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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