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26 CHAPTER 4 Work Plan: Analytical Methodologies The goal of Task 3 for the NCHRP 12-70 Project was to iden- motion demand. The NCHRP 20-07 Project recommended tify analytical methodologies that would be developed to adoption of the 1,000-year return period for the extreme address the knowledge gaps and problems presented in the limit state (that is, an event having a 7 percent probability previous chapters. The discussion of the work plan for analyti- of exceedance in 75 years). The NCHRP 20-07 guideline cal methodology developments is presented under four major also focused its approach on the spectral acceleration at headings: 1second period (S1). This was an important development prompted by the observation that PGA is not a good param- Seismic ground motions eter to correlate with historical damage to structures. Measures Retaining walls of ground shaking at some intermediate period range (say Slopes and embankments spectral accelerations around 1 to 2 seconds) are a better indi- Buried structures cator of displacement demand related to historical damage and hence more important for characterizing ground shaking The discussion of seismic ground motion follows earlier for design. This is also true for designing retaining walls, discussions about the importance of the ground motions to slopes and embankments, and buried structures. the overall Project. As noted previously, decisions on seismic In general, PGV is closely related to spectral accelerations ground motion levels depended to a certain extent on conclu- at intermediate periods and, therefore, is a more appropriate sions reached during the NCHRP 20-07 Project, which was con- measure of ground motion displacement demand than PGA, ducted as a separate contract. One of the principal investigators especially for cross correlation to the amplitude of ground for the NCHRP 12-70 Project served as a technical advisor to deformations or permanent slope displacements. Also, re- the NCHRP 20-07 Project, enabling the NCHRP 12-70 Project cent seismological research suggested that lower levels of to keep abreast of the ground motion recommendations and spectral acceleration at intermediate periods for CEUS com- other components of the NCHRP 20-07 Project that could pared to WUS, and these reductions are relevant to Project affect the NCHRP 12-70 Project. requirements. Historically, due to the absence of strong motion data from CEUS sites, seismic design criteria for projects in CEUS 4.1 Developments for Seismic have generally been developed by applying the small PGA Ground Motions values from the CEUS sites to empirical WUS spectral shapes The first area of development involved the ground motions to define the target design spectrum for CEUS conditions. used during the seismic design of retaining walls, slopes and However, studies such as NUREG/CR-6728 conducted by the embankments, and buried structures. The LRFD design pro- Nuclear Regulatory Commission (NRC) for nuclear power cedure involves comparing the capacity of the design element plant applications (NUREG, 2001) have shown that the dif- to the seismic demand for various limit states (that is, strength, ferences in CEUS seismological conditions not only result in service, and extreme). Establishing the seismic ground motion lower shaking levels (that is, lower PGA), but also result in was a necessary step when defining the expected demand dur- much lower long-period content for CEUS sites. The NUREG/ ing seismic loading. CR-6728 studies have been adopted by the NRC in recogni- The Project followed the recommendations from the tion of the fundamental difference between requirements NCHRP 20-07 Project in the definition of the seismic ground for seismological studies in CEUS versus historical WUS

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27 Figure 4-1. Boundary between WUS and CEUS. practice. Figure 4-1 presents the WUS and CEUS geograph- CR-0098 spectral shape shown in Figure 4-2 is based on New- ical boundary following the USGS seismic-hazard mapping mark's recommendation using historical strong motion data program. The boundary basically follows the Rocky Moun- from WUS, while the spectral shape for CEUS was developed tains passing through Montana, Wyoming, Utah, Arizona, using procedures described in the NUREG/CR-6728 report then bending east through southern Colorado, New Mexico, based on up-to-date techniques for CEUS endorsed by NRC. and western Texas. The Regulatory Guide 1.60 is the historical design spectral shape Figure 4-2 presents results from a major study funded by originally used for designing nuclear power plants, now consid- NRC to identify differences in ground motion characteristics ered overly conservative. In this figure both spectral displace- between WUS and CEUS for horizontal motions representa- ment (RD) and peak spectral acceleration (PSA) at 1 second tive of magnitude 6.5 events for generic soil sites. The NUREG/ are normalized by PGA. Figure 4-2. Spectral curve shapes for generic sites covering both WUS and CEUS (Sandia, 2004).