Environmental Engineering Reference
In-Depth Information
16.9.3.7
Should vertical ground motion be included?
There is no reason that vertical ground motion should not be included in any analysis. Given
that the response effects to vertical ground motion are likely to be very small, the simple
approach of reducing “gravity” by the peak vertical acceleration would be appropriate for
the pseudo-static methods to account for this motion. For the more complex FEM analyses,
the engineer could either reduce “gravity” or apply to the model a measured vertical
accelerogram along with its companion horizontal accelerogram. Remember that the verti-
cal motion does not get applied to an “added” mass.
16.9.3.8
Reservoir level variation
If a reservoir only gets to full supply level infrequently, a lesser reservoir level may be jus-
tified for analysis.
16.9.3.9
What do the results of analyses mean?
The engineer doing analysis of a concrete dam must sooner or later face up to this ques-
tion. “Should I check the stability of the dam at its peak displacement?” “What does a
permanent shear displacement of 100-200 mm mean?”
In the case of the pseudo-static methods, if reasonably high pgas are used, the dam will
inevitably be shown to be unstable. The method, by its very nature, demands that the dam's
stability be checked.
FERC (2000) states that:
“
In a departure from the way the FERC has previously considered seismic loading,
there is no longer any acceptance criteria for stability under earthquake loading. Factors
of safety under earthquake loading will not longer be evaluated.
Acceptance criteria are
based on the dam's stability under post earthquake static loading considering damage likely
to result from the earthquake. The purpose of considering dynamic loading is to determine
the damage that will be caused so that this damage can be accounted for in the subsequent
post earthquake static analysis.”
and later:
“Because of the oscillatory nature of earthquakes, and the subsequent structural
responses,
conventional moment equilibrium and sliding stability criteria are not valid
when dynamic and pseudo dynamic methods are used.
The purpose of these investiga-
tions is not to determine dam stability in a conventional sense, but rather to determine
what damage will be caused during the earthquake, and then to determine if the dam can
continue to resist the applied static loads in a damaged condition with possible loading
changes due to increased uplift or silt liquefaction.”
FERC (2000) approach is that the analyses are to show if and where there could be
problems, such as permanent displacement, and how these problems might affect the
dam, for example, extensively cracking the dam, seriously damaging prestressed anchors,
completely cutting foundation drains and so on. From these results, one would decide
what weaknesses have been created and whether or not this weakness could influence the
dam's post-earthquake condition. Partial or full loss of prestressed anchors, change in
drainage conditions, extensive cracking and loss of shear strength would be typical of the
impacts in a post-earthquake analysis.
The FERC recommendations have much merit. However it should be recognised that a sim-
ple structure subjected to ground shaking could collapse during the earthquake, not just after-
wards. Perhaps the key point is that most gravity dams are not simple structures; for example,
there could be 3-D action or significant differences, even reversals, in displacement along the
axis at any particular time interval. The author's preference for new dams would be to have a
