EP 1110-2-12
30 Sep 95
(1) Dams greater than 100 feet in height located
control points on a tripartite plot. Table 5-1 presents
at a site classified as a "high seismic risk site."
the spectrum in equation format so it is easily devel-
oped for any damping value. The standard design
(2) Dams greater than 100 feet in height located
spectrum shown in Figure 5-2 and defined in equation
in Seismic Zone 2B, 3, or 4 even though the site is
format in Table 5-1 is normalized to 1.0 g PGA. The
not classified as a "high seismic risk site."
standard spectrum shall be anchored to the PGA for
the OBE and the MCE by using the appropriate scal-
(3) Dams not greater than 100 feet in height
ing factors provided in Table 5-2. The correct scal-
located in Seismic Zone 2B, 3, or 4 when the site is
ing factors are selected based on the seismic zone
classified as a "high seismic risk site."
location of the site using the seismic zone map shown
in Figure 5-1.
d. Standard design response spectra. Standard
design response spectra are based on fixed spectral
5-6.
Horizontal and Vertical Design
shapes established for very general site classifications
Response Spectra
such as rock or soil site. They ignore the effects of
earthquake magnitude and distance, and the specific
a. Site-specific design response spectra. When
foundation characteristics at the site. The standard
design spectra are usually "anchored" to the estimated
site-specific design response spectra are required in
peak ground acceleration (PGA) established for the
accordance with paragraph 5-5c, two independent
design earthquake. The fixed spectral shape is usu-
design response spectra shall be developed, one to
ally presented such that it is normalized to a 1.0 g
define the horizontal component of ground motion,
value of maximum ground acceleration. This normal-
and the second to define the vertical component. The
ized value can be easily checked by observing the
vertical component of ground motion usually contains
spectral acceleration value from the spectrum plot for
much higher frequency content than the horizontal
frequencies above about 50 cps where the response
component, therefore the spectral shape is quite dif-
and the maximum ground acceleration coincide.
ferent than that of the horizontal component. The
Standard design response spectra are adapted to the
PGA associated with the vertical component will also
severity of ground motion associated with the OBE or
be different than the PGA of the horizontal compo-
MCE by using the PGA as a scaling factor. The
nent. Both values of PGA are dependent on the dis-
standard design response spectra can be used for:
tance from the source, but for short distances, the
PGA of the vertical component may actually exceed
(1) Dams greater than 100 feet in height located
the PGA of the horizontal component.
in Seismic Zone 0, 1, or 2A when the site is not
classified as a "high seismic risk site."
b. Standard design response spectra. When it
is acceptable to use standard design response spectra
(2) Dams not greater than 100 feet in height
to define the design earthquakes, the horizontal com-
located in Seismic Zone 0, 1, or 2A.
ponent of ground motion shall be defined by anchor-
ing the standard design response spectra for the
(3) Dams not greater than 100 feet in height
appropriate damping factor developed from Table 5-1
located in Seismic Zone 2B, 3, or 4 when the site is
with the scaling factor provided in Table 5-2. The
not classified as a "high seismic risk site."
vertical component of ground motion shall utilize the
same standard design response spectrum used for the
e. Required design spectrum. When it is
horizontal component, but it shall be scaled using the
acceptable to use a standard design response spectrum
appropriate ratio of the PGA for the vertical compo-
to define the design earthquakes, the standard design
nent to the PGA for the horizontal component as
spectrum shown in Figure 5-2 shall be used (Applied
provided in Figure 5-3. This ratio is based on the
Technology Council 1984). This spectrum is consid-
site to source distance (R) and the fundamental natu-
ered conservative but reasonable for essential struc-
ral period of vibration of the structure.
tures such as dams. It is fully described by only five
5-3
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