34 Perspective on Cyclic, Impact, and Dynamic Loads

Chapter 34, authored by John D. Stevenson, deals with perspectives on cyclic, impact, and impulse dynamic loads. John notes that dynamic loads applied to the design of mechanical systems and components are of three basic types: cyclic, impulse, and impact. In addition, there is a fourth potential cyclic-type load in the vibratory motion category. Although vibratory motion is not usually considered in the original design basis, it may be observed during steady-state or transient operations to cause premature fatigue or ratchet failure of metal components. The original Section III definition of a plant's operating life includes design-basis normal, abnormal, emergency, and faulted plant- or system-operating conditions, as defined in the Design Specification. John notes that these operating conditions should not be confused with Service Levels A, B, C, and D currently defined in the Code for design purposes. It is possible to have different Service Level design conditions for the same operating condition, depending on the required response of a component.

John covers in-depth Nonmandatory Appendix N (Dynamic Analysis Methods) of Section III. For completeness, he covers other types of dynamic loads not addressed explicitly by Appendix N that the designer of pressure-retaining nuclear components must consider. He also discusses the ASCE Standard used for defining earthquake motions to a building foundation and for supporting the mechanical system or component; these two references deal primarily with earthquake cyclic—type dynamic loads. In addition, commentary is provided on the guidelines used for dynamic impulse and impact loadings provided in the ANS, ASME Appendix B, and B31.1 Code Standards. A discussion is provided about the ASME Operation and Maintenance (O&M) Standard, used for determining the effect of operational vibratory motion independent of the cause of vibration.