Failure modes of piping under seismic loads which have both load and displacement controlled characteristics

Abstract

Evaluation for BDBE requires best estimation based on realistic failure phenomena. However failure modes of piping under seismic loads are not clear, because seismic load is alternate one and has both load- and displacement-controlled characteristics. For design and seismic PRA (probabilistic risk assessment), the failure mode is assumed to be collapse under PGA (peak ground acceleration). However, this conservative assumption is not appropriate for risk assessment with best estimation and uncertainty. The objective of this study is the clarification of realistic failure modes of piping under seismic loads.Previous experimental studies showed ratcheting deformation and fatigue as the most probable piping failure modes. Collapse cases were seldom. Authors pay attention to the analogy of failure modes between thermal alternate loads and seismic alternate loads. Ratcheting and collapse under thermal loads can be evaluated by the famous Bree diagram, where main parameters are constant pressure and alternate thermal loads. The Bree diagram was extensively applied to piping ratchet and collapse with such parameters as constant gravity force and alternate seismic loads. The frequency of seismic loads was also introduced as an additional parameter.As the results of experimental and analytical studies, ratcheting and collapse under seismic loads can be evaluated by the Bree-like diagram, where main parameters are constant gravity force, dynamic loads and their frequencies. Furthermore, frequency dependent characteristics of seismic loads were understood. They act like load controlled when frequency is lower than natural frequency. In the case of higher frequency than natural frequency, they become closer to displacement controlled. When higher than twice of natural frequency, they are less damageable than displacement-controlled.