EP 1110-2-12
30 Sep 95
established allowables and the analysis method pro-
recommended that the refinements be considered in
vides a reasonably accurate or conservative represen-
the following order:
tation of project conditions. Should the analysis
method utilize an extremely simplified representation
(1) 3-D representation. Consider refining the
of project conditions, the response may not neces-
analysis from two to three dimensions when the accu-
sarily be conservative and will likely be of relatively
racy of the response from a 2-D analysis cannot lead
low order of accuracy. However, the response may
to a confident judgment that the response is
still be judged acceptable without pursuing more
acceptable.
refined analyses on the basis that the tensile stresses
are far enough below the established allowables to
(2) Time-history analysis. Consider defining the
clearly infer that the response satisfies the require-
design earthquakes with appropriate ground motion
ments and criteria described above. Refer to para-
time-history records, and performing a time-history
analysis when additional insight into the structural
graphs 2-2e, 2-2f, and 2-2g for information on allow-
behavior beyond that provided by the response spec-
able tensile stress criteria for various methods of
trum analysis is needed. A time-history analysis
analysis.
yields additional information regarding the excursions
of tensile stress cycles beyond the allowables and
b. Modifying the design of a new dam. When
provides a better understanding of the response. This
the response from a dynamic stress analysis for a new
applies both to existing dams or to the design of a
dam is judged not acceptable, consideration shall be
new dam when all practical and economical modifi-
given to modifying the design, adjusting the computer
cations to the design of a new dam have been
model to reflect the modifications, and reanalyzing.
exhausted.
Modifications include:
(3) Nonlinear analysis. The analysis based on
(1) Modify geometric configuration.
nonlinear material behavior represents the greatest
possible refinement and it produces the most accurate
(2) Superior mixes. Use richer, higher strength
results. However, it is also the most complex and the
superior RCC mixes in overstressed areas.
most costly. It requires time-history ground motion
input, direct integration solution, a large main frame
(3) Reducing aggregate size. Increase tensile
computer, specialized computer programs, and a
strength by reducing the maximum size aggregate.
considerable amount of computer time. As such, it is
the last recourse in the attribute refining process. The
(4) Mortar bedding. Provide mortar bedding to
nonlinear analysis should only be undertaken under
increase tensile strength at lift joints.
the guidance of an expert in the field of fracture
mechanics and finite element methods.
(5) Zone boundaries. Adjust the zone boundaries
of the superior RCC mixes to better fit the tensile
stress pattern.
8-4.
Preliminary Design of New Dams
c. Refining the dynamic analysis methods.
Preliminary design includes engineering and design
When the response from a dynamic stress analysis of
through the Feasibility Phase, or through the General
an existing dam is judged not acceptable, the next
Design Memorandum (GDM) phase if a GDM is
step in the procedure shall be to reanalyze using an
prepared for the project.
analysis method with more refined attributes. In
contrast to this, there is no clearly defined point in
a. Initial dynamic analysis. The initial dynamic
the design procedure for new dams that indicates
stress analysis shall use the simplest analysis method
when the analysis method should be refined. The
which is identified by the following attributes:
design conditions and results of the design procedure
(1) linear-elastic material behavior, (2) 2-D represen-
already completed must be evaluated to determine
tation, and (3) design response spectrum definition of
when it is appropriate to suspend the design modifica-
the design earthquake. The analysis shall be per-
tion process, and pursue a more refined analysis of
formed using the cross-section of the critical trans-
the latest modified design. When the attributes of the
verse element of the dam which usually consists of a
dynamic analysis method are to be refined, it is
8-5
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