The Correlation Analysis Of Ground Motion Intensity Index Based On AMD And The Nonlinear Responses Of RC Frames

Selecting earthquake input reasonably is one of the greatest difficulties of promoting the time-history analysis method. After years of development some good controlling indexes and methods for choosing earthquake waves are proposed, but analysis results of structures may have large differences sometimes. Researchers have not reached a consensus on parameters of earthquake ground motion, so it is of gteat significance to research the intensity index of ground motion which is used to express the effect of earthquake ground motion on structures.Analytical Mode Decomposition(AMD) is a new method of time-frequency analysis proposed recently, which is used to separate the component of high frequency and low frequency conveniently and accurately. The theory of AMD is firstly applied to the research for seismic characteristic parameters in this paper, which is the main innovation point of the research. The controlling component of earthquake ground motion for the nonlinear response of structures is extracted by AMD and the ground motion intensity indexes are built. Through the correlation analysis between intensity indexes and nonlinear responses of reinforced concrete frames, the fact that the proposed intensity indexes can reflect effects of ground motion on the structural nonlinear response exactly is verified. The major works of this paper are listed as follows:(1) 60 earthquake waves are selected according to certain principles, which are divided into two groups. Four frames of different periods and seismic fortification intensity of 8 and 9 degree are designed according to requirements of design codes. And OpenSees models of the frames are built to calculate the nonlinear responses under the action of these earthquake waves.(2) The low frequency ground motion component below the fundamental frequency of frames, which is called S1, is extracted through AMD, and the nonlinear responses of frames under the action of S1 are calculated to compare with the results under the action of earthquake waves. Through the comparison, the conclusion is proposed that for the structures of relatively short period and mainly first mode, the controlling component of ground motion for its nonlinear response is the low frequency component below the fundamental frequency of structures.(3) As the improvement of S1, the low frequency wave S is proposed by means of considering some high frequency component of ground motion by the weight coefficients, which are the ratio of the structural modal participation factors, based on the low frequency component below the fundamental frequency of structures. The comparison between the nonlinear responses of frames under the action of S and earthquake waves shows that S is the controlling component of ground motion for its nonlinear response of the structures which are influenced greatly by high-order vibration modes.(4) The maximum peak value of S1 and S are defined as characteristic peak one and characteristic peak two. The correlation analysis between characteristic peaks and the maximum nonlinear responses of four reinforced concrete frames shows that both of them are almost simultaneous, and have a remarkable linear positive correlation. And the effect of characteristic peak two is better than characteristic peak one.(5) Another 30 earthquake waves are chose for the 4 frames, and the analysis results are the same as the conclusions above. All these indicate that the characteristic peak one and characteristic peak two proposed in this paper both can reflect the influence of earthquake ground motion on nonlinear responses of frames to a certain extent and be the intensity index when selecting earthquake input reasonably. And in comparison, characteristic peak two is a better choice.