DBE Response Spectra at High Frequencies

DOE-STD-1020-2002

motion. Such spectral shapes are necessary in order to provide practical input. DBE ground

motion at the site is defined in terms of smooth and broad frequency content response spectra in

the horizontal and vertical directions defined at a specific control point. In most cases, the

control point should be on the free ground surface. However, in some cases it might be

preferable to define the DBE response spectra at some other location. One such case is when a

soft (less than 750 feet/second shear wave velocity), shallow (less than 100 feet) soil layer at the

ground surface is underlain by much stiffer material. In this case, the control point should be

specified at the free surface of an outcrop of this stiffer material. Wherever specified, the

breadth and amplification of the DBE response spectra should be either consistent with or

conservative for the site soil profile, and facility embedment conditions.

Ideally, it is desirable for the DBE response spectrum to be defined by the mean uniform

hazard response spectrum (UHS) associated with the seismic hazard annual frequency of

exceedance, PH, over the entire frequency range of interest (generally 0.5 to 40 Hz). However,

currently considerable controversy exists concerning both the shape and amplitude of mean

UHS. (See DOE-STD-1023)

Preferably, the median deterministic DBE response spectrum shape should be

site-specific and consistent with the expected earthquake magnitudes, and distances, and the site

soil profile and embedment depths. When a site-specific response spectrum shape is unavailable

then a median standardized spectral shape such as the spectral shape defined in IBC 2000

NUREG/CR-0098 (Reference C-15) may be used so long as such a shape is either reasonably

consistent with or conservative for the site conditions.

C.3.2.1 DBE Response Spectra at High Frequencies

For PC-1and PC-2 SSCs, earthquake loading is evaluated from the base shear equation

in accordance with IBC 2000 seismic provisions. IBC 2000 includes a typical design/evaluation

spectra, which is plotted showing spectral acceleration as a function of natural period.

In the seismic design and evaluation criteria presented in Chapter 2, for PC-3 and PC-4 SSCs,

DBE spectra are used for dynamic seismic analysis. However, in accordance with Reference

C-5, for fundamental periods lower than the period at which the maximum spectral acceleration

occurs, spectral acceleration should be taken as the maximum spectral acceleration. For higher

modes, the actual spectrum at all natural periods should be used in accordance with

recommendations from Reference C-5. This requirement is illustrated in Figure C-5. Note that

this requirement necessitates that response spectrum dynamic analysis be performed for building

response evaluation. Alternately, the actual spectrum may be used for all modes if there is high

confidence in the frequency evaluation and F is taken to be unity. The actual spectrum at all

frequencies should be used to evaluate subsystems mounted on the ground floor; and to develop

floor response spectra used for the evaluation of structure-supported subsystems.

The basis for using the maximum spectral acceleration in the low period range by both

the Reference C-2 and C-5 approaches is threefold: (1) to avoid being unconservative when

C-16