Analysis On Characteristics Of Earthquake Response And Influence Of High-Order Vibration Modes For Long-Period Structrue

The long-period ground motion has the characteristics of relative low peak acceleration, relativel high peak velocity and peak displacement, long duration and variety of long-period ingredient. Historical earthquake disaster investigations show that larger and severer damage occurred when the long-period structures were hit by long-period ground motions than general ground motions. With the accelerated process of urbanization, more and more super high-rise with long vibration period have been constructed and put into operation. It is of great significance to research the seismic response characteristics of long-period structures under long-period ground motions. Based on the time-frequency characteristics of long-period ground motions and the seismic response of long-period structures, the main contents in this paper as follows:(1) Based on the ground motions of the Mw9.0 Tohoku Japan Earthquake, through analyzing the time-frequency characteristics of the far-field ground motions and the influence of epicentral distance for the time-frequency parameters of the far-field ground motions, the study on long-period characteristics of the far-field ground motions was carried out. The analysis results show that with the increase of spreading distance, the effective duration of ground motions does not deteriorate but indicates an relative increasing trend; The predominant period Tp and mean period Tm all indicate an increasing trend. Compared to the quike decline of the peak acceleration PGA, the peak displacement PGD deteriorates relatively slowly. The results prove that, the far-field ground motions under large magnitude earthquake have such characteristics as long duration, low acceleration and relative large displacement, showing obvious long-period characteristics. Based on the fourier-spectrum, ? is taken as the boundary parameter between long-period ground motion and general ground motions. The influence of magnitude, epicentral distance and site condition for long-period ground motion was studied through a large number of long-period ground motions chosen by the parameter ?.(2) A comparative study was made to research the seismic responses of a frame-core tube structural model under the general ground motions and long-period ground motions in the use of finite element analysis software ETABS and time-history analysis method. The results prove that, the horizontal displacement, story-drift, base shear force and overturning moment of the model were bigger under the action of long-period ground motions than general ground motions. In addition, the calculate results with the mean of response spectrum method are also smaller than that. Under long-period ground motion, the most severe responses of long-period structure usually appear at the middle or end of the ground motion. The structure show severe seismic responses in most of the acceleration time history, which is harmful for the earthquake-resistant of structrues.(3) Based on the research of the influence of all modes for base shear force of long-period structure, the result shows that, the contribution to base shear force of vibration mode has close relationship with the product of the modal mass participating coefficient r and the amplitude of the design response spectrum amplitude a. A method of engineering ground motion selection considering the influence of high-modes is proposed in this paper, and the effectiveness of which is verified in the case of a long-period structure model by ETABS program through the comparison of the time-procedure analysis and the response spectrum analysis.(4) Based on the vibration mode decomposition analysis method, the seismic response of the long-period structure with different modal combinations was calculated to research the influence of high-order for long-period structure. It was shown that, high-order modes of long-period structure affect the internal force responses more than displacement responses, and the influence of high-order modes has close relationship with the dynamic characteristics of the strcture and the frequency-spectrum characteristics of the ground motions.