Study On Characteristics Of The Duration Of Horizontal Components Of Ground Motions

In earthquake, the destruction of most of buildings(structures) is caused by strong earthquakes, and the degree of damage mainly depends on the three elements(amplitude, spectrum and duration).On the basis of generalization and summary of the contemporary domestic and foreign research status and trend of motion duration, this paper probes into the main influence factors and spatial variation of ground motion, along with its relationship with site conditions, providing a scientific gist of earthquake fortification.The main content of the paper are as follows:1. On the basis of inducing and summarizing the contemporary domestic and foreign research status and trend of ground motion duration, the characteristics and shortcomings of four different kinds of widely used duration(namely bracket duration, uniform duration, significant duration, effective duration) were analyzed and discussed. Besides, the main factors of ground duration is analyzed and discussed. The results showed that,(1) with a simple and strait forward definition, bracket duration is mainly used in the ground motion duration prediction equation, the cumulative damage of structures, the assessment of site liquefaction and the research of response spectrum. Uniform duration is based on bracket duration and is usually regarded as supplement for bracket duration;(2) significant duration and effective duration are both based on Arias intensity. At present, 5-95% significant duration is most widely applied in the engineering practice. The effective duration is principally applied in elastoplastic model of single degree of freedom system.(3) Duration is mainly affected by the magnitude, fault distance, site conditions and relative position of stations and thresholds.2. Ground motion records of main shock of the Wenchuan earthquake from 87 stations were used to explore spatial variation of the four different durations. The results showed that,(1) Brackets duration, uniform duration and effective duration, which use absolute threshold, decreased with the increase of fault distance; while the significant duration, which adopts absolute threshold, increased with the increase of fault distance;(2) In general, if the fault distance of the hanging wall area and the footwall area are same, the effective duration of the former is bigger than the latter, while the significant duration of the former the less than the latter.(3)Where the fault distance is less than 80 km, the bracket duration, uniform duration, and effective duration in hanging wall area are higher than those of the footwall area. Where the fault distance is greater than 80 km, the law is opposite.(4) The effect of direction on ground motion duration is quite significant. In forward area, bracket duration, important duration and effective duration are all less than those of places in the backward area.3. Ground motion records from 108 sites of Japan Ki K-net during the main shock of 311 earthquake were used in the study of the relationships between the ratio of duration at surface and bedrock, and the relationship between wave velocity and thickness of soil layer, equivalent shear velocity, PGA, peak acceleration ratio of the surface and the bedrock. The results showed that:(1) the general characteristics of the ratio between surface and bedrock duration are: uniform duration ratio > effective duration ratio > brackets duration ratio.and were greater than 1.0, which are mainly distributed in more than 2.5, 1.7 and 1.5; important to than to less than 1.0, accounting for 79.6%.(2) Ratios of brackets duration, uniform duration, effective duration between surface and bedrock, decreases with the increase of equivalent shear wave velocity, and increased with the increase of peak acceleration, but the correlation is not obvious.(3) the ratios of bracket duration, uniform duration, effective duration and significant duration between surface and bedrock, have no significant correlation with soil thickness and PGA.4. Records of 127 earthquakes from 4 typical sites in Japan Ki K-net were used in the analysis of the relationship between the surface-to-bedrock duration ratio and the site amplification coefficient corresponding to the fundamental frequency. The results showed that:(1) the discreteness of the relationship between surface-to-bedrock duration ratio and the site amplification coefficient corresponding to the fundamental frequency is quite obvious. This means that the duration amplification effects of the site has nothing to do with the site amplification coefficient.(2) When serious nonlinear response occurs, the brackets duration ratio and uniform duration ratio between surface and bedrock decreased.