Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 30
20 The survey also asked the respondents about the software to achieve statistically significant results. Many respondents verification and validation procedures (see Table B15). Their were also aware of the need for special considerations for answers ranged from none to extensive evaluation to com- ground motions, including directivity, and velocity pulses to parisons between software. A number of users conducted capture near-fault effects. equivalent-linear analyses and compared them with non- linear analyses. Others performed analyses using multiple The respondents had varying responses to the question of programs and looked for consistency. Verification and vali- how they handle uncertainty in the selected ground motions, dation are complex and challenging tasks. Although exten- but one common theme was that the motions were allowed sive studies are available for 1-D, there is a dearth of 2-D and to vary to some degree about the target response spectrum. 3-D verification studies. Some said that the degree of deviation was within 50% for the spectrum, whereas others stated that it was within 84 percentile of the ground motion. Clearly, the responses do SEISMIC HAZARD AND MOTION INPUT REQUIRED FOR not reflect a consensus by the respondents. SITE RESPONSE ANALYSIS The respondents generally agreed that the topic of input The seismic ground motion at a site is commonly defined ground motions needs more work and guidance is needed in as a target response spectrum corresponding to a given this area. A few of the respondents were aware of the signifi- desired hazard level. Such a spectrum can be derived using cance of using Conditional Mean Spectrum (CMS) in gen- deterministic or probabilistic seismic hazard analysis. erating input ground motion. Respondents also indicated the More recently, conditional mean spectrum (CMS) has also need for guidance on generation of ground motions for deep been used (Baker and Cornell 2006; Baker 2011). The tar- basins where rock depth is great. A respondent indicated that get response spectrum is developed at an equivalent rock the development of the PEER ground motion toolbox (http:// outcrop and then site factors are used to include the effect peer.berkeley.edu/peer_ground_motion_database/) is an of local conditions on the ground motions. More recently, important step forward. with the use of NGA attenuation relationships, the site factors are embedded in the equations through the use of Vs30 (average shear wave velocity of top 30 meters) as an SOIL PROFILE INPUT INFORMATION REQUIRED FOR input parameter (Abrahamson and Silva 2008; Boore and SITE RESPONSE ANALYSIS Atkinson 2008; Campbell and Bozorgnia 2008; Chiou and Youngs 2008; Idriss 2008; and USGS 2008). However, the Site response analyses require information on the soil pro- user needs to be familiar with important limitations when file, including stratigraphy, shear wave velocity, location of using these equations. the water table, and dynamic soil properties. A series of questions were posed related to the seismic Most of the survey respondents indicated that they directly hazard input used by respondents in the site response analy- measure shear wave velocity--an important parameter. This sis. Many respondents rely on U.S. Geologic Survey maps reflects an important positive development in the profession. and web tools or their adaptations in various code provisions Nevertheless, a substantial number of respondents reported (AASHTO, ASCE7-05, IBC). However, a significant num- that they obtain the shear wave velocity (Vs) profile from cor- ber of respondents use computer software (EZ-Frisk, RISK- relations of penetration resistance. Significant uncertainty is Engineering 2009, Haz-38) or spreadsheets to program associated with these correlations, and studies are needed ground motion prediction equations (GMPE). on their validity and alternatives for direct measurement of shear wave velocity. Selection of spectrally compatible ground motion time histories is the next important step in developing input for A number of respondents said that they performed labora- site response analysis. Many of the respondents selected nat- tory tests on soil samples to develop dynamic soil properties; ural time histories; significantly fewer used synthetic ground however, more respondents indicated that they used modulus motions. Many of the respondents used rigorous spectral reduction and damping curves available in the literature. In matching procedures. RspMatch is by far the most widely addition to the Darendeli, Vucetic-Dobry, and Seed-Idriss used program for development of these motions. curves, the respondents also used EPRI curves. A number of respondents use specialized custom-developed curves. Although a number of respondents indicated that they used up to three ground motions in the spectral matching The respondents were very much aware of the need to process, many said that they use seven pairs (two horizontal account for uncertainty in soil properties, but there was no directions) for site response analyses. A few indicated that consensus on the best approach. Several use upper, lower they use more. For some projects, respondents said that they bound soil profile approaches, and a few use systematic needed to use a greater number of motions (say, 15 to 20) randomization approaches (Toro and Silva 2001; Romero