with 5 percent probability of exceedance in 50 years
The
(i.e., a return period of about 1,000 years). This
structure
is
assigned
to
document prescribes three-quarters of the MCE as
Performance Objectives 2B or 3B.
Ground Motion B for the design of essential
The site of the structure is within 10
buildings. As indicated in Paragraph 3-2, a direct
comparison of the ground motion at of MCE, with
kilometers of an active fault.
that based on 5 percent probability of exceedance in
The structure is located on Type F soils.
50 years, can only be made on a site-specific or
regional basis. The pragmatic intent of of MCE
A time history response analysis of the
was the specification of a ground motion for
structure will be performed.
enhanced performance objectives that would be
comparable to that specified in prior military
The structure is to be designed with
documents.
base isolation or energy dissipation.
b.
Design Values.
The design spectral
Site-specific determination of the ground motion
shall be performed only with prior authorization of
Ground Motion B, shall be in accordance with the
the cognizant design authority.
If a site-specific
following:
spectrum is determined for the design ground
motion, the spectrum is permitted to be less than the
SDS = SMS
(3-28)
general response spectrum given in Figure 3-2, but
not less than 70 percent of that spectrum.
SD1 = SMI
(3-29)
b.
Required Expertise.
Multi-disciplinary
Other design parameters, as defined in Paragraph 3-
expertise is needed for the development of site-
2 for the ELF or modal analysis procedures, shall be
specific
response
spectra.
Geological
and
calculated using the above values of SDS and SD1 and
seismological
expertise
are
required
in
the
a response modification factor, R, of 1.0.
characterization of seismic sources. The selection of
appropriate attenuation relationships and the conduct
of site response analyses requires expertise in
3-4.
Site-Specific Determination of Ground
Motion.
geotechnical
engineering
and
strong-motion
seismology. Conduct of probabilistic seismic hazard
a.
General. The site-specific determination of
analyses requires expertise in probabilistic modeling
ground motion may be used for any structure, and
and methods. A team approach is therefore often
should be considered where any of the following
appropriate for site-specific response spectrum
apply:
development. It is important that the team or lead
3-19
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