Nuclear Power Plants (NPPs) are designed to withstand postulated earthquake events. Seismic loads induced by a seismic event on essential structures such as a piping are typically evaluated with two different load categories: inertia loads and deformation loads which are also called as load-controlled loads and displacement-controlled loads, respectively. The inertia force is still believed to govern failure mode of piping components as almost of all design codes for NPPs give weight at qualifying the inertia loads as primary stress components for piping. The first paper PVP2015-45287 [22] anticipated a structure excited by a lower frequency than the natural frequency which is considered as an excitation at Rigid condition could result in plastic collapse because of a minimal recovering force counteracting the deformation. However, the second paper PVP2016-63363 [23] which applied an elastic-plastic analysis showed the different conclusion that a single mass cantilever structure at Rigid condition finally behaved as Soft condition which was anticipated as a stable condition in the paper along the progress of the plastic deformation on the structure. This result implies that the current design codes which assume elastic-behavior may include significant over-conservatism to ensure the adequacy of structures under seismic condition. As many experimental results are showing, very large seismic loads which excessively exceed the design limit barely caused failure in piping components. This paper investigates the relationship between inertia forces and element forces on a single mass cantilever model applying a bi-linear material property against several random time-history loads which are adjusted to represent the said excitation conditions. This paper also clarifies the correlation between deformations due to the excitations and the inertia/element forces observed on the models. This study takes over the previous researches published as PVP2015-45287 and PVP2016-63363.

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