Date of Award
Doctor of Philosophy (PhD)
This thesis aims to assess the influence of long duration shaking during earthquakes on structural collapse. It is well accepted that ground motion intensity and its frequency content affects structural seismic response; however, the influence of duration of ground shaking on structural damage remains a topic of debate. According to past research, duration of ground motion is found to influence structural damage when damage measures based on cumulative energy are used, and when the damage measures are based on maximum response, duration is generally found to be insignificant.
In the first part of the thesis, building models are subjected to ground motions with varying duration in order to investigate the influence of duration on the intensity of ground motion at which collapse occurs. Results indicate that the ground motion characteristics of duration and structural characteristics of fundamental period and ductility capacity are significant predictors of structural collapse capacity. Long duration records are associated with larger number of cycles of loading and impart more energy to the structure. As a result, the probability of collapse for a structure is found to be higher on being subjected to long duration ground motion having same intensity as a short duration ground motions. Therefore, the study shows that it is important to consider the expected duration of ground motions at the site for seismic design of structures in addition to ground motion's intensity and frequency, which are already explicitly accounted for in building codes.
In the second part of the thesis, the collapse capacity of structures is assessed for seismic events in which the structure is subjected to long duration shaking: (1) subduction earthquakes and (2) mainshock-aftershock earthquake sequences. Ground motions from subduction zone earthquakes are characterized by long duration of shaking and a have distinct frequency content. The study of subduction earthquakes focuses on the imminent danger of large magnitude subduction earthquake in the Cascadia subduction zone, which lies off the coast of the Pacific Northwest region of the U.S In order to assess the influence of subduction ground motions on collapse of reinforced concrete buildings, nonlinear dynamic analysis utilizing crustal and subduction ground motions is carried out on a portfolio of buildings in the cities of Seattle and Portland. The results indicate significantly higher collapse risk of buildings on being subjected to long duration subduction earthquake shaking as compared to crustal earthquakes. This higher risk affects all structures analyses, although the impacts are more significant for more ductile structures.
Structures that are exposed to mainshock-aftershock sequences also experience longer duration shaking as compared to an individual mainshock event. It is important to understand the aftershock fragility of the buildings because of the possibility of further damage in aftershock for a building already damaged in mainshock. This thesis assessed the aftershock damage and collapse risk of mainshock damaged modern reinforced concrete buildings in California. The study shows that low to moderate levels of mainshock damage do not significantly alter a building's capacity to withstand subsequent shaking, but that more substantial damage during mainshock can significantly reduce its collapse resistance during aftershock. The findings also suggest that residual interstory and roof drifts are strong predictors of reduced ability to withstand subsequent shaking and should remain a substantial part of post-earthquake visual safety assessments, like those described in ATC-20.
Raghunandan, Meera, "Influence of Long Duration Ground Shaking on Collapse of Reinforced Concrete Structures" (2013). Civil Engineering Graduate Theses & Dissertations. 467.