EP 1110-2-12
30 Sep 95
Chapter 6
a statistical basis to have a reasonable chance of
occurrence at the time of the design earthquake.
Earthquake Load Cases
(1) Flood frequency data from project flood flow
and flood routing studies provide a basis for estab-
6-1.
Load Combinations
lishing reasonable high pool elevations. Each dam
must be evaluated based on its own set of unique
The cyclic and oscillatory nature of vibratory
conditions.
response can cause critical tensile stresses to occur in
either the upstream or the downstream face of the
(2) The conservation pool elevation for the proj-
dam. Therefore, the earthquake load cases must
ect shall be used for earthquake load cases involving
consider combinations of the design earthquake load-
low pool conditions. If there is no established con-
ing with other loads which lead to critical tension in
servation pool, use the lowest average pool elevation
both the upstream and downstream faces. Usually
that can best be judged to exist for a 30-day period in
two or more OBE load cases and two or more MCE
a normal yearly flow cycle.
load cases must be evaluated. The discussion of
earthquake load cases that follows refers to seismic
(3) Where tailwater is applicable for an earth-
criteria regarding ground shaking and foundation fault
quake load case, the elevation shall be selected which
displacement as discussed in paragraphs 2-2 and 2-3,
increases the response while being consistent with the
respectively, and not stability criteria described in
reservoir conditions.
paragraph 2-1. Load case requirements for stability
are covered in EM 1110-2-2200.
against either face of the dam has both a static and
dynamic load effect during an earthquake. These
6-2.
Dynamic Loads To Be Considered
loads shall be included in all earthquake load cases.
Static loading shall be based on at-rest pressures.
The design earthquake imposes several types of dy-
Dynamic loading may be approximated by the
namic loads on the dam. The greatest dynamic load
Mononobe and Okabe method utilizing the inertia
is the inertia load caused by the response of the con-
force acting on the Coulomb sliding wedge in the
crete mass to ground motion accelerations. Next is
appropriate direction as discussed in EM 1110-2-
the hydrodynamic load created by a high reservoir
2502. For finite element analyses the dynamic effect
and tailwater condition. Hydrodynamic forces are
may be approximated by added mass based on the
imposed on the dam due to motions of the dam react-
Coulomb sliding wedge.
ing with the surrounding water, and motions of the
reservoir bottom. Finally, backfill or silt deposits
c. Siltation load. During the life of the dam,
against the faces of the dam will interact with the
silt may build up against the upstream face to a depth
structural mass of the dam in a manner similar to the
which may cause a moderate increase in the tensile
hydrodynamic load.
stresses in load cases where tension in the upstream
face is critical. For these load cases, siltation loading
shall be considered based on the full depth expected
6-3.
Static Loads To Be Considered
during the life of the dam. In load cases where ten-
sion in the downstream face is critical, the siltation
The effects on the dam structure due to static loads,
load will decrease the tensile stresses. For these load
as discussed below, are determined by conventional
cases a zero depth of silt shall be assumed. When
static analysis methods. The results of the dynamic
silt is included, both static and dynamic loading
and static analyses are combined by superposition to
effects should be incorporated using the same meth-
determine the total stresses for the earthquake load
ods as discussed for backfill loads.
case.
d. Gravity loads. Gravity loads shall include
a. Reservoir and tailwater loads. Load cases
the weight of the RCC, weight of backfill or silt on
shall be included to cover both the highest and the
battered faces of the dam, and weight of equipment if
lowest reservoir pool elevations that can be judged on
significant.
6-1
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