Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments (2008) / Chapter Skim
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From page 35... ...
. At the time the NCHRP 12-70 Project work was being performed, preliminary feedback from the AASHTO T3 subcommittee was very favorable towards use of the 1,000 year return period and the NEHRP spectral shape concept.
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are used to anchor a spectral curve shape. Figure 5-1 shows the resultant design acceleration response spectrum after adjusting the referenced soft rock spectrum for site soil effects.
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design approach also multiplies the resulting spectrum by a 2/3rd factor to account for the "reserve capacity" against collapse within most buildings.) The AASHTO procedure also involves anchoring the design spectrum at zero period (PGA)
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The purpose of the study was to establish the range in ground shaking levels that must be considered during the seismic design of retaining walls, slopes and embankments, and buried structures -- based on the recommendations given in the NCHRP 20-07 Project. The 1,000-year hazard spectra used in this sensitivity study were generated by making use of the USGS interactive website, rather than the results of the USGS 1,000-Year Mapping Program.
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The three spectral curve shapes define an upper bound (UB) , lower bound (LB)
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Spectral curve shapes from spectra presented in Figure 5-2. Figure 5-4.
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5.2.1 Approach for Updating Newmark Charts One major step in establishing performance criteria for design purposes is to estimate the displacement of a retaining structure or slope due to the design earthquake. When a time history of the design earthquake is available, earthquakeinduced displacements can be calculated using the Newmark's sliding block method.
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5.2.3 Permanent Displacement Data Permanent displacement is a characteristic of the strong motion record, as well as the ratio of the structure yield acceleration to peak ground acceleration in the sliding mass (ky/kmax) of the subject structure.
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Results in Figures 5-8 and 5-9 show the following trends: • Records with higher magnitudes generally have higher amplitude in the long-period range. • Records for WUS and CEUS generally have different spectral shapes.
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kmax = the maximum seismic acceleration in the sliding block, A = peak ground acceleration (in/sec2) , and V = peak ground velocity (in/sec)
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Figure 5-9. Average normalized spectral acceleration for soil records.
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For this Project kmax is used rather than As to be consistent with the common practice in geotechnical earthquake engineering of using k as the seismic coefficient during seismic earth pressure and slope stability evaluations. 5.2.8 Comparison Between Correlations A comparison between correlations for different regions and site conditions has been performed.
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Figure 5-11. Comparison between WUS-Rock and WUS-Soil correlations for PGV = 60 kmax.
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Figure 5-14. Comparison between WUS-Rock and CEUS-Rock correlations for PGV = 30 kmax.
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PGV = 60 × kmax, respectively, with respect to the mean design curve given by Equation (5-8)
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The initial approach taken to develop the PGV-S1 correlation involved performing statistical studies of the USNRC database. However, the resulting correlation exhibited considerable scatter.
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CEUS records. The NUREG/CR-6728 strong motion data, as discussed in Section 5.2.4, was used to evaluate the validity of the Abrahamson PGV equation shown above.
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Figure 5-21. Comparison between Abrahamson PGV equation with only NUREG/CR-6728 CEUS rock data.
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Comparison between Abrahamson PGV equation with only NUREG/CR-6728 WUS soil data.
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As shown in this chapter, the recommended 1,000-year return period is a significant change from the existing AASHTO Specifications, in terms of PGA and spectral shape for WUS and CEUS locations. These differences need to be considered when conducting seismic analysis and design for retaining walls, slopes and embankments, and buried structures, and therefore these ground motion discussions form an important component of the overall NCHRP 12-70 Project.
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