Effects of Ground Motions on Seismic Response of Structures

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Some of the lessons from recent significant earthquakes have shown the need for further research on design areas that have not been studied in detail. The subduction earthquakes in Sumatra, Indonesia (Mw 9.1) in 2004, Maule, Chile (Mw 8.8) in 2010, and Tohoku, Japan (Mw 9.0) in 2011 are some of the largest earthquakes ever recorded. The Cascadia subduction zone in Western Canada and the Pacific Northwest of the U.S. has the potential of generating very large earthquakes, as well. A recent study commissioned by the Insurance Bureau of Canada (Air Worldwide, 2013) estimate that the economic losses from a magnitude 9 in the region could be as high at 75 billion dollars, of which about 49 billion dollars would be due to ground shaking and the rest due to tsunami, fire-following, and liquefaction and landslides effects. So there is now strong motivation to reduce economic losses due to ground shaking. For several years, the applicant and his research team have been studying how the ground motions from different types of earthquakes affect the seismic response of structures. These studies included analytical studies, shake table and cyclic tests, and field testing and monitoring of instrumented structures. The proposed research will address three topics of significant relevance to the understanding of structural response during earthquakes. This, in turn, will contribute to the reduction of seismic risk in Canada: 1) Soil-foundation-structure interaction (SFSI) in buildings during earthquakes is characterized by several aspects including the variation between the free-field and foundation motions. The analytical studies conducted so far show that amplification occurs when the system period is close to the dominant period of the input motion. When these two periods are far apart, the motions at the slab are reduced with respect to the field. The basis of code recommendation should be re-examined in light of the fact that it appears in contradiction to the findings from field data and analytical results. 2) Pulse-like near fault ground motions can occur on sites in close proximity to a causative fault. In such cases consideration of flexibility of the foundation system is very important. The purpose of this research is to study the nonlinear response of structures to pulse-like near ground fault. 3) Subduction ground motions are important for Western Canada and their effects on the response of structures needs to be investigated in detail. This study is intended to provide a deep insight into subduction earthquakes by carrying out a comprehensive analysis of ground motions recorded during this type of event. The effects of duration and long period components of the ground motions on the linear and nonlinear response of structures will be investigated. To date, most of the research quantifying building and bridge response and safety has focused on crustal, California-type ground motions. The distinct geologic characteristics of subduction earthquake sources and attenuation, cause ground motions from subduction earthquakes to have different duration and frequency content when compared to crustal ground motions. The goal of this research is to quantify the impact of these ground motions on the response of buildings and bridges and to develop information that can be used by practicing engineers to design, or retrofit, structures that may be subjected to this type of ground shaking. Recommendations for incorporation of subduction ground motion effects on the National Building Code of Canada, and the Canadian Highway and Bridge Design Code will be developed.