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OCR for page 39
39 Table 5-3. 1000-year soft rock spectral ordinates. 5% Damped Spectral Acceleration (g) WUS Sites EUS Sites San Francisco Salt Lake City Los Angeles Minneapolis New Madrid Charleston Columbus Evansville New York Memphis Phoenix Seattle Period (Second) 0.01 0.607 0.593 0.443 0.492 0.051 0.952 0.397 0.200 0.406 0.101 0.040 0.015 0.10 1.107 1.306 0.861 0.986 0.091 1.995 0.916 0.474 0.910 0.240 0.094 0.031 0.20 1.431 1.405 0.985 1.139 0.116 1.687 0.746 0.407 0.713 0.184 0.090 0.033 0.30 1.361 1.393 0.856 1.034 0.102 1.368 0.588 0.326 0.547 0.132 0.077 0.030 0.50 1.102 0.998 0.647 0.776 0.071 0.920 0.391 0.220 0.348 0.078 0.059 0.024 1.00 0.686 0.671 0.328 0.433 0.039 0.437 0.191 0.113 0.158 0.038 0.038 0.016 2.00 0.363 0.247 0.149 0.194 0.021 0.190 0.085 0.052 0.066 0.017 0.021 0.010 Deag Magnitude at 7.9 7.9 7.2 7.0 6.6 7.7 7.7 7.7 7.3 7.0 7.7 7.7 1-Sec Deag Distance 11.5 12.0 7.0 1.7 171.0 17.2 59.7 164.2 23.5 413.9 616.6 939.3 (Km) Note: Spectral values shown in bold correspond to points SDS and SD1 in Figure 5-1. are less than 25 percent of what would be expected for The spectral shapes shown in Figure 5-3 reflect the varia- Memphis and Charleston (without considering the much tions in spectral shapes (that is, response spectra after nor- higher shaking at the epicenter location at New Madrid). malizing by the design PGA) across the United States for a ref- erenced soft rock condition classified as Site Class B by the The relationship between spectral accelerations at 1 second USGS. However, for sites where deposits of soil occur, the soft and the PGA also is observed to differ between the CEUS and rock spectra need to be modified to local site soil conditions. the WUS. A good rule-of-thumb is to assume that for the For typical soil sites (commonly encountered in practical de- Class B soft rock ground shaking, PGA is related to S1 by the sign conditions), there tends to be a higher level of amplifica- following relationship: (1) WUS Class B Rock Sites, PGA S1; tion for the intermediate period of response around 1 second. and (2) CEUS Class B Rock Sites, PGA 2S1. The effects of local soil amplification on the spectral shapes shown in Figure 5-3 also were evaluated. Following the NCHRP 20-07 Project guidelines, adjustments were made to 5.1.3 Variation in Spectral Shapes for Soil the spectral ordinates at 0.2 (short) and 1-second (long) pe- and Rock Sites in WUS versus CEUS riods. For this evaluation an adjustment factor for Site Class E The design response spectra shown in the previous section site conditions (loose sand or soft clays with Vs < 650 ft/sec.) were developed from the USGS Hazard Mapping website for was used to evaluate the maximum potential effects of soil the referenced soft rock conditions. Figure 5-3 presents the amplification on the spectral shapes. At lower shaking levels normalized spectral curve shapes for the spectra shown in where maximum site amplification occurs, the site adjustment Figure 5-2. factors were 3.5 and 2.5, respectively, for the short-period and The differences between the spectral curve shapes for long-period adjustment factors. CEUS (shown in dashed lines) versus WUS (shown in con- Figure 5-4 shows three spectral curve shapes developed tinuous lines) is quite evident in this figure. Beyond approx- from the above discussed sensitivity studies. These three imately 0.3 seconds, the ordinates for CEUS sites are gener- curves are used to illustrate variations in the spectral curve ally about half of the ordinates from WUS sites for the same shapes after allowing for differences between CEUS and WUS period, with the exception of the Columbus, Ohio and the ground motions, as well as between rock and soil site effects. Minneapolis, Minnesota sites. These sites are extremely far The three spectral curve shapes define an upper bound (UB), from known seismic sources and are of extremely low design lower bound (LB), and intermediate (Mid) spectral shape-- shaking levels. representing the combination of seismological variations

OCR for page 39
40 Figure 5-3. Spectral curve shapes from spectra presented in Figure 5-2. (that is, between WUS and CEUS) and potential soil condi- The Mid spectral curve shape is the soft rock spectrum tions variations (that is, Category B, C, D, and E sites). directly developed for San Francisco The physical representation of the three shapes shown in Figure 5-4 is: The spectral curve ordinates at 1-second period now reflect about a factor of 4.5 variation between the UB versus the LB The LB spectral curve shape was developed from the soft shaking conditions reflecting amplification of the intermedi- rock spectrum for the New York City site, a CEUS site. ate period (that is, about 1 second) motion due to site soil re- The UB spectral curve shape was developed for a San Fran- sponse effects. As discussed later, spectrum-compatible mo- cisco site, a WUS site, after applying the Site Class D soil fac- tions will be generated for the three spectral curve shapes tor to the San Francisco reference soft rock spectrum. that then will be used for slope and retaining wall scattering Figure 5-4. Spectral curve shapes adopted for further ground motion studies.