Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 220

Thanks to this method, the experimental modes of the bridge deck and the tower
were predicted with an average accuracy of 5%. The first four torsion and bending
frequencies, the experimental and numerical modes, together with the corresponding
frequencies are illustrated in Figure 6.15. The determining factors for the calculation
of eigenvalues and vectors in this case were found to be the cable tensions and the
density of the highway
6.6. Application to large dams
The owners of large dams are increasingly concerned about the seismic safety of
their works, and the best methods to assess them formed one of the main themes of
the ICOLD Congresses (Large Dam International Commission).
In situ dynamic tests are acknowledged as the most reliable ways to assess the
dynamic properties of dams [HAL 88, COL 90]. With recent advances in finite
element modeling techniques that take complex interaction phenomena between
different sub-structures into account (dam, foundation, reservoir, ice covering),
compiling a reliable database on the dynamic behavior of dams is now greatly
simplified. Ambient and forced vibration tests are also quite useful, but are carried
out at significantly lower loading levels.
Recording accelerometer networks are already in place on some large dams
([DAR 95], [DAR 01]) and allow the behavior of these works to be studied in real
time, especially during reservoir filling and emptying cycles. It is widely known that
hydrodynamic pressures and wave absorption at the bottom of the reservoirs play an
important part in the dynamic response of concrete dams, using hydrodynamic
pressure measurements made during forced vibration tests on gravity and arch dams
([DUR 88], [PRO 97]).
The following sections describe three forced vibration test campaigns undertaken
to assess the seismic behavior of large concrete dams, and highlight the use and
efficiency of the non-destructive dynamic techniques used.
6.6.1. Assessment of a response spectrum on the crown
The Beauharnois gravity dam is located near Montreal (Quebec, Canada) and
runs across the Saint-Laurent river. This 20m high concrete structure includes 36
alternators, and has a crown that is almost 1 km long. It was constructed in three
phases in 1930, 1950 and 1960. The gravity dam is a channel-head with the power