Recommended Interim Procedure. - Continued

DOE-STD-1024-92

this slope6 varies at most from 3.7 to 2.9. Over this range of b values, the factor

above ranges from 1.4 to 1.55, for the suggested hazard ratio of 3.6. We

conclude that due to the insensitivity of these results to the observed site-to-site

variations, it is satisfactory to simply adopt a single factor, say 1.5 for all cases. In

this case, Step 2 is followed immediately by:

Step 3 (revised; replacing Steps 3, 4, and 5 above): Multiply the "composite" PGA

corresponding to the median target probability of exceedance by 1.5 to obtain the

"composite" PGA corresponding to the target mean probability of exceedance.

Examples of applying this proposed procedure are given in Table 2 of the Interim Position

Report.

Note that if LLNL 5GX were to be used as a basis, rather than 4GX, the results change

as follows:

The LLNL/EPRI ratio of 2 increases to about 3.7 (see footnote 3). The geometric

mean of the two ratios increases from 5 to ( 3.7)(3.5) or 6.7. The mean/median

factor increases from 3.6 to exp {(In 6.7)2} = 6.1. The corresponding factor on

the PGA increases from 1.5 to, say, (6.1)1/3.3 = 1.7.

Finally, it is recommended that this PGA value be used, at least as an interim basis,

simply as the "ZPA" (zero period acceleration) at which to anchor either a standard broad band

spectral shape, such as that in NUREG 0098, or a site-specific spectral shape (developed by

any of several familiar means, properly allowing for local soil conditions). It is suggested that this

be a median spectral shape (where median refers to the median of a sample of shapes from a

representative suite of ground motion records.) It might be argued that, because this median

shape is not being anchored to a spectral ordinate obtained from the seismic hazard analysis but

rather to a PGA, it therefore fails to capture the record-to-record variability in the Sv/PGA ratio

(D.A.F.) (or "peak-to-valley" variability within a typical rough spectrum). This variability is

included in spectral velocity hazard analyses, but when combined with the larger PGA variability

the net effect is only a small increase.

[wplet2\doe.rpt]

Kennedy specifies the slope through the ratio of the PGA at 10-5 hazard to the PGA at

10-3 hazard, a ratio he calls a5/ac. His suggested range for EUS is within 3.5 to 5.0. It is

easily shown that a5/ac = (0.01)-1/b or b = 2/log10 (a5/ac).

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