Rocking and Sliding of Unanchored Bodies Subjected To Seismic Load According To Conventional and Nuclear Rules

Abstract eng:
In nuclear and nonnuclear facilities unanchored components like storage casks for spent fuel may cause unacceptable damage in case of an earthquake. It must be proven that the unanchored body will not tip over and/or slide impermissible distances. Rocking and sliding are highly nonlinear phenomena which cannot be completely described analytically. Therefore, the rocking and sliding motion of a rigid body under various conditions will be analysed analytically and numerically by numerical integration of the equations of motion and by using the Finite Element Method (implicit time integration). It is shown that a finite element program with an implicit time integration method can solve the impact problem. By using the Finite Element Method it is possible to investigate the influence of the properties of the base, the contact conditions and slightly deviations from the ideal geometry on the motion of the body and especially on the coefficient of restitution. Additionally, the influence of occasionally used anti-slip pads below the rocking body is investigated. In practice most unanchored bodies are not slender and therefore this paper studies a compact body with an aspect ratio of 2. One focal topic is the discussion of the coefficient of restitution. It is shown that more realistic finite element models, which take into account the above mentioned influences, lead to higher coefficients than those derived analytically from simplified models. The agreement between results from finite element analyses and the numerical integration of the equations of motion is very good if the coefficients of restitution are based on best estimate values from finite element analyses. The dynamic investigations consider an excitation by sine, triangle or rectangle vibrations and time histories from earthquake ground accelerations. It is remarkable, that in the some cases of the amplitudes will increase significantly over time until overturning. The criteria and methods given in European (German) and US standards with regard to overturning and impermissible sliding are presented. The safety margins against overturning and impermissible sliding will be discussed.

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