Duration Of Ground Motion Based On NGA-West2 Strong Earthquake Data

Ground motion input is of great significance in the research of of engineering seismic fortification.Due to the limited number of natural earthquakes,especially near-field ground motion records,it often contains complex site effects,which is not suitable for direct use in other engineering sites.Therefore,synthetic ground motion is an effective method to provide seismic input time history.In synthetic ground motion time history,it is often focused on how to meet spectrum characteristics and amplitude,while the selection of duration is relatively simple.In fact,the duration has a great influence on the nonlinear response of the structure,but compared with the amplitude and frequency spectrum characteristics,the research on the duration of ground motion is still less.Based on the NGA-West2 strong earthquake database of the Pacific Earthquake Engineering Research Center and the ground motion records of several earthquakes with magnitude greater than 7.0 in recent years,the main work is as follows:(1)According to the definition of ground motion duration,the characteristics and shortcomings of bracketed duration,uniform duration and significant duration are analyzed and discussed:the value of bracket duration is greatly influenced by the threshold and may exaggerate the description of duration;uniform duration cannot describe the continuous period;When the energy accumulation is too fast,acceleration value at the beginning and end of the time history may be ignored for significant duration,then a smaller duration value may be obtained.(2)The main influencing factors of ground motion duration,and the correlation between significant duration and bracket duration with magnitude and distance were discussed.As the magnitude increases,ground motion duration will increase;bracket and uniform duration were negatively correlated with the distance,but the significant duration was positively correlated with distance.Site conditions can significantly affect the duration of ground motions;the value of significant duration is affected by the shape of acceleration time history,and the significant duration of two horizontal components of the same ground motion record may be quite different.Significant duration was significantly positively correlated with distance,but not significantly correlated with magnitude.The value of bracket duration obviously increases with the increase of magnitude and decreases with the increase of distance.(3)Based on the 77 earthquakes of PEER NGA-West2,a total of 2917 pairs of seismic data were subjected to multi parameter nonlinear regression analysis considering random effects,and the duration prediction equation was obtained.In addition,the ground motion records of Wenchuan earthquake(M_w=7.9)and Fukushima earthquake(M_w=9.0)in Japan are supplemented,and regression analysis was carried out again to modify the prediction equation,and the change of the corrected time-dependent prediction equation is discussed.Based on the duration prediction equation obtained by regression,the prediction results of significant duration and bracket duration are compared:in the nearer distance range,the predicted value of bracket duration is greater than that of significant duration,especially for large earthquakes.(4)Referring to the research on the envelope function of ground motion,the duration values of the prediction equation and the strong earthquake stable segment were compared,and the definition and value of duration were discussed.It was found by comparison that in the near-fields of large earthquakes and great earthquakes,bracket duration(0.01g and0.05g)are greater than those of strong earthquake section duration(t_s).