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35 CHAPTER 5 Seismic Ground Motions This chapter summarizes the results of ground motion probability of exceedance in 75 years, which corresponded studies completed for the Project. The primary objectives of approximately to a 1,000-year return period; and (2) a change the ground motion studies were to in the shape of the 5 percent damped response spectrum in the longer period range. The discussion of these seismic load- · Provide a consistent basis for establishing ground motion ing criteria in this section begins with a review of the update to use during the seismic analysis of retaining walls, slopes to the current AASHTO LRFD Bridge Design Specifications. and embankments, and buried structures; This review is followed by a summary of the ranges of ground · Update Newmark charts for estimating permanent ground motions that can be expected in various regions of the United displacements of retaining walls and slopes to be consistent States and then the variation in response spectra for CEUS ver- with the results of ground motion studies for CEUS and sus WUS based on approaches recommended by the NCHRP WUS; and 20-07 Project. · Establish correlations between PGV and spectral accelera- tion at a period of 1 second (S1) for use in the seismic analy- 5.1.1 Update to AASHTO Seismic ses of retaining walls, slopes and embankments, and buried Ground Motion Criteria structures. Seismic loading criteria used by the NCHRP 12-70 Project Information in this chapter serves as input for the seismic were taken from the criteria being developed for the seismic response studies discussed in Chapters 6 through 9. These re- design of bridges within the NCHRP 20-07 Project Recom- sults also form the basis of sections in Volume 2 containing rec- mended LRFD Guidelines for the Seismic Design of Highway ommended specifications and commentaries in the AASHTO Bridges (Imbsen, 2006). At the time the NCHRP 12-70 Proj- LRFD Bridge Design Specifications. ect 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 5.1 Seismic Loading Criteria shape concept. Rather than taking a separate approach or The seismic design of bridges in the then current (2006) conducting a dual development, the NCHRP 12-70 Project AASHTO LRFD Bridge Design Specifications was based on the assumed that the NCHRP 20-07 recommendations would be peak ground accelerations and an appropriate response spec- adopted at the AASHTO meeting in 2007. AASHTO mem- trum for the site. This same general approach was reviewed bers later adopted the ground motion changes during a vote during the NCHRP 12-70 Project for the seismic analyses of in July of 2007. retaining walls, slopes and embankments, and buried struc- There were several good reasons for using the criteria de- tures. However, criteria in the AASHTO LRFD Bridge Design veloped for the NCHRP 20-07 Project for the seismic design Specifications were expected to change based on recommen- of retaining walls, slopes and embankments, and buried dations from the NCHRP 20-07 Project. Key changes recom- structures. First, it would be consistent with the approach mended by the NCHRP 20-07 Project included (1) a change being used by most transportation agencies and already used in the return period of the ground motion used for bridge de- in part within the current AASHTO LRFD Bridge Design sign from the existing 10 percent probability of exceedance in Specifications. Secondly, by using the same criteria as devel- a 50-year period (that is, 475-year return period) to a 7 percent oped for the NCHRP 20-07 Project, there was less chance for
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36 confusion between guidelines being used for different parts ground shaking criteria. The spectrum was defined on the of a project. Lastly, retaining walls, slopes and embankments, basis of spectral acceleration (Sa) at three periods: 0.0, 0.2 and buried structures are all components of the transportation and 1.0 seconds corresponding to the 1,000-year uniform- network and by using the same criteria used by bridges, there risk spectrum for a referenced soft rock condition. The was a common basis for judging risk to the transportation three periods defined the PGA, short-period spectral ac- system. celeration (Ss), and the spectral acceleration at 1 second Key aspects of the NCHRP 20-07 Project related to ground (S1), respectively. These spectral values are for soft rock motion criteria are summarized below: site conditions where the average shear wave velocity within the upper 100 feet of geologic profile ranges from 1. The safety level earthquake was based on the USGS/ 2,500 to 5,000 feet per second (ft/sec), which is referred as AASHTO seismic hazard mapping program. The recom- Site Class B. mended ground motion hazard level was a 7 percent prob- 4. The above three spectral ordinates (that is, at 0.0, 0.2 and ability of exceedance in 75 years, corresponding roughly 1.0 seconds) are used to anchor a spectral curve shape. to a 1,000-year return period. The USGS was contracted Figure 5-1 shows the resultant design acceleration re- by AASHTO to provide 1,000-year hazard maps and an sponse spectrum after adjusting the referenced soft rock implementation CD. spectrum for site soil effects. The adjustments for site ef- 2. The map and implementation CD, with the proposed fects account for amplification or deamplification of the specifications developed by the NCHRP 20-07 Project referenced rock motion for soil conditions at the site. This team, were used by various state bridge departments for method of determining the spectrum is generally the trial designs. These trials were carried out in 2006 and same as that proposed earlier in the NCHRP 12-49 Proj- balloting for adoption by AASHTO was held in July of ect (NCHRP Report 472, 2002) and has been used in 2007. As noted above, this meant that much of the both the 2003 and 2006 International Building Code NCHRP 12-70 Project had to proceed on the basis that (IBC) for regulating the design of new buildings. The the NCHRP 20-07 recommendations would be adopted primary difference with the new approach adopted by by AASHTO. AASHTO in July of 2007 from a ground motion stand- 3. The approach recommended in the NCHRP 20-07 Proj- point is that it is using the 1,000-year return period, ect report involved developing a free-field ground surface versus the 2,475-year return period recommended in design spectrum that served as the basic benchmark NCHRP 12-49 and IBC 2003 and IBC 2006. (The IBC Figure 5-1. Design response spectrum constructed with the three- point method.
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37 Table 5-1. Values of Fa as a function of site class and mapped short-period spectral acceleration. design approach also multiplies the resulting spectrum by factors to be applied to the two spectral ordinates for a 2/3rd factor to account for the "reserve capacity" against other site soil/rock categories. Table 5-1 tabulates site collapse within most buildings.) The AASHTO procedure coefficients (Fa) at the short period range (that is, at also involves anchoring the design spectrum at zero pe- 0.0-second and 0.2-second periods), and Table 5-2 tabu- riod (PGA) based on a 1,000-year return period hazard lates site coefficients (Fv) at the 1-second period. (AASHTO level. This approach compares to the IBC which assumes subsequently adopted a separate table for Fpga to be ap- that the PGA is equal to 0.4 times the spectral acceleration plied to PGA. Values of Fpga are the same as Fa. Note also at 0.2 seconds (that is, the short period spectral accelera- that AASHTO normalizes PGA to be dimensionless. The tion, Ss). The site coefficient used by AASHTO to adjust current version of AASHTO shows the same Fa and Fv the PGA value (Fpga) for various soil classifications is iden- values but without the units of gravitational acceleration tical to the coefficient used for the 0.2-second, short pe- (g).) The two site coefficient factors are applied to the riod site factor (Fa) recommended by the NCHRP 12-49 three spectral ordinates from the new AASHTO 1,000- Project and used by IBC. year maps and implementation CD for various site cat- 5. Similar to NCHRP 12-49 and IBC 2006, the NCHRP egories in relation to the reference USGS Site Class B 20-07 document provided two tables for site modification condition. Table 5-2. Values of Fv as a function of site class and mapped 1 second period spectral acceleration.