Seismic Responses Analysis Of Steel Framed Structures Subjected To Near-fault Ground Motions

Near-fault earthquake has been paid more and more attention by seismologists all over the world since some ruinous earthquakes broke out in last decade of last century.Recently,the research in near-fault ground motions has developed a lot.However ,as the characteristics of near-fault ground motions is so complex and so many factors affect it,much more research should be needed in order to understand deeply the characteristics of near-fault ground motions and seismic responses of structures under near-fault ground motions.In this thesis,the historic and current researches are reviewed and summarized,including characteristics of near-fault ground motions,responses of structures subjected to near-fault ground motions and so on.It is found that the research in attenuation relationship of particular factors of near-fault ground motions is so little. During the process of investigating seismic response under near-fault ground motions, mostly reinforced concrete models have been selected domestically,and the research on steel structures is not enough.Furthermore,there is still no answer to whether the affecting levels of the structures with different heights by near-fault ground motions are same. As we know,there are many factors affecting the performance of structures under near-fault ground motions.But,how to find out new factors which can describe the performance of structures under near-fault ground motions still needs a lot of work.Firstly,after collecting a number of near-fault earthquake records,the ratios of vertical PGA(peak value of ground acceleration) to horizontal PGA for each record are summarized.It is revealed that some values have been higher than 2/3.Then,the factors affecting this ratio are discussed furtherly,including site classification and fault distance. It is illustrated that when site classification is similar,this ratio becomes lower and lower as fault distance increases;and when fault distance is similar,the value becomes lower and lower as site classification transforms from soft soil to rigid soil.In order to investigate the attenuation relationship of PGA,PGV,PGD of near-fault ground motions,729 typical horizontal records are selected and classified according to M_W,site classification and fault distance respectively.It is concluded that the higher M_W is,the more obviously horizontal PGA,PGV,PGD changes along fault distance.The conclusion also illustrates horizontal PGA,PGV,PGD becomes higher and higher as site classification transforms from A to D,and these relationships are a little different under different site classifications.On the basis of this research,the definition of near-fault within 15 km is confirmed.Secondly, 19 near-fault earthquake records with typical velocity pulse and 5 general earthquake records are selected,and the responses of three Benchmark steel frame models which can stand for low,medium,high buildings respectively are calculated by nonlinear time history analysis.It shows that responses of these three models subjected to near-fault ground motions are all higher than those subjected to general ground motions.Under near-fault ground motions,the value of maximum displacement difference between two close floors changes more prominently along the height than that under general records,and for high buildings,this value becomes lower and lower as height becomes bigger and bigger,no such relationship for low and medium buildings.Compared with low and medium buildings,the displacement of top floor in the high building is higher,while there is no such relationship in maximum fundus shear force and the value of maximum displacement difference between two close floors.In the end, after 21 typical near-fault earthquake records are selected and the 9-floor Benchmark steel frame model is chosen as the typical structure with multiple degrees of freedom,the seismic responses of this model subjected to near-fault ground motions are obtained.Then,the relationship between the responses and some affecting factors are discussed furtherly,including PGA,PGV/PGA,site classification,fault distance and the ratio of period of velocity pulse to basic period of the structure.It indicates that it is better to use PGA and PGV/PGA at the same time while selecting them to measure responses of the structure under near-fault ground motions.When PGA is similar, the higher PGV/PGA is,the bigger the responses are.When PGV/PGA is similar,the higher PGA is,the bigger the responses are.The responses of the structure under near-fault ground motions are so affected by site classifications that site should be classified during the process of earthquake resistant design,but there is no need to classify the fault distance once the area of near-fault is fixed.The period of velocity pulse can be selected to estimate whether the structure has resonated or not. When this period is close to the basic period of structure,the responses are so high. Furthermore, when the ratio of the period of velocity pulse to the basic period of structure is higher than 1 ,the responses are higher than that under the condition of the ratio lower than 1.