EP 1110-2-12
30 Sep 95
Chapter 4
(2) The level of cracking is considered to be
"minor cracking" when the tensile stresses are less
Design Earthquakes
than 1.25 f ′t as shown in Figure 3-7.
c. Allowable tensile stress. The allowable ten-
4-1.
Definition
sile stresses ft (allowable) for the OBE are established
below. The formulae apply to the calculation of both
The term "design earthquake" refers to the specifica-
allowable tensile stress of the parent material and
tion of the free field ground motion that would be felt
allowable tensile stress of the lift joints. DTS =
at the dam site due to a particular seismic event that
Dynamic Tensile Strength, and f ′t = direct tensile
is used as the basis for earthquake resistant design of
strength.
new RCC dams, or to evaluate the response of exist-
ing RCC dams.
(1) Existing dams:
ft (allowable) = 1.25 DTS = 1.875 f ′t
4-2.
Operating Basis Earthquake (OBE)
(2) New dams in seismic zones 0, 1, 2A,
The OBE is defined as the earthquake producing the
and 2B:
greatest level of ground motion that is likely to occur
at the site during the service life of the dam. The
ft (allowable) = 0.60 DTS = 0.90 f ′t
service life shall be taken as 100 years for both new
dams and existing dams. The seismic risk or adverse
(3) New dams in seismic zones 3 and 4:
consequences of failure of an existing dam is not
reduced as long as the dam is in operation; therefore,
ft (allowable) = 0.90 DTS = 1.35 f ′t
the "remaining service life" of an existing dam shall
not be substituted for the 100-year service life speci-
d. Damping. Studies on dams under severe
fied above. The OBE is determined using probab-
ground motion which cause stresses in the upper
ilistic methods and, as such, is defined as the earth-
reaches of the elastic range indicate a dampened
quake with a 50 percent chance of exceedance in the
response which corresponds to a damping factor of
service life of the dam.
about 5 percent of critical. On this basis the OBE
shall be analyzed using a damping ratio equal to
a. General performance requirements. All
5.0 percent of critical damping for the concrete dam
structural, mechanical, and control equipment used to
structure only. This factor must be modified as out-
regulate the reservoir shall be capable of remaining
lined in paragraph 7-3 to account for foundation
fully operational during and after an OBE. New
damping.
RCC dams located in low seismic regions shall be
designed to prevent the initiation of cracking in the
concrete structure. Tensile cracking in new RCC
4-3.
Maximum Credible Earthquake (MCE)
dams located in high seismic regions and in existing
dams in all seismic regions is allowed; however, it
The MCE is defined as the largest possible earthqu-
shall be limited to only "minor cracking" that requires
ake that could reasonably occur along the recognized
little or no repair.
faults or within a particular seismic source. Often
several fault sources must be investigated to deter-
b. Structural criteria. The following general
mine which will produce the critical site ground
structural criteria shall be the basis for satisfying the
motion. By definition the MCE has a very low prob-
concrete cracking performance requirements stated
ability of occurence. Ground motion associated with
above.
the MCE is established using the deterministic
approach.
(1) Initiation of cracking is prevented when the
tensile stresses are less than 0.60 f ′t as shown in
Figure 3-7.
4-1
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