Occurrence of plastic collapse under ratcheting due to gravity and seismic loading

The most dominant failure mode of piping components under seismic loading is fatigue failure with ratcheting. While it was confirmed via the experimental tests in the past, the Primary stress limit is applied to seismic loading to prevent plastic collapse. The plastic collapse due to seismic loading was first confirmed at Pipe-Fitting Dynamic Reliability Program (PFDRP) conducted by EPRI in 1980s. But, the mechanism and occurrence condition of this failure has not been clarified yet. In this research, a composite failure mode of the ratchet-induced collapse, which represents the behavior of the plastic collapse failure induced by ratchet deformation, is introduced. The transition of the failure modes along ratcheting is explained with the seismic failure mode map which identifies the occurrence condition of ratcheting and first-excursion failure, and the X-Y trajectory, which explains the excitation condition of structures under ratcheting, is introduced to project the transition. With the X-Y trajectory and the occurrence condition of the plastic collapse, this study conceptually proposes the prediction approach of the ratchet-induced collapse without the simulation analyses.