RESONANT FREQUENCY OF CONCRETE ARCH DAM EVALUATED FROM OBSERVATIONAL IN-SITU RECORDS AND EFFECTS OF CONTRACTION JOINTS ON THESE FEATURES

Resonant frequencies of concrete dams are very important property because they influence earthquake responses of concrete dams. For two existing arch dams, resonant frequencies are evaluated from acceleration records during earthquakes at the upper and lower parts of dams. And those are also determined from the results of forced vibration tests and ambient measurements. The relations between resonant frequencies and water levels of the reservoir are presented and it is shown that resonant frequencies at low water range decrease with the reduction of water depth. It is also shown that resonant frequency corresponding to large peak acceleration of dam response is lower than that corresponding to small peak acceleration. It is discussed that these features are attributed to effects such as stress of dam which can affect the contact conditions in the contraction joints.