Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 226

Figure 6.20. Frequency responses on the crown
Comparing both series of tests reveals a new mode near the first resonance. This
is mostly visible with responses obtained inside inspection galleries, as illustrated by
the diagrams in the middle of Figure 6.20. Amplitudes are drawn at the same scale
to highlight the response reduction in passing from one gallery to another one. The
double peak is clearly identified in the case of the first gallery, 20 m below the
crown.
Ice cover also causes modifications similar to hydrodynamic pressures. The
diagrams at the bottom of Figure 6.20 show responses along the upper bank of the
reservoir (actually, 3 m away from the bank in the winter tests) at respective depths
of 15 m and 65 m. The damping increase and resonance shift are obvious, as in the
case of acceleration responses: the new mode can be observed as well. It should be
remembered that the water level difference between both series of tests can be
neglected and cannot account for the resonance frequency variation.