EP 1110-2-12
30 Sep 95
depends on mix consistency and placement and
c. Types of design spectra. Design response
compaction methods as well as mix proportions.
spectra for the OBE are usually developed using a
Tensile strength of both the lift joint and the parent
probabilistic approach, and design response spectra
concrete shall be determined from cores taken from
for the MCE are developed using a deterministic
test fill placements for new dam design and from the
approach. Design response spectra are further
in-place RCC for existing dams. Although splitting
classified into two types: (1) site-specific or
tensile tests may be used, the test results shall be
(2) standard. The seismic zone location of the site,
adjusted to reflect direct tensile strength. From the
the height of the dam, and the proximity to active
direct tensile strength, the allowable design tensile
faults are the factors used to determine if it is
stresses shall be established for both lift joints and
necessary to develop a site-specific design response
parent concrete by applying adjustment factors to
spectra or if the standard spectra may be used in the
account for high strain rate associated with dynamic
dynamic analysis. When standard design response
loading and certain nonlinear characteristics of the
spectra are acceptable, Chapter 5 provides the
stress/strain curve. Adjustment factors shall be
appropriate spectra along with the PGA values to be
selected to maintain serviceability during an OBE and
used for scaling. These standard design spectra are
to produce a reasonable safety factor for a MCE.
based on the mean level of the ground motion
parameters for the records selected in the
development of the standard spectra.
1-8.
Important Factors
d. Ground motion time histories. The more
Discussed below are recommendations regarding
refined analysis methods require a ground motion
factors which are important because they have a
time history representation of the design earthquakes.
significant impact on the dynamic response.
These may be developed using actual past earthquake
Recommendations that differ from those contained in
ground motion records, synthetically, or by modifying
ETL 1110-2-303 and ER 1110-2-1806 are identified.
an actual record. Ground motion time histories are
developed so their response spectrum closely matches
a. Effective damping. The material and
the site-specific design response spectrum.
radiation damping of the foundation contribute
significantly to the damping of the combined
dam-foundation system, and must be considered in
1-7.
Acceptance Criteria
the analysis. This requires calculating an effective
viscous damping ratio to reflect the damping
contribution of both the dam and the foundation.
produced during a seismic event can cause cracks to
This will result in a considerably higher damping
occur in an RCC dam. As cracking progresses,
ratio for a foundation having a very low modulus
serviceability is eventually impaired. If ground
than the damping ratio used previously.
shaking is extremely severe, or if strong ground
shaking combines with a foundation fault displace-
b. Hydrodynamic effect. Added mass shall be
ment, it is conceivable that continued propagation of
calculated using standard hydrodynamic pressure
the system of cracks could eventually lead to a failure
function curves which consider compressibility of the
mechanism where the dam is no longer capable of
water, stiffness characteristics of the dam, and
containing the pool. This EP establishes acceptance
reservoir bottom absorption (Fenves and Chopra
criteria which maintain serviceability during an OBE,
1986). Appendix D provides an example showing the
and provide a reasonable safety factor against
required procedure.
developing a failure mechanism during a MCE.
Because of the complexity and the great number of
c. Mode combination methods. The complete
variables involved in seismic design, the EP criteria
should be supplemented with the judgment of
quadratic combination method (CQC) of combining
structural engineers experienced in seismic design.
modes shall be used for final design of dams under
critical seismic design conditions and for evaluation
of existing dams. Critical conditions are considered
b. Direct tensile strength. The direct tensile
to exist when site-specific design response spectra are
strength of the RCC is the design parameter used for
required by this EP. Either the square root of the
establishing the acceptance criteria. Unlike
sum of the squares method (SRSS) or the CQC
conventional concrete, tensile strength of RCC
1-2
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