NEESR-II: Towards Developing an Engineering Procedure for Evaluating Building Performance on Softened Ground

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ABSTRACTRecent earthquakes have provided countless examples of the damaging effects of liquefaction. Ob-servations of building performance included building punching, bearing failure, and lateral shifting of buildings. Although much has been learned, significant uncertainty exists in the evaluation of these case histories, which has hampered back-analyses and the development of improved analytical methods. Key sources of uncertainty are the characteristics of the ground motions, lack of documentation of the se-quence of liquefaction, ground failure and building performance, and the inherent variability of ground conditions. Centrifuge testing, where the input motion, ground conditions, and ground/structural response can be carefully tracked, followed by advanced back-analyses of these models are warranted to advance the profession's understanding of ground failure and its resulting impact on structures. Through the proposed research, a series of well documented model "case histories" of building per-formance at sites undergoing severe and moderate ground failure are developed by using the UC Davis large centrifuge. Soil-foundation-structure-interaction is a key NEESR research priority that is addressed through this centrifuge testing of different building systems on various ground conditions. It is of para-mount importance, because ground failure appeared to be caused in part due to the dynamic response of the overlying structures, and building damage was clearly linked to ground response. Additionally, the poorly understood seismic response of silt will be explored in some of these ex-periments, as opposed to continuing efforts over the past decade to test just clean uniform sands in centri-fuge experiments. The relative importance of the thickness of the liquefiable soil layer and its density will be explored, while examining the interacting effects of different buildings and foundation conditions on the response of the liquefiable soil and building performance. As part of this project, a focused "Student Earthquake Engineering Symposium" (SEES) that reaches out to high school students, with an emphasis on minority students, is created. The outreach program utilizes a strategy that has worked well for the sciences for the last 43 years in Northern California. Many high school students already participate in their schools' science fairs. By channeling their interests and their teachers' interests on the challenging problems involved in earthquake hazard mitigation, we can excite the next generation of earthquake engineers and scientists. As emphasized in NRC(2003), "NEES should be used to move past the prediction of free field liquefaction to the next level, which would be the ability to predict deformations ... for structures ... by considering the timing, sequence, and location of soil strength loss in the vicinity of the constructed feature." The first step towards this goal is the generation of well documented "case histories" of building response on liquefied ground, which these carefully performed centrifuge model tests will achieve. These illustrative model studies will also offer emergency managers and teachers a dramatic alternative to currently existing static pictures of tilted buildings. Video clips augmented with animated measured response data will allow the seismic hazard of liquefaction and its effect on buildings to come alive.