Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 225

Figure 6.19 illustrates the typical forced vibration signals obtained. The
hydrodynamic pressure is shown in the first two graphs ((a) and (b)) for two
different depths along the upper bank, whereas the acceleration response obtained on
the lower bank is illustrated in (c). A 5 V pulse is emitted for each mass rotation
(graph (d)), which allows the exact rotation frequency of the exciter to be assessed (a
little over 8 Hz in this case).
Figure 6.19. Examples of signals measured under forced vibrations
The signals were used to calculate the frequency responses for each measuring
position and to extract the dynamic properties of the dam. Both graphs at the top of
Figure 6.20 show the responses obtained on the crown for two exciter positions. By
comparing the amplitudes and phases for the summer (dotted lines) and winter tests
(solid lines), it is clear that amplitudes are far smaller when there is ice, and that
resonance frequencies are generally decreased. Therefore, the ice cover imparts an
additional stiffness to the reservoir-dam set and also increases modal damping
indicated by smoothing of the peaks.
Figure 6.20 also illustrates the displacement effect of the excitation point. For
results obtained in summer, the first resonance (which corresponds to a symmetric
mode) is more loaded in a central position (block H) than in the quarter point
position (block M). Conversely, the second resonance (associated with an anti-
symmetric mode) is almost invisible at the central stimulation position and highly
stressed at the other.