Stress classification approach for failure modes of piping elbows under seismic loads

Takito, Kiyotaka ; Furuya, Osamu*; Nakamura, Izumi*; Okuda, Yukihiko  ; Yamazaki, Tatsuhiro

For seismic design of piping system, stress limits are set to prevent different types of failure through stress classification. The primary stress limit prevents ductile fracture and plastic collapse, and the secondary stress limit prevents ratchet deformation. Furthermore, the primary, secondary and peak stress limits are employed to prevent fatigue. Fatigue is often the dominant failure mode for piping subjected to seismic loads, so these limits are evaluated when determining the allowable state of the system. Nevertheless, further research on piping failure mechanisms i.e., fatigue, ratcheting, and plastic collapse, under dynamic loads that exceed the design basis event (BDBE) is still required. Consequently, the authors investigated elbow; piping failure modes by means of shaking table tests. The test results showed that failure modes depended on the dead weight and the level of input acceleration. An evaluation method was then developed to interpret the test data and to quantify the relationship between load combinations and failure modes. The method adopts a stress classification approach that incorporates an elastic follow up factor, q. The proposed method was applied to elbows that exhibited ratcheting deformation and plastic collapse during the shaking table tests. By using the stress classification approach, the method successfully reproduced the observed failure modes. This approach can therefore provide valuable insight for improving seismic fragility assessments of piping systems.