| During the seismic design of engineering structure, response spectrum is often used to express seismic action. In general, the response spectrum is determined according to the seismic zoning map and site conditions. With the accumulation of the strong ground motion records, the understanding of the site conditions’ effects on ground motion is getting deeper and deeper, from adjusting intensity according to site conditions and the height of design response spectrum, to only adjusting characteristic period of the response spectrum without the height of the short-period part, and the amplitude of the short and long period of the response spectrum(equivalent to adjusting the characteristic period). Based on the second edition of strong ground motion database of a new generation of ground motion attenuation projects of the Western United States(NGA-West2), which is released by the Pacific Earthquake Engineering Research Center in 2013, this paper mainly stydies the effect of site conditions on the seismic response spectrum, through statistical analysis, a preliminary plan of the site coefficient of the seismic design code is proposed.The NGA-West2 database has greatly increased in number and been broaden in coverage.This paper selects the 4435 sets of seismic records of 1509 stations’ 238 earthquakes, whose magnitude is 4 or more, PGA is 20 gal or more, from the NGA-West2 database, which contains 21539 sets of seismic records of 607 earthquakes; including 4435 sets of horizontal(average) ground motion records, and 4435 sets of vertical ground motion records, which are one to one correspondence; the maximum moment magnitude is 7.9, and the minimum moment magnitude is 4.2, the records between M4.7 to M7.5 are distributed evenly; the maximum epicentral distance is 1162.50 km, and the minimum epicenter distance is 0.44 km. The epicenter distance of small and moderate earthquake is between several kilometers to tens of kilometers mostly, otherwises, strong earthquake’s epicenter distance is between several kilometers to t wo or three hundred kilometers; the number of the records of Class C site and Class D site accout for 95%, Class B site and 6 sets of the records of Class A account for 2%, the other 3% is Class E site.NGA-West2 database provides every station’s site classification of the United States, primarily accroding to the average shear wave velocity of 30 m depth(Vs30), however, site classification in Chinese seismic design code is based on Vs20 and the overburden thickness. In order to analyse the site coefficient of seismic design codes of China with NGA-West2 database, this paper analyses and compares the site classification of the two countries, based on the two classifications of 366 sites of Sichuan and the United States, whose shear wave velocity is available, summarizes the general relationship between every two site classifications of Class A~F site and Class I~IV site. Class A and Class B site correspond to Classe I site, Class C site corresponds to Class I and Class II site, Class D site generally corresponds to Class II and Class III site, Class E and Class F site correspond to Class IV site. With the empirical relations, calculate the Vs30 corresponding to the seismic design codes of China section by section, this paper suggest a proposal for NGA that replace the site classification with Class I to Class IV.Through the study of some examples, this paper emphasizes the importance of data packet. In order to increase the the level of detail and stability, this paper improve the proposal of data packet using the magnitude and distance. Seclect five thresholds of horizontal PGA packet and magnitude thresholds, through the NGA-West2 CB13 attenuation, calculated a serie of the magnitude M and the distance R for each packet, establishs the proposal of data packet on the M-R plane, smooths the packet boundaries and suggests a proposal of data packet.For each data packet, calculate the average spectrum of all the horizontal ground motion response spectrum, standailize the spectrum according to the given form of seismic design codes, extract maximum value of the spectrum and the characteristic periods; for each site category, integrated standardization of the average spectrum of the taking six packets of ground motion, extract two parameters, or given characteristic period of seismic design code and extract maximum value of the spectrum; for each A packet, integrated standardization of the average spectrum of the taking four packets of site category, extract two parameters, or given characteristic period of seismic design code and extract maximum value of the spectrum. Take the ratio of maximum value of the spectrum of four site categories of each A packet and the relevant value of the Class II site as the estimated site coefficient, summarize the relationship between the maximum values of different site category from statistics, furtherly summarize and suggest the proposal of ho rizontal site coefficient. Follow the same step, calculate the average spectrum of all the vertical ground motion response spectrum; standailize the spectrum according to the given form of seismic design codes in three methods, extract two parameters, or given characteristic period of seismic design code and extract maximum value of the spectrum, take the ratio of maximum value of the spectrum of four site categories of each A packet and the relevant value of the Class II site as the estimated site coefficient, summarize and suggest the proposal of horizontal site coefficient.Based on the same response spectrum formula, calculate two tables of parameters, suggest a proposal of the site coefficient of Guidelines for Seismic Design of Highway Bridges, including horizontal site coefficient table and vertical site coefficient table. |
