Most if not all of the dams are located near or on the faults, and severe ground shaking from nearfaultmay cause significant damage to dams. Such damage usually is associated with changes instructural stiffness. Consequently, the actual performance of the dam can be predicted only bynon-linear analysis. Moreover standard earthquake analyses of civil engineering structures useuniform ground motions even though considerable variations in both amplitude and phase canoccur along the foundation interface for long-span bridges and large dams. Non-uniform motion ofthe canyon is major cause of large concrete stresses in the dam body, particularly near theabutments. The main purpose of this study is the identification of vibration properties, effect ofinteractions on these properties, and finally detection of nonlinearity in the dam-foundation rockreservoirsystem by applying system identification methods on seismic response data from PacoimaDam. The response of Pacoima dam in Northridge earthquake was recorded by a network ofCDMG accelerometers. The strong motion accelerograms provide a unique opportunity toexamine actual behavior of the dam subjected to a large earthquake by using on-line and off-lineidentification methods. For the identification of dam properties from seismic response data during1994 Northridge earthquake, the multiple inputs from the abutment of the dam to represent thenonuniform excitation of seismic input are considered, and ARX models are applied using thetime-invariant and time-variant or recursive approach to identify the dynamic characteristics of thedam-foundation rock-reservoir. The system modal dampings, natural frequencies and frequencyresponse functions are identified. The identified resonant frequencies and corresponding dampingratios have been compared with the results obtained from forced vibration measurements andanalytical methods. All of the results are fairly in good agreement. It has also been shown thatassumption of uniform input is not realistic and could lead to erroneous results. In addition timevaryingmodal parameters have been estimated. The method is useful for obtaining vibrationproperties of dam-foundation rock-reservoir system. These properties are essential in safetyevaluation of existing dam structures. The results of this method can be used in calibration ofanalytical methods and experimental results. Moreover such kind of approaches are helpful in
展开▼
