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38 The spectral ordinate at 0.2 second defines a flat plateau Figure 5-2 shows the results from this analysis; Table 5-3 with a constant spectral acceleration. This constant accel- tabulates these results. The figure shows the distinctly dif- eration branch of the spectral curve starts at 0.2 Ts where ferent shapes of the response spectra in CEUS versus the Ts is defined by the ratio of Sa at 0.2 seconds to Sa at 1 sec- WUS. In this figure, spectral curves for sites located in the ond. The long-period limit of the spectrum is governed more active WUS are shown by continuous lines, and sites by the intersection of the constant acceleration branch of for the less active CEUS are denoted by dashed lines. The the curve and the decreasing spectral acceleration branch difference between WUS and CEUS occurs along a distinc- of the response spectrum curve anchored at the 1-second tive boundary (see Figure 4-1) along the US Rocky Moun- ordinate. tains. West of this boundary is referred to as the more seis- The long-period range (decreasing spectral accelera- mically active WUS, and east is the less active CEUS. In tion) is defined by the spectral ordinate at 1 second general, ground shaking is higher in WUS as compared to along with the assumption that the curve shape is in- CEUS, especially at longer periods (for example, 0.5 seconds versely proportional to period (T); that is, Sa 1/T. This or more). 1/T decrease is consistent with an assumption of con- Other observations regarding the variation in ground mo- stant spectral velocity. It also corresponds with a spec- tion intensity between CEUS and WUS also were made from tral displacement that increases linearly with the period the sensitivity study, as summarized here. These observations of motion. (Note that the current IBC 2006 has a further are keyed to the spectral demand at the 1-second period, fol- provision where the 1/T decrease changes to a 1/T 2 de- lowing the approach taken in the NCHRP 20-07 Project, which crease. The period of this change differs across the makes use of spectral demand at 1 second for quantification of United States, ranging from 4 seconds to 16 seconds. the seismic design category. The change from 1/T to 1/T 2 was introduced for the de- sign of long-period structures, such as multistory build- 1. In general, the expected ground motion shaking level at ings, and for sloshing of large-diameter water reservoirs. 1-second period (S1), as measured by the 5 percent damped A similar approach has not been taken by AASHTO for spectral acceleration for WUS typically ranges from 0.3 to the design of long-period bridges. The maps in IBC 2006 0.6g. In contrast for CEUS, the shaking level is much lower are not applicable because they represent a return period for S1--typically no more than 0.2g, even for relative active of 2,475 years as opposed to the 1,000-year return period seismic areas near the cities of Memphis and Charleston. being recommended within the new AASHTO maps. It For many of the population centers, including New York is presumed that the seismic design of long-span bridges and Boston, S1 is well below 0.1g--often being 0.05g or less. would use site-specific evaluation methods in the ab- 2. There appears to be a larger range in ground shaking for sence of maps similar to those in IBC 2006.) CEUS sites as compared to WUS. For example, the design S1 for Seattle or Salt Lake City is approximately 50 percent 5.1.2 Range of Ground Shaking Levels in the of San Francisco and Los Angeles, the most active regions. In United States for Referenced Soft Rock contrast for CEUS, the population centers in the Northeast A sensitivity analysis was conducted during the NCHRP 12-70 Project to determine the ground shaking levels for the 1,000-year return period at various locations in the United States. Site Class B soft rock reference condition was used for conducting this analysis. The purpose of the study was to establish the range in ground shaking levels that must be con- sidered during the seismic design of retaining walls, slopes and embankments, and buried structures--based on the rec- ommendations 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 web- site, rather than the results of the USGS 1,000-Year Mapping Program. Although the USGS program was very close to completion at the time of this work, the results of the 1,000-year data were not available at the time the analyses were con- ducted (Fall 2005). Appendix C provides background infor- Figure 5-2. Variation in the 1,000-year benchmark mation on the USGS interactive website. soft rock spectra over the United States.