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Suggested Citation:"References." 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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Suggested Citation:"References." 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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Suggested Citation:"References." 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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Suggested Citation:"References." 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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Suggested Citation:"References." 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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Suggested Citation:"References." 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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Suggested Citation:"References." 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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17 CHAPTER THREE APPROACH TO SURVEY OF CURRENT PRACTICE BACKGROUND characterization of properties of the soil column. The survey then presents a series of questions about the nature of site To obtain a better understanding of which practices, pro- response analysis (equivalent-linear, total stress nonlinear cedures, and site response models are used in engineering analysis, effective-stress nonlinear analysis) and the process practice, a formal survey was developed and posted on a of model setup and development of model input parameters. website specializing in this type of application. The draft Respondents are asked about their approach to dealing with version of the survey was pretested by a select group of DOT uncertainties in the analysis process. Finally, the survey representatives. The formal invitations to participate were asks respondents about how they evaluate the results of site sent through TRB to a number of individuals that practice in response analyses and how they use the output from site the areas of geotechnical earthquake engineering and struc- response analyses in further engineering analyses. tural engineering (structural dynamics). In particular, the invitations were sent to the DOT representatives and consul- TABLE 3 tants identified by DOTs. In the second solicitation for par- MAIN SURVEY TOPICS ticipation in the survey, invitations were sent to consultants Topic No. Topic Description identified in the first round by survey participants and the principal investigators (PIs), and also to select domestic and 1 General Practice international researchers and software developers identified 2 Criteria and Programs Used in Site Response Analysis by PIs. 3 Dimensions, Analysis, and Model Type 4 Seismic Hazard Motion Input Required for Site Response It should be noted that the list of participants was not Analysis developed with any explicit consideration of strict statisti- 5 Soil Profile Input Required for Site Response Analysis cal concepts. The results of the survey, therefore, may be 6 Site Response Analysis (Procedures, Models, Programs, biased and unrepresentative of the distributions of site etc.) response models and practices that are in use. Nevertheless, 7 Consideration of Uncertainties in Site Response Analysis the survey fulfilled its original goal of identifying practices, 8 Evaluation and Use of Results procedures, and site response models used in practice and provided additional insights into the manner in which engi- neers use those models. Although most of the survey's 37 questions were multiple choice, many required that respondents assign percentages to various choices and others requested that users indicate OVERVIEW OF THE SURVEY multiple selections where appropriate. Several questions asked respondents to provide additional information within The survey covered eight main topics (see Table 3). The top- the multiple choices. The respondents were also asked to ics and their sequence were designed to reflect the steps that provide other information if the listed choices were not rep- a practicing engineering would take in addressing the topic resentative of their practice. Finally, the respondents were of site-specific evaluation of earthquake ground motions. A encouraged to comment on the general subject matter and number of the questions and topics are similar to those in the on issues that they believed were important to advancing the survey of Kramer and Paulsen (2004), but the current sur- practice of seismic site response analysis. (See Appendix A vey goes into greater detail. The survey starts by asking the for a copy of the survey.) respondents questions about their general practice and about the guidelines and manuals they use for site-specific evalu- ations. The survey then asks respondents about the criteria SURVEY RESPONDENTS they use to determine when a site response analysis using computer software is required. The next questions are about Thirty-seven of the 70 people who were invited to partici- the development of input required for a site response analy- pate in the survey responded. An additional two respondents sis, including (1) ground motions and seismic hazard and (2) provided incomplete yet useful responses that were included

18 in the analysis, increasing the statistical database to 39. The abbreviated responses provided by e-mail, received from four state DOTs, were not included in statistical processing but were considered as well where appropriate. We received responses from 16 states, including all T-3 states and 5 of the 6 T-3+ states. The balance of respondents who completed the study were consultants identified by DOTs of T-3+ states, including states that do not perform FIGURE 10 Geographic distribution of survey respondents as generated by the survey program. site response analyses in-house and researchers that were contacted primarily to obtain relevant information about the programs and/or models they developed. This study does not further identify the respondents to preserve the prom- SURVEY RESPONSES ised anonymity. The full details of the survey responses appear in Appendix The respondents represented DOTs/firms with a wide B. The authors have examined the responses for regional dif- variety of sizes; some were describing their own practices ferences or differences between DOTs of T-3 states and their and others the practices of their DOTs or firms. Some indi- consultants. The survey responses did not show trends that cated that their responses represented their own views and warrant the separation of the survey into subcategories by practices, and others indicated that their responses repre- respondent type. The following chapter presents a synthesis sented the practice of DOTs and/or firms with many engi- of the survey findings and are used to evaluate the current neers. Figure 10, generated by the survey program, shows state of practice. the geographic distribution of the survey respondents.

19 CHAPTER FOUR SURVEY RESPONSES AND RELEVANT LITERATURE GENERAL PRACTICE site response analysis is relied upon extensively in engi- neering practice. Most respondents said that they use site The first set of questions asked the respondents about the response analysis when they anticipate F and E site classes, nature of their practice, the number of projects they are soft ground conditions, and liquefaction. Site response anal- involved with, and the design guidance documents they use yses are required where a hazard level is high, with varying in site response analysis. Most of the respondents provided definitions of what is considered high. Analyses were also responses based on their agency or firm practice, with size required when important or critical structures were being varying from a single person practice to large firms with up considered, again with a range of definitions as to how a to 500 engineers. Many of the respondents participated in structure is classified. more than seven projects a year involving site response anal- ysis while a significant number of respondents were involved with only one to two projects a year. DIMENSIONS, ANALYSIS, AND MODEL TYPE A number of respondents use guidelines for their seismic The responses showed that 1-D equivalent-linear analyses practice. Information from these documents are shown in are by far the most commonly used method in contemporary Appendix B, Table B4. The most commonly used document geotechnical earthquake engineering practice. When T-3+ is the AASHTO guideline itself or state DOT adaptations respondents and consulting firms were asked to estimate of the guideline. However, these guidelines do not provide the percentages of different types of analyses (relative to the detailed procedures for conducting a site response analysis. total number of site response analyses performed), both said that they used 1-D equivalent-linear analyses much more fre- quently than other types of analyses (Table B13). CRITERIA AND PROGRAMS USED IN SITE RESPONSE ANALYSIS Some respondents pointed out that site response efforts are usually controlled by budget and time constraints. Oth- A number of approaches are available to engineers for con- ers expressed concern that equivalent-linear analyses were ducting a site-specific evaluation of ground motions. These often used for soft clay and liquefiable sites and for very include (1) empirically derived site factors and (2) computer strong levels of shaking where their inherent assumptions software for site response analysis. about material behavior are least valid. What is revealing in the responses is that half of respondents perform nonlinear The survey respondents were asked what factors trig- site response analyses, indicating the significant rise in pop- gered the use of site response analysis versus the use of stan- ularity of this type of analyses. This is a marked difference dard code-based factors. The respondents were also asked to from the findings of Kramer and Paulsen (2004), where few identify conditions in which the use of code-based factors used nonlinear site response analyses. was acceptable (Tables B5 and B6). All respondents indi- cated that they use code-based factors, especially in prelimi- The computer software used to perform these analyses nary design. Code-based factors are used for class sites other are listed in Table B14. For equivalent-linear analyses, the than F, including sites suspected to have high soil liquefac- most commonly used program is one of the many avail- tion potential. able flavors of the program SHAKE. A few respondents indicated they used the program DEEPSOIL. For nonlin- The respondents were asked to provide detailed descrip- ear analyses, D-MOD (i.e., D-MOD2000) was the most tions of conditions whereby the use of site response analysis commonly used software followed by DEEPSOIL and is required. The respondents provided detailed input on the then FLAC. Several respondents also listed multipurpose various options provided in the survey, and this feedback analysis software such as ABAQUS, PLAXIS, and DYNA- in provided in Table B7 through Table B12. A total of 10 FLOW. It is worth noting that this reflects U.S. practice. of 32 respondents indicated that they perform site response CyberQuake (Modaressi and Foerster 2000) is widely used all the time, which is a significant number indicating that in a number of European countries.

20 The survey also asked the respondents about the software to achieve statistically significant results. Many respondents verification and validation procedures (see Table B15). Their were also aware of the need for special considerations for answers ranged from none to extensive evaluation to com- ground motions, including directivity, and velocity pulses to parisons between software. A number of users conducted capture near-fault effects. equivalent-linear analyses and compared them with non- linear analyses. Others performed analyses using multiple The respondents had varying responses to the question of programs and looked for consistency. Verification and vali- how they handle uncertainty in the selected ground motions, dation are complex and challenging tasks. Although exten- but one common theme was that the motions were allowed sive studies are available for 1-D, there is a dearth of 2-D and to vary to some degree about the target response spectrum. 3-D verification studies. Some said that the degree of deviation was within 50% for the spectrum, whereas others stated that it was within 84 percentile of the ground motion. Clearly, the responses do SEISMIC HAZARD AND MOTION INPUT REQUIRED FOR not reflect a consensus by the respondents. SITE RESPONSE ANALYSIS The respondents generally agreed that the topic of input The seismic ground motion at a site is commonly defined ground motions needs more work and guidance is needed in as a target response spectrum corresponding to a given this area. A few of the respondents were aware of the signifi- desired hazard level. Such a spectrum can be derived using cance of using Conditional Mean Spectrum (CMS) in gen- deterministic or probabilistic seismic hazard analysis. erating input ground motion. Respondents also indicated the More recently, conditional mean spectrum (CMS) has also need for guidance on generation of ground motions for deep been used (Baker and Cornell 2006; Baker 2011). The tar- basins where rock depth is great. A respondent indicated that get response spectrum is developed at an equivalent rock the development of the PEER ground motion toolbox (http:// outcrop and then site factors are used to include the effect peer.berkeley.edu/peer_ground_motion_database/) is an of local conditions on the ground motions. More recently, important step forward. with the use of NGA attenuation relationships, the site factors are embedded in the equations through the use of Vs30 (average shear wave velocity of top 30 meters) as an SOIL PROFILE INPUT INFORMATION REQUIRED FOR input parameter (Abrahamson and Silva 2008; Boore and SITE RESPONSE ANALYSIS Atkinson 2008; Campbell and Bozorgnia 2008; Chiou and Youngs 2008; Idriss 2008; and USGS 2008). However, the Site response analyses require information on the soil pro- user needs to be familiar with important limitations when file, including stratigraphy, shear wave velocity, location of using these equations. the water table, and dynamic soil properties. A series of questions were posed related to the seismic Most of the survey respondents indicated that they directly hazard input used by respondents in the site response analy- measure shear wave velocity--an important parameter. This sis. Many respondents rely on U.S. Geologic Survey maps reflects an important positive development in the profession. and web tools or their adaptations in various code provisions Nevertheless, a substantial number of respondents reported (AASHTO, ASCE7-05, IBC). However, a significant num- that they obtain the shear wave velocity (Vs) profile from cor- ber of respondents use computer software (EZ-Frisk, RISK- relations of penetration resistance. Significant uncertainty is Engineering 2009, Haz-38) or spreadsheets to program associated with these correlations, and studies are needed ground motion prediction equations (GMPE). on their validity and alternatives for direct measurement of shear wave velocity. Selection of spectrally compatible ground motion time histories is the next important step in developing input for A number of respondents said that they performed labora- site response analysis. Many of the respondents selected nat- tory tests on soil samples to develop dynamic soil properties; ural time histories; significantly fewer used synthetic ground however, more respondents indicated that they used modulus motions. Many of the respondents used rigorous spectral reduction and damping curves available in the literature. In matching procedures. RspMatch is by far the most widely addition to the Darendeli, Vucetic-Dobry, and Seed-Idriss used program for development of these motions. curves, the respondents also used EPRI curves. A number of respondents use specialized custom-developed curves. Although a number of respondents indicated that they used up to three ground motions in the spectral matching The respondents were very much aware of the need to process, many said that they use seven pairs (two horizontal account for uncertainty in soil properties, but there was no directions) for site response analyses. A few indicated that consensus on the best approach. Several use upper, lower they use more. For some projects, respondents said that they bound soil profile approaches, and a few use systematic needed to use a greater number of motions (say, 15 to 20) randomization approaches (Toro and Silva 2001; Romero

21 and Rix 2005). Cost was often cited as a reason for not this is not the case among their consultants and research- conducting parametric studies, but a number of the survey ers. The most commonly used program for equivalent-linear respondents indicated that they subcontract this work to analysis is SHAKE2000, followed by DEEPSOIL. specialists/experts. Nonlinear Total Stress Analysis SITE RESPONSE ANALYSIS The survey responses reflect a broad range of opinions as to when a nonlinear (NL) site response analysis is needed. Equivalent-Linear Models Most respondents concur that a site response analysis is needed when dealing with softer soil sites (i.e., with classes The survey responses reflect a broad range of opinions as E and F), though, as with equivalent-linear analyses, others to when an equivalent-linear (EL) site response analysis is reported that they use them for site classes C and D classes as needed. Most respondents concurred that a site response well. There were no clear criteria regarding what PGA input analysis is needed when dealing with softer soil sites (classes would trigger a site response analysis. Some respondents E and F); some indicated that they use them for C and D indicated that they use it only for high PGA levels (PGA > classes as well. There were no clear criteria on the input PGA 1.0 g) whereas others indicated a lower PGA threshold (0.4­ that would trigger a site response analysis. Some respon- 0.5g). Most respondents recognized that for more important dents said that they used it for low levels of seismic hazard, structures (e.g., base isolated and long structures), an NL while others reported that they used it when the hazard is site response analysis is warranted. There was a consensus high. Most recognized that a nonlinear (NL) site response amongst respondents that an NL site response analysis could analysis is warranted for more important structures (e.g., be used when computed shear strains exceed 1%. A num- base isolated or long structures). ber of respondents said that NL analyses are used to reduce demand on a structure. Equivalent-linear analysis is a robust approach that has been used in the profession for the past 40 years. The lit- The responses reflect the occasional confusion about the erature (e.g., Hashash et al. 2010) indicates that 1-D EL use and role of site response analysis and the need for greater site response analysis can always be performed regardless understanding and guidelines. The use of strain-based crite- of whether the higher dimension or nonlinear site response ria for switching to nonlinear analysis is reasonable as this analyses are included. One-dimensional EL site response reflects when nonlinear soil behavior becomes important. analyses provide the key characteristics of a site and the However, a 1% shear strain threshold is likely too high, as propagated ground motion. They can be used to flush out many soils would be at or near failure at this level. Studies errors that might inadvertently be introduced in more by the authors have shown that nonlinearity in soil behavior advanced site response analyses. can affect site response at strains as small as 0.1% or lower. A PGA or ground intensity measure on its own probably will Equivalent-linear analyses require characterization of not be sufficient because strain levels in soft soils can be dynamic soil properties by modulus reduction and damping quite high even for what appears to be a low level of shaking. curves. A number of relationships for such curves are avail- Better guidance is clearly needed on when a nonlinear site able in the literature. In the 1970s, separate curves were pre- response analysis is necessary. sented for sands and clays before the effects of soil plasticity and effective confining pressure on soil behavior were well Nonlinear site response analyses use constitutive models understood. The responses showed, however, that the origi- that represent nonlinear soil behavior. Models are param- nal sand and clay curves are still widely used in contempo- eterized in different ways, so comparison is much more rary practice. Other widely used curves include Vucetic and difficult than for equivalent-linear site response analyses. Dobry (1991), EPRI (1993), and Darendeli (2001). The survey responses indicate that the choice of nonlinear soil model was closely tied to the choice of nonlinear site The available literature does not provide a systematic evalu- response software, because some nonlinear software does ation of the various modulus reduction and damping curves and not offer multiple soil models. Modified hyperbolic models their applicability. Indeed, the rigorously documented curves of with Masing rules were the most widely reported nonlin- Darendeli (2001) and Menq (2003) have not been yet published ear soil models. The respondents indicated that they tried to in the refereed literature. All these curves are generic in nature calibrate their models to match both modulus reduction and and can be selected based on soil index properties and stress damping curves. history. Only one respondent indicated that they directly mea- sure soil dynamic properties in the laboratory. Nonlinear Effective-Stress Analysis The survey indicates that 1-D, equivalent-linear analyses The survey responses reflect a broad range of opinions as to are by far the most commonly used by DOTs surveyed, but when an NL effective-stress site response analysis is needed.

22 Most respondents concurred that a site response analysis is this evaluation. They looked for reasonableness of the results needed for softer soil sites (classes E and F). There were no and compared the results with empirical correlations such as clear criteria regarding the input PGA; however, most of the existing code factors for similar site classes. They also used respondents used a lower PGA threshold for NL effective- multiple software and analysis approaches. stress analysis than the threshold used in the NL total stress analysis. For more important structures, an NL site effec- Use of Results tive-stress response is warranted. There was no consensus among respondents on the pwp ratio threshold for which NL Site response analyses provide a range of information that effective-stress analysis results might be considered. can be used for engineering analyses. Most respondents use the resulting computed surface response spectrum and com- The choice of nonlinear effectives stress soil model was puted surface time histories. The ground motion is presum- closely tied to the choice of nonlinear site response software. ably used in the dynamic analysis of the bridge structure. The For 1-D site response, Matasovic's (1993) pwp model is respondents also said that they use profiles of PGA, shear used (D-MOD2000, DEEPSOIL), OpenSees users apply the strains, and shear stresses in their engineering analyses. One Elgamal model, and FLAC users employ UBC. respondent reported that the type of output used depends on the type of engineering analysis (structure response versus soil liquefaction evaluation). EVALUATION AND USE OF RESULTS A number of respondents indicated that they use the Consideration of Uncertainties in Site Response Analysis shear strain profile as an input for pile analysis or for tunnel analysis. A few respondents reported that they did perform The results of site response analyses, like those of all other base line corrections on the output motion. However, many analyses, are influenced by uncertainties in input parame- respondents indicated that they either did not perform site ters. The survey attempted to determine the users' percep- response analyses or had specialists/consultants perform tions of where uncertainties influenced the results of site these analyses for them. response analyses most strongly and how these uncertainties were accounted for in the design process. For the simplified method of soil liquefaction assessment, most respondents reported that they use surface PGA from The survey listed a series of input parameters to typical equivalent-linear analysis. Few respondents, though, indi- site response analyses and asked which were considered most cated that they use computed surface PGA from nonlinear important. Respondents could (and did) indicate more than total stress analysis. A number of respondents said that they one parameter. Many of the respondents consistently indi- did not use cyclic stress ratio from site response analysis. cated that uncertainties in input motions were most impor- Some respondents indicated that they performed effective- tant. Several users expressed particular concern in eastern stress nonlinear site response analyses. These responses North America where few strong-motion records are avail- reflect significant confusion and lack of guidance as to the able. Uncertainties in material properties (e.g., soil stiffness use of site response analysis for soil liquefaction evaluation. and damping at high strain levels) were also considered to be important. Few respondents were concerned with damping Many respondents indicated that they use PGA, or a per- at small strains even though the literature review highlighted centage of it, in a pseudo-static-type slope stability analysis its importance in an analysis. About one-third of respondents and for the simplified Newmark-type sliding block analy- considered geometry and bedrock properties to be among the sis. One respondent indicated a preference for a 2-D site most important uncertainties. A similar number of respon- response analysis. dents emphasized material properties and input motions. Most respondents said that they use pwp generation to A number of different methods for accounting for uncer- evaluate the occurrence of liquefaction in a soil profile. One tainties in design were reported. The most common method respondent indicated that the analyses may mask liquefac- was the use of sensitivity analysis; details on how the results tion potential in the top layers owing to soil softening in the of sensitivity analyses were interpreted were not requested. lower layers. Another respondent said that it was difficult to About one-third of the total number of respondents based determine pwp parameters required in an analysis. Another their analyses on best-estimate values of input parameters indicated the need for further guidance on this issue. and then applied some degree of conservatism to the results. Evaluation of Validity of Site Response Analysis Results GENERAL COMMENTS ON THE SURVEY Survey participants were concerned about evaluating the Among the wide range of comments were areas requir- validity of their results and used a number of approaches in ing additional guidance. These included (1) vertical site

23 response, (2) SFSI analysis, (3) 3-D effects, and (4) 2011 independent review panels composed of both highly quali- Tohoku, Japan Earthquake data analysis. Most of those sur- fied practitioners and academics. Many were also interested veyed recognize the need for significant project oversight by in receiving the results of this survey and synthesis.

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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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