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3 CHAPTER ONE INTRODUCTION AASHTO specifications for seismic design, including both being used in practice, including methods that account for the 2009 Interim AASHTO Load and Resistance Factor deep soil basin effects and for pore water pressure genera- Design (LRFD) Bridge Design Specifications and the 2009 tion. Significant expertise is required to conduct and inter- Guide Specifications for LRFD Seismic Bridge Design, pret the results from these newer methods, often leading to mandate site-specific evaluation of the earthquake design questions about the validity of results. For instance, experi- ground motions (i.e., the acceleration response spectrum) ence with the newer nonlinear analysis methods show that for ground conditions termed Site Class F. In the AASHTO strains (and hence stiffness reduction) may become more specifications (AASHTO 2010a), Site Class F soils are soft localized than in an equivalent-linear total stress analysis. clay sites. These AASHTO specifications also allow discre- As a result, details of the soil profile, particularly soft lay- tionary site-specific analyses for other ground conditions ers and impedance contrasts, can have a larger effect on the and a reduction in mapped ground motions by as much as results of a nonlinear analysis than they do on the results 33% if justified by a site-specific ground motion analysis. of an equivalent-linear analysis. Furthermore, all available Some state departments of transportation (DOTs) are tak- methods of site response analysis (including equivalent- ing advantage of this site response reduction provision, linear total stress analysis) require significant expertise particularly in cases where pore pressure generation could and numerous discretionary decisions. For example, the lead to liquefaction. Furthermore, there is some evidence, analysis requires selection of an appropriate suite of time part of which is based on the authors' experiences, that histories and a determination as to whether the small strain the AASHTO site factors, used to adjust mapped values of modulus and other soil properties should be measured in the design ground motions for local ground conditions, may be field and/or laboratory or obtained using correlations. The inappropriate under some conditions. For example, they may analysis also requires decisions on the extent of sensitivity not be appropriate for short period structures (fundamental analyses and what modulus reduction and damping curves period of the structure, To < 0.5 sec) at shallow bedrock sites to use. More expertise and discretionary decision making is (that is, depth to bedrock less than 100 ft), and for structures required with nonlinear methods than with equivalent-lin- with a relatively long predominant period (To > 1.0 sec) at ear analysis and is greatest with analyses that consider pore deep soil basin sites [e.g., depth to bedrock greater than 500 pressure generation and dissipation. Commentary within ft (Park and Hashash 2005a, 2005b)]. Site-specific analyses the AASHTO specifications cautions the reader of potential are also being used in these circumstances as an alternative issues when conducting site-specific ground motion studies, to the use of AASHTO site factors. but the commentary does not provide guidance on the nature of these issues, and how or when to consider these potential For years, the equivalent-linear total stress approach, as issues. This lack of guidance raises concerns as to whether programmed in one-dimensional (1-D) site response analy- appropriate estimates of site-specific ground motions are sis codes, has been the primary method used to evaluate being made for design, potentially resulting in either exces- the influence of local ground conditions on earthquake sive project construction costs when ground motion response design ground motions on a site-specific basis. However, is overestimated or unacceptable risk to the public when this type of analysis has limitations: (1) for strong shak- ground motion response is underestimated. ing at some sites owing to nonlinear site response effects resulting in large shear strain response; (2) at sites where This synthesis study identifies and describes current prac- there is a potential for significant seismically induced tice and available methods for site-specific analysis of earth- pore water pressure buildup, including soil liquefaction, quake ground motions. The study is primarily concerned because it cannot consider the effects of pore pressure gen- with the response of soil deposits to strong ground shaking eration; and (3) at soft clay sites subject to moderate inten- and, as such, does not address representation of structural sity/long-duration motions as it cannot consider the effects response. The study's primary focus is on one-dimensional of cyclic degradation. (1-D) analyses as this represents both the majority of site response analysis work to date and current state of practice. A number of nonlinear site response analysis methods Two-dimensional (2-D) and three-dimensional (3-D) analy- have become available over the past decade and are now ses are discussed, but at a limited level.
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4 This study starts with a discussion of current knowledge pated in the survey. DOT's invited to participate in the survey based on a review of technical literature and contacts with included T-3 states (Alaska, Arkansas, California, Illinois, select publishers and software authors (researchers) for clar- Indiana, Missouri, Montana, Nevada, Oregon, South Caro- ification. As a part of the documentation of this literature lina, Tennessee, Washington), plus DOTs of Georgia, Hawaii search, an attempt was made to identify and explain key (T-3+), Massachusetts, New York, Rhode Island, and Utah, concepts involved in current site response analysis practice. and their consultants. The respondents represented DOTs/ The study also summarizes experience gained in developing firms of various sizes; some were describing their own prac- and employing these methods, including challenges in their tices and others the practices of their DOTs or firms. application and perceived advantages and disadvantages of the different methods. As appropriate, the synthesized survey results are related to findings from a review of current knowledge. The research The literature search is followed by a survey of current and development needs identified through the work and sur- practice. Most of the survey participants were from state vey responses documented here are provided at the end of DOTs and their consultants. Selected researchers also partici- this study.