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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.