Experimental Dynamic Response of Free-Standing Bodies Under Near Field and Far Field Ground Motions

Abstract eng:
This study is a part of a research project evaluating seismic response of free-standing dry storage casks under long-term seismic events. Free-standing structures are highly susceptible to rocking and sliding during seismic events. Characterization of motion of three dimensional free-standing structures is a highly nonlinear problem, especially when rocking and sliding motion occur simultaneously. Response of such free-standing bodies depend on many factors like aspect ratios (radius to centroidal height ratio, r/hcg), coefficient of friction and ground motion characteristics. The main purpose of this article is to present the effect of ground motions with a single large pulse and those with multiple smaller pulses (NFGMs and FFGMs, respectively) on the dynamic response of free-standing bodies. For this purpose, experiments with Near Field and Far Field ground motions were conducted using a six degree-of-freedom (6DOF) shake table at the University of Nevada, Reno. Four scaled free-standing bodies with four different aspect ratio: 0.39, 0.43, 0.55 and 0.62; were studied in the experimental tests. However, results for free-standing bodies of three aspect ratios (0.39, 0.43, and 0.62) are discussed in this article. Ground motion records used in the study were spectrally matched to the seismic hazard level or spectral accelerations developed for earthquake events of 10,000 and 30,000 year return periods. The same system and ground motion cases were used several times to investigate whether the response of free-standing bodies is repeatable under scenarios. The experimental results show that the ground motions containing multiple pulses (FFGMs) produces more lateral displacements and rocking angle compared to NFGMs. The results also show that the casks’ response lacks repeatability under random ground motions. Finally, three chaotic analysis methods are used to investigate the reason behind such lack of repeatability.

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