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DOE-STD-1020-2002
to be appropriate because facilities designed by these criteria are intended to reach strains
beyond yield level if subjected to the design/evaluation level earthquake ground motion, and
such damping levels are consistent with other seismic analysis provisions based on Reference
C-5.
The Response Level 1 and 2 damping values given in Chapter 2 are to be used to
evaluate in-structure response spectra or displacements to be used in seismic interaction
evaluations. In these cases, it is important to use damping which is consistent with stress levels
reached in the majority of the lateral force resisting system so as to not be unconservative in the
evaluation of input to structure-supported components or input for interaction considerations.
Even though seismic design is performed in accordance with these criteria which permit limited
inelastic behavior, actual stress levels in structures may be relatively low due to unintentional
conservatism introduced during the design process or because the design may be governed by
loads other than earthquake loads.
C.4.4.2 Inelastic Behavior
Energy absorption in the inelastic range of response of structures and equipment to
earthquake motions can be very significant. Figure C-7 shows that large hysteretic energy
absorption can occur even for structural systems with relatively low ductility such as concrete
shear walls or steel braced frames. Generally, an accurate determination of inelastic behavior
necessitates dynamic nonlinear analyses performed on a time-history time step integration basis.
However, there are simplified methods to approximate nonlinear structural response based on
elastic response spectrum analyses through the use of either spectral reduction factors or inelastic
energy absorption factors. Spectral reduction factors and inelastic energy absorption factors
permit structural response to exceed yield stress levels a limited amount as a means to account
for energy absorption in the inelastic range. Based on observations during past earthquakes and
considerable dynamic test data, it is known that structures can undergo limited inelastic
deformations without unacceptable damage when subjected to transient earthquake ground
motion. Simple linear analytical methods approximating inelastic behavior using spectral
reduction factors and inelastic energy absorption factors are briefly described below.
1.
Spectral reduction factors - Structural response is determined from a response
spectrum dynamic analysis. The spectral reduction factors are used to deamplify the
elastic acceleration response spectrum producing an inelastic acceleration response
spectrum which is used in the analysis. The resulting member forces are combined
with concurrent non-seismic member forces and compared to ultimate/limit-state level
stresses to determine structural adequacy.
2.
Inelastic energy absorption factors - Structural response is determined from either
response spectra or time history dynamic analyses with the input excitation consistent
with the elastic response spectra. The resulting elastically computed member forces
are reduced by member specified inelastic energy absorption factors to give the
inelastic demand. The inelastic demand is combined with concurrent non-seismic
C-29


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