IEEE 344:2004 pdf free download – IEEE Recommended Practice for Seismic Qualification of Class 1E Equipment for Nuclear Power Generating Stations
6.1 Introduction
Damping is the generic name ascribed to the numerous energy dissipation mechanisms in a system. Inpractice, damping depends on many parameters， such as the structural system,mode of vibration,strain,normal force, velocity, materials, joint, and slippage. In linear vibration theory, the simplifying assumptionis made that damping is purely viscous or proportional to the relative velocity of moving parts. Therefore,when a value of damping is associated with a practical system, it is usually assumed to be equivalent toviscous or linear. This is a convenient simplification relating real-world hardware bchavior,which may benonlinear to some degree, with theoretical concepts that normally utilize linear methods of analysis.
For equipment composed of an assembly of components,there is usually no single value of damping.Damping is associated with every part of the equipment，ranging from bolted or welded construction touniform material. The value of damping may vary from place to place,depending on numerous factors.Therefore, when a value of damping is ascribed to equipment, it is common to give a range of typical values.Since each mode of vibration of a structure can, and most often does, have a different value of damping, auseful practice in analysis is to associate a value of damping to each mode of vibration of the equipment thatis in the frequency range of interest.
6.2 Measurement of damping
Linear vibration theory indicates that there are numerous methods for measuring damping.Considerablecare must be exercised in making the correlation between an idealized model and a practical system. Forexample, it is rarely possible to locate precise points in equipment that have exact correspondence with thelumped mass elements in a model. Some methods of calculating modal damping, such as the Q method, relypurely on SDOF assumptions.
Damping calculated directly from the maximum response at the resonance peak measured at any point in theequipment and the magnitude of the sine-sweep input excitation is not generally acceptable since theresponse of points in equipment is usually determined by the mode shape vector and the participation factorfor each vibration mode. The following methods for evaluating damping are commonly used but otherjustifiable methods may be used. They assume that a single mode of vibration can be excited in theequipment and that motion transducers are mounted at positions other than at a point of zero motion. In allcases,care should be exercised to determine whether damping nonlinearity with response amplitude issignificant.
6.3.1 Application of damping in analysis
In analysis,a mathematical model is made of the equipment so as to predict the response to the seismicmotion. The value of damping used in this model should correspond to the actual energy dissipation in theequipment to enable the response to be accurately predicted.An alternative approach is to use a conservativevalue of linear damping to obtain a conservative estimate of response. In any case, there is a need to knowthe ranges of damping for the specific equipment and the nature of nonlincarities and their effect on theresponse.Appropriate values of damping may be obtained from tests or other justifiable sources.
lt is reasonable to state that actual damping is nonlinear by nature. In most equipment, it is a function ofresponse amplitude owing to such factors as internal friction within material, or at connections betweencomponents,or Coulomb-type sliding friction.For analytical purposes, these energy dissipation dampingmechanisms may often be treated in terms of linear damping approximations if proper consideration is givento the fact that these approximations vary，sometimes significantly,with increasing response.As anexample, the use of low-impedance testing to determine damping must be exercised with caution since atstrong motion shaking, indicative of significant earthquakes, the aforementioned factors may cause the realdamping to be quite different and higher than that measured at low levels.