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PRACTICEs AND PROCEDURES FOR SITE-SPECIFIC EVALUATION OF EARTHQUAKE GROUND MOTIONS SUMMARY The current AASHTO specifications for seismic design mandate site-specific evaluation of the earthquake design ground motion (i.e., the acceleration response spectrum) for ground conditions termed Site Class F. In the AASHTO specifications, Site Class F soils include soft clay sites. These AASHTO specifications also allow discretionary site-specific analy- ses for other ground conditions and a reduction in mapped design ground motions of as much as 33% if justified by a site-specific ground motion analysis. Some state departments of transportation (DOTs) are taking advantage of this site response reduction provision, particularly in cases where pore pressure generation could lead to soil liquefaction. For Site Classes C, D, and E, AASHTO's tabulated site response adjustment factors (site factors) are typically used to adjust mapped values of ground motions. However, as stipulated in AASHTO's recommendations, site-specific site response analyses have also been used in these circumstances as an alternative because the site factor approach may be inappropriate under some conditions. Of particular concern is the adequacy of the site factor approach in evaluating the response of short period structures (fundamental period of the site, To < 0.5 sec) on shallow bedrock sites (i.e., depth to bedrock less than 100 ft), and of longer period structures (To > 1.0 sec) at deep soil basin sites (e.g., depth to bedrock greater than 500 ft). For years, the equivalent-linear total stress approach, as programmed in one-dimen- sional (1-D) site response analysis codes (e.g., SHAKE) has been the primary method used to evaluate the influence of local ground conditions on earthquake design ground motions on a site-specific basis. However, this type of analysis has limitations: (1) for strong shak- ing at some sites owing to nonlinear site response effects resulting in large shear strain response, which is not properly captured in the equivalent-linear soil models; (2) at sites where soils have the potential to develop significant seismically induced excess pore water pressures, including soil liquefaction; and (3) at soft clay sites subject to moderate inten- sity/long-duration motions, because the analysis cannot take the effects of cyclic degrada- tion into consideration. This synthesis study 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 study focuses on evaluating the response of soil deposits to strong ground shaking, and therefore does not address representation of structural response. In accordance with current practice, the study's primary focus is on one-dimensional (1-D) site response analysis (both equivalent-linear and nonlinear). Two-dimensional (2-D) and three-dimensional (3-D) analyses are discussed, but at a more limited level. The synthesis study consists of a literature review and a survey of current practice. The literature review revealed significant developments in the area of nonlinear site response over the past decade, including studies to define usage protocols of total stress nonlinear site response analyses. New generations of generic dynamic material property sets (e.g., modulus reduction and damping curves, and seismic pore water pressure gen- eration model parameters) have also been developed and are now available for use in site
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2 response analyses. The review also showed increasing availability of commercial 2-D analy- sis software for site response analysis with pore water pressure generation. However, there is a lack of agreed-upon protocols for both 1-D and 2-D nonlinear site response analysis with soils that have potential for significant pore water pressure generation. The survey identifies survey participants, survey questions, methods of processing sur- vey responses, and relevant findings of the survey. Most of the survey participants were from state DOTs and their consultants. Selected academic researchers also participated in the sur- vey. DOTs invited to participate in the survey included the AASHTO Highway Subcommit- tee on Bridges and Structures, Technical Committee T-3 states (Seismic Design) (Alaska, Arkansas, California, Illinois, Indiana, Missouri, Montana, Nevada, Oregon, South Caro- lina, Tennessee, Washington), plus DOTs of Georgia, Hawaii (T-3+), Massachusetts, New York, Rhode Island, and Utah, and their consultants. The respondents represented DOTs/ firms of various sizes; some described their own practices and others the practices of their DOTs or firms. The survey questions encompassed criteria used by the respondents to deter- mine when a site response analysis (through computer software) is required; development of input parameters required for a site response analysis; nature of site response analysis (equivalent-linear, total stress nonlinear, effective-stress nonlinear) performed; and the pro- cess of model setup and development of model input parameters. The survey also asked about how uncertainties are dealt with in the analysis process and the practice for evaluating the results of site response analyses, as well as the use of site response analyses in further engineering analyses. The survey reveals widespread use of both equivalent-linear and nonlinear site response analyses by DOTs and other participants. A significant portion of the DOTs' analyses use nonlinear site response analyses, including analyses with pore water pressure generation for liquefaction evaluation. Use of 2-D numerical models in site response evaluation, although limited, appears to be increasing. The issues associated with the use of both nonlinear effec- tive-stress and 2-D software include development of adequate input parameters and ground motions, verification and validation of the results, evaluation of uncertainty, and vertical site response. The survey respondents provided a wide range of answers on a number of key issues in site response analysis, including input motion, material properties, analysis procedures, and use of results. The responses illustrate the lack of consensus and the need to develop guidance on these important issues. The study concludes with a list of research topics suggested by the survey participants and identified from the literature review. The topics include benchmarking of site response models with pore water pressure generation, shear wave velocity correlation evaluation, benchmarking of 2-D and 3-D analysis codes, vertical site response, and calibration with recent (i.e., the 2011 Tohoku, Japan, earthquake) data. Research in these topics will facilitate future reliable use of advanced analysis techniques in highway engineering practice.