Study On Stiffness Damage Test Of Plate Bearing Under Reciprocal Load And Simulation Of Damage Form Of Grid Structure Under Disaster Earthquake

The construction of the future city requires that the gymnasium,railway station,bus station and exhibition hall of the grid structure should not only meet the function of daily public activities,but also provide a safe emergency shelter for the victims when natural disaster earthquakes come.As an important part of connecting the upper grid frame and the lower support system,the bearing force of the grid frame support is very complicated when the earthquake occurs.The earthquake damage investigation of the previous disaster earthquakes found that some of the supports of the grid structure were damaged to varying degrees,and some even caused the collapse of the grid structure,which directly hindered the emergency rescue function of the building of the grid structure for the shelter.Therefore,it is of great significance to study the stress process of grid support under strong earthquake,considering the influence of stiffness degradation of support node and damage of bar on the damage form of grid structure,to improve the safety of public buildings and to build a resilient city.This article is part of the research content of the National Natural Science Foundation of China "Research on Ultra-low Cycle Fatigue Performance and Evaluation Method of Bolt Ball Grid Structure under Strong Earthquakes".The main work of this article is as follows:(1)According to the existing loading equipment in the laboratory,a two-way loading device was designed and the ultra-low cycle fatigue test of the plate bearing under constant vertical loading and horizontal reciprocating cyclic loading was carried out.This paper focuses on the analysis of bearing slip,rotation around axis,bending of anchor bolt,crack and propagation of anchor bolt during loading,as well as the study of the characteristics of anchor bolt section,stiffness change of support and bearing capacity degradation.According to the results,the hysteretic curves of the bearing specimens are Z shape and poor full,and the overall energy dissipation capacity is general.The stiffness variation of the bearing specimens is as follows: non-sliding "stepwise" descent-smooth decline in the slip phase-the anchor bolt contact hole wall slowly increases-the anchor bolt cracking rigidity drops sharply.After the slippage,the strengthening effect of the bearing capacity in the early stage of thespecimen is greater than the damage effect.With the increase of the number of cycles,the damage accumulation of the anchor bolt makes the overall bearing capacity damage effect greater than the strengthening effect.The bearing capacity of the specimen decreases after reaching the peak point until the anchor broke.(2)Taking Lushan Middle School Gymnasium damaged in Ya’an earthquake as an example,the dynamic elastic-plastic time history analysis method is adopted by SAP2000 software to simulate its failure form,and the analysis method of grid structure failure form under earthquake action is discussed.The actual seismic record and unit plastic hinge model of Ya’an earthquake are used in the analysis,and the support model refers to the results of this paper.Under the action of strong seismic waves,both the rod and the support will repeatedly enter the elastic and plastic state,resulting in continuous damage accumulation and a serious impact on its damage form.In view of the fact that there is no mature plastic hinge damage model,this paper approximates the impact of plastic damage by changing the support stiffness and plastic hinge stiffness reduction coefficients,and macroscopically approximates the damage form in earthquakes.Through the comparative analysis of the simulation results and the actual damage form,the shortage of the calculation model is discussed,and the reasons for the difference between the two are analyzed to lay the foundation for the subsequent improvement research of the calculation model.The results of the study show that the failure of the support during the earthquake caused the release of horizontal constraints and the number of failures of the surrounding members decreased,while the failure of the members around the support adjacent to the failed support continued to increase.The support of the grid was considered under the earthquake Stiffness degradation has a significant effect on the formation of locally softened areas;considering the stiffness damage coefficient of the rod has a significant effect on the failure form of the grid,the grid begins to extend to the mid-span after the local failure,and the damage has continuity;comprehensive consideration of the support The stiffness degradation and the stiffness damage coefficient of the members are similar to the distribution of the failure members in the actual earthquake damage,and the failure form has a good similarity to the actual earthquake damage.The main reasons for the difference between the simulation results andthe actual earthquake damage are the failure to obtain the real ground motion and the directional relationship with the structure,the failure to consider the specific impact of the weld seam cracking on the stiffness,and the simulation process Failure to consider the effect of the end of the rod and the fracture of the connecting bolt will cause the simulation results to be different from the actual earthquake damage.(3)Taking Lushan Middle School Gymnasium as an example,this paper discusses the measures to improve the anti-collapse damage ability of flat-plate grid structure under disaster earthquake by changing the type of support and increasing the section of key members.SAP2000 element plastic hinge dynamic increment method is used for calculation and analysis.The results show that the critical failure bearing capacity of the grid is improved significantly by replacing it with rubber bearing and increasing the cross section of the key member.In addition,by taking shock-absorbing measures,the plasticity of the bar is further developed,and the ductility of the structure is also greatly improved.