found that only when the cushion is thick enough and the elastic modulus attenuation exceeds50%,the natural frequency of the frame will decrease.Finally,this article attempts to explain the complex sensitivity of the dynamic response of the wooden frame to different input conditions through nonlinear dynamics,and draws a phase diagram of the free vibration section of the structure to show the state of the structure under natural vibration.It is found that the attenuation of the elastic modulus of the cushion,the increase of the thickness of the cushion and the increase of the mass of the column top will accelerate the energy dissipation of the structure under the condition of natural vibration.