Inelastic Behavior

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.

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.

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