Damage Analysis Of Ancient Masonry Tower Subjected To Mainshock-Aftershock Sequences

The ancient masonry pagoda is an outstanding representative of ancient Chinese architectural art.It has the characteristics of large self-weight,low material strength,and complex tower structure,and easily damaged under the action of human or natural hazards.Actual seismic observation data show that aftershocks usually occur after a strong earthquake.The time intervals among the main shock and followed aftershocks are always very short,the pagoda damaged under the action of the main shock cannot be repaired in time.Thus,the ancient pagoda is "incremental damaged" after the main shock.In order to study the damage law of the masonry pagoda under the ground motion of the mainaftershock sequence,taking the Zhongjiang South Pagoda as the research object,combined with small shaking table test and numerical simulation method,this paper studies the variation of natural vibration frequency and damage factor of the masonry pagoda structure,the seismic damage evolution of the pagoda body,dynamic response characteristics of the structure,and the influence of aftershocks on the potential damage possibility of the damaged structure.The main work completed is as follows:（1）Determination of similarity relationship and making of similarity test model.Taking the Zhongjiang South Pagoda,located in Zhongjiang County,Sichuan Province,as the prototype,the geometric size and geometric similarity ratio of the test model are determined according to the size of the small shaking table and the load limit of the table.Combined with the physical and mechanical parameters of the material of the experimental model,the similarity ratio of elastic modulus and the similarity ratio of density are determined,and then the similarity ratio of acceleration is determined according to the similarity law.Finally the similarity ratio of the physical quantities required for the test are obtained.Design and make a test model,and obtain the natural vibration frequency of the test model through modal analysis,The results compared with the original structure show that the test model can reflect the dynamic characteristics of the original structure well.（2）Damage analysis of test model under main-aftershock sequence.The damage evolution,mode,and mechanism of the masonry pagoda under the main-aftershock are obtained.The variation characteristics of the structural dynamic response under the mainaftershocks are analyzed,and the causes are analyzed.The variation law of the first threeorder natural frequency and damage factor of the structure with PGA of the main shock and VPGA of the aftershock is studied.The results show that under the action of the mainaftershock sequence,the doors and windows of each floor of the pagoda are damaged,and horizontal cracks occur on the first and sixth floors.The reason is that the vibration response of the structure is dominated by the second-order and third-order vibration modes.In addition,for ancient pagodas with moderate damage and above,aftershocks with ▽PGA greater than 0.2 will cause further damage to damaged structures.（3）Damage analysis of finite element model under main-aftershock sequence.The material constitutive model is determined based on the material property test,and then the finite element model of the structure is established.The dynamic characteristics of the test model and the finite element model are compared.The damage and dynamic response of the structure under the main-aftershock sequence are studied and compared with the test results.The results show that the finite element model can better reflect the dynamic characteristics of the structure,and the structural damage and dynamic response are consistent with the test results.Combined with the results of test and numerical analysis results,suggestions for improving the performance of the Zhongjiang South pagoda are put forward.（4）Vulnerability analysis of the prototype structure of the Zhongjiang SouthPagoda.The finite element model of Zhongjiang South Pagoda is established,and the incremental dynamic analysis（IDA）is carried out when ▽PGA is 0.2 and 0.5 respectively.The structural vulnerability curves under the main-aftershock sequence and the main shock are obtained,and compared.The results show that when the PGA of the main shock is greater than 0.3g,the aftershocks with VPGA of 0.2 will increase the possibility of structural collapse;When the main shock PGA is greater than 0.15g,the aftershocks with VPGA of 0.5 will increase the possibility of serious damage to the structure.In addition,aftershocks with VPGA of 0.5 also significantly increase the probability of structural collapse.The above conclusions are consistent with the trend reflected by the small shaking table test results.