Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard- and Risk-consistent Ground Motion Spectra Guidelines (NUREG/CR-6728)
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Manuscript Completed: May 2001
Date Published: October 2001
R. K. McGuire1, W. J. Silva2, C. J. Costantino3
1Risk Engineering, Inc., Principal Contractor
4155 Darley Avenue, Suite A
Boulder, CO 80305
2Pacific Engineering & Analysis
311 Pomona Avenue
El Cerrito, CA 94530
3Carl J. Costantino, Consultant
4 Rockingham Road
Spring Valley, NY 10977
R. M. Kenneally, NRC Project Manager
NRC Job Code W6248
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
Recommendations for seismic design ground motions for nuclear facilities require a consistency with both observed strong motion data and with seismological theory on the characteristics of strong shaking. Different recommendations are appropriate for various regions of the US, because both earthquake source characteristics differ and the earth's crustal properties vary with region.
A database of recorded time histories forms the foundation of empirical recommendations for spectral shapes. This database includes motions recorded as recently as the 1999 Turkey and Taiwan earthquakes. Empirical attenuation equations derived primarily from California strong motion data form the basis for spectral shape recommendations for western US (WUS) sites on rock, and these spectral shape recommendations are confirmed and supported by the empirical database.
For the central and eastern US (CEUS), a well-validated, simple model of strong motion allows quantification of the difference between WUS and CEUS motions, accounting for differences in both the seismic source and in path and site attenuation. This model adjusts the WUS empirical soft-rock spectral shapes to CEUS hard-rock conditions. These spectral shape recommendations are made for both the 1-corner and 2-corner seismic source model for the CEUS, which are competing models that imply different spectral shapes for design.
Selecting the appropriate design spectrum or spectra requires a probabilistic seismic hazard analysis (PSHA) at the site for rock conditions. The seismic hazard is deaggregated at 10 and 1 Hz to determine the dominant magnitudes and distances at those frequencies. Two sets of spectral shapes are developed for those magnitudes and distances: one from the recommended functions, and a second from the attenuation equations used in the PSHA. In the CEUS, the designer will use both the 1- and 2-corner earthquake source models to develop weighted spectral shapes, both from the recommended functions and from the PSHA attenuation equations. The spectral shapes are scaled to match the uniform hazard spectrum (U-HS) amplitudes at 10 and 1 Hz, typically at the 10-4 annual frequency of exceedence level. The two sets of spectral shapes provide a consistency check with the UHS.
For design recommendations, the UHS is modified by a scale factor to a Uniform Reliability Spectrum (URS). This scale factor achieves a relatively consistent annual frequency of plant component failure across the range of plant locations and structural frequencies. It does this by accounting for the slope of the seismic hazard curve, which changes with structural frequency and site location. For some sites and natural frequencies the URS exceeds the UHS, and at other sites and frequencies it lies below the UHS.
For design purposes the spectral shapes determined from the attenuation equations are scaled to the 10 Hz and 1 Hz URS amplitudes. The URS must be matched within certain tolerances by the scaled spectral shapes, but the use of two (or more) design shapes allows a more accurate representation of the seismic threat, for example when a broad-banded spectrum is unlikely.
The database of recorded time histories on rock is divided into magnitude and distance bins, and three component records (two horizontals and one vertical) are archived on a CD-ROM for both the WUS and CEUS. We augmented available recorded rock motions for the CEUS by modifying WUS rock records to account for differences in seismic source and crustal properties between the two regions. This database allows designers to select one or a set of records from the appropriate magnitude and distance range and to adjust those records to match a rock design spectrum, for the derivation of detailed input motions.
For these artificial motions, we recommend criteria for matching their spectra to the target (scaled) spectra. The matching criteria lead to mean-based fits, with half of the spectral values above the target and half below, within specified limits. The matching is done with the response spectrum at 5% of critical damping, obviating the need to meet a minimum power spectral density requirement or to match at multiple dampings. However, checks are required of peak motion parameters, duration of shaking, and directional correlation.
For soil sites, a PSHA is conducted for rock conditions to determine spectra scaled to the 10 Hz and 1 Hz UHS amplitudes, as discussed above. These spectra represent control motions input to a soil model that calculates soil response and that accounts for uncertainties in soil properties. The soil analysis gives the mean soil amplification, its uncertainty, and its slope with increasing rock amplitude. These factors allow the engineer to estimate the soil UHS at 10-4 and 10-5 annual frequencies of exceedence, from which the 10-4 URS can be determined for that soil. Generic soil spectral shapes are not derived here because the soil spectra should be obtained from a site-specific analysis. The site-specific soil amplification studies yield spectral shapes that are scaled to the UHS (for a consistency check) and to the URS (for design purposes).
The database of recorded time histories includes motions at WUS and CEUS soil sites, divided into magnitude and distance bins, and these three-component motions are archived on a CD-ROM. The CEUS soil site motions were derived from WUS soil motions by modeling differences in seismic sources and crustal properties between the two regions. These archived records allow designers to select one or a set of records from the appropriate magnitude and distance range and to adjust those records to match a soil design spectrum, for the derivation of detailed input motions.
We demonstrate the procedures for developing design spectra for rock conditions and for four soil profiles in the WUS and in the CEUS, using as example sites a location in the Mojave desert, California, and Columbia, South Carolina. To demonstrate that the URS gives reliability-consistent design amplitudes, we examine eleven sites across the US and use three ground motion parameters at each. These results indicate that the URS, as calculated here, provides reliability-consistent designs over a range of site locations and structural frequencies.