| "The Japan Commission on Large Dams (JCOLD) collected acceleration time histories recorded at dam sites, and published them in 2002(1). The earthquakes compiled into this publication spans from January 1st, 1978 to October 6th, 2000, including 1978 Miyagi-Oki Earthquake with the magnitude of 7.4, 1995 Kobe Earthquake with the magnitude of 7.3, and 2000 Tottori Earthquake with the magnitude of 7.3. The number of the dams where acceleration time histories were recorded is 84. The dam type includes concrete gravity, concrete arch, rockfill, and earth dam. The publication and CD-ROM were distributed at the ICOLD Montreal Congress. After this publication, four major events including 2003 Tokachi with the magnitude of 7.8 occurred and have provided additional information regarding the seismic performance of dams. We analyzed these data and tried to summarize the characteristics of foundation motions, natural frequency of dams, and amplification of dams to foundation motions.Now sophisticated methodology is able to predict the performance of dams subjected to strong motion. However, the modern analytical procedures entail complex and incompletely quantifiable factors, and hence the final seismic safety evaluation should be based on and compared with the performance of similar structures during earthquakes. Therefore, we presented the analysis and summary of recorded performance of dams during earthquakes.First, peak ground accelerations (PGA) measured at the rock foundations and soil ground surface were compared. Attenuation relations of PGA are shown with the distance to the causative fault, and marked difference was found between PGA of rock motions and soil motions.Secondly, the natural period of the dam was analyzed by using the frequency transfer function computed from Fourier spectra of base and crest records, since strong motion acceleration records were obtained at the both of base and crest of most dams. The natural period is shown in relation with the dam height and each dam type. Thirdly, we describe the amplification ratio of PGA of dam crest to foundation. The decrease of amplification ratio with the increase of PGA is given in the figure, which indicates the nonlinear properties of dam structures and materials." |
