Quantification of Basement Geometry Effects on the Free Surface Ground Motion

Abstract

The dissertation titled "Quantification of the basement focusing effect on free surface ground motion’’.It means studying how the characteristics of the basement, such as its depth and material properties, affect the vibrations felt on the ground surface. The aim is to understand how these basement features impact the amplification or reduction of seismic waves, which in turn affect how buildings and structures respond during earthquakes. Aims to investigate the basement focusing effect, a significant phenomenon in engineering. This effect refers to the concentration or amplification of seismic energy within a building's foundation due to the interaction between the underlying soil layers and surface waves generated by earthquakes. Understanding this phenomenon is crucial for engineers to assess the potential impact of ground motion on structures, identify vulnerable areas, and design resilient buildings to mitigate damage risks during earthquakes. The primary focus of this thesis is to determine the causes of selective damage observed in the Nisqually earthquake in western Seattle, Washington. Specifically, the research aims to understand the physics behind the damage specifically affecting chimneys and analyze how the extent of damage varies with changes in the physical parameters of the basement. These critical questions have been thoroughly addressed and answered through the comprehensive research conducted in this dissertation. The study employs numerical modeling using the finite difference method, utilizing a code developed by Prof. J.P. Narayan and their team. Both bounded and unbounded media are considered, taking into account variations in basement shape ratio (SRM1, SRM2, and SRM3) and impedance contrast (ICM1, ICM2, and ICM3). The analysis also includes the examination of four different building heights (H85, H27.5, H20, and H12.5), evaluating their responses in the time and frequency domains within the specified scenarios. The obtained inferences strongly support the explanation for the occurrence of selective damage. In the last chapter, the amplification factors of building response were calculated at different locations above the free surface. These amplification factors were then compared to the velocity spectrum values derived from code IS 1893 Part 1, which specifies the extent of damage to a building located in the basement when subjected to a DBE (Design Basis Earthquake) and explains the reason behind the selective damage that happened in Nisqually earthquake in western Seattle.