地震応答を生じる構造物の塑性崩壊に関する考え方

抄録

Design codes for nuclear power plants usually require qualifying structures under seismic loading for Primary Stress limit which is meant to prevent plastic collapse. However, several experimental researches conducted by EPRI, NUPEC, and NIED in 1980s through early 1990s revealed that the failure mode of piping structures under excessive seismic loading is actually fatigue failure with ratchet deformation. No observation of plastic collapse on the piping specimens left a doubt that the actual failure mode could different from what the design codes specify with Primary Stress limit. The characteristic of dynamic response of structure was considered to be one of the factors which altered the anticipated failure mode, as Primary Stress limit was established based off the equilibrium of a static force on structures.

In this paper, the characteristic of dynamic response in inelastic region on a simplified structure is investigated based on the correlation of the natural frequency and dynamic input loads using analytical solutions by changing the plastic properties such as yield stresses and secondary slopes in bi-linear approximation for the material so as to clarify the possibility of plastic collapse which the Primary Stress limit means to prevent beyond elastic region. Three conceptual conditions, Rigid, Resonant, and Soft, are defined in accordance with the correlation in frequency and the analyses found the yield stresses and the secondary slopes slightly affect the dynamic responses of the structure, the essential trend that structures at Soft or Resonant condition against input loads settle down a stable condition as the plastic deformation progresses and Rigid condition may end up with falling into the unstable condition which is plastic collapse. This trend implies that structures at the Rigid condition may need to be qualified by Primary Stress limit but it may not be required for any other conditions to prove the structural integrity.