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5
CHAPTER TWO
CURRENT STATE OF EVALUATION OF SITE EFFECTS ON
GROUND MOTIONS
INTRODUCTION SITE RESPONSE EVALUATION APPROACHES
Site-specific evaluation of earthquake ground motions Three general approaches can be used to evaluate soil (i.e.,
includes a number of contributors such as soil stratigraphy, site) effects on ground motions: (1) the attenuation relation-
basin effects, regional geology, topographic relief, and soil- ship approach, (2) the code-factor approach, and (3) the site
foundation-structure interaction (SFSI). The study of basin response analysis approach.
effects, regional geology, and topographic relief impacts on
ground motions are primarily in the domain of engineering The attenuation relationship approach uses attenua-
seismology and remains primarily in the realm of research. tion relationships or ground motion prediction equations
Code-based factors (e.g., Eurocode 8; EC8 2000) have been (GMPE) that consider local site effects, including soil con-
introduced to account for these effects, but site-specific evalu- ditions. While older attenuation equations distinguish only
ation of these effects for highway facilities is rare and will between soil and rock, recently published Next Generation
not be discussed in this study. Attempts have been made to Attenuation (NGA) relationships (e.g., Abrahamson et al.
capture some of these effects on a more limited basis through 2008) can provide ground motion prediction as a function of
the use of 2-D and 3-D analyses and will be briefly addressed shear wave velocity (Vs), including velocities based on ASCE
in that context only. The study of SFSI effects is an area under 7-type site classes that are also adopted by IBC 2006 (Inter-
rapid development, mostly by structural engineers. An over- national Code Council 2006). In this approach, a response
view of these effects relevant to geotechnical engineers can spectrum is developed and can be used directly in a spectral
be found in Kramer (1996) and more recently in Kramer and analysis. If needed, ground motions would have to be sepa-
Stewart (2004) and is beyond the scope of this study. rately generated through some form of spectral matching,
which will be discussed later.
This study focuses on evaluation of local soil deposit-
related site effects as illustrated in Figure 1. The presentation The second approach for assessing soil effects on
is primarily focused on horizontally layered soil deposits, ground motions computes rock outcrop (surface rock)
including other soil deposits and earthen structures that can response spectrum using a rock attenuation equation and
be approximated as the horizontally layered soil deposits. then modifies the rock spectrum by generic soil amplifica-
Both total and effective stress conditions are addressed. The tion factors such as the FPGA (used in AASHTO); Fa, and
focus of this study is on practical applications relevant to Fv factors in Tables 11.4-1 and 11.4-2 of ASCE 7; or other
design and analysis of highway facilities. published sources such as EPRI (Electric Power Research
Institute 1993), Rodriguez-Marek et al. (2001), or Stewart
et al. (2003). As in the first approach, a response spectrum
is developed and can be used directly in a spectral analy-
sis. If needed, ground motions can be generated separately
through some form of spectral matching, which will be
discussed later.
The third approach calls for evaluation of local site
effects by conducting a detailed site response analysis using
computer software. The site response analysis approach for
evaluation of site effects on ground motions is widely used
(see also the survey synthesis section, chapter four). This
approach is favored by Geotechnical Earthquake Engineers
as it takes into account the unique geotechnical character-
istic (i.e., "seismic signature") of a site. The approach uses
back analysis of numerous case histories and works well
FIGURE 1 Framework of site response analysis. when the profile has significant impedance contrast and
