Study On Selection Of Ground Motion Records In Code For Seismic Design Of Buildings

With the continuous accumulation of ground motion records in China,based on the background of the popularization of structural elastoplastic time history analysis,domestic engineering designers pay more attention to the selection of ground motion records,and selecting appropriate and reasonable ground motion records has also become a research hotspot in the direction of engineering earthquake.Chinese seismic code GB50011-2010 has corresponding provisions and instructions for the selection of seismic records,however,there are some differences and shortcomings compared with the current foreign seismic codes,especially the ASCE series codes in the United States.In this dissertation,the similarities,differences and advantages of record selection between Chinese code and American code were compared,the selection of different ground motion parameters of horizontal ground motion was studied,the input mode of two-dimensional ground motion in time history analysis was analyzed,and the record selection of isolated structures was explored to a certain extent.Finally,the study of the selection of ground motion records in Chinese building seismic code was completed.The specific contents are as follows:(1)The differences in selection of records between Chinese code and American code were compared.Ground motions of National Strong-Motion Obersvation Network System from 2007 to 2015 and NGA-West1 of Pacific Earthquake Engineering Research were used,the primary conditions were obtained according to attenuation relationship in Chinese fifth generation zoning map,magnitude,distance and site conditions.Based on frame structure models with different natural periods,taking the same set seismic conditions and the same fortification level as the starting point respectively,the records of the primary record library were selected according to the provisions of Chinese code GB50011-2010 and American code ASCE7-16,and the record selection result and time history analysis results were compared and analyzed.Under the same set earthquake conditions,the number of natural records selected by the U.S.code is too small,which is greatly affected by engineers;records selected by Chinese code mostly are from the same earthquake event and record library,which is not representative.Under the same fortification level,the ground motion conversion parameters in Chinese and American codes are given;the dispersion of time history analysis results of records selected by Chinese code is large;the records selecting by Amrican code have clearer physical meaning,but they are more conservative because of the specified lower limit.Therefor,there are obvious differences between Chinese and American codes in subjective and objective,physical meaning,economy and security.(2)Different parameters and sensitivity of horizontal two-way ground motion records were analyzed.Based on the different provisions on the selection of horizontal two-way ground motion records in different foreign seismic codes,the maximum direction spectrum(Rot D100),root mean square spectrum(SRSS)and geometric mean spectrum(GM)were used for spectrum matching of horizontal two-way ground motion,which were input into frame structures with different natural periods for time history analysis,and structural response,scaling factor,frequency spectrum and duration characteristics of ground motion were compared,and the sensiticities were analyzed.The results show that different ground motion parameters have a significant impact on record selection results;compared with SRSS and GM,Rot D100 has less standard deviation and discreteness of structural response,scaling factor and ground motion characteristics;the results of Rot D100 at medium and short periods are slightly larger than the target spectrum value;the correlation between amplitude,duration characteristics and spectrum characteristics is obvious adopting Rot D100.Since Rot D100’s conservatism at medium and short periods and higher safety,it is recommended to use Rot D100 for horizontal two-way records selection in the project.At the same time,factors such as effective peak acceleration can be properly considered in practice,which can provide ideas for horizontal two-way records selection in Chinese code.(3)The effects of different input modes of ground motion on structural response were analyzed.The peak acceleration ratio of the two horizontal orthogonal components and the angle between the station layout direction and the fault strike are closely related,thus the ratio of 1:0.85 in Chinese code is not convincing.In the dissertation,the ground motion in the Rot D100 parameter horizontal bidirectional recording library was rotated with an accuracy of 5 ° in the range of 0 ～ 180 ° according to the principle of vector decomposition.The regular plane structure system with the same,different aspect ratio and the irregular plane “L” structure system with the same,different centroid input point distance were established,and the records before and after rotation were input into the structure for time history analysis.The results show that the input mode of ground motion has no obvious effect on the structures with different natural periods;the structural response of regular plane structure system with different aspect ratio has an obvious linear relationship with the input mode;there is no obvious correlation between the structural response of irregular plane “L” structural systems with different centroid input point distances and the input mode.Therefore,different ground motion input modes can be selected according to the actual situation in the project.Therefore,it is recommended to choose different input methods according to different structure types.(4)The selection of ground motion records of isolated structures was studied.The ground motion records mostly used in isolated structures are classical records with little selectivity.According to the definition of near fault ground motion,the preliminarily selected ground motion library was divided into near fault record library,far-field record library and hybrid record library.Two different scaling methods before and after the structure is equipped with isolation bearings were selected,and 7 ～ 14 records of different quantities were selected.The structural response was compared and analyzed with the single degree of freedom simplified model,and 8-story isolated frame shear wall structure in a project was taken as the checking model.The results show that for the record selection of isolated structures,the sensitivity of the selected number is low,so it is sufficient to select 7 records in the project;the results of the selection method after the structure is equipped with isolation bearings are slightly small that those before the structure is equipped with isolation bearings,and its dispersion is smaller;the dispersion and difference of the selection of hybrid record library are between near fault record library and far-field record library.In conclusion,when selecting records for isolated structures,it is recommended to select 7 records from the hybrid record library,which not only meets the requirements of smaller dispersion and higher matching degree of the selection results,but also more in line with the engineering significance.At the same time,the records shall be selected using the method after the structure is equipped with isolation bearings.However,when the difference between the selection results and the target spectrum is too large,the method before the structure is equipped with isolation bearings can be carried.