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Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions (2012)

Chapter: CHAPTER THREE Approach to Survey of Current Practice

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Suggested Citation:"CHAPTER THREE Approach to Survey of Current Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Page 17
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Suggested Citation:"CHAPTER THREE Approach to Survey of Current Practice." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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7 structural response. The AASHTO Guide Specification for period of interest. Bommer and Acevedo (2004) present a Seismic Isolation Design, 3rd Edition (AASHTO 2010b) series of recommendations applicable to the selection of real requires the use of three sets of time histories (a single set records for engineering analysis. Kottke and Rathje (2007) consists of two horizontal components and a vertical com- developed a semi-automatic procedure that facilitates the ponent). If three time-history analyses are performed, then selection of a suite of motions from a user-provided library of the maximum response is used for design. If seven or more records, and the scaling of the selected motions so that their time-history analyses are performed, then the average of the average fits a target response spectrum. Baker et al. (2011) response parameter of interest may be used for design, per (http://stanford.edu/~bakerjw/gmselection.html) developed the same code. a similar procedure, with time histories scaled up or down to match the target spectrum at a predominant period of the In general, regardless of the source cited, the approach facility of concern. for the development of site-specific design ground motions (acceleration time histories and/or acceleration response For the spectrum matching approach, a carefully spectra) considers two steps: initial selection of time histo- selected time history (seed motion) is adjusted either in the ries, and modification of time histories. frequency domain by varying the amplitudes of the Fou- rier amplitude spectrum, or in the time domain by adding The initial selection of time histories includes records wavelets in iterations until a satisfactory match to the target that closely match the site tectonic environment, controlling spectrum is obtained. Spectral matching is considered to be earthquake magnitudes and distances, local site conditions, an "art" (Abrahamson 2008) as it requires certain skills to response spectral characteristics, and, for geotechnical eval- produce a single time history that typically replaces three to uations, duration of strong ground shaking. Both recorded four natural records. At a minimum, magnitude, distance, time histories from past earthquakes and carefully gener- spectral content, and rupture directivity need to be con- ated synthetic time histories may be used. Multiple time sidered when selecting a seed motion. An example of time histories are considered; the number of records depends on history successfully matched to design (target) spectrum is the type of analysis and the modification method used (dis- shown in Figure 2. cussed below). The most popular sources of this information on the Internet are the PEER Strong Motion Database (www. peer.berkeley.edu), COSMOS Virtual Data Center (http:// db.cosmos-eq.org), and the KiK-net Digital Strong-Motion Seismograph Network (www.kik.bosai.go.jp). PEER pro- vides records mostly for crustal seismic events and offers a search tool that facilitates the selection of records based on a number of site and seismogenic parameters. For subduc- tion events, the COSMOS website provides records from a number of subduction zones around the world. Records for Japan are available from the KiK-net network. For areas in the Central and Eastern United States that lack an adequate number of recorded events, synthetic accelerograms are generated from the hazard deaggregation at the site, per- FIGURE 2 Spectrum matching approach for selection of formed through the U.S. Geological Survey national seis- design time histories. mic hazard mapping project website (http://eqint.cr.usgs. gov/deaggint/2002/index.php). The spectrum matching approach is gaining acceptance as the structural design is steadily moving away from a code- Modification of time histories is required because the based response spectrum approach to a response spectrum initially selected time histories often differ from the design developed as a part of site response analysis. This approach motions in terms of shaking peak amplitude and response is especially convenient for engineers because it calls for spectral ordinates; they would need to be modified for use analysis based on matching of suites of ground motions to a in analysis. Two modification methods are commonly used single spectrum, hence no "enveloping" of shear forces and in practice: simple scaling approach and spectrum matching moments generated by multiple time histories is required. It approach. is also adopted for geotechnical applications (e.g., Greater Vancouver Liquefaction Task Force 2007). "User friendly" In the simple scaling approach, the entire time history software for modifying the seed record in the time domain is is scaled up or down so that its spectrum approximately available, either in stand-alone form (e.g., the original Rsp- matches that selected for design (target spectrum) over the Match by Abrahamson 1992; RspMatch 2005 by Hancock et

8 al. 2006; RspMatch 2010 by Al Atik and Abrahamson 2010) site. The shear wave velocity profile and its variability across or as part of software suites (SHAKE2000/D-MOD2000, the site is another key parameter. Direct measurement of the Ordonez 2000; Matasovic and Ordonez 2007). shear wave velocity is now commonplace using techniques such as the seismic cone penetration test (sCPT), down- For sites near faults, the ground motion time history needs hole logging, suspension logging, Refraction Microtremor to include additional characteristics beyond spectral match- (ReMi), and spectral analysis of surface waves (SASW) ing. These include directivity, velocity pulses, and fling techniques. The details of field investigations to character- effects (e.g., Somerville 1998; Munfagh et al. 1999; Bray et al. ize the soil profile is beyond the scope of this report and can 2009). Significant specialized expertise is required to prop- be found in other sources (e.g., Kavazanjian et al. 1997a,b; erly represent these effects in ground motion time histories; Sabatini et al. 2002). discussion of these issues is beyond the scope of this report. Development of Soil Properties Typically, the input time histories for site response anal- ysis are specified as rock outcrop acceleration time histo- Site response analysis also requires index properties such ries that are then modified within the program to represent as density, Atterberg limits, and relative density of the vari- time histories in bedrock underlying the site. For nonlinear ous layers. Strength properties such as friction angle and analysis, an "outcropping" motion is used in a simulation by undrained shear strength are important input properties, introducing a layer representing an elastic half space (trans- especially for soft soils and areas with high levels of shak- mitting boundary) at the base of the soil column. Similarly, ing. In addition to these properties, dynamic soil properties an "in-hole" (i.e., "within") motion, commonly obtained of the soil layers need to be defined. Laboratory tests using from a downhole array, is used in a simulation by using a cyclic triaxial, cyclic direct simple shear (DSS), and reso- rigid half-space. Some site response analysis programs, nant column devices are by far the most common devices for such as PLAXIS, OpenSees, and ABAQUS, allow the input defining the dynamic behavior of soils at a given site. These motion to be entered as acceleration, velocity, or displace- tests hinge on the availability of high-quality undisturbed ment time history. On the other hand, FLAC allows the input samples, which might be available for cohesive soils, but are motion to be entered only as a velocity time history. difficult to obtain for cohesionless soils. In areas where "competent rock" is too deep (e.g., Mis- Cyclic laboratory tests are therefore not as commonly sissippi embayment where competent rock is at depths of 1 available, and engineers often rely on standardized dynamic km or more south of Memphis), significant debate remains soil response curves in the form of normalized modulus as to what depth could be used in site response analysis, what reduction and damping curves as a function of shear strain material/shear wave velocity needs should represent "com- that approximate the nonlinear hysteretic soil behavior. petent rock," and what type of design motion can be used at Figure 3 shows the hysteretic stress strain behavior of soils such depth (e.g., simulated motion, bedrock outcrop motion, under symmetrical cyclic loading by (1) an equivalent shear or deconvoluted motion). modulus (G ) that corresponds to the secant modulus through the endpoints of a hysteresis loop; and (2) equivalent viscous Recent research at the Pacific Earthquake Engineering damping ratio (), which is proportional to the energy loss Research Center (PEER) by Haselton (2009) and Baker et from a single cycle of shear deformation. Both G and are al. (2011) provides a detailed review on the topic of ground functions of shear strain amplitude (), as can be inferred motions selection and scaling with a focus on structural appli- from Figure 3a. Plots of shear modulus normalized by the cations. Baker et al. (2011) introduce a new ground motions maximum shear modulus and damping as a function of shear selection procedure whose response spectra match a target strain are then developed, as shown in Figure 3b. mean and variance. The procedure avoids the use of spec- tral matching approaches by taking advantage of availability Over the years, a number of standardized families of of the large PEER ground motion data base. The literature curves have been developed and used in the practice. These review and experience clearly indicate that ground motion include Seed and Idriss (1970), Vucetic and Dobry (1991), selection, scaling, and matching for site response analysis EPRI (1993), and more recently, Darendeli (2001) and Menq remain unsettled issues and more studies are needed in this (2003). These curves are used in both equivalent-linear and area to provide better guidance to practicing engineers. nonlinear site response analysis. Definition of Subsurface Stratigraphy EQUIVALENT-LINEAR SITE RESPONSE ANALYSIS A site response analysis requires detailed information on subsurface stratigraphy. A thorough field investigation is The equivalent-linear analysis approach was first introduced required to understand the geologic history of the soil depos- by Seed and Idriss (1970) and has remained substantially the its and define the soil and rock units and water level at a same since. The wave propagation through the soil deposit is

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 428: Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions identifies and describes current practice and available methods for evaluating the influence of local ground conditions on earthquake design ground motions on a site-specific basis.

The report focuses on evaluating the response of soil deposits to strong ground shaking.

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