Shake Table Tests And Methods Of Seismic Damage Assessment On Ancient Masonry Pagoda

As for ancient buildings in China,masonry pagoda occupies a considerable proportion.They are not only the witnesses of Chinese thousands of years of history,but also the typical representative of ancient high-rise buildings and the embodiment of ancient architectural level.However,after years of natural disasters and wars,some of the ancient tower has collapsed,and others,which are still standing,also have the issues of the material strength reduction,part of the wall damage,tower tilt and etc...It is hardly to withstand the earthquake again.Thus the seismic damage assessment is urgently needed,and disaster prevention and protection measures should be adopted accordingly to ensure the seismic reliability of the existing pagoda.At present,there are few studies on damage assessment and simulated shake table tests of this kind of structure in China,and the corresponding protection theory and implement method are still in the initial stage.In this paper,based on the field survey data of typical masonry pagoda,the Xi'an Small Goose Pagoda,the seismic damage assessment theory and its engineering application of the masonry pagoda structure are studied by simulating shake table test.(1)Based on the historical references,the material composition and the main damage characteristics of the Small Goose Pagoda were analyzed by the investigation of both the current damage status and historical repair events.The results show that although the overall performance of the pagoda is intact,but there are some issues including decline of mechanical properties of the main material,the key parts of the wall outside weathering off,dense cracks distributed in the body part,and loess exposed in the internal walls.All these issues indicate that certain damage existed in the pagoda.Also,according to the field tests of this pagoda,its main dynamic characteristic was obtained,i.e.the basic period is 0.74 s and damping ratio is 0.902.(2)The model of Small Goose Pagoda,scaled 1/10 and made of ancient black brick and sticky rice mortar,was designed and made on the basis of filed survey and historical references.Then the shake table tests were carried out with various imputing of ground motions.The test observations and results were analyzed,and the damage distribution was also studied consequently.The results reveal that there was some minor cracks appeared at the bottom of the model pagoda and vicinity of the upper holes under the minor earthquake of seismic fortification level of eight;the existing cracks widened and new cracks appeared under the frequently earthquake of same level,also apparently shaking of upper pagoda can be detected,illustrating the severe damage of the structure.(3)Due to the particularity of the ancient pagoda structure and the complexity of the damage study,it is difficult to describe the damage of the ancient tower structure under the action of the earthquake.Consequently,in this paper,the damage identification method of the identifiable parameters,which has been proved to have certain accuracy,can be used to judge the injury of every single layer of the pagoda through the analysis of the three levels of damage theory based on the material,the component and the structure.The data of shake table tests show that the damage was first generated at the bottom of the model structure under the action of frequently earthquakes.With the increase of the seismic intensity,the damage development at the bottom and the top of the structure was more obvious.In addition,according to the damage value of each layer from the test results,the corresponding structural damage value was obtained by using the weighting coefficient method.This value was compared with the result,which was attained by the overall method of structural damage.The comparison shows good agreement,indicating that the damage identification method is precise enough,which can be used for damage assessment of studied structures.(4)The finite element software ANSYS was adopted to model and analyze the prototype structure of Small Goose Pagoda.The results of modal analysis show that the main modal parameters are very close to the measured values,indicating that the finite element modeling method can be used to simulate the damage of the prototype structure.According to the experimental results and the similarity theory,the finite element analysis model of the prototype structure was established.Model analysis was carried out for the purpose of comparing results.The comparison shows that the damage identification method is close to the overall method,which means analytical results are reliable.(5)Dynamic time analysis of the prototype structure of the small goose pagoda was carried out with the ground motions of EL-Centro wave and Taft wave.Based on the main test results of the model structure,the damage degree of the prototype structure under the earthquake was evaluated by the maximum inter-story drift as the division.The results show that the prototype structure is in a state of slight damage under minor earthquake of the seismic fortification level of eight,and it is in the severe damage state under the frequently earthquake and nearly collapse under the rare earthquake.Consequently,it is urgent to take effective measures to protect and prevent the earthquake in order to make sure the seismic reliability of Small Goose Pagoda Structure.