
 DOESTD102395
The distance of the controlling earthquake from the site is next determined from
the following (or similar) equation:
M N
M N
log R(1) =
log(rj) H(mi, rj) /
H(mi, rj)
i=1 j=1
i=1 j=1
Step 5:
Select, from the sitespecific PSHA results, the mean seismic hazard curve for
the ground motion parameter SA(12.5), i.e., the average spectral acceleration at
1 and 2.5 Hertz, and use the same PH and Steps 1 through 4 as above to
determine the magnitude m(2) and distance r(2) that control the SA(12.5).
Step 6:
Develop the median normalized response spectrum shape for m(1):r(1) and, if
necessary, m(2):r(2). Acceptable methods are described after Step 8.
Step 7:
Scale the normalized median spectrum shape for m(1):r(1) to the mean SA (5
4
10) with the appropriate annual probability (e.g., 1x10 for sites not located
near tectonic plate boundaries, containing facilities with SSCs in PC 4).
Step 8:
Determine if the scaled spectrum shape for m(1): r(1) envelops the 1 to 2.5 Hz
region of the m(2): r (2) spectrum shape. If not, envelop the two resulting
spectra to create a single response spectrum. The engineer/designer shall either
use the above single envelope spectrum or analyze twice, one for each m:r
combination, and use the more conservative result for design purposes. It is
intended that the resulting envelope will be a smooth, broad spectrum without
significant gaps in spectral ordinates when compared to the mean UHS.
After the controlling earthquake magnitudes and distances are determined, the site specific
response spectra shapes are developed by any combination of the following methods:
(1)
Statistical analysis of ground motion records
The median response spectrum shape is derived from a suite of actual ground motion
records judged associated with site similar magnitudes, distances, and soil profiles.
When a sufficient amount of suitable ground motion records is not available, the
response spectrum shape may be approximated by scaling in accordance with the
method suggested by Heaton, et. al. (1986).
(2)
Attenuation of spectral ordinates
The median response spectrum shape is derived from regression equations defining
median spectral amplifications at various natural frequencies as a function of
magnitude, distance, and site soil profile. Recent data shall be used when available.
A6

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