Vibration Test of Richard B. Russell Concrete Dam before Reservoir Impoundment

摘要

This forced vibration test of Richard B. Russell Dam was conducted before an appreciable reservoir is impounded. Hence a rare opportunity was seized to experimentally measure the dynamic properties of a concrete gravity dam without hydrodynamic interaction. The structure was excited at three locations by a crest mounted 17,000-pound inertial mass which was driven by an electrohydraulic servo-controlled actuator. The force input to the dam was computed as the product of the measured acceleration and the 17,000-pound mass. Servo accelerometers with a sensitivity of 0.25 volts/g measured the horizontal crest accelerations of the tallest 31 monoliths and the distribution of horizontal acceleration with elevation in the three drive point monoliths. All tests were 1/2 decade/minute sine sweeps from 25 to 1 Hz. Analog signals were recorded on magnetic tapes and later digitized and processed by a Structural Dynamics Analyzer. The results reveal the linear and the nonlinear characteristics of the complex structure. The natural frequencies and mode shapes are consistent with results of two- and three-dimensional finite element analyses when foundation flexibility is considered. Modal damping ratios were greater than independently measured concrete specimen values but were scattered between 1.4 and 5.2 percent. Relative joint motion was observed above a low response threshold.