Failure modes of piping systems under seismic motions were discussed for several decades if the fatigue failure is dominant or there is some possibility that the plastic collapse could occur. A handful of ratchet-buckling failure observed in Pipe Fittings Dynamic Reliability Program by EPRI was frequently taken up as the evidence of the plastic collapse, and inclusion of seismic response on structures into the Primary stress evaluation for piping systems in the code evaluation was considered to be conventionally justified. Although prevention of the plastic collapse type failure is the purpose of imposing the Primary stress evaluation, the other experimental tests conducted in several countries for decades were unable to represent the plastic collapse of piping components exposed to seismic loading and the discussion was abandoned for a while. However, the drastically increased design seismic motions for nuclear power plants due to several huge earthquake occurred in Japan reminded us of exploring the fact of the plastic collapse and the necessity of the Primary stress evaluation.

The load classification concept proposed by the authors introduces 3 conceptual force terms from the equation of motion to clarify the seismic loading from the aspect of the correlation of the said force terms. Based on the finding from the concept that the input force amplitude is to be evaluated for Primary stress, the gross-plastic deformation on a single cantilever with elastic-plastic analyses using multiple of single-cycle sinusoidal forcing functions was compared with the input force term. When the plastic collapse is defined as a gross-plastic deformation, the level of plastic collapse was found to be possibly anticipated with a static force evaluation that can be substitute for the conventional Primary stress evaluation with the dynamic response analysis.

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