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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
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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.